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Phased Array inspection system applied to complex geometry Carbon Fibre Reinforced Polymer parts André Filipe da Conceição Cereja Thesis to obtain the Master of Science Degree in Mechanical Engineering Supervisor: Prof. Maria Luísa Coutinho Gomes de Almeida Examination Committee Chairperson: Prof. Rui Manuel dos Santos Oliveira Baptista Supervisor: Prof. Maria Luísa Coutinho Gomes de Almeida Members of the Committee: Dr. Nuno Miguel Carvalho Pedrosa Prof. Telmo Jorge Gomes dos Santos May 2015
Transcript of Phased Array inspection system applied to complex geometry ... · (Este resumo está escrito de...
Phased Array inspection system applied to
complex geometry Carbon Fibre Reinforced Polymer parts
André Filipe da Conceição Cereja
Thesis to obtain the Master of Science Degree in
Mechanical Engineering
Supervisor: Prof. Maria Luísa Coutinho Gomes de Almeida
Examination Committee
Chairperson: Prof. Rui Manuel dos Santos Oliveira Baptista
Supervisor: Prof. Maria Luísa Coutinho Gomes de Almeida
Members of the Committee: Dr. Nuno Miguel Carvalho Pedrosa
Prof. Telmo Jorge Gomes dos Santos
May 2015
“Success consists of going from failure to failure without loss of enthusiasm.”
Winston Churchill
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Acknowledgments
First and foremost, a word of gratitude goes to Professor Luísa Coutinho, my scientific advisor, who has
challenged me from the very first class I had with her, to become a better engineer, and a better person.
The opportunity that I was given to develop this work must be mentioned, as well as the motivation,
counselling and revision efforts she placed into this MSc thesis.
Secondly, an acknowledgment to Dr. Nuno Pedrosa, who welcomed me with open arms in ISQ.
Dr. Nuno Pedrosa gave me the tools that enabled me to develop this work: from him I learned about
various sorts of subjects I had no previous contact with, during the months I spent in ISQ. Whenever I
had a doubt, or when I was unsure on how to proceed when facing a challenging situation, he always
had a guiding word to give.
To the rest of the staff in ISQ, namely the engineers Daniel Leitão, Liliana Santos and José Pedro
Sousa, a sincere word of appreciation for all the guidance and support they gave when I was developing
this work. Their technical knowledge, which they were so kind to share during my stay in ISQ proved
vital when I was faced with difficulties along the way.
To the company EXTENDE, namely Caroline Quintanilha, for being so kind as to supply me with a CIVA
license, so that I could develop my work.
To all the friends I met during my stay in ISQ, specifically João Amorim, João Borges, João Nabais,
Gonçalo Silva and Paulo Meyrelles, for all the help and cheerful moments.
To all the friends I was fortunate enough to make during my years in IST, both those I made during my
Mechanical Engineering studies and during my enrolment in Projecto FST Novabase, namely the
FST 04e team, thank you for sharing the most important moments in my life so far, that contributed to
making me the man I am today.
Last but definitely not least: there are not enough words to thank my parents, José Manuel and Maria
Valentina and my brother, João, for all the unconditional love they always gave me, together with the
confidence they always had in me. Only we and God know how hard they worked so that I could be
where I am today. Finally, my last word goes to my special someone, my girlfriend Joana Prata, for
reading this document and for always being there when I needed the most.
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Abstract
Following the growing use of composite materials in the aerospace industry, particularly carbon fibre
reinforced polymer (CFRP), arises the need to develop procedures to guarantee the monitoring and
evaluation of CFRP components. Moreover, the cost associated with destructive testing should be
eliminated, using methods that do not harm the parts’ service life.
Ultrasonic testing methods, namely the advanced phased array technique, can be a useful tool for
dealing with the challenges engineers face, when having to perform evaluation procedures for
components subject to fatigue and/or impacts.
A simulation software, CIVA, is used before the physical inspection of both planar and complex geometry
parts is performed. With CIVA, the phased array probes are selected, together with the inspection
parameters, e.g. probe aperture and focal laws. A study is performed on the methods CIVA employs to
calculate the acoustic field in the parts, as well as the homogenization algorithms behind the handling
of the anisotropy of CFRP. The difficulties expected when performing the physical inspection, namely
attenuation, are predicted. For the complex geometries, an evaluation is done on the potential to inspect
these components.
Afterwards, physical inspections are performed. Three components are analysed: a testing specimen
with embedded defects, a component subjected to fatigue loading, and a reinforcing omega-stringer.
The defects (delaminations) in the test specimen are identified and characterized, in size and depth.
Lacks of resin and debondings are discovered in the fatigue specimen. The stringer, impacted with
known energies, is analysed and the resulting flaws identified and measured.
Keywords
Carbon fibre reinforced polymer
Non-destructive testing
Phased array ultrasonic testing
Self adaptive algorithm
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Resumo
Acompanhando o uso crescente de materiais compósitos na indústria aeroespacial, particularmente
fibra de carbono (CFRP), surge a necessidade de desenvolver procedimentos que assegurem a
correcta monitorização e avaliação destes componentes. Ainda, o custo associado a ensaios
destrutivos deve ser eliminado, recorrendo a métodos que não prejudiquem a vida útil das peças.
Ensaios com ultrassons, nomeadamente a técnica avançada phased array, podem ser uma ferramenta
útil para lidar com os desafios que os engenheiros enfrentam, para avaliar componentes sujeitos a
fadiga e/ou impactos.
O programa de simulação CIVA é empregue antes da realização dos ensaios de peças de geometria
plana e complexa. Com o CIVA, seleccionam-se das sondas phased array, junto com a definição dos
parâmetros de inspecção, e.g. abertura da sonda e leis focais. Estudam-se os métodos empregues
pelo CIVA no cálculo do campo acústico, como o algoritmo de homogeneização responsável pelo
tratamento das características anisotrópicas do CFRP. As dificuldades esperadas aquando dos testes
físicos, nomeadamente a atenuação, são previstas. Para as peças complexas, é feita uma avaliação
do potencial de inspeccionar estes componentes.
Posteriormente são realizados os ensaios, a três componentes diferentes: um componente de teste
com defeitos inseridos no material, uma peça sujeita a fadiga, e uma longarina de reforço em ómega.
Os defeitos (delaminações) no componente de teste são identificados e caracterizados, em dimensão
e localização. No componente sujeito a fadiga são descobertas faltas de resina e descolagens. A
longarina, após sofrer impactos com energias conhecidas é analisada, identificando e medindo os seus
defeitos.
(Este resumo está escrito de acordo com o antigo acordo ortográfico.)
Palavras-chave
Fibra de carbono
Ensaios não destrutivos
Ensaios de ultrassons phased array
Algoritmo adaptativo
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Table of Contents
Acknowledgments .....................................................................................................................................i
Abstract.................................................................................................................................................... iii
Keywords ................................................................................................................................................. iii
Resumo ....................................................................................................................................................v
Palavras-chave .........................................................................................................................................v
Table of Contents .................................................................................................................................... vi
List of Figures ...........................................................................................................................................x
List of Tables .......................................................................................................................................... xii
List of Symbols and Acronyms .............................................................................................................. xiv
1 Introduction ...................................................................................................................................... 1
1.1 Scope ....................................................................................................................................... 1
1.2 Work contribution ..................................................................................................................... 2
1.3 Objectives ................................................................................................................................ 2
1.4 Document Structure ................................................................................................................. 2
2 State of The Art ............................................................................................................................... 4
2.1 Historical background .............................................................................................................. 4
2.1.1 Composite Materials in the Aerospace Industry .............................................................. 4
2.1.1.1 Manufacturing of CFRP ............................................................................................... 4
2.1.1.2 Applications of CFRP on the Aerospace Industry ....................................................... 5
2.1.2 History of non-destructive evaluation .............................................................................. 5
2.1.2.1 NDE: the beginning ..................................................................................................... 5
2.1.2.2 Importance of NDE ...................................................................................................... 6
2.2 Composite Fundamentals ........................................................................................................ 7
2.2.1 Materials used in PMCs ................................................................................................... 7
2.2.1.1 Matrix ........................................................................................................................... 7
2.2.1.2 Reinforcement ............................................................................................................. 8
2.2.2 Composite manufacturing processes ............................................................................ 10
2.2.3 Mechanical Properties of CFRP .................................................................................... 11
2.2.3.1 Anisotropic Constitutive Model .................................................................................. 11
2.2.3.2 Orthotropic Constitutive Model .................................................................................. 12
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2.2.3.3 Transversely Isotropic Constitutive Model ................................................................. 13
2.2.3.4 Rule of Mixtures ......................................................................................................... 14
2.2.4 Damage Mechanisms in Composite Materials .............................................................. 16
2.2.4.1 Fatigue damage ......................................................................................................... 16
2.2.4.2 Impact Damage ......................................................................................................... 16
2.2.4.3 Manufacturing defects ............................................................................................... 16
2.3 Ultrasonic non-destructive testing ......................................................................................... 17
2.3.1 Fundamentals of sound propagation ............................................................................. 17
2.3.1.1 Wave types ................................................................................................................ 17
2.3.1.2 Wave velocity ............................................................................................................. 20
2.3.1.3 Acoustic intensity and attenuation ............................................................................. 21
2.3.1.4 Frequency and defect detection ................................................................................ 23
2.3.1.5 Interface influence on sound propagation ................................................................. 23
2.3.2 Ultrasonic Inspection ..................................................................................................... 24
2.3.2.1 Transducer ................................................................................................................. 24
2.3.2.2 Beam characteristics ................................................................................................. 25
2.3.2.3 Inspection technique .................................................................................................. 25
2.3.2.4 UT data representation .............................................................................................. 26
2.3.3 Phased array advanced technique ................................................................................ 26
2.3.3.1 Probes ....................................................................................................................... 26
2.3.3.2 Beam control .............................................................................................................. 27
2.3.3.3 Adaptive Algorithm..................................................................................................... 28
2.3.3.4 Advantages and limitations of PAUT compared to other methods ............................ 29
2.4 Chapter summary .................................................................................................................. 31
3 Acoustic Model .............................................................................................................................. 32
3.1 CIVA software overview ........................................................................................................ 32
3.2 CFRP test specimens ............................................................................................................ 33
3.2.1 Calibration specimen ..................................................................................................... 33
3.2.2 Fatigue test panel .......................................................................................................... 35
3.3 Material properties ................................................................................................................. 37
3.4 PAUT probes ......................................................................................................................... 40
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3.4.1 Probe selection criteria .................................................................................................. 41
3.4.2 Characteristics of the selected probes .......................................................................... 41
3.5 Parameter case studies ......................................................................................................... 42
3.5.1 Signal characterization .................................................................................................. 42
3.5.2 Attenuation influence on the acoustic field .................................................................... 44
3.5.2.1 Attenuation case study 1: 5 MHz probe .................................................................... 45
3.5.2.2 Attenuation case study 2: 3,5 MHz probe ................................................................. 47
3.5.3 Aperture influence on the acoustic field ........................................................................ 48
3.5.4 Multi-point focusing technique ....................................................................................... 51
3.6 Amplitude differences between interface and backwall echo ................................................ 53
3.7 Defect characterization .......................................................................................................... 54
3.8 Complex geometry part inspection ........................................................................................ 56
3.9 Chapter summary .................................................................................................................. 57
4 Physical Inspection ........................................................................................................................ 58
4.1 Overview of the system ......................................................................................................... 58
4.1.1 Hardware ....................................................................................................................... 58
4.1.2 Software ......................................................................................................................... 60
4.2 General physical studies ....................................................................................................... 61
4.2.1 Gain ............................................................................................................................... 62
4.2.2 Maximum inspection velocity ......................................................................................... 62
4.2.3 Inspection step............................................................................................................... 63
4.2.4 Total inspection time ...................................................................................................... 64
4.3 Acoustic Impedance studies .................................................................................................. 65
4.4 Attenuation coefficient measurements .................................................................................. 66
4.5 SAUL algorithm testing .......................................................................................................... 67
4.6 Calibration specimen testing results ...................................................................................... 67
4.7 Fatigue panel and omega stringer testing results ................................................................. 71
4.7.1 Flat panel ....................................................................................................................... 71
4.7.2 Omega stringers ............................................................................................................ 74
4.8 Chapter summary .................................................................................................................. 76
5 Conclusions ................................................................................................................................... 77
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6 Future Work ................................................................................................................................... 79
References ............................................................................................................................................ 80
Appendix A: Maple code ........................................................................................................................ 85
A.1 Single Ply Composite ............................................................................................................ 85
A.2 Multiple ply composite ........................................................................................................... 86
Appendix B: Immersion tank dimensions .............................................................................................. 87
Appendix C: MultiX datasheet ............................................................................................................... 88
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List of Figures
Figure 1.1 – Materials used in the Airbus A350 XWB [3] ........................................................................ 1
Figure 2.1 – Carbon fibre manufacturing schematic [11] ........................................................................ 4
Figure 2.2 – Detail of a satellite antenna reflector, courtesy of INVENT GmbH [15] .............................. 5
Figure 2.3 - Investigators examine flight 232 engine [23] ....................................................................... 6
Figure 2.4 – Tensile test results for some fibres and an epoxy resin [29] ............................................... 8
Figure 2.5 – Nonwoven unidirectional plies stitched together [28] .......................................................... 9
Figure 2.6 – Airplane wing panel manufacturing stages ....................................................................... 10
Figure 2.7 – Unidirectional lamina coordinate system .......................................................................... 12
Figure 2.8 – Maximum and minimum values for a composite’s Young modulus calculated with Voigt
and Reuss models. [36] ......................................................................................................................... 15
Figure 2.9 – Common CFRP defects [40] ............................................................................................. 17
Figure 2.10 – Elastic material model representation [43] ...................................................................... 18
Figure 2.11 – Longitudinal wave [44] .................................................................................................... 19
Figure 2.12 – Transverse wave [44] ...................................................................................................... 19
Figure 2.13 – Rayleigh wave propagating through a solid [46] ............................................................. 19
Figure 2.14 – Lamb waves in plates: (a) symmetric, (b) asymmetric [24] ............................................. 20
Figure 2.15 – Sound intensity evolution from the source point [51] ...................................................... 21
Figure 2.16 – Signal attenuation and exponential decay law. Adapted from [53] ................................. 22
Figure 2.17 – Normally incident wave, reflection and transmission ...................................................... 23
Figure 2.18 – Oblique incident wave, reflection and refraction ............................................................. 24
Figure 2.19 – Schematic of a piezoelectric transducer, courtesy of Beijing Ultrasonic [55] ................. 25
Figure 2.20 – Near field and far field in an ultrasound beam, courtesy of Olympus Corporation [56] .. 25
Figure 2.21 – Different types of UT data representation [58] ................................................................ 26
Figure 2.22 – 3 different types of phased array probes. Adapted from [57] .......................................... 26
Figure 2.23 – Electronic, depth and sectorial scanning techniques. Adapted from [57] ....................... 28
Figure 2.24 – Principle of the SAUL algorithm [64] ............................................................................... 29
Figure 2.25 – Schematic of a thermography setup via external irradiation [72] .................................... 31
Figure 3.1 – CIVA software [73] ............................................................................................................ 32
Figure 3.2 – CFRP calibration specimen specifications. All dimensions are in millimetre. ................... 33
Figure 3.3 – Defects’ positioning and numbering .................................................................................. 34
Figure 3.4 – Calibration specimen transversal section ......................................................................... 35
Figure 3.5 – Fatigue test specimen’s overall dimensions and impact locations ................................... 36
Figure 3.6 – Generic shape for an omega stringer (dimensions in millimetre) ..................................... 37
Figure 3.7 – Evolution of the Young’s modulus evolution vs Vf ............................................................ 39
Figure 3.8 – Evolution of the shear modulus evolution vs Vf ................................................................ 40
Figure 3.9 – Evolution of the Poisson’s ratio evolution vs Vf ................................................................ 40
Figure 3.10 – Gaussian and Henning signals ....................................................................................... 43
Figure 3.11 – Influence of the relative bandwidth in the electric signals ............................................... 43
Figure 3.12 – Electric signals with different frequencies ....................................................................... 44
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Figure 3.13 – Attenuation plots obtained from CIVA ............................................................................. 45
Figure 3.14 – A-scan, attenuation comparison for the 5 MHz probe .................................................... 46
Figure 3.15 – Focal spot without and with attenuation, 5 MHz probe ................................................... 47
Figure 3.16 – A-scan, attenuation comparison for 3,5 MHz probe ....................................................... 47
Figure 3.17 – Focal spot without and with attenuation, 3,5 MHz probe ................................................ 48
Figure 3.18 – Overview of the case study a) ......................................................................................... 49
Figure 3.19 – Acoustic pressure results for the aperture variation case studies .................................. 50
Figure 3.20 – Amplitude response for the aperture variation case studies ........................................... 50
Figure 3.21 – Acoustic fields for the 3-shot scenario, with the 3,5 MHz probe ..................................... 51
Figure 3.22 – Acoustic fields for the 3-shot scenario, with the 5 MHz probe ........................................ 52
Figure 3.23 – Acoustic field variation for the deeper focal point ........................................................... 52
Figure 3.24 – CFRP test specimen with modelled defects ................................................................... 54
Figure 3.25 – A-scan and B-scan for defect number 19 ....................................................................... 55
Figure 3.26 – Overlaid A-scans for defect number 19, from the 3,5 and 5 MHz probes....................... 55
Figure 3.27 – Multi-point focalization inspection technique applied to the reinforcing omega stringer . 56
Figure 3.28 – Acoustic field results for the stringer ............................................................................... 57
Figure 4.1 – ISQ’s immersion tank ........................................................................................................ 58
Figure 4.2 – Raster scan pattern ........................................................................................................... 59
Figure 4.3 – Laptop and MultiX ............................................................................................................. 60
Figure 4.4 – Immersion tank touch screen interface ............................................................................. 60
Figure 4.5 – Gain needed to IF echo @80% screen ............................................................................. 62
Figure 4.6 – Maximum inspection velocity ............................................................................................ 63
Figure 4.7 – Inspection step .................................................................................................................. 63
Figure 4.8 – Total inspection time ......................................................................................................... 64
Figure 4.9 – Attenuation measurements ............................................................................................... 66
Figure 4.10 – Example of the SAUL algorithm application .................................................................... 67
Figure 4.11 – PAUT of the CFRP calibration specimen - complete results .......................................... 68
Figure 4.12 – DAC curve for the 10 MHz probe and CFRP calibration specimen ................................ 69
Figure 4.13 – Physical testing of the flat panel and omega stringer ..................................................... 71
Figure 4.14 – Flat CFRP panel with sensors and impact locations....................................................... 71
Figure 4.15 – PAUT results for the flat panel. On the left, amplitude results; on the right, time of flight
results. ................................................................................................................................................... 72
Figure 4.16 – Debonding defect ............................................................................................................ 74
Figure 4.17 – Colour highlights for the several sections of the omega stringer .................................... 74
Figure 4.18 – Stringer inspection performed without and with the SAUL algorithm.............................. 75
Figure 4.19 – PAUT results for the omega stringer. On the top, amplitude results; below, time of flight
results. ................................................................................................................................................... 75
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List of Tables
Table 2.1– List of most used NDE methods [24] ..................................................................................... 6
Table 2.2 – Physical quantities in sound waves .................................................................................... 18
Table 2.3 – Mechanical properties of some materials [49],[50] ............................................................. 21
Table 2.4 – Parameters of a phased array probe ................................................................................. 27
Table 3.1 – Defects in the CRFP calibration specimen ......................................................................... 34
Table 3.2 – Specifications of the impact tests ....................................................................................... 36
Table 3.3 – Epoxy and carbon fibre material specifications from the CIVA library ............................... 37
Table 3.4 – Specifications of the linear probes ..................................................................................... 41
Table 3.5 – Specifications of the 2-D matrix probes ............................................................................. 41
Table 3.6 – Theoretical attenuation values ........................................................................................... 45
Table 3.7 – Amplitude differences between interface and backwall echoes ......................................... 53
Table 3.8 – Amplitude variations for the worst case scenario defects between the two 2D-matrix
probes .................................................................................................................................................... 56
Table 4.1 – Specifications of the immersion tank .................................................................................. 59
Table 4.2 – Acoustic impedance calculation results ............................................................................. 65
Table 4.3 – Attenuation coefficients, α .................................................................................................. 66
Table 4.4 – CFRP calibration specimen defect results ......................................................................... 70
Table 4.5 – CFRP calibration specimen deviation values analysis ....................................................... 70
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List of Symbols and Acronyms
ISQ – Instituto de Soldadura e Qualidade
XWB – Xtra Wide Body
NDE – Non-destructive Evaluation
NDT – Non-destructive Testing
CFRP – Carbon Fibre Reinforced Polymer
SPA – Sampling Phased Array
RTD – Research and Technical Development
QREN – Quadro de Referência Estratégica Nacional
PAN – Polyacrylonitrile
NASA - National Aeronautics and Space Administration
ASNDT – American Society for Non-destructive Testing
AE – Acoustic Emission Testing
ET – Electromagnetic Testing
LM – Laser Testing Methods
MFL – Magnetic Flux Leakage
PT – Liquid Penetrant Testing
MT – Magnetic Particle Testing
RT – Radiographic Testing
IR – Thermal/Infrared Testing
UT – Ultrasonic Testing
VA – Vibration Analysis
VT – Visual Testing
PMC – Polymer Matrix Composite
CMC – Ceramic Matrix Composite
MMC – Metal Matrix Composite
ABS – Acrylonitrile Butadiene Styrene
Tg – Glass transition temperature
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Ei – Young’s modulus in direction i
Pa – Pascal
ATL – Automatic Tape Laying
ATP – Automatic Fibre Placement
CLPT – Classical Laminated Plate Theory
σi– Stress component in direction i
Cij – Stiffness matrix coefficients
εi – Strain components in direction i
(x1, x2, x3) – Cartesian coordinate system
Sij – Compliance matrix coefficients
νij – Poisson’s ratio in the xixj plane
Gij – Shear modulus in the xixj plane
Ec – Young’s modulus of a composite
Vf – Fibre volume fraction
Vm – Matrix volume fraction
n – Number of composite layers
Af – Areal weight of the fabric
ρf – Fibre density
t – Laminate thickness
BVID – Barely Visible Impact Damage
Hz – Hertz
f – Frequency
λ – Wavelenght
Cl – Longitudinal wave velocity
Ct – Transversal wave velocity
I – Intensity
Z – Acoustic impedance
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dB – Decibel
R – Reflection coefficient
D – Transmission coefficient
α – Attenuation coefficient
αe, αr, αt – Incidence, reflection and refraction angles
elts – Probe elements
POD – Probability Of Detection
SAUL – Self Adaptive Ultrasonic Technique
TOF – Time Of Flight
S/N – Signal-to-noise ratio
J – Joule
CAD – Computer Aided Design
NA – Non-applicable
(X, Y, Z) – Immersion tank linear axis
(φ, ω, Ω) – Immersion tank rotational axis
DAC – Distance Amplitude Correction
IF – Interface echo
ttotal – Total inspection time
Vmax – Maximum inspection velocity
S – Inspection step
σ – Standard deviation
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1 Introduction
1.1 Scope
Following today’s transportation industry design requirements, namely in the aerospace sector, an
increase is observable in the use of composite fibre materials. The demand for carbon fibre reinforced
polymer (CFRP) is forecasted to double by 2015, following the aim for an improved fuel efficiency and
the need to use high-strength lightweight materials [1]1.
Specifically, in many commercial airplane models developed nowadays, the amount of fibre composites
being used has increased. As an example, the model A350 XWB (Xtra Wide Body) from the company
Airbus, in Figure 1.1, which had its first flight on June 2013, has a composite structure weight of 53% [2].
This amount is mostly due to CFRP.
Composite materials allow engineers to design components with the aim of increasing performance,
while maintaining or even reducing weight. This is possible given the high stiffness to weight ratio of
CFRP, much higher than steel, for example.
Figure 1.1 – Materials used in the Airbus A350 XWB [3]
Allied with the use of CFRP, comes the need to mitigate the risk of catastrophic component failure. It is
crucial to evaluate the failure modes associated to the use of composites, either coming from the design,
fabrication or service phase.
Non-destructive testing (NDT) activities come as the most widely used option to identify and characterize
defects. NDT was valued in 2014 as a $1400 Million industry. [4] Without these methods, the flying
safety would decrease, and the cost of maintaining and flying an aircraft would increase.
The present document and the work described in it, comes upon the development of an RTD project
within the company Instituto de Soldadura e Qualidade (ISQ) [5], titled CompInspect – Development of
Technologies and Capabilities to Inspect Aerospace Composite Structures (project number 30311). This
project has received support from FEDER [6], and the initial work has been addressed by Amorim, J.
(2014). The consortium is composed by ISQ International – Inspecções Técnicas SA, Optimal Structural
Solutions, Lda. [8] and ISQ – Instituto de Soldadura e Qualidade.
1 Author’s note: along this document, several hyperlinked cross-references are used, to provide an easier reading. To use them, simply click on the desired link.
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1.2 Work contribution
This MSc Thesis intends to respond to the need to analyse internal defects in CFRP parts, within ISQ,
specifically in the complex geometries which are very common in the aeronautical industry. The
successful development of this work allows for future inspections of such parts, establishing the correct
procedures depending on specific material characteristics, component geometry and inspection
equipment used.
Thus, the aim is to further develop the inspection procedures existent at ISQ, in order to improve the
composite inspection associated knowledge of the company, while assuring value creation for potential
clients.
1.3 Objectives
The objectives of this dissertation begin with a review of the state of the art, crucial to acquire knowledge
in composite materials and non-destructive inspection techniques, particularly ultrasonic testing.
Information regarding the composite material internal structure and how it influences sound propagation
has to be understood.
Secondly, an acoustic model must be developed, implemented and validated, using the phased array
technique. This model has to account for several parameters, including probe type, frequency, aperture,
delay law, among others.
Considering the existence of non-flat parts in the aerospace industry, an adaptive control method has
to be employed, which is able to adjust the inspection equipment settings to the part profile.
1.4 Document Structure
The present work is divided into six chapters, with the following structure:
Introduction
State of The Art
Acoustic Model
Physical Inspection
Conclusions
Future Work
After this introductory chapter, the state of the art review is done in Chapter 2. There are disclosed the
historical breakthroughs both in carbon fibre composites use and non-destructive inspection methods.
On a more technical note, detail is given on the physical and acoustic properties of CFRP. Also, the
physics of ultrasonic inspection are discussed, both conventional and advanced methods.
In Chapter 3, a computational model for the sound propagation is developed. Generally, this model
considers the influence of test specimen material and geometry, probe, delay laws and inspection
parameters. The major output from the model is the information about the acoustic field within the part.
This model allows for the tuning of a wide range of parameters, with the goal of assuring a high
probability of detection of defects.
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After the computational work, physical tests are executed, using electronic acquisition equipment. This
work is described in Chapter 4. Several part geometries are analysed, together with a range of probes,
aiming to validate the results from the computational model. A detailed analysis of the acquisitions is
performed.
In Chapter 5, the set of results from the work is disclosed, regarding the successful implementation of
the developed methodology. The limitations encountered during the inspections of aerospace-grade
parts are also discussed.
Finally, in Chapter 6, a list of possible future developments is added, aiming to allow for the continuous
improvement of the inspection procedures.
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2 State of The Art
In this chapter, the fundamentals of composite theory and non-destructive evaluation are reviewed.
2.1 Historical background
2.1.1 Composite Materials in the Aerospace Industry
Carbon fibre composites appeared for the first time in a very common object: a light bulb. Edison, T.
(1880) used carbon fibre filaments in some of his early light bulb designs. These fibres initially came
from the carbonization of cotton and bamboo fibres. After this, in the 1960’s, Bacon, R. produced the
first high performance carbon fibres, using a polymer based on petroleum [10]. These developments
paved the way for the introduction of this novel material in several engineering products.
2.1.1.1 Manufacturing of CFRP
The industrial manufacturing process of carbon fibre is divided into mechanical and chemical setups.
Historically, the raw material, or precursor, has been mostly rayon, petroleum pitch or PAN
(polyacrylonitrile). Considering PAN, the material is stretched and converted into a thermoset, with
heating to very high temperatures; it is drawn into long strands, with no presence of oxygen. This way,
the fibre does not burn, and the heat energy originates violent vibrations in the atoms, until most non-
carbon atoms are expelled, in a process called carbonization. A sequence of other processes (to
improve the surface wettability and future adhesion to the matrix) is also deemed necessary so the fibre
becomes ready to be woven and available for part manufacture (see Figure 2.1).
Figure 2.1 – Carbon fibre manufacturing schematic [11]
Despite the fact that Bacon’s fibres were still an academic study, shortly after “Curry Ford and Charles
Mitchell patented a process for making fibres and cloths by heat-treating rayon to high temperatures, up
to 3,000 ºC. They had produced the strongest commercial carbon fibres to date, which led to the entry
of carbon fibres into the “advanced composites” industry in 1963.” [12]
- 5 -
2.1.1.2 Applications of CFRP on the Aerospace Industry
These developments led the way into commercial applications of carbon fibre composites. Its initial use
came from the military (US Fighter Aircrafts) and space (NASA) industries. “Initial applications of
composite materials to aircraft structures were in secondary structures such as fairings, small doors and
control surfaces. As the technology matured, the use of composite materials for primary structures such
as wings and fuselages has increased.” [13]
In several aerospace applications, CFRP has replaced metal parts in both primary and secondary
structures. Also, satellites are another application for the pitch-based fibres, thanks to their high thermal
conductivity, that allows minimum thermal expansion in space. [14]
Figure 2.2 – Detail of a satellite antenna reflector, courtesy of INVENT GmbH [15]
2.1.2 History of non-destructive evaluation
According to the American Society for Non-destructive Testing (ASNDT), “non-destructive testing (NDT)
is the process of inspecting, testing, or evaluating materials, components or assemblies for
discontinuities, or differences in characteristics without destroying the serviceability of the part or
system.” [16] Non-destructive evaluation has been used industrially since the beginning of the 20th
century, and its evolution has been a continuous one. From the very first visual inspection of parts, to
the nowadays real-time acquisition of data from embedded sensors, the developments and relevance
of this engineering field are evident.
2.1.2.1 NDE: the beginning
The first reference to NDE comes from pottery makers from around 6500 B.C., who identified the faulty
clay pots based on the cracking sound they emitted when allowed to cool too quickly. Despite this, only
centuries later, around the 1930s and 40s, NDE methods were regarded as a valuable tool. Before the
Second World War, engineers had to base their component designs on very conservative approaches,
since there were no reliable procedures to assure part quality during its life span. A set of important
events were the failures of several ships, namely the Liberty Class ships operating in the cold waters of
the Northern Atlantic. [17],[18] These ships operated at low water temperatures, that caused the steel
used in the hull to behave as a brittle material; this, associated with stress concentration in the welding
joints, caused the steel structure to fail suddenly due to cracking.
- 6 -
These incidents were a milestone in identifying the need to access part quality, during both the
manufacturing and service life of components: this is the scope of NDE. It is important to state that
sometimes parts with defects can continue to be used: knowing the evolution of defect size is another
aspect of NDE. This is known as damage tolerant design, and can be considered when monitoring
fatigue crack growth, for example. [18] With this approach, cost can be reduced, since a part is only
removed from service when a defect becomes critical.
2.1.2.2 Importance of NDE
The use of NDE procedures on components is one of the options available to mitigate the risks of
material and life losses due to accidents. Unfortunately, throughout the years some accidents could
have been avoided if potential failure causes had been detected. Some examples can be listed: a
pressure vessel explosion in a shoe factory in Massachusetts killed 58 people and injured 150 [20], the
Columbia Space Shuttle accident, due to the failure of panels in the wing and nose, killed the seven
crew members [21], the crash of United Airlines flight 232 in Iowa, due to cracks in the engine rotor,
caused the death of 111 of 296 passengers (see Figure 2.3). [22]
Figure 2.3 - Investigators examine flight 232 engine [23]
As a reference, some important NDE methods used nowadays are listed in the table below.
Table 2.1– List of most used NDE methods [24]
Technique Acronym
Acoustic Emission Testing AE
Electromagnetic Testing ET
Laser Testing Methods LM
Magnetic Flux Leakage MFL
Liquid Penetrant Testing PT
Magnetic Particle Testing MT
Radiographic Testing RT
Thermal/Infrared Testing IR
Ultrasonic Testing UT
Vibration Analysis VA
Visual Testing VT
This work is focused on UT, specifically on the phased array advanced technique. Despite this,
comparisons with other testing methods may be used throughout this document.
- 7 -
2.2 Composite Fundamentals
The composite material terminology relates to a certain material that “can be defined as a combination
of two or more materials that results in better properties that those of the individual components used
alone.” [25] The most commonly manufactured composite materials nowadays are usually divided into
three main groups:
Polymer Matrix Composites (PMCs): these materials use a polymer-based resin as the matrix,
and fibres as the reinforcement. These are also known as fibre reinforced composites.
Ceramic Matrix Composites (CMCs): materials for high temperature applications, consisting of
a ceramic matrix and short fibre reinforcements (e.g. made from boron nitride).
Metal Matrix Composites (MMCs): with an increasing use in the automotive industry [26],
these materials use fibres or particles such as silicon carbide to reinforce a metal matrix,
commonly aluminium.
The PMCs are the study subject of this work, and will be the ones addressed from this point onwards,
specifically CFRP monolithic2 panels.
2.2.1 Materials used in PMCs
2.2.1.1 Matrix
The function of the resin matrix is to hold the fibres together, ensuring a strong bond between them. It
also protects the fibres from external damage, while transferring the applied loads to and from them.
Depending on the application, the matrix also plays a role in the composite’s performance, being it
regarding ductility, toughness or electrical/fireproof insulation. [27] Component service temperature is
one of the important design parameters when selecting the matrix, either because of the degradation of
the matrix itself, or because of undesirable reactions in the fibre-matrix interface.
The polymer resins are divided into two classes: thermoplastics (polyamide, polypropylene, etc.) and
thermosets (phenolic, polyester, epoxy, polyurethane, etc.). Thermoplastics are materials that allow
temperature cycles of heating beyond the melting point (associated with material softening) followed by
cooling (causing the hardening of the polymer) without affecting the material properties.
Probably the most used thermoplastic polymer is acrylonitrile butadiene styrene (ABS), which can be
reinforced with short fibres, known as whiskers. These can have a diameter less than 1µm and length
up to several millimetre. On the other hand, thermoset resins originate from a chemical reaction, after
mixing a resin and a hardener or catalyst. The reaction that follows is not reversible, and originates a
product used to embed the dry fibres, that hardens after the cure is complete. If enough energy in the
form of heat is supplied to a thermoset matrix composite, it will cause the matrix to overcome its glass
transition temperature, Tg. This will lead to a drop in the matrix’s Young’s Modulus (Em), diminishing its
stiffness, and the overall composite’s shear and compressive strength.
2 Opposed to sandwich panels, monolithic composites are only composed by a fibre and matrix, i.e. they do not possess a core material.
- 8 -
For components required to have high performance characteristics, the most commonly used resins are
the epoxies, due to their superior mechanical properties; epoxy resin, when compared to other options,
provides better stiffness and strength, excellent adhesion to fibres, good chemical resistance, and a low
shrinkage after curing (which minimizes internal stresses and maintains a good surface contact in the
fibre-matrix interface). [28]
To assure that the tensile stresses applied to a composite component are supported by the fibre, the
matrix has to be able to deform at least the same amount as the fibre. The cure process can be
performed at room temperature; the cure temperature range can vary from 5 ºC to 180 ºC. One of the
main disadvantages of the resins lies in the fact that they tend to degrade due to the absorption of
moisture: despite also suffering from this issue, an epoxy laminate placed underwater and compared to
a polyester one, will maintain about 90% of its inter-laminar shear strength.
2.2.1.2 Reinforcement
The reinforcing materials used in PMCs are fibre filaments and particles. Since the fibres possess
superior mechanical properties than those of the matrix, they are the major contributors to the overall
properties of the laminate. Fibres are characterized for having a dimension larger than the other two by
at least an order of magnitude. The fibres used in CFRP are designated long fibres, since the properties
of the laminate do not vary if the fibre length is increased. These fibres also possess better mechanical
properties than those of the short fibres. Generally, the long filaments can be glass, carbon or aramid
(known by the commercial brand Kevlar™). Natural fibres (hemp, jute, etc.) and ceramic fibres (e.g.
basalt) are also subject of study nowadays, due to their sustainability, for example.
Figure 2.4 shows the curves for a tensile test of several fibres, performed until failure. For reference,
information on the same test applied to an epoxy resin is also provided. The analysed fibres are: high
strength carbon (E<265 GPa), aramid, E-glass (the most common glass fibre) and stiff-glass (the glass
fibre with the best mechanical properties).
Figure 2.4 – Tensile test results for some fibres and an epoxy resin [29]
- 9 -
It is clear that the fibres can sustain a great amount of stress until they break. Oppositely, the resin does
not present the same behaviour, although its elongation to break is a lot superior: this confirms the
requirement for deformability of the matrix.
Fibres can be acquired either in a dry state or pre-impregnated with resin. The latter are known as
prepregs. Generally, both forms can be found in a unidirectional setup, known as tape, or in a
bidirectional arrangement, known as woven fabric. The woven fabric type consists of thousands of
filaments woven together and, when compared with unidirectional fibres, they adapt better to complex
structure designs. On the other hand, unidirectional composites offer the engineer the possibility of
aligning the fibres following a known load case, saving unnecessary product weight and reducing cost.
For this reason, unidirectional fibres are often used in the aerospace industry.
Another arrangement is also possible, with a group of nonwoven unidirectional plies oriented in different
directions, and then stitched together. This example of multiaxial fibres can be seen below, in Figure
2.5.
Figure 2.5 – Nonwoven unidirectional plies stitched together [28]
Notice the orientation of the upper fibres, aligned with the loading direction: this is the 0º direction. The
objective of this ply arrangement is to produce a laminate with similar properties in several directions,
similarly to an isotropic material: as not all possible directions are covered (one can only consider the
laminate plane), this laminate is designed a quasi-isotropic composite. [30] A unidirectional ply is by
definition anisotropic, i.e. it presents different mechanical properties in different directions. The several
orientations are used to react to different loads:
{0°: axial loads
±45°: shear loads90°: side loads
The nomenclature used to define the laminate above is as follows:
[0°/90°/+45°/90°/−45°]ns
Where:
n is the number of repetitions of the sequence
s refers to a symmetric laminate
- 10 -
Fibre arrangements with [0°/90°] orientations are designated cross-ply laminates. The stitching of these
fibres is performed by numerically controlled high precision machines. In comparison with woven fabrics,
this sort of laminate possesses superior mechanical properties, since it is possible to achieve a higher
fibre density, reducing the amount of resin used and because there are no stress points due to warping
of the woven fabric.
This setup also favours an improved fabrication speed, minimizing lay-up time, since it reduces the
overall number of plies being laminated individually. Final cost is a compromise, since the reduced
fabrication time is weighed against a higher cost for this type of multiaxial fabric.
2.2.2 Composite manufacturing processes
Within the aerospace industry, several industrial manufacturing processes are used, to fabricate a wide
range of parts. Each fabrication method possesses its own characteristics, mainly concerning the type
of material used and the processing technique. The list of processes used in CFRP fabrication for the
aerospace industry includes: automatic tape laying (ATL), automatic fibre placement (ATP), filament
winding, resin transfer moulding (RTM) and hand lay-up.
Automatized processes such as ATL, AFP and filament winding are mostly used when fabricating large
structures, such as wing panels and fuselage sections. Taking, for example, one of the largest structures
of an airplane, a wing panel, the several stages of manufacturing are considered in Figure 2.6.
Figure 2.6 – Airplane wing panel manufacturing stages
Mould Preparation
In order to prepare the mould for a new manufacturing cycle, it has to be cleaned and coated with demoulding agents. This machined mould creates the outer surface of the wing.
Automatic Tape Laying
This step consists on the placement of prepreg CFRP tape on the mould. Since the layed tapes are narrow, they adapt easily to a curved surface.
Stringer placement
As the layed plies of CFRP are not strong enough to deal with the wing's bending stresses, longitudinal CFRP stringers are glued to the wing section.
Vacuum bagging
After all the plies are in place, a plastic film is placed over the composite, and is then sealed. To consolidate the laminate, vaccum is then applied to the bag, assuring a distributed pressure.
Autoclave curing
The curing cycle of the prepreg's resin begins after the component is placed inside the autoclave. Besides the vaccuum from the plastic film, the autoclave supplies extra pressure (up to 5 bar) and heat to the laminate.
Nondestructive testing
After the cure is finished, the component is removed from the bag, and is inspected through NDT, to assure that no manufacturing defects exist.
Milling and painting
If the component is approved, its rivet holes and other features are then milled. After the wing is painted is then available for assembly with the other components.
- 11 -
It is important to notice that only the mould side of the laminate is smooth; the plastic film side has a
rough finish. This aspect will be addressed again in Chapter 5.
2.2.3 Mechanical Properties of CFRP
Although it is not the main scope of this work, the author feels the need to introduce some concepts
regarding the design criteria for composites. The importance of some mechanical properties comes into
play when applying certain NDT procedures, which see their process characteristics influenced. The
next pages exhibit some of the main concepts of the Classical Laminated Plate Theory (CLPT) applied
to CFRP laminates.
2.2.3.1 Anisotropic Constitutive Model
Due to their material constitution, composites, unlike many other materials, do not possess the same
mechanical properties in all directions, i.e. they are not isotropic materials. Since they are mostly formed
by a resin reinforced with long, continuous fibres, the mechanical properties are dependent on the
direction in which the load is applied. Generally, this type of material is described as an anisotropic
material, meaning that it possesses different properties in different directions.
The generalized Hooke’s law for an elastic anisotropic material in the Voigt-Kelvin notation is:
{σi} = [Cij]{εi}
Where each item is decomposed in:
{σi} =
{
σ1σ2σ3σ4σ5σ6}
=
{
σ11σ22σ33σ23σ13σ12}
[Cij] =
[ C11 C12 C13 C14 C15 C16C21 C22 C23 C24 C25 C26C31 C32 C33 C34 C35 C36C41 C42 C43 C44 C45 C46C51 C52 C53 C54 C55 C56C61 C62 C63 C64 C65 C66]
{εi} =
{
ε1ε2ε3ε4ε5ε6}
=
{
ε11ε22ε33ε23ε13ε12}
We have that:
{
{σi} − stress components
[Cij] − material coefficients
{εi} − strain components
All the indexes refer to an orthogonal Cartesian coordinate system, (x1, x2, x3).
- 12 -
Generically, an anisotropic material is characterized by 21 independent constants, since the stiffness
matrix is symmetric (Cij = Cji). If a material has one or more planes of symmetry for its properties, the
number of constants is further reduced. [31]
The compliance matrix is obtained from the stiffness matrix and is its inverse: [Sij] = [Cij]−1
. In short
notation:
{εi} = [Sij]{σi}
In matrix form:
{
ε1ε2ε3ε4ε5ε6}
=
[ S11 S12 S13 S14 S15 S16S21 S22 S23 S24 S25 S26S31 S32 S33 S34 S35 S36S41 S42 S43 S44 S45 S46S51 S52 S53 S54 S55 S56S61 S62 S63 S64 S65 S66]
{
σ1σ2σ3σ4σ5σ6}
2.2.3.2 Orthotropic Constitutive Model
If one considers a unidirectional CFRP lamina, it is noticeable that it can have a plane of symmetry
perpendicular to the fibre alignment, and planes of symmetry also parallel to the fibre direction. The axis
x1 is placed parallel to the fibre alignment, while the x2 is perpendicular to the fibre, in the laminate
plane, and the x3 is also perpendicular to the fibre direction, but perpendicular to the laminate plane, as
shown in Figure 2.7.
Figure 2.7 – Unidirectional lamina coordinate system
This type of material is denominated orthotropic, and its independent material constants are reduced to
nine. These form the (also symmetric) stiffness matrix given by the material coefficients Cij.
[Cij] =
[ C11 C12 C13 0 0 0C12 C22 C23 0 0 0C13 C23 C33 0 0 00 0 0 C44 0 00 0 0 0 C55 00 0 0 0 0 C66]
The compliance matrix becomes:
[Sij] =
[ S11 S12 S13 0 0 0S12 S22 S23 0 0 0S13 S23 S33 0 0 00 0 0 S44 0 00 0 0 0 S55 00 0 0 0 0 S66]
- 13 -
To obtain the material properties in the elastic region, it is usual to perform simple laboratory tests, such
as a uniaxial tensile or compression tests and shear tests. Hence, there is a direct relation between the
stiffness and compliance material coefficients and the engineering constants. Here will only be
presented the final results of the deduction steps, for details please refer to Reddy, J. (2004). The
compliance matrix for orthotropic materials is:
[Sij] =
[ 1
E1−ν21E2
−ν31E3
0 0 0
−ν12E1
1
E2−ν32E3
0 0 0
−ν13E1
−ν23E2
1
E30 0 0
0 0 01
G230 0
0 0 0 01
G130
0 0 0 0 01
G12]
From linear algebra comes that the inverse of a symmetric matrix is also a symmetric matrix, so νij
Ei=
νji
Ej,
for i = 1,2,3. From this matrix, the nine independent material constants are:
E1, E2, E3, G12, G23, G13, ν12, ν23, ν13
The stiffness coefficients matrix can be obtained from the inverse of the compliance matrix:
[Cij] =
[ 1 − ν23ν23E2E3Δ
ν21 − ν31ν23E2E3Δ
ν31 − ν21ν32E2E3Δ
0 0 0
ν12 − ν32ν13E1E3Δ
1 − ν13ν31E1E3Δ
ν32 − ν12ν31E1E3Δ
0 0 0
ν13 − ν12ν23E1E2Δ
ν23 − ν21ν13E1E2Δ
1 − ν12ν21E1E2Δ
0 0 0
0 0 0 G23 0 00 0 0 0 G31 00 0 0 0 0 G12]
Δ =1 − ν12ν21 − ν23ν32 − 2ν21ν32ν13
E1E2E3
Ei is the Young’s modulus in the xi direction, νij is the Poisson’s ratio and Gij is the shear modulus in the
xixj plane. [31]
2.2.3.3 Transversely Isotropic Constitutive Model
There is a special derivation of the orthotropic constitutive model that is characterized by having similar
properties in a plane, and different properties in the normal direction to this plane. [32] This constitutive
model, in which materials are called transversely isotropic, allows for the reduction of the independent
material constants to 5. This model is applicable, for example, to a composite laminate, with several
unidirectional layers in the same plane, oriented in multiple directions (0°, 90°, ±45°). It can also be
applied to a single fibre, having similar properties in the cross sectional plane and different ones in the
- 14 -
longitudinal direction. If these transversely isotropic fibres are used in a unidirectional lamina, it can also
be considered transversely isotropic (properties in directions x2 and x3 of Figure 2.7 are equal).
Usually, the use of this model allows for a reduction of the number of tests necessary to find the material
constants. These five constants are:
E1, E3, ν13, ν21, G13
The Poisson’s ratio in the plane x2, x3 relates to the other constants over the relation:
ν12 =E22G23
− 1
For this constitutive model, the compliance matrix from Equation (2.12) becomes:
[Sij] =
[ S11 S12 S13 0 0 0S12 S11 S13 0 0 0S13 S13 S33 0 0 00 0 0 S44 0 00 0 0 0 S44 00 0 0 0 0 2(S11 − S12)]
=
[ 1
E1−ν21E1
−ν13E3
0 0 0
−ν21E1
1
E1−ν13E3
0 0 0
−ν31E1
−ν31E1
1
E30 0 0
0 0 01
G130 0
0 0 0 01
G130
0 0 0 0 02(1+ν21)
E1 ]
Due to the symmetry, this constitutive model follows the relations ν21 = ν12 and ν31
E1=
ν13
E3. [33]
Once again, the stiffness matrix is obtained from the inverse of the compliance matrix.
2.2.3.4 Rule of Mixtures
Besides conducting physical tests to obtain the mechanical properties of a composite, there is another
approach, valid only for monolithic laminates, called the rule of mixtures. Using this method, the Young’s
modulus of the composite, Ec, can be obtained, based on the weighted mean of the properties of the
matrix and fibre.
Considering an axial loading of a unidirectional panel, with the fibres aligned with the load direction, the
model used is the Voigt model. This model gives an estimate for the maximum value for Ec. Its
formulation is:
Ec = EfVf + EmVm
- 15 -
The indexes f and m concern the fibre and matrix, respectively. This model assumes the deformation of
the fibre is equal to the matrix (εf = εm = εc). [34]
The Vf constant is known as the fibre volume fraction and the Vm as the matrix volume fraction. Vf is a
function of the:
number of layers, n
areal weight of the fabric, Af
density of the fibre, ρf
thickness of the laminate, t
Vf =nAfρft
Also, the fibre volume fraction plus the matrix volume fraction give the unity:
Vf + Vm = 1
Generally, the higher the Vf, the better the properties of the laminate. Depending on the type and
arrangement of the fibre, usually the maximum value for Vf is around 65%. [35]
There is another model, called the Reuss model, which provides an estimate for the minimum value for
Ec. This model takes into account the stress on the lamina, considering it is the same in the fibre and
matrix (σf = σm = σc). [34] It has the following formulation:
1
Ec=VmEm
+VfEf
It should be noted that neither model considers any imperfections in the matrix-fibre interface, such as
manufacturing defects or void inclusions. They assume a perfect interface connection. Nevertheless,
they provide useful information, as the actual value for the Young’s modulus lies between both results.
If the maximum and minimum theoretical results are computed, the plots in Figure 2.8 are obtained:
Figure 2.8 – Maximum and minimum values for a composite’s Young modulus calculated with Voigt and Reuss
models. [36]
- 16 -
2.2.4 Damage Mechanisms in Composite Materials
Since the failure of an important component of an aircraft during its life service is considered
unacceptable by the aerospace industry, it is important to have full knowledge of the defects these
structures can have. During both the manufacturing stage and the service life of a composite structure
used in an airplane, it is necessary to recognize the mechanisms involved in the damage of the
composite. It is important to know how this damage affects the composite structure, minimizing its
performance, and thus affecting the safety of the entire structure.
2.2.4.1 Fatigue damage
A fatigue failure occurs below the ultimate strength limit of the material, when stresses have been
repeated a very large number of times. [19] This is the most dangerous failure mode of a structure
because, if not detected, it occurs suddenly and in a catastrophic manner. Moreover, in a composite
structure, due to its inhomogeneous and anisotropic internal structure, it is not easy to evaluate fatigue
life. If the component’s material is not manufactured correctly (e.g. if there are areas with no resin), or if
it collects damage (e.g. internal defects) during its service, these issues act as factors that reduce the
part’s performance.
In a CFRP laminate under a cyclic loading, failure starts in the matrix, with cracks propagating until they
reach the interface fibre-matrix, causing a debonding. With these cracks acting as stress risers, a further
increase in the area of the debonding may cause the laminate to be unable to sustain the imposed
stresses, causing a failure of the component.
2.2.4.2 Impact Damage
Many types of impact may happen to an airplane structure, be it a bird impact during takeoff, hail or
lightning strikes, or even a maintenance worker dropping a wrench on top of a wing. Some of these
impacts may leave a visible superficial defect, although many do not. The latter are designated barely-
visible impact damage, BVID. Research has shown that the damage modes associated with BVID are
rather complex. Richardson and Wisheart (1996) were able to describe matrix cracking and debonding
between fibres as the initial damage after an impact takes place. The cracks in the matrix may have
several orientations: if bending stresses exist, the orientation of the crack is parallel to the impact
direction, whereas oblique orientations come from the shape of the impactor itself. The next phenomena
are delaminations, “in the resin-rich area between plies of different fibre orientation” [37]
Recently, Appleby-Thomas et al. (2011) performed post-impact NDT analysis of the ballistic response
of CFRP to ice projectiles, considering multiple impacts and energies. [38] Through this work it was
possible to identify a range of six categories of composite damage, “ranging from no apparent surface
damage, to penetration accompanied by complete lay-up disruption.”
2.2.4.3 Manufacturing defects
Besides the damage created from fatigue and impacts, manufacturing induced defects are also possible.
The most common are probably porosity and voids, where small to medium-sized cavities exist in the
matrix. This may be due to an inconsistent resin distribution, or air leaks during the vacuum bagging, for
example. After a certain point, a laminate’s porosity may become critical. During manufacturing, foreign
- 17 -
bodies inclusions are possible, from plastic film pieces to grease from a worker’s fingers, who failed to
use gloves. Irregularities in the automated fibre placement can also cause lay-up misalignments, as well
as delaminations and broken fibres, which start at the hole boundary of drilling and machining
operations. [39] Furthermore, components in the aerospace industry are commonly joined together via
adhesive bonding. This manufacturing process has its own inherent difficulties: for example, if the
surfaces to be bonded are not duly cleaned and degreased, after the part is stressed a disbond may
initiate.
To summarize, the most important defects evaluated by ultrasonic inspection are: internal voids,
delaminations, porosity and disbonds. Many of these defects are parallel to the composite surface. It
must be said that it is important to develop inspection methods to both establish pass/fail criteria for
parts, but also to determine the size and location of potential defects.
Figure 2.9 – Common CFRP defects [40]
2.3 Ultrasonic non-destructive testing
This sub-chapter focuses on the most widely used NDT procedure nowadays for the inspection of CFRP
structures, ultrasonic testing (UT). This method is based on the transmission of high frequency sound
waves; in the industry, these are usually between the 500 kHz and the 20 MHz range. Note that human
hearing is more or less within the 20 Hz to 20 kHz range. [41] The use of this testing method targets the
detection and characterization of defects in the composite, providing information about their location,
depth, size and orientation.
2.3.1 Fundamentals of sound propagation
It is important to address the physical behaviour of the sound wave, why and how it propagates through
a component. A wide range of properties associated with sound propagation influence the probability of
detecting a defect.
2.3.1.1 Wave types
Sound is a mechanical wave, meaning that it needs a physical, elastic medium to propagate. Sound
cannot propagate in vacuum. Acoustics is the science that studies sound propagation through materials;
this propagation is generally obtained via the microscopic vibrations of the particles that compose
matter, around their equilibrium positions. When these vibrations occur in a continuous mode, the
associated elastic energy is translated into a wave propagating in the medium. This event exists due to
- 18 -
the elastic behaviour of materials (similar to a particle mesh, joined together by springs – see Figure
2.10). The amplitudes of these vibrations impose minimal stress to the material, much below the yield
limit, so no permanent deformation of the component occurs. [42]
Figure 2.10 – Elastic material model representation [43]
The ultrasonic waves resulting from the small displacement of the particles create different types of
waves. In a NDT scope, there are four wave types commonly used: longitudinal, transverse or shear,
surface or Rayleigh, and Lamb waves. There are different quantities associated with the physical
phenomena of sound waves. The most important ones are:
Table 2.2 – Physical quantities in sound waves
Physical quantity Symbol Unit Physical meaning
Sound pressure p Pa Alternating pressure in relation to the ambient atmospheric pressure
Amplitude of sound
pressure p Pa Maximum deviation from the normal pressure
Wavelength λ m Distance between 2 planes in which the particles are in the same
state of motion
Frequency f Hz Number of oscillations of a particle per second
Velocity c m. s−1 Rate of position change of a sound wave
Intensity I W.m−2 Sound power per unit of area
Impedance Z Pa.m−3 Opposition of a material to sound flow
For all waves, the wave velocity is directly proportional to the wavelength and the frequency:
c = fλ
The movement of longitudinal waves (also called compression waves), is characterized by a sequential
compression and rarefaction of particles, propagating in the same direction as these particle oscillations.
Longitudinal waves propagate both in fluids and solids. An example of a longitudinal sound wave
travelling from left to right can be seen in Figure 2.11.
- 19 -
Figure 2.11 – Longitudinal wave [44]
In the transverse wave, the particles move in the perpendicular direction to the wave propagation. This
wave is also called a shear wave, due to the fact that the movement comes from a shear force
transmitted to the particles. Since “a solid can resist a shear stress by a static deformation; a fluid
cannot”, White, F. M. (2003), it is not possible to transmit properly a transverse wave through a fluid,
e.g. air or water, unless this fluid has a high viscosity coefficient, and a thin layer between the source of
excitation and the material to be inspected. A schematic of a transverse wave can be observed in Figure
2.12.
Figure 2.12 – Transverse wave [44]
Rayleigh or surface waves are another type of wave used for the inspection of parts, namely for thick
components. This wave type travels along the surfaces of the parts, in the solid-gas interface; their
propagation is based on the elastic cohesion differences between both mediums. Rayleigh waves are
used because of their sensitivity to surface defects and because they can follow a surface around bends.
Simplifying, the oscillation of the material particles follows an elliptical orbit, characterized by having the
large ellipse axis perpendicular to the component surface, and the small ellipse axis parallel to the
direction of wave travel.
Figure 2.13 – Rayleigh wave propagating through a solid [46]
Also commonly used in NDT are the Lamb or plate waves. They were first reported by Horace Lamb in
1916. [47],[48] These are waves that propagate along the thickness of a plate with free boundaries, and
they can exist in several different modes, although the most typical are symmetric or asymmetric. These
waves have the ability to propagate along the tested piece for long distances. Lamb waves are
particularly useful when detecting cracks in thin sheet materials and tubular components, such as
products in aerospace, pipe and transportation industries. An example of symmetric and asymmetric
Lamb waves can be seen in Figure 2.14.
- 20 -
Figure 2.14 – Lamb waves in plates: (a) symmetric, (b) asymmetric [24]
2.3.1.2 Wave velocity
It is possible to calculate the velocities of propagation of the various types of waves discussed before.
The sound wave velocity is a characteristic of the medium where it is travelling, and is thus dependent
on parameters obtainable from the [Cij] matrix, such as the Young’s modulus, E, and the Poisson’s ratio,
ν. It is also dependent on the density of the material, ρ. The stress state and temperature of the
component also affect the wave velocity, but not as much as the other parameters.
The theoretical formulas of the several wave velocities are given below.
Longitudinal waves:
Cl = √E
ρ
1 − ν
(1 + ν)(1 − 2ν)
Transverse waves:
Ct = √E
ρ
1
2(1 + ν)
Rayleigh waves (approximate formula):
CR =0.87 + 1.12ν
1 − ν√E
ρ
1
2(1 + ν)
For Lamb waves, the calculation of the velocity is increasingly difficult, since it depends not only on the
material properties, but also on the plate thickness. This means that these waves can also be directly
applied to thickness measurements and monitoring of parts that are exposed to thickness degradation
or erosion.
An aluminium component, for example made out of the Al 7075-T6, a grade commonly used in the
aerospace industry, can be analysed. This grade has E = 71,7 GPa, ν = 0,33 and ρ = 2810 kg/m3. The
theoretical longitudinal and transverse velocities are:
{Cl = 6123 m/sCt = 3084 m/s
Table 2.3 shows experimental data for some materials used/inspected in NDT.
- 21 -
Table 2.3 – Mechanical properties of some materials [49],[50]
Material 𝐄 [𝐆𝐏𝐚] 𝛒 [𝐤𝐠/𝐦𝟑] 𝛎 𝐂𝐥[𝐦/𝐬] 𝐂𝐭[𝐦/𝐬]
Mild steel 210 7700 0,28 5900 3230
Stainless steel 194 8000 0,3 5740 3130
Copper 111 8890 0,37 4700 2140
Al 7075-T6 71,5 2700 0,34 6350 3100
Epoxy resin 35,9 1340 0,39 2400 1100
Water - 1000 - 1480 -
Oil - 870 - 1740 -
As shown in the table above, the transverse waves travel approximately at half the speed of the
longitudinal waves. A direct comparison between the theoretical and experimental results: in the Al
7075-T6 case, it can be observed that in comparison with the theoretical results, the longitudinal
velocities are around 3,6% higher, and the transverse velocities are 0,5% lower. It has to be mentioned
the fact that the formulas described before were developed based on the assumption of a homogeneous,
isotropic material. Considering the case of an anisotropic material, such as CFRP, it is not possible to
calculate the wave velocities so easily, because the [Cij] matrix produces different velocities for different
directions. Methods to calculate the wave velocities on these cases will be addressed on Chapter 4.
2.3.1.3 Acoustic intensity and attenuation
Considering a model of a spherical wave front, as sound travels in a medium, its intensity (power per
unit of area) decreases accordingly to an inverse law. This means that the sound intensity at a given
point is reduced by 50% as its distance from the source doubles (see Figure 2.15). This intensity in
ultrasonics is measured in decibel (dB), and it is considered to have a level of 0 dB for the limit of human
hearing. Intensity is correlated to the amplitude of a signal, in the instruments used to carry out NDT.
Figure 2.15 – Sound intensity evolution from the source point [51]
Considering two different acoustic intensities, the ratio between them is given by:
dB = 10 log (I
I0)
- 22 -
I0 is the threshold of hearing, i.e. the minimum sound pressure perceptible by an average ear with
normal hearing, and it is at placed around 0,98 x 10−12 W/m2.
For two different acoustic pressures, p1 and p2, the ratio in dB is given by:
dB = 20 log (p1p2)
Another important aspect to take into account when performing an ultrasound inspection, is the sound
attenuation in the material. This attenuation is a decay in sound pressure that is caused by three main
reasons: wave spreading, scattering and absorption effects.
The scattering effects come from non-homogeneity of the materials, causing a reflection of the wave in
the material boundaries, for example in the fibre-matrix interface. In the case of CFRP, the absorption
comes mainly from the visco-elastic losses in the epoxy matrix. This attenuation losses can be compared
to a damping effect. Take, for example, a car damper: the higher the velocity applied to the damper, the
more energy it dissipates into heat. The very same happens to materials, and these losses increase
with the increase in the sound wave frequency. As for the wave spreading, this is a characteristic of the
sound wave itself and it relates to the near and far field concepts, discussed in Chapter 2.3.2.2.
The sound pressure, seen as an amplitude, decays in the form of an exponential function. The
parameters that influence the decay are the attenuation coefficient α, and the sound pressures at the
beginning and end (p0, p) of a given section of length d. The attenuation coefficient, given in dB/m, is a
characteristic of: the material, the frequency of the sound wave, the wave form (longitudinal waves are
less attenuated than transverse) and the temperature (increasing with temperature rise). Usually,
attenuation is dealt with via an increase in the gain applied to a signal that generates the sound wave.
This, however, also leads to a decrease in the signal-to-noise ratio (S/N), which is a measure of how
much a signal has been corrupted by noise. [52] Figure 2.16 shows the attenuation effect on a signal,
and the exponential decay law.
Figure 2.16 – Signal attenuation and exponential decay law. Adapted from [53]
- 23 -
2.3.1.4 Frequency and defect detection
As explained before, the attenuation effect on sound can also be reduced when using waves with lower
frequency. However, this action cannot be taken lightly, since it poses a compromise between the
benefits of lesser attenuation versus the detectability of defects.
The problem that lies in decreasing the frequency is that, due to the fact that the wavelength increases,
it is more difficult to detect small sized flaws or defects. This ability to detect discontinuities in a
component is named sensitivity. Also, the maximum depth on a component where defects can be found
also decreases when the frequency rises.
One more parameter to take into account is the resolution, which is the ability to detect flaws placed
close together, or near the surfaces of the component. Resolution also increases with the increase in
frequency, so it is another important issue to consider when finding the middle ground between
frequency and defect detection.
2.3.1.5 Interface influence on sound propagation
Ultrasonic waves reflect part of their energy in boundaries across their path. These boundaries may
come from the existence of discontinuities within the component, such as a delamination between two
CFRP plies. The reflection echo from the boundaries exists due to the differences in acoustic
impedance, Z [kg.m−2. s−1], of the CFRP and the air in the delamination. Z is defined as the material’s
density multiplied by the velocity of the sound of the wave propagating in it.
When an ultrasonic pressure wave with an amplitude pe is normally incident on an interface, two
phenomena occur: part of the energy is reflected in the form of another wave of pressure pr and the rest
is transmitted with a pressure pt. This concept is displayed in Figure 2.17.
Figure 2.17 – Normally incident wave, reflection and transmission
These wave amplitudes define the pressure reflection and transmission dimensionless coefficients, R
and D. Considering two mediums, with acoustic impedances Z1 and Z2:
R =prpe=Z2 − Z1Z2 + Z1
- 24 -
D =ptpe=
2Z2Z2 + Z1
D − R = 1
If the incident sound wave is not normal to the surface, refraction also happens. Considering two
different materials, the first being a fluid and the second a solid, and longitudinal incident waves, they
will originate two wave types, one longitudinal and another transverse (see Figure 2.18). Universally,
this phenomenon is described by Snell’s law: [54]
sin αeCl1
=sin αtLCl2
=sin αtTCt2
αe = αr
Figure 2.18 – Oblique incident wave, reflection and refraction
To calculate the refracted angles in material 2, one must use the longitudinal or transverse sound
velocities accordingly. The creation of a transverse wave from an incident longitudinal wave is named
mode conversion: if the sound velocity in material 2 is higher than that in material 1, then above certain
angles refraction will be accompanied by mode conversion, more frequently from a longitudinal wave to
a transverse wave, although surface waves can also be created with increasing αe angles.
2.3.2 Ultrasonic Inspection
In this sub-chapter some remarks are made about the instrumentation used to perform an ultrasonic
inspection, the several techniques used, and the information obtained from the inspection.
2.3.2.1 Transducer
To produce the ultrasonic waves, an instrument that transforms electric energy into mechanical
vibrational energy is required. This conversion is made through the use of a piezoelectric transducer,
like the one schematically displayed in Figure 2.19. When receiving an echo, the opposite conversion
happens, from mechanical to electric energy. It is this electric signal that is analysed to account for
defects in the part being inspected.
- 25 -
The piezoelectric transducer, also called element, is enclosed by a metallic casing, and the assembly is
named probe. The probes can contain a single transducer, or several. Probes containing several
elements are called phased array probes and are object of discussion in Chapter 2.3.3.1.
Figure 2.19 – Schematic of a piezoelectric transducer, courtesy of Beijing Ultrasonic [55]
2.3.2.2 Beam characteristics
The sound waves emitted from the transducer come, not from a single point in the output face, but from
several points. These emitted sound waves are then subjected to constructive and destructive effects
along their path in the medium they propagate in. There are two distinct areas in the sound beam of the
emitted waves: the Fresnel region or near field, and the Fraunhofer region or far field (Figure 2.20). The
near field is the region close to the probe, where there are intense fluctuations in the sound pressure.
In the transition between the Fresnel-Fraunhofer regions, the beam has its maximum intensity, and
reflexions at this distance from the probe produce the strongest echoes. Past this area, the beam
spreads, losing energy along its path.
Figure 2.20 – Near field and far field in an ultrasound beam, courtesy of Olympus Corporation [56]
2.3.2.3 Inspection technique
From the range of available UT techniques (contact/immersion testing, water jet probe, air-coupled
transmission, laser generated ultrasounds, straight/angled beam, single transducer/phased array,
pulse-echo, time-of-flight diffraction, etc.), this work focuses on pulse-echo immersion testing with
phased array straight beams. Unlike, for example, the inspection of welding defects, there is no explicit
need to use angle beams, since the defects in composites are well oriented (as referred in Chapter
2.2.4.3). As for the option to use immersion testing, this is related to the availability of a numerically
- 26 -
controlled immersion inspection system in ISQ, developed by (Amorim, J., 2014). Contrary to contact
testing, the immersion technique is better suited to deal with an automatic control, programmable to
account for non-flat parts with a complex geometry. The medium to transmit the ultrasonic waves
between the transducer and the component (known as coupling medium) is a water column. More details
on the system used to perform the inspection of the CFRP parts can be found in Chapter 4.
2.3.2.4 UT data representation
Ultrasonic data is analysed in three different forms: A, B and C-scans (see Figure 2.21). The simplest
and most common is the A-scan, a plot of energy vs time at a given point of the component being
inspected. A B-scan is a representation of a series of A-scans along a dimension. If a second spatial
dimension is added, a C-scan is obtained.
Figure 2.21 – Different types of UT data representation [58]
2.3.3 Phased array advanced technique
This section focuses on the phased array advanced ultrasound technique (PAUT), where multi-element
probes are used, together with different control algorithms. The principle behind the phased array
technique is “to activate for each shot all or some of the transducer elements which, with the adapted
delay laws, contribute collectively to the generation of the beam”. [59] Contrary to the mechanical
translation of a single element probe, this technique allows to perform electronic scanning steps, with a
single transducer position. Moreover, both electronic and mechanical steps can be combined, increasing
the overall inspection efficiency, reducing the overall inspection time, and thus, cost.
2.3.3.1 Probes
Conceptually, a phased array probe is nothing more than a set of single element probes joined together
and controlled electronically. Probes can be acquired in different shapes, sizes, number and positioning
of elements, etc. Figure 2.22 shows a range of types of phased array probes, for different applications.
Figure 2.22 – 3 different types of phased array probes. Adapted from [57]
- 27 -
Besides the displayed probe types, some others are available. Note the difference between the linear
and concave probe: the latter is characterized by producing a mechanical focalization of the ultrasound
waves, or designed specifically to suit a certain component geometry. The common number of elements
(elts) on a phased array probe ranges from 16 to 256 elts. Moreover, probes can be also designed in a
2-D matrix shape, to allow beam control in 2 directions.
The main parameters that characterize a phased array probe are shown in Table 2.4.
Table 2.4 – Parameters of a phased array probe
Parameter Description
Element size The size of the element affects the shape of the sound field transmitted by the probe.
Nominal
Frequency
An ultrasonic transducer does not radiate sound waves in a single frequency. Instead, it emits a
range of frequencies. The nominal frequency is the mean frequency of probes of the same type.
A probe’s frequency influences greatly the detection and sizing of defects. Penetration increases
with lower frequencies (0,5MHz – 2,25MHz), whereas resolution and focal sharpness increase
with higher frequencies (15MHz – 25MHz).
Bandwidth The range of frequencies centred at the nominal frequency is named bandwidth. The industry
standard is to specify this bandwidth at the -6dB below the maximum signal amplitude.
Near field
length
The near field length value is where the unfocused sound beam possesses the highest amplitude
echoes. There are formulas available to calculate this value, based on the probe’s frequency,
element size and sound velocity. Focusing can only be done inside the near field.
Element
number
The number of elements can vary between 16 and 256. Probes with a higher number of elements
allow for a more precise control of the acoustic beam.
Aperture
The aperture of a probe is the size of its active area. In a phased array probe, it is divided into
the active and passive aperture, one being the total active probe length and the other the
element’s width.
Gap and pitch
The gap and pitch characterize the distances between the consecutive elements of a probe. The
gap is the width of the acoustic insulation while the pitch is the distance between element centres.
Dimensions are given in millimetre.
2.3.3.2 Beam control
Through the use of constructive and destructive wave interaction, it is possible to steer and focus the
sound beam. This is the great advantage of PAUT probes when compared to the conventional single
element probes. The wave interaction is achieved through the use of delay laws, where the elements
are excited at different times, both for emission and reception. Computer programs calculate these delay
laws, based on the probe specifications, medium(s) where the sound will travel and the inputs of the
user. There are several types of scanning patterns available, with the three most used being:
Electronic scanning: scanning is performed along the elements of the probe, by 𝑛 elements at
a time. The same emission and receptions laws are used. This technique reduces the number
of mechanical passes needed to inspect a given area.
Depth focusing: different focal depths are used, to maximize the sound beam energy in different
depth spots of the part being inspected.
- 28 -
Sectorial scanning: also called S-scans, in this technique the sound beam is swept between a
minimum 𝛼𝑁 and a maximum 𝛼𝐼 angles, dictated by the probe and the physical laws of sound.
Figure 2.23 – Electronic, depth and sectorial scanning techniques. Adapted from [57]
It is noticeable that the use of this advanced technique possesses several advantages when compared
with conventional ultrasound. Inspection time, and thus costs, can be reduced; there is an increase in
the probability of detection (POD) of defects, being well oriented in relation with the beam or not; smaller
defects can be detected and sized due to the advanced beam control, which has a superior precision.
The negative side of this technique consists in the fact that both the phased array probes and
instrumentation are more expensive (around the hundreds of thousands of Euros), as well as the fact
that the technicians that perform the evaluations have to possess a greater knowledge when compared
to conventional ultrasonic inspections.
2.3.3.3 Adaptive Algorithm
Complex part geometries are commonly present in the components of the aerospace industry, being
them curved sections or reinforcing stringers, amongst others. To inspect these parts, the previously
mentioned techniques do not suffice, since the sound beam focusing cannot adapt to the complex
geometries. The first mention of an adaptive technique comes from (S. Mahaut et. al.) in 1998. [60] After
that, a control algorithm has been developed by (S. Robert et. al.) in 2012, [61] which “enables the
transmission of an incident wave-front parallel to any complex surface”. This Self-Adaptive Ultrasonic
Technique (SAUL), implemented by the company M2M in their PAUT systems [62], measures the time
an ultrasonic wave takes to hit a reflector and to return to the probe, known as time of flight (TOF), for
the entire range of elements being used. Then, this reception law is applied as an emission law, so that
the emitted sound wave matches the shape of the part to inspect. This process is done in real-time,
iteratively, until the incident wave matches the front surface, and the iterations converge. [63] The SAUL
algorithm needs a maximum of four iterations to converge, and it can adapt to both concave and convex
shapes. The expressions used to implement the algorithm in the control software are detailed next.
- 29 -
For the emission laws:
Ei(j+1)
= Ei(j)−ti(j)
2− Min (Ei
(j)−ti(j)
2) ,
i, j ∈ ℕ
For the reception laws:
Ri(j+1)
= Max (Ei(j)−ti(j)
2) − (Ei
(j−1)−ti(j)
2) ,
i, j ∈ ℕ
Where j + 1 represents a given transmission or reception law related to the previous shot j, while i
represents the element being considered. The use of this algorithm in a corner radius of a CFRP
component gives a better S/N ratio, mainly on the backwall echo, since the adaptation of the sound
wave to the part’s geometry provides echoes with large increases in amplitude. The incident wave is
normal to the profile, as can be seen in Figure 2.24.
It must also be referred that, since this is an adaptive technique, it not only adapts to the geometries of
the component, but it can also correct the sound beam if, for example, a probe is not perfectly laterally
aligned with the component being inspected along its length.
Figure 2.24 – Principle of the SAUL algorithm [64]
The limitation behind this method is that, for now, it is not possible to perform simultaneously beam
focalization or sweeping, which would improve the inspection results even further. Despite this, the
SAUL algorithm proves its usefulness, allowing to cover a larger area of any radii, with great sound
quality. Further details about the implementation of this algorithm can be found in Chapter 4.
2.3.3.4 Advantages and limitations of PAUT compared to other methods
The NDT technologies are in constant mutation and evolution, and different solutions are available to
solve the same industry problems. Concerning ultrasonic inspection, there are more techniques than
the ones stated before. Examples are air-coupled transmission, laser generated ultrasounds and
acoustography. [65],[66] Besides the ultrasonic method of inspecting composites, there are other
alternatives available and in development, based on different physical concepts.
- 30 -
Addressing not on the time of flight of an ultrasonic wave, but on the frequency changes of a beam
passing through a component, the ultrasonic spectroscopy technique is a valid alternative, for example,
to detect porosity in a composite material. The existence of porosity attenuates the high frequency
waves, but not the low frequency. This is accomplished by generating waves where the highest
frequency wavelength is of the same order of magnitude as the pores in the resin. Also, when compared
to pulse-echo, this technique may offer an advantage if used in thin laminates, since there is no need to
evaluate the reflections on the front and backwalls. On the other hand, it essential that the material’s
attenuation coefficient 𝛼 is well known, to account for the attenuation of sound energy. Also, it is more
difficult to achieve a good S/N ratio, when compared with PAUT. [67]
Different wave types, namely electromagnetic waves, have been studied and applied to composite
inspection. From these, it is important to refer microwaves, infrared and terahertz waves. Concerning
the microwave technique, according to Qaddoumi (1998), the changes in the dielectrical properties of a
material after being excited with a microwave could be used to predict discontinuities within the
component. Later, Abu-Khousa et al. (2003) developed a model to interpret the information on the
magnitude and phase interactions between microwaves and defects. Nowadays, some companies are
already applying this technique to fibre composites, mainly to detect delaminations and disbonds. [68]
When compared with PAUT, this technique has the advantage of being able to precisely evaluate
material specifications such as the state of cure of an adhesive or resin based on the dielectric
properties, at the cost of suffering from interference from other electromagnetic sources and posing a
safety hazard when power is increased.
Terahertz waves are located in the electromagnetic spectrum between the microwaves and the infrared
waves. Their frequency is around 1012 Hz. Anbarasu (2008) performed tests in composites using
continuous waves (oppositely to pulsed waves), being able to detect voids and delaminations with a
much higher resolution and accuracy than those of ultrasonic methods. Despite this potential advantage,
this young technique still needs more development, as the one being done by Ospald et al. (2014) since
there are no fast and affordable industrial solutions available in the market yet. [69]
Thermography is another method currently being tested for application on the inspection of aerospace
structures. Testing on the applicability to CFRP has been done recently by Yang and Wang (2009), and
Georges (2013). This technique has some variants, depending on the source of the thermal field. If the
thermal field is applied to the part externally, for example with a lamp projector, it is considered a thermal
pulse technique. On the other end, vibrotermography uses the heat generated within the part by an
imposed stress. Thermography is a method that relies on the different temperature rates of variation on
different areas of the component being inspected. For example, an infrared camera can be used to
monitor the temperatures across a part irradiated with thermal energy; contrast on the recording is seen
on internal discontinuities, such as a delamination, shown as a lesser rate of temperature increase. The
information recorded on camera is then processed via software, and a colour map like the one in Figure
2.25 is obtained. This method has the potential to identify different defects, from internal voids to impact
damage cracks, and is considered one of the major alternatives to PAUT, since the ratio between the
area/inspection time is higher (seconds vs minutes). The thermography’s main limitation consists on the
- 31 -
difficult interpretation of the results generated by anisotropic materials; also, the heat source must
provide a uniform energy input, or the results may be influenced; external heat sources, such the room
illumination, must be uniform. Furthermore, the detection of defects in thicker components requires an
increase in the thermal energy given to the part: there is a compromise since the resin must be able to
withstand the heat load without affecting its properties. [67],[70],[71]
Figure 2.25 – Schematic of a thermography setup via external irradiation [72]
Surely, there are more methods available that deserve to be compared with PAUT, but the last one that
is mentioned here is radiography. As it is known, the applications of radiography testing are quite
different, ranging from medicine to scanning devices at airports. X-rays are also used to inspect
components: ionizing radiation is used to bombard the part, going through its thickness and is then
absorbed by a radiation sensitive film. Zones with different radiation absorption properties are revealed
as having different shades of grey in the developed film. Advanced techniques also exist, such as
tomography, which produces a 3D representation of flaws. When compared with PAUT, radiography
produces results with better resolution, and is able to inspect thicker components. One of the major
drawbacks of its use is the hazard it poses to human health, due to the radiation ionizing effect. Besides
this, via the use of radiography, the depth of the defects cannot be characterized.
To conclude, the use of each one of these methods is a compromise between several constraints, and
the decision depends largely on the method’s inherent cost, time necessary to perform the inspection,
and the criteria defined to characterize the defects.
2.4 Chapter summary
Along this chapter, several remarks are made about the current state of the art in composite technology
and inspection in the aerospace industry. The historical background on the use of composites is
depicted, as well as the applications of NDT. Regarding CFRP components, the author discusses the
manufacturing processes, and presents an overview to the CLPT model, together with the main damage
mechanisms in composites. Regarding ultrasonic non-destructive testing, the fundamentals of sound
propagation are discussed, together with the equipment, parameters and associated obtained data.
Advanced methods and algorithms such as PAUT and SAUL are presented, together with comparisons
to other inspection methods. This chapter highlights the added value of this thesis: it aims to surpass
the difficulties in inspecting the critical CFRP components used in the aerospace industry, using a
method that can be adapted to the characteristics of this anisotropic material and to its complex shaped
geometries.
- 32 -
3 Acoustic Model
Within the third chapter of this work, focus is given to the study of the ultrasonic wave propagation inside
a CFRP test component. The author addresses the main factors that influence the way sound
propagates, is attenuated, and is used to discover defects. From the work described in this chapter, it is
possible to evolve to a physical inspection of composite structures, thus confirming the methodologies
developed.
3.1 CIVA software overview
The commercial simulation and analysis software CIVA NDE 11, developed by the French company
EXTENDE [73], is a tool developed specifically for NDT applications. This tool allows the user to study
different inspection methods, decide which one to use, and then adapt the chosen one to the problem
at hand.
Figure 3.1 – CIVA software [73]
The software consists of three distinct modules, each one adapted to different inspection methods:
ultrasonic inspection (UT), radiography (RT) and Eddy current3 (ET). In this work only the UT module is
addressed.
The main advantage in the use of CIVA is the reduction of the time it takes to decide on the several
parameters needed for an inspection: probe selection; sound beam specifications, depending on the
focal law(s); likelihood of detecting and characterizing defects. When software such as this is not used,
experimental setup testing has to be made, increasing the amount of time it takes to prepare an
inspection procedure.
The UT module allows the computation of complex inspection scenarios, with the PAUT technique. The
software gives the user a wide range of options to control. It is possible to simulate contact and
immersion testing, single transducer or phased array. Various types of probes can be studied, giving
the possibility to decide what probe to use, or to buy, to suit an inspection procedure. In this work, the
probes that are assessed are part of the wide range of equipment available at ISQ. CIVA, based on a
semi-analytical algorithm, has the physical laws that rule sound propagation as the foundation of the
simulation tool, contributing to establish direct validation and comparison with the physical tests.
3 Inspection method only valid for conductive materials.
- 33 -
3.2 CFRP test specimens
3.2.1 Calibration specimen
In order to prepare the procedures to inspect several sections of an airplane, it is necessary to account
for the characteristics of the parts that might need to be inspected. One of the ways to do this, is via a
calibration specimen, with various features. For the CompInspect project, a monolithic CFRP calibration
specimen is developed and manufactured by Optimal Structural Solutions, Lda. This calibration
specimen consists of several CFRP plies stacked together and cured to form a composite with several
thicknesses, that simulate the various sections of a given airplane model. A 90º L-shaped stringer is
bonded to a section of the test specimen, using epoxy adhesive. Several artificial rectangular defects,
made from Teflon®, are embedded in the calibration specimen. These defects are placed between the
plies, at several depths and locations, to simulate delaminations. There are five distinct areas in the
component, displayed in Figure 3.2.
Figure 3.2 – CFRP calibration specimen specifications. All dimensions are in millimetre.
Due to intellectual property rights, detailed information about the fibres and resin cannot be disclosed in
this work. However, it can be said that the fibre is a woven prepreg, with the mat having an areal weight
of 0,204 kg/m2 and a thickness of 0,234 mm. The fibre volume fraction is 61,5% and the stacking
sequence is [+45°/−45°]n. The fibre and resin are laid up in a metallic planar mould, and vacuum
bagged with an autoclave cure cycle. Note that both P4 and P5 have the same thickness; the only
difference is that the latter has the glued stringer. The Teflon® inserts have a thickness of 1mm, and
are placed in the component according to the information in Figure 3.3 and Table 3.1. The defects mimic
delaminations, at several depths. Dimensions are the defects’ width and length.
- 34 -
Figure 3.3 – Defects’ positioning and numbering
Table 3.1 – Defects in the CRFP calibration specimen
Defect ID Area ID Plies
Design values
Dimensions Depth
W [mm] L [mm] [mm]
1
P1
28 plies
5,7 5,2 1,7
2 5,5 5,2 3,5
3 5,0 4,9 5,2
4 2,9 4,4 1,7
5 3,0 4,5 3,5
6 3,2 3,1 5,2
7
P2
38 plies
3,3 4,3 3,0
8 7,8 8,3 3,0
9 4,9 5,3 3,0
10 3,4 3,2 6,0
11 8,3 8,0 6,0
12 5,4 5,6 6,0
13
P3
48 plies
3,8 4,6 2,9
14 9,8 9,1 2,9
15 5,6 5,8 0,4
16 5,1 4,6 8,7
17 3,2 4,4 8,7
18 8,4 8,3 8,7
19 5,3 5,0 5,8
20
P4
18 plies
4,9 6,1 3,5
21 5,3 4,9 2,3
22 5,6 5,4 1,2
23 8,8 8,7 3,5
24 9,0 8,4 2,3
25 8,3 8,5 1,2
- 35 -
The first step to model the calibration specimen in CIVA is defining the geometry. For this component,
as it has a plane of symmetry along its width, the transversal section is drawn (Figure 3.4).
Figure 3.4 – Calibration specimen transversal section
The coloured lines represent the different interfaces between the part and the medium surrounding it,
as well as discontinuities within the part itself. In detail:
1. Entry wall for the acoustic beam
2. Interface between distinct areas of the component
3. Side wall
4. Back wall
After introducing the defects in the correct locations, and extruding the part along the width, the shape
of the component is ready to be analysed. Is it necessary to divide the component into the several areas,
P1-5, to account for the different material characteristics (number of plies, for example).
3.2.2 Fatigue test panel
Besides the calibration specimen, another component is also evaluated in this work. A representative
section of an airplane, in the form of a CFRP square, planar panel, having a set of glued reinforcing
longitudinal omega stringers (named after their characteristic shape), is chosen to access the successful
implementation of the PAUT in composites. Particularly, the execution of the SAUL algorithm is tested
in the complex-shaped stringers. The overall shape and dimensions of the component are shown in
Figure 3.5.
In this Figure are also shown the several locations where the panel suffers impacts. The impacts are
performed before the panel is stressed in several fatigue tests. The omega stringer analysed in this work
is the one where impacts number 1 and 2 are done.
- 36 -
Figure 3.5 – Fatigue test specimen’s overall dimensions and impact locations
The specifications for the impact tests, impact position and energy, are shown in Table 3.2.
Table 3.2 – Specifications of the impact tests
Impact Nr. Position
Energy [J]
X [mm] Y [mm]
1 110 130 13
2 590 130 10
3 400 255 9
4 720 255 12
5 250 280 15
These are small energies, and the indentations usually cannot be quantified by visual inspection, since
the damage may be more critical in the inner layers of the composite.
For this component, due to confidentiality arrangements, no specific material specifications are
mentioned in this work. Moreover, the real geometrical details of the omega stringers are not disclosed
as well. But, as an analysis has to be performed for the inspection of the omega stringers, a hypothetic
shape is modelled in the CAD software DS SOLISWORKS©, to be imported to CIVA for analysis. This
geometry is shown in Figure 3.6.
- 37 -
Figure 3.6 – Generic shape for an omega stringer (dimensions in millimetre)
3.3 Material properties
CIVA allows the user to select a wide range of materials, available from an internal library. Since no
information is given about the specific fibre and resin, it is decided to use the generic existing material
properties. The used default specifications for the epoxy resin and carbon fibre filaments are listed in
Table 3.3.
Table 3.3 – Epoxy and carbon fibre material specifications from the CIVA library
Epoxy Carbon Fibre
Density 1230 kg/m3 1670 kg/m3
Material constitutive model Isotropic Transversely Isotropic
𝐂𝐥 2488 m/s NA
𝐂𝐭 1134 m/s NA
Besides these parameters, CIVA also requires other inputs. It is defined: Vf = 61,5% and a fibre filament
diameter = 0,005 mm.
To obtain the stiffness matrix of the composite laminate, essential for the calculation of the acoustic
field, two steps are necessary: first, CIVA uses a homogenization algorithm that treats the information
available from the material lamina properties, to calculate the [Cij] entries for this single ply. Afterwards,
and considering the information inserted about the stacking sequence, a second stiffness matrix is
calculated via a second algorithm.
The first algorithm, described in the paper submitted by Lonné et al. (2004), is based on a model that
couples viscoelastic and scattering losses of energy of the sound beam travelling through a fibrous
composite. This model treats the complex wave behaviour, due to a composite’s anisotropy, and
considers the attenuation occurring in the matrix (viscoelastic losses) and in the fibres (scattering
losses). The material densities, fibre volume fraction and wave frequency are the main parameters that
are computed to calculate a complex-valued wavenumber. The real part of this wave number is related
to the dependence on the frequency of the sound velocity, while the imaginary part commands the
attenuation-frequency relation. This complex wave number is then used to calculate the entries of the
stiffness matrix. Detailed information can be found in [74].
- 38 -
For laminates with several plies, CIVA uses another homogenization method, developed by Deydier et
al. The purpose of this method is to simplify the properties of the laminate by reducing it to a
homogeneous equivalent whose elastic constants are determined by a semi-analytical method. In short,
this method traces the direction of the energy ray path as it propagates through the laminate, then
synthesizes the equivalent slowness curves on the basis of the orthogonality between slowness
surfaces and the direction of energy rays. It is subsequently possible to deduce an orthotropic elasticity
matrix that characterizes the equivalent material. For further details, please refer to [75].
Using both CIVA’s default parameters and the information available from the fabrication of the
component, the transversely isotropic stiffness matrix for the single CFRP ply is obtained:
[Cij] =
[ 145,312 5,316 5,316 0 0 05,316 12,615 5,847 0 0 05,316 5,847 12,615 0 0 00 0 0 3,384 0 00 0 0 0 5,28 00 0 0 0 0 5,28]
(3.1)
Also, for the laminate, following the orthotropic material model, CIVA calculates the following stiffness
matrix:
[Cij] =
[ 76,422 115,387 5,468 0 0 0115,387 76,401 5,478 0 0 05,468 5,847 12,615 0 0 00 0 0 4,332 0 00 0 0 0 4,332 00 0 0 0 0 36,824]
(3.2)
Several parameter studies are carried out to understand the variations of the entries of the stiffness
matrix, both for the single ply, and for the laminate. From this information, and using a symbolic
calculation software, Maple™ 17, a calculation method is developed to obtain the engineering constants.
The code is available in the Appendix A: Maple code. This method takes the theory compliance matrix,
inverts it to obtain the stiffness matrix and then equals the latter to the numerically filled matrix from
CIVA. The Maple™ software symbolically manipulates the expressions to solve the non-linear equation
system, obtaining the engineering constants. Considering the single ply, the following values are
calculated:
{
E1 = 142,250 GPaE3 = 9,848 GPaν13 = 0,288ν21 = 0,0199G13 = 5,28 GPa
(3.3)
The anisotropic behaviour of the single CFRP ply can be observed, notorious due to the difference
between 𝐸1 and 𝐸3. The values of the engineering constants of the complete laminate, with the
previously mentioned specifications, are also calculated via Maple™:
- 39 -
{
𝐸1 = −98,489 𝐺𝑃𝑎𝐸2 = −98,449 𝐺𝑃𝑎𝐸3 = 12,303 𝐺𝑃𝑎𝐺12 = 36,824 𝐺𝑃𝑎𝐺23 = 4,332 𝐺𝑃𝑎𝐺13 = 4,332 𝐺𝑃𝑎𝜈12 = 1,527𝜈23 = −0,227𝜈13 = −0,23
(3.4)
It is important to stress the fact that the values in the orthotropic laminate results cannot be directly
compared with mechanical test data for composites! For example, negative values for the Young’s
modulus, or Poisson’s ratios greater than 0,5 do not cope with the theoretical defined limits. Usually,
E > 0 and −1 < ν < 0,5. The main reason behind the work developed with the software Maple™ is to
understand the evolution of the stiffness matrix values, when imposing parameter variations, such as
the Vf.
The results from the single ply, transversely isotropic composite studies are represented in Figures
Figure 3.7, Figure 3.8 and Figure 3.9.
Figure 3.7 – Evolution of the Young’s modulus evolution vs Vf
- 40 -
Figure 3.9 – Evolution of the Poisson’s ratio evolution vs Vf
From the above plots, all constructed based on the single ply composite, it is possible to observe the
linear variations of E1, E2 and ν12. As for G12 and G23, these values vary exponentially. The obtained
results are realistic, since it can be correctly inferred that a single laminate, having a higher fibre volume
fraction, will possess higher mechanical properties, such as the Young’s modulus.
The author also evaluates the complete laminate, but the results are not so promising. Due to the fact
that the obtained mechanical properties lie outside the common theoretical knowledge, further work
would have to be developed as to understand the influence of parameter variation in the stiffness matrix
and consequently, in the mechanical properties. Since this effort is not the main focus of this work, it is
left as a future task. For more details, please refer to Chapter 6.
3.4 PAUT probes
In this sub-chapter, some details on the most relevant PAUT probes used in this work are given. Even
though other probes are tested (such as a 128 elts linear probe), they are not detailed here, as the
obtained results are not so relevant.
Figure 3.8 – Evolution of the shear modulus evolution vs Vf
- 41 -
3.4.1 Probe selection criteria
From the range of probes available in ISQ, a set of four PAUT are chosen for this project. The
established criteria for the probe selection is:
1. Immersion probes
2. Wide frequency range
3. Linear and 2-D matrix probes
4. Probes with a planar active area, with no mechanical focusing
The above requirements come from the need to use ISQ’s automatic immersion inspection system,
prepared to perform fast and precise acquisitions. Also, since CFRP proves to be a challenging material
to inspect, a broader range of frequencies is addressed, to account both for attenuation and high
definition in defect characterization. Finally, due to the need to inspect complex structures, linear and
2-D matrix, planar probes are required when applying adaptive algorithms.
3.4.2 Characteristics of the selected probes
The four selected PAUT probes are all from the French manufacturer IMASONIC. [76] Their main
characteristics are disclosed in Table 3.4 and Table 3.5.
Table 3.4 – Specifications of the linear probes
IMASONIC 6091 IMASONIC 6553
Type Linear Linear
Number of Elements 32 elts 32 elts
Nominal Frequency [MHz] 10 4
Bandwidth (-6 dB) ≥ 60% ≥ 60%
Aperture [mm] 9,87 x 7 25,45 x 8
Table 3.5 – Specifications of the 2-D matrix probes
IMASONIC 10977 IMASONIC 10978
Type 2-D Matrix 2-D Matrix
Number of Elements 16 x 4 = 64 elts 16 x 4 = 64 elts
Nominal Frequency [MHz] 5 3,5
Bandwidth (-6 dB) ≥ 50% ≥ 50%
Aperture [mm] 12,7 x 4,7 19,95 x 7,05
- 42 -
The probes described in these tables are all modelled in CIVA, allowing the evaluation and creation of
inspection setups for CFRP structures.
In the following pages, more emphasis is given on the presentation of data relating to the 2-D Matrix
probes, despite the fact that several CIVA parameter iterations are also performed with the linear probes.
This author’s decision comes from the fact that it is necessary to account for the complex geometry
CFPR parts, for which the goal to obtain the best possible results is grasped via probes with an inherent
potential to provide results with better quality. Hence, the 2-D matrix probes are used for the inspection
of planar and complex geometries, and the linear probes only for planar geometries.
3.5 Parameter case studies
When working with any engineering software, the user must first try to understand what are the basic
principles applied to calculate and produce results. The calculated results cannot be taken as a universal
truth; an engineer has to be able to criticise and comprehend why and how these results appear. With
this aspect in mind, in the following topics some tests performed in CIVA are described, to evaluate the
changes caused by certain parameter variations. This is not an exhaustive list, only parameters
considered to be the most important are addressed.
3.5.1 Signal characterization
After introducing the probe specifications in the programme, it is necessary to properly define the electric
signal used to excite the piezoelectric elements. CIVA offers the user the possibility to apply three
different signal types: Gaussian, Henning, and a user-defined, imported signal.
To properly create the signal, the software requires the following inputs:
Type of signal
Nominal frequency of the probe
Bandwidth (relative) at -3, -6 or -12 dB
The sampling frequency for the signal can be user-defined, but it is also possible to request the software
to perform an automatic calculation, assuring the frequency is high enough for a reliable result.
The signal plotted is an amplitude as function of time. In the next three figures, Figure 3.10, Figure 3.11,
and Figure 3.12, the signal variations when modifying the inputs above are exemplified.
Starting with the differences between the two mentioned signal types: when using the same sampling
frequency, bandwidth and frequency, the Hanning signal presents a more damped oscillatory behaviour.
- 43 -
Figure 3.10 – Gaussian and Henning signals
Analysing the effect of the relative bandwidth, if all the remaining parameters are kept equal, it can be
observed below that, with the increase in absolute values of the reference value, the overall damping of
the wave signal decreases.
Figure 3.11 – Influence of the relative bandwidth in the electric signals
The last figure of this sub-chapter contains the two plots of signals with distinct frequencies. The different
wavelengths can be seen, as well as the fact that the peaks and valleys have always the same amplitude
and number of repetitions.
-100,0
-80,0
-60,0
-40,0
-20,0
0,0
20,0
40,0
60,0
80,0
100,0
1,5 1,7 1,9 2,1 2,3 2,5 2,7 2,9
Am
plit
ud
e [
%]
Time [ms]
Gaussian
Hanning
-100,00
-80,00
-60,00
-40,00
-20,00
0,00
20,00
40,00
60,00
80,00
100,00
1,5 1,7 1,9 2,1 2,3 2,5 2,7 2,9
Am
plit
ud
e [
%]
Time [ms]
-3dB
-6dB
-12dB
- 44 -
Figure 3.12 – Electric signals with different frequencies
Following the parameter variation tests above, and also referring to the probe’s manufacturer
information, the reference signal to use during the simulations has the following specifications:
Signal type: Gaussian
Frequency: dependent on the probe
Bandwidth: 60% at -6 dB
Phase: 0º
3.5.2 Attenuation influence on the acoustic field
After modelling the shape and material of the CFRP calibration specimen, it is possible to obtain
information from CIVA regarding the attenuation of the sound beam. Attenuation, i.e. a drop in the
signal’s amplitude as the sound waves travel along the part, is calculated in CIVA from the information
of the material’s stiffness matrix, using the algorithm referred before, from by Deydier et al. The
attenuation has a frequency dependence; the computational calculated values for the minimum and
maximum frequencies used are shown in Figure 3.13.
-100,00
-80,00
-60,00
-40,00
-20,00
0,00
20,00
40,00
60,00
80,00
100,00
0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4
Am
plit
ud
e [
%]
Time[ms]
3,5 MHz
5 MHz
- 45 -
Figure 3.13 – Attenuation plots obtained from CIVA
The remaining values that can be obtained from the linear behaviour of the plot are shown in Table 3.6.
Table 3.6 – Theoretical attenuation values
Frequency [𝐌𝐇𝐳] Attenuation [𝐝𝐁/𝐦𝐦]
3,5 0,709
4 0,811
5 1,015
10 2,034
Some tests are performed to evaluate the signal’s amplitude loss, in order to confirm if the attenuation
effect should be accounted for or not. Both 2-D matrix probes are used: all simulation parameters for
the two scenarios are equal, only the probe’s specifications are changed. The sound beam is focused
on a single point, placed at a 5 mm depth of the surface of the component.
3.5.2.1 Attenuation case study 1: 5 MHz probe
For the 5 MHz probe, considering the same point in the component, it is observable a reduction of 6,2 dB
in the signal amplitude when the attenuation effect is taken into account. This can be seen in the A-scan
plot of Figure 3.14, where the default black signal is compared with the attenuated red signal.
- 46 -
Figure 3.14 – A-scan, attenuation comparison for the 5 MHz probe
CIVA also provides information concerning the distribution of the energy of the sound beam of the
section being evaluated, via a colour plot. In Figure 3.15, in the image on the left, no attenuation exists.
A large focal spot can be seen, marked by the rectangle surrounding the area with the higher acoustic
pressure (in light blue). The focal spot is calculated in relation to a -6 dB amplitude loss. It is also visible
that the sound beam in the cross-section on the left has a better penetration.
Looking at the right image in Figure 3.15, where the attenuation is considered, two changes can be
observed: there is a reduction of 9,22 mm2 in the focal spot size, and an upwards shift of the maximum
acoustic pressure zone, causing a discrepancy with the initial objective of having a focal spot at a 5 mm
depth on the part.
- 47 -
Figure 3.15 – Focal spot without and with attenuation, 5 MHz probe
3.5.2.2 Attenuation case study 2: 3,5 MHz probe
A comparison is made between the 5 MHz and the 3,5 MHz probe, in order to understand the influence
of the frequency-dependent attenuation. In the 3,5 MHz probe, the reduction in the signal amplitude is
smaller: 4,6 dB. This effect can be seen in Figure 3.16.
Figure 3.16 – A-scan, attenuation comparison for 3,5 MHz probe
Analysing the focal spot in Figure 3.17, a decrease of 5 mm2 is visible, as well as the same upwards
movement of the maximum acoustic pressure zone, although less than before.
- 48 -
Figure 3.17 – Focal spot without and with attenuation, 3,5 MHz probe
Summing up, the two case studies confirm CIVA’s capability to account for the attenuation effect,
considering different frequencies. This test also confirms the need to account for the attenuation in
upcoming simulations, providing a better accuracy in results, at the expense of an increased
computational time.
However, as will be in Chapter 4, the real attenuation values are even higher than the values calculated
by CIVA, and measures have to be taken to deal with this issue.
3.5.3 Aperture influence on the acoustic field
The probe aperture, i.e. the number of elements used, and how it affects the quality of the expected
results, is also subject of study. The goal is to understand how the variation in aperture affects the
capacity a probe has to focus the sound beam energy at a single point, comprehending at the same
time how the acoustic pressure changes with the decrease of the number of active elements.
A point at 0,5 mm of depth is considered and analysed via four case studies:
a) 64 elements
b) 48 elements
c) 32 elements
d) 16 elements
Only the emission laws are affected by the element reduction in the case studies; in the reception, the
entire aperture is always used. Also, the discarded elements are always the ones in the outer part of the
probe. The decision to study a point so close to the entry surface, is due to the fact that it is complicated
to detect discontinuities in this area. Since in the entry surface exists an interface echo, it is often difficult
- 49 -
to differentiate this first echo, and the echo of a near delamination, for example, especially when having
a low signal-to-noise ratio.
In Figure 3.18 is exemplified a 2-D matrix probe element numbering, as well as frontal and isometric
views of the inspection setup, used on the P1 area of the CFRP calibration specimen. This study uses
this self-developed numbering setup, to easily reduce the number of elements being excited: case a)
uses the entire range, case b) uses the element range 17-64, etc.
Figure 3.18 – Overview of the case study a)
In the figure:
1. Arrangement and numbering of the elements
2. Delay law representation
3. Probe
4. Component
5. Computation zone
6. Focal spot
For this test, the 3,5 MHz probe is used, with a 25 mm water path; it is assured focalization is performed
inside the near field zone. Considering the aforementioned case studies, the obtained results for the
acoustic pressure are displayed in Figure 3.19. It can be observed that, when the full aperture is used,
the probe has a better ability to focus on the desired spot. When using only 16 elements, the sound
beam is not so efficient in concentrating the energy; this can be seen by the spreading of the colour
gradient, particularly in pink and red.
- 50 -
Figure 3.19 – Acoustic pressure results for the aperture variation case studies
To better quantify the effect of the aperture in the amplitude response, CIVA gives the user the option
to compare directly the four case studies. In Figure 3.20 are represented the amplitudes obtained for
the studied focal spot. The black line represents the reference amplitude, when using the entire probe
aperture. The red, blue and green lines represent cases b), c) and d), respectively. The three latter are
comparisons with the reference black line. The 2,25 mm value represents, in fact, the 0,5 mm depth
mentioned earlier.
Figure 3.20 – Amplitude response for the aperture variation case studies
As expected, it can be observed that the amplitude drops when decreasing the number of elements
being used. The difference when using up to half the probe’s elements is not critical, but when using
only one fourth, 16 elements, this effect is more notorious, as it translates into a drop of around 4 dB. It
must be mentioned that the deeper the chosen focal spot is, the larger is the effect the probe aperture
has, due to the attenuation.
- 51 -
Also, it is important to notice that, only by using a water column, via an immersion inspection technique,
it is possible to focalize in a point so close to the interface surface. If a contact technique was
implemented, focusing would not be possible in such an upper focal spot. This is a big advantage when
using immersion inspection systems.
3.5.4 Multi-point focusing technique
To be able to successfully detect discontinuities along the thickness of a composite, it may be necessary
to use a multi-point depth focusing technique, instead of doing a simple electronic scanning (refer to
Figure 2.23). This more advanced technique uses several focal spots at different component depths, in
an attempt to maximize the acoustic energy, and thus, defect response.
With CIVA, an analysis is performed, considering the thickest zone in the calibration specimen, P3, with
11,4 mm. Both 2-D matrix probes are used and, although more focal points could be used, the choice
came down to 3, to decrease the computational effort. The points are located near the entry interface,
in the middle of the section, and near the backwall. The full aperture is used, and attenuation is
accounted for.
For the 3,5 MHz probe, the computation results are shown in Figure 3.21.
Figure 3.21 – Acoustic fields for the 3-shot scenario, with the 3,5 MHz probe
It can be seen that, in the colour plot on the left, the area with the greater sound pressure, in light blue,
is directly on top of the focal point. Due to the effect of the attenuation, this no longer occurs in the
middle and lower focal spots.
- 52 -
This event is more noticeable with the higher frequency probe, with 5 MHz. The results for this probe
are below, in Figure 3.22.
Figure 3.22 – Acoustic fields for the 3-shot scenario, with the 5 MHz probe
Since this probe with a higher frequency is more attenuated, it becomes increasingly difficult to receive
a good quality signal from points located in deeper zones of the component.
A comparison of the acoustic field sizes is done, to determine the differences between both probes,
applying the -6 dB drop criteria; the focal spot is the deepest one. Observing Figure 3.23, a reduction of
15 mm2 in the acoustic field is noticeable, when comparing both probes. Moreover, neither probe can
effectively penetrate the part in its total thickness: the signal loss in the desired focal spot will always be
higher than -6 dB.
Figure 3.23 – Acoustic field variation for the deeper focal point
- 53 -
3.6 Amplitude differences between interface and backwall echo
It is important to evaluate the effect of the energy loss along the component’s thickness, particularly the
difference between the interface echo and the backwall echo amplitudes. Based on this information, the
loss in amplitude of the backwall echo can be predicted: if this attenuation is very high, in a limit situation,
it may cause the complete disappearance of this echo in when performing an inspection, even more
probable if the signal has a low signal-to-noise ratio. In Table 3.7 there are some results obtained from
computational studies performed in CIVA.
Table 3.7 – Amplitude differences between interface and backwall echoes
3,5 MHz 5 MHz 10 MHz
Full aperture, no focalization -7,6 dB -9,9 dB -12,8 dB
Half aperture, no focalization -7,6 dB -9,9 dB -12,8 dB
Full aperture, with focalization +3,8 dB -0,9 dB -1,5 dB
Half aperture, with focalization -1,3 dB -7,1 dB -8,6 dB
The focalization is done considering a point very near to the backwall (at 11,3 mm depth), to attest for
the minimum possible differences between both echoes.
The awareness of these losses is important when setting up the inspection parameters during the
physical testing, namely the amplitude gain adjustments for the screen height of the interface echo -
commonly set at 80% of full screen height (FSH) - and the positioning of the software acquisition gates.
Despite considering full aperture scenarios, when performing the acquisition it is not common to use this
setup. The reason concerns the fact that using a complete aperture increases exponentially the
inspection time, since the PA probe behaves similarly to a conventional single element probe, and
acquires the results along a single line, instead of returning results from the electronic scan complete
area.
Analysing the results it is noticeable that, when a null delay law is used, the relative differences between
the interface and backwall echoes are equal. Since the signal type and the material used are the same,
the aperture changes do no influence the individual results. However, if the signals are compared
directly, the large aperture signal has a higher energy, and as such appears larger in the A-scan.
The increase in the strength in the backwall echo when performing focalization is also evident. There is
a stronger backwall echo when focalization is used and, as such, the differences between interface and
backwall echo are alleviated, thus obtaining a better signal. For this reason, whenever necessary,
focalisation will be used, particularly for thicker component sections and deeper defects.
- 54 -
3.7 Defect characterization
After the parameters that influence the sound field are understood, the next step is to address the
characterization of the embedded defects in the CFRP test specimen. These are modelled in CIVA,
following the project information from Figure 3.3 and Table 3.1.
Figure 3.24 – CFRP test specimen with modelled defects
The objective is to realize if, with the parameters previously studied, it is possible to detect the defects
embedded in the component. For simplicity, here is only considered a set of defects that compose a
worst-case scenario: defects located in P3, with small dimensions, and located at different depths. If
these defects are well characterized, it can be assumed that larger defects, and defects located in
thinner parts of the component, will also be found and sized.
Hence, the defects that are tested in the development of the inspection setups are the numbers 15, 16
and 19. These are located in near the interface surface, near the backwall and in the middle of the part,
respectively.
In Figure 3.25 are displayed the A-scan and B-scan for the defect number 19. These results are obtained
with the 5 MHz 2-D matrix probe, using the simple electronic scanning technique, with half aperture for
the emission, and full aperture for the reception. No focalization is used. When comparing visually with
the colour plots for the 3,5 MHz probe, there are almost no visible differences. For a better interpretation
of the results, the several echoes are labelled. Since no focalization is used, as expected the higher
echo response comes from the interface water-CFRP, seen either in the large amplitude (in absolute
value) of the echo, or by the light blue colour in the B-scan.
- 55 -
Figure 3.25 – A-scan and B-scan for defect number 19
To better analyse the results, it is useful to compare the responses of the defects with the A-scan plot.
This representation allows for a direct comparison between inspections of the same defect with the two
matrix probes. In Figure 3.26 are overlaid the responses of defect nº 19, located at a depth of 5,8 mm.
The black plot represents the 3,5 MHz probe, while the red plot represents the 5 MHz probe.
Using the available analysis tools in CIVA, it is possible to compare the amplitude variations between
the two results. The figure below shows that the higher frequency probe has a loss in amplitude of
9,6 dB, for the same defect.
Figure 3.26 – Overlaid A-scans for defect number 19, from the 3,5 and 5 MHz probes
- 56 -
Identical analysis are performed on the two remaining defects, numbers 15 and 16. The results obtained
for the variations in amplitude, when comparing the 5 MHz probe with the 3,5 MHz, are summarized in
Table 3.8. These values represent the gain that is needed for the 5 MHz probe to achieve the same
signal amplitude as the 3,5 MHz probe.
Table 3.8 – Amplitude variations for the worst case scenario defects between the two 2D-matrix probes
Amplitude variation [dB]
Defect nº 15 -8,6
Defect nº 19 -9,6
Defect nº 16 -10,8
From this data, it can be confirmed that the deeper is located a given defect in the part, the higher the
amplitude loss for the higher frequency probe will be. This result is in accordance with the information
about a probe’s attenuation previously presented.
3.8 Complex geometry part inspection
The SAUL algorithm is not implemented in CIVA. Despite this, the generic omega stringer from Figure
3.6 is modelled in CIVA, in an attempt to better understand the inherent difficulties in inspecting complex
geometries.
The 2D-Matrix 5 MHz probe is used with half aperture, applying a multi-point focalization technique in
nine points located at half thickness. An overview of the inspection setup is shown in Figure 3.27, with
the first emission law for the first point, coincident with the first step of the electronic scanning.
Figure 3.27 – Multi-point focalization inspection technique applied to the reinforcing omega stringer
The black lines represent the several incident shots. It can be seen in this figure that, since the
component has surfaces that are not parallel to the probe, most of the reflected sound waves do not
return to the probe.
- 57 -
When CIVA finishes calculating the averaged acoustic field for all the shots, the obtained results are
those of Figure 3.28. It is very difficult to obtain good results for a complex geometry without the use of
a solution that adapts the sound waves to the part’s geometry. Even when focalizing the sound in specific
points, the results obtained show that no backwall echo exists. The absence of the backwall echo harms
the successful inspection of any component, rendering it impossible to know, for example, the depth of
defects.
Figure 3.28 – Acoustic field results for the stringer
From the above results, it becomes evident that the PAUT focusing methods developed so far do not
suffice when inspecting complex geometries, and thus the need to apply an adaptive inspection
algorithm, such as SAUL, is completely justified.
3.9 Chapter summary
Throughout this third chapter, the author presents and discusses the results obtained for the evaluation
of the main parameters that influence the PAUT of CFRP parts. Both linear and 2D-Matrix probes are
considered, as well as different test components. Among the analysed factors that influence the sound
wave propagation and ultimately, its interaction with the component, the following can be mentioned:
part shape and material, specifications of the used probes, signal, attenuation, aperture, and inclusion
of defects.
Knowledge is acquired on the development of an acoustic model for composite inspection, validated via
the physical inspection of three different components, which is the object of discussion in the next
chapter.
The importance of this chapter lies in the fact that, without previously understanding or studying the
methods of sound propagation in the components being inspected, it proves to be increasingly difficult
to correctly perform the physical inspection and to obtain reliable results. Therefore, the use of a
configurable computational tool, such as CIVA, contributes to saving time and reducing the costs
associated to performing NDT of CFRP components.
- 58 -
4 Physical Inspection
In this chapter, the immersion system developed in ISQ is used to perform PAUT acquisitions for the
CFRP calibration specimen, the fatigue panel and the omega stringer. Descriptions are given for the
overall system, the several acquisitions performed, and the tested parameters.
4.1 Overview of the system
4.1.1 Hardware
The inspection system used in this work is composed by three main components:
The immersion tank;
The MultiX PAUT acquisition unit;
A laptop for displaying results.
Starting with the first item, the immersion inspection tank: this instrumented, numerically controlled
system, is available at ISQ mainly for the development of testing procedures, which are used for
subsequent in-situ inspections. Depending on the type and size of a part, final inspections can be
performed as well.
Specifically in this work, it is used for the physical inspection of the CFRP components: the calibration
specimen, fatigue panel and omega stringer. Hence, it is applied in non-destructive testing of both the
planar sections and the complex geometries.
In Figure 4.1 the main components of the immersion tank are labelled, as well as the reference axis, for
linear and circular motion. For more details on the shape and dimensions of the immersion tank, please
refer to the Appendix B: Immersion tank dimensions.
Figure 4.1 – ISQ’s immersion tank
- 59 -
The general characteristics of the immersion system are summarized in Table 4.1.
Table 4.1 – Specifications of the immersion tank
Specification Value
Overall internal dimensions 690 x 2400 x 1250 mm
Maximum inspection area 2000 x 1300 mm
Maximum water capacity 2070 l
Number of linear axis 3 (X,Y,Z)
Number of rotation axis 3 (φ,ω,Ω)
Maximum scanning velocity 200 mm/s
The component to be inspected is placed inside the tank, on a levelled table, assuring the parallelism
with the scanning axis. The six degrees of freedom, all controlled numerically, to which is added a probe
holder that is designed to clamp probes of several sizes and shapes, give the system the flexibility
needed to inspect complex geometry parts. The X-Y inspections are the most used when dealing with
planar components, usually applying a raster scan pattern, as the one in Figure 4.2. For circular objects,
which are not within the scope of this work, the Ω-Z axis would be a proper choice. The control system
is also prepared to execute interpolation between several points, using all the motion axis: this feature
is used when inspecting the complex parts, creating an inspection pattern accordingly to the component
shape. The main advantage of using a system as this one, is the ability to assure a high precision of
control of the probe placement and motion, namely a constant water path, while guaranteeing high
accuracy and reproducibility of the motion.
Figure 4.2 – Raster scan pattern
The second component of the inspection system is the MultiX PAUT acquisition unit. This is a product
manufactured by the French company M2M, who specializes in equipment for NDT. [77] This unit is
prepared to comply with complex 2-D matrix probes, with up to 128 channels (probe elements), and to
apply the SAUL algorithm. The MultiX communicates with both the control system of the immersion tank
and the laptop, while being responsible for sending and receiving the signals to and from the probe.
With information on the location of the probe in relation to the component being inspected, the MultiX
calculates, in real time, the changes in the delay laws for the adaptive algorithm, while presenting the
information for A, B or C-scans to the user.
- 60 -
This information is visualized on the laptop, where the control of the MultiX is performed, and the results
are visualized. In Figure 4.3 are shown both the laptop used and the MultiX unit.
Figure 4.3 – Laptop and MultiX
4.1.2 Software
The immersion system uses a software developed in ISQ. This control software is created in LabVIEW©,
specifically for this purpose, and is tested by the author during the execution of this work, both for
debugging purposes and to assure user friendliness. The touch screen interface of the software is shown
below, in Figure 4.4.
Figure 4.4 – Immersion tank touch screen interface
- 61 -
In the lower left corner of the touch screen display are located the main control buttons: Start, Pause
and Stop, as well as the Home button, to zero out the system coordinates. Four coloured frames are
added to the image to highlight the main features.
Inside the light blue frames are the positions of the motion axis, as well as indicators for the limit stops
of a given axis. To use a local reference frame, reset buttons for the coordinate system are also included.
Inside the red frame are the buttons used to actuate the motion axis (linear or rotation), for both
directions of movement. The axis can be moved either in a normal velocity mode, or in a more accurate,
slow mode. The tabs on top change the scanning mode between the shown manual mode and an
automatic mode based, for example, on start and finish inspection points, with a raster scan between
them. It is in this latter mode that three-dimensional trajectory interpolations are performed.
The last frame, in yellow, gives the user visual information on the trajectory of the probe; in this example,
the interpolation between three points, in the form of a parabola, is shown (the interpolation calculation
uses the cubic spline method). Here are also included indicators for the connectivity status of the axis,
knobs for the velocity control, either in auto or manual mode, and a virtual LED that shows when the
immersion tank’s emergency switch is engaged.
The software used in the laptop is also developed by M2M, and is specific for their products: it is named
Multi2000. This software provides the user with a set of three main menus:
Configuration: here the user inputs the characteristics of the part being inspected, such as
material and overall dimensions, as well as the specifications of the probe. The delay laws for
the inspection setups studied and defined with CIVA are loaded here as well.
Parameters: after loading the delay laws, the user can apply a set of tools to prepare the
inspection in this window, particularly digital or analogue gain adjustments, placement of the
acquisition gates, Distance Amplitude Correction (DAC), among others. An A-scan, updated in
real time, is the main visualization method to understand and interact with the modifications in
the probe signal. It is also in this menu that the communication with the immersion tank is
accounted for, via the characterization of the position encoders attached to the motion axis. This
is undoubtedly the menu where more time is spent, in order to achieve good results in the
inspection;
Acquisition: in this last menu, a wide set of options are available to observe in real time the
acquisition being performed. The user can select the A, B or C-scans, available for a single
acquisition step of the raster scan, or for the entire component. Visualizations for time of flight
or amplitude are also available.
4.2 General physical studies
With the objective to improve the inspection procedure, a set of acquisitions is done on the CFRP
calibration specimen. These early acquisitions are performed with the linear, 10 MHz probe, easier to
configure initially. The set of results obtained can generally be extrapolated to the 2-D matrix probes.
The several iterations consist on a set of acquisitions, performed with different probe apertures (varying
- 62 -
the elements used in the emission law, in steps of four). The goal of this particular work is to acquire
data on the evolution of four parameters, disclosed below.
4.2.1 Gain
The plot in Figure 4.5 displays the evolution of the gain needed to bring the interface echo (abbreviated
as IF), between the water and the CFRP part, to a screen height of 80%. The more gain is needed and
applied, the greater the increase in the signal noise as well, harming the target to have a good signal-
to-noise ratio.
Figure 4.5 – Gain needed to IF echo @80% screen
As the energy of the emitted sound beam increases with the number of elements used, the gain values
decrease until a convergence behaviour is observed, from 16 elements upwards.
4.2.2 Maximum inspection velocity
When a lower number of elements is used in a phased array electronic scanning, the number of
iterations needed for that scanning to cover the entire probe aperture is larger. This number of iterations
is equal to the number of shots that need to be calculated and controlled by the MultiX acquisition unit.
Hence, as can be seen in the plot of Figure 4.6, to achieve a higher inspection velocity when performing
a scan, it is necessary to increase the number of elements being used in the electronic scanning. When
using few elements and a high velocity, blank sections appear in the result, representing losses of
information, due to the limit in the acquisition rate of the MultiX.
From the use of 20 elements onwards, the velocity converges to a maximum: the MultiX is working
below its acquisition rate limit, but the linear axis (usually X) is working at its top velocity (165 mm/s).
0,0
5,0
10,0
15,0
20,0
25,0
30,0
4 8 12 16 20 24 28 32
Gai
n
Number of Elements
- 63 -
Figure 4.6 – Maximum inspection velocity
4.2.3 Inspection step
To assure the entire component is covered in the inspection, it is advisable to have a certain overlay
between two consecutive passes in the raster scan. When the raster scan is performed as shown in
Figure 4.2, this overlay refers to the value entered for the Y inspection step. To obtain the value, the
following formula is used:
Inspection step [mm] = number of used elts × total aperture [mm]
total number of elts (4.1)
This formula provides an estimate for an initial value to use for the inspection step; actual measurements
on the maximum possible step value are performed: when the step is maximized, the total number of
raster scan paths decreases, leading to a lower final inspection time. The behaviour of the inspection
step evolution is documented in Figure 4.7.
Figure 4.7 – Inspection step
0,0
20,0
40,0
60,0
80,0
100,0
120,0
140,0
160,0
180,0
4 8 12 16 20 24 28 32
Insp
ecti
on
vel
oci
ty [
mm
/s]
Number of Elements
0,0
1,0
2,0
3,0
4,0
5,0
6,0
4 8 12 16 20 24 28 32
Insp
ecti
on
ste
p [
mm
]
Number of Elements
- 64 -
The plot presents a convergence when using up to 16 elements; after this value, the inspection step
value decreases continuously, until a minimum value of 0,5 mm is obtained for a full aperture. This
minimum value can be explained easily: when using a full aperture, the probe behaves as a conventional
single element probe, not performing an electronic scanning at all.
4.2.4 Total inspection time
The final test to be performed is the calculation of the total inspection time, in this case for the CFRP
calibration specimen, having the dimensions of 200 x 465 mm. The values displayed in Figure 4.8 are
calculated considering the part dimensions, the inspection step and the maximum inspection velocity.
The developed formula, for a rectangular component, is the following:
ttotal =W
Vmax(L
S+ 1)
(4.2)
Where:
ttotal [s] is the total inspection time;
W [mm] is the plate width along the Y axis;
Vmax [mm/s] is the maximum inspection velocity;
L [mm] is the plate length, along the X axis;
S [mm] the maximum inspection step.
Analysing the results, it can be observed that the minimum values are within the 16 to 20 elements
range. The minimum value, of 2 minutes and 12,3 seconds, is obtained when using 20 elements. Albeit
the difference between 16 and 20 is only 4,5 seconds, it must be mentioned that this is an inspection of
a small component: the larger the component, the more important this difference is.
Figure 4.8 – Total inspection time
0,00
50,00
100,00
150,00
200,00
250,00
300,00
4 8 12 16 20 24 28
Tota
l in
spec
tio
n t
ime
[s]
Number of Elements
- 65 -
To conclude, it is the author’s opinion that when using the mentioned 10 MHz probe to inspect a flat
component, the ideal parameters to use are:
Aperture of 20 elements;
Gain of around 6 dB;
Maximum velocity settings for the raster scan;
Inspection step of 4,3 mm.
As said before, similar studies can be done for the remaining probes.
This setup provided some of the results analysed ahead in this work.
4.3 Acoustic Impedance studies
With the knowledge of the thickness of the CFRP test specimen, and information displayed in the MultiX
A-scan view it is possible to accurately calculate the longitudinal wave velocity in the component. For
this material characteristics, the obtained value is: Cl = 3300 m/s.
After obtaining this value, and knowing the density of the material, its acoustic impedance can be
calculated. With the knowledge of the water properties as well, a quick estimation for the energy losses
when performing the inspection can be obtained.
In Table 4.2 the results of the calculations for two cases are given, one for the water/CFRP interfaces,
and another for the water/CFRP/epoxy interfaces.
Table 4.2 – Acoustic impedance calculation results
Water CFRP Epoxy
ρ [kg/m3] 1000 1501 1103
Cl [m/s] 1483 3300 2488
Z [Ns/m3] 1483000 4953300 2744264
R water/CFRP 0,29 R CFRP/water 0,29
T water/CFRP 0,71 T CFRP/water 0,71
R CFRP/Epoxy 0,08 R Epoxy/CFRP 0,08
T CFRP/Epoxy 0,92 T Epoxy/CFRP 0,92
Water->CFRP->Water
% sound returning to probe 14,6%
Decibel drop in backwall echo -16,70
Water->CFRP->Epoxy->CFRP->Water
% sound returning to probe 10,4%
Decibel drop in backwall echo -19,68
- 66 -
Since the presence of the epoxy glue between the 90º L-shaped stringer constitutes another interface,
it is expected that sound in this area has a larger reduction in the backwall echo. Added to these losses,
one must also take into account the attenuation of the sound beam along the component thickness.
4.4 Attenuation coefficient measurements
Aiming to completely characterize the attenuation suffered by the sound beam in the CFRP, a range of
measurements is performed, using both 2D matrix array probes. These measurements are performed
observing the decay in amplitude of the repetitions of the echoes of a signal, as shown previously in
Figure 2.16. To characterize the attenuation in function of the part thickness, measurements are done
in the whole range of thicknesses (P1 to P4) of the CFRP test specimen. The results obtained are
displayed in Figure 4.9.
Figure 4.9 – Attenuation measurements
Using the information above, the attenuation coefficients in this material for the two considered probes
are obtained and displayed in Table 4.3.
Table 4.3 – Attenuation coefficients, α
Probe frequency
f [MHz]
Attenuation coefficient
𝜶 [dB/mm]
3,5 2,1
5 2,6
y = 2,6x + 3,2R² = 0,99
y = 2,1x + 3,0R² = 0,98
10
15
20
25
30
35
4 5 6 7 8 9 10 11 12
Att
enu
atio
n [
dB
]
Tickness [mm]
5 MHz 3,5 MHz Linear (5 MHz) Linear (3,5 MHz)
- 67 -
4.5 SAUL algorithm testing
To account for the effectiveness of the adaptive algorithm, in order for the author to familiarize himself
with the way SAUL works, a simple experiment is performed. Using a round aluminium pipe, and the 5
MHz 2-D matrix array probe directly above its transversal section, the objective is to guarantee the
correct implementation and tuning of the algorithm parameters. After all the control parameters
associated with the SAUL algorithm (gain control, acquisition gates, elementary channels saturation,
etc.) are correctly calibrated and understood, it is possible to visualize the algorithm in action, as seen
in Figure 4.10.
Figure 4.10 – Example of the SAUL algorithm application
In the first B-scan shot, the adaptive algorithm is turned off. The probe’s entire aperture is being used,
with a null delay law (all elements are excited at the same time). It can be observed that only the central
part of the pipe, more parallel to the probe, is reflecting some energy back to the probe. The elements
on the edges of the probe are not receiving any energy at all.
As the SAUL adaptive algorithm is engaged, the sound beam frontal wave is shaped with the delay
laws, thus simulating the pipe’s round shape (as seen in Figure 2.24). The amount of energy that is
returning to the probe is considerably higher, as represented by the hotter colours. As expected, it takes
the information from four shots for the algorithm to successfully maximize the sound response, and to
present the user with a flattened view of the component.
This simple exercise proves the efficiency of the algorithm, and leads the way to the more complex
acquisitions evaluated in Chapter 4.7.2.
4.6 Calibration specimen testing results
In this sub chapter are shown the final acquisitions performed for the CFRP calibration specimen. The
probe used is the 10 MHz linear probe, with the parameters selected before. Although not publicised
here, several other acquisitions are performed, both with this and the other probes, to gather information
and analyse parameter variations, acquisition quality and the reliability of the inspection system.
Figure 4.11 contains the complete PAUT acquisition results for the CFRP calibration specimen. The
figure contains three views: two C-scans and a B-scan. The top C-scan reproduces the signal’s
amplitude, with the colour plot considering the screen height relative to the maximum echo value. The
- 68 -
B-scan uses the same scale, and derives from the A-A section cut indicated in the figure. Blue represents
no echo, and the scale varies up to red, for echoes above 80% of the recorded maximum. The middle
C-scan contains information on a time basis, with the recording of the time of flight of the sound beam.
Figure 4.11 – PAUT of the CFRP calibration specimen - complete results
In the above results, it is possible to see that all the embedded defects are visible. Due to the high
attenuation of the 10 MHz probe, at first the defects 16 and 19, located deeper in the P3 section, could
not be detected. Due to this, an analogue DAC is implemented in the software, slaved to the gate
detecting the interface echo, which allows the increase of the signal’s strength, thus providing better
results. This DAC has two different slopes, as displayed in Figure 4.12: the first slope is to account for
- 69 -
the acoustic impedance mismatch of the water and the CFRP, while the second slope targets the
attenuation. The need to use a DAC arises following the predictions regarding energy losses that are
made before, both the attenuation calculated in CIVA and the acoustic impedance losses.
Figure 4.12 – DAC curve for the 10 MHz probe and CFRP calibration specimen
Analysing the results in detail, the locations of the defects can clearly be seen, both in plane view and
along the thickness (represented in the B-scan). In the middle C-scan, the different colours of the defects
characterize their position: as can be understood when observing the colour scale, blue defects are
located near the top surface, while red defects are placed near the backwall.
Around the defects, a white zone can be seen: this white colour represents a zone with no recorded
echo. Due to the thickness of 1 mm of the defects, the fibres are deformed in this area, and the sound
beam is reflected in such a way that it does not return to the probe. The same effect can be seen for the
interface between the several sections of the part.
Another aspect worth mentioning is the effect the glued L-shaped stringer has on the echo’s amplitude:
the fact that another interface exists, with its associated impedance, decreases the signal strength in
the P5 section, as can be observed by the green colour. As the impedance of the epoxy adhesive is
more similar to that of the CFRP, when compared with the water, more sound energy passes along the
adhesive, with a lower acoustic pressure returning to the probe. Of course, the middle C-scan is,
nevertheless, displaying the same time of flight, since the thickness of the section where the stringer is
glued to is identical to P4’s.
Based on the obtained results, and using the criteria for an echo loss of -6 dB, represented in the colour
scale by the 40% yellow value, the real dimensions of the defects can be compared to the theoretical
ones from Table 3.1. The defect dimensions are calculated using a measuring tool embedded in the
Multi2000 software. The values obtained can be consulted in Table 4.4 and Table 4.5, including
maximum, minimum, average and standard deviation results.
0
10
20
30
40
50
60
0 2 4 6 8 10
Am
plit
ud
e [d
B]
Time [μs]
- 70 -
Table 4.4 – CFRP calibration specimen defect results
Defect ID Area ID Plies
Design values Real values Deviation
Dimensions Depth Dimensions
W [mm] L [mm] [mm] W [mm] L [mm] W [mm] L [mm]
1
P1
28 plies
5,7 5,2 1,7 6,2 5,4 0,5 0,2
2 5,5 5,2 3,5 5,1 6,1 0,4 0,9
3 5,0 4,9 5,2 5,6 5,0 0,6 0,1
4 2,9 4,4 1,7 3,2 4,2 0,3 0,2
5 3,0 4,5 3,5 3,2 4,6 0,2 0,1
6 3,2 3,1 5,2 3,2 3,6 0,0 0,5
7
P2
38 plies
3,3 4,3 3,0 4,2 4,2 0,9 0,1
8 7,8 8,3 3,0 7,7 8,2 0,1 0,1
9 4,9 5,3 3,0 5,4 5,2 0,5 0,1
10 3,4 3,2 6,0 3,4 4,3 0,0 1,1
11 8,3 8,0 6,0 8,1 8,6 0,2 0,6
12 5,4 5,6 6,0 6,7 5,8 1,3 0,2
13
P3
48 plies
3,8 4,6 2,9 4,0 5,0 0,2 0,4
14 9,8 9,1 2,9 9,0 9,7 0,8 0,6
15 5,6 5,8 0,4 5,5 5,9 0,1 0,1
16 5,1 4,6 8,7 5,0 5,7 0,1 1,1
17 3,2 4,4 8,7 4,0 5,5 0,8 1,1
18 8,4 8,3 8,7 9,0 8,0 0,6 0,3
19 5,3 5,0 5,8 4,0 6,0 1,3 1,0
20
P4
18 plies
4,9 6,1 3,5 5,7 6,4 0,8 0,3
21 5,3 4,9 2,3 5,4 5,1 0,1 0,2
22 5,6 5,4 1,2 6,0 5,8 0,4 0,4
23 8,8 8,7 3,5 9,4 9,2 0,6 0,5
24 9,0 8,4 2,3 9,8 9,0 0,8 0,6
25 8,3 8,5 1,2 8,9 9,1 0,6 0,6
Table 4.5 – CFRP calibration specimen deviation values analysis
Dimension deviations
W [mm] L [mm]
Maximum 1,3 1,1
Minimum 0,0 0,1
Average 0,5 0,5
σ 0,4 0,3
- 71 -
Analysing the results, the maximum deviation from the design values corresponds to 1,3 mm, while the
minimum deviation that is possible to identify is as small as 0,1 mm. This value highlights the quality
and resolution of the results that are possible to obtain via the use of PAUT.
4.7 Fatigue panel and omega stringer testing results
The next challenge for the PAUT system is the inspection of the fatigue and impact damage imposed to
the CFRP shown previously in Figure 3.5. The study of this panel is divided in two parts: the analysis of
the flat part of the panel, and the analysis of the glued omega stringer. In the latter, the efficiency of the
SAUL algorithm is put to the test. Figure 4.13 shows the PAUT of the flat panel and of the omega
stringer.
Figure 4.13 – Physical testing of the flat panel and omega stringer
4.7.1 Flat panel
The flat panel is instrumented with a set of sensors in both sides, as can be seen in Figure 4.14.
Figure 4.14 – Flat CFRP panel with sensors and impact locations
- 72 -
In the previous figure is indicated the reference frame used for the acquisitions. Only the inside section
of the panel is considered, which has a thickness of 2 mm. The frame around this section, having a set
of drilled holes, is used to secure the panel to the fatigue test rig, and has a 6 mm thick section. On side
A, the panel’s frontal side, two types of monitoring sensors can be seen: the first type consists of 10
optic fibre sensors, indicated by 1. The other type of sensor consists of two sensors (2.1 and 2.2),
embedded in the composite, and indicated by the green rectangles.
On the back side of the panel it can be seen a series of electrical wires, held in place with tape, and
used to collect the information from a set of eight strain gauges attached to the panel with adhesive.
These strain gauges are positioned inside the blue rectangles; some are placed near the impact
locations, marked by the red “x” symbols (the impact energies are indicated in Table 3.2). The stringer
in the bottom is analysed next in this work, together with the impact damage it sustains, indicated by 4.1
and 4.2.
After the impact tests are done, the panel is submitted to a series of fatigue tests. The panel is stressed
in sequences of 5000 tensile-compression tests, with loads of ±10000 N. These tests are performed
four times, with PAUT done between each test. Hence, the total fatigue cycles add up to 20000.
For the sake of keeping this work concise, only the final results for the flat panel are disclosed here.
Along the four iterations, it is possible to distinguish an increase in some of the defect types that are
detected. These defects are: delaminations, debondings, dents due to the impacts and areas with
insufficient resin.
The final acquisition, after the complete fatigue testing is performed, is presented in Figure 4.15. This
acquisition is performed with the 5 MHz 2-D matrix probe, with a null delay law.
Figure 4.15 – PAUT results for the flat panel. On the left, amplitude results; on the right, time of flight results.
- 73 -
In the latter image, built from the backwall echo signal, the signal’s amplitude measurements are
displayed on the left. On the right, the treated information from the time of flight is presented: the scale
shows the thickness measurements, for the calculated Cl = 3300 m/s.
Several defects are identified and labelled:
1. This defect matches the impact number 4, with an energy of 12 J. The PAUT results prove the
impact caused a delamination in the inner layers of the composite panel. Along the several
fatigue tests, an increase in the overall area of the delamination is observable, due to the stress
concentration in this area. From the initial value, prior to the fatigue tests, to the final PAUT
results, the identified delamination area increases about 245 mm2.
2. The identified area, with the shape of a ring, is characterized by having a large attenuation in
the backwall echo. Analysing the amplitude scale, this ring displays an attenuation of more than
40% in the yellow areas, corresponding to a loss of more than 6 dB. In the green areas, the
attenuation value is even higher. Both via visual inspection, and analysing the NDT results, it
can be assumed that this area is affected by manufacturing problems, causing the used CFRP
prepreg to have insufficient resin. Another cause could be a poor vacuum pressure in this area,
but since the overall thickness is the same (as seen by the green colour on the right), this theory
can be rejected. So, in this area, the amount of resin is too low when compared with the rest of
the composite, and it causes the sound beam to suffer a strong attenuation, mainly due to
energy scattering in the fibres, and to be affected by a poor sound transmission along the
composite. Throughout the fatigue testing cycles, no particular increase is identified in this area.
3. Around the embedded sensor number 2.1, a delamination can be identified, at about half of the
composite’s thickness. The presence of the sensor inside the composite acts as a stress riser,
affecting the material around it. Throughout the fatigue tests, this area increases around 1600
mm2. This large delamination could be observed from the very first PAUT, prior to the fatigue
cycles, thus pointing to a manufacturing problem, possibly a bad fibre compaction around the
sensor.
4. Around the embedded sensor 2.2, no particular delamination is observed prior to the fatigue
loading. However, for the same motive as of point number 3, the stress concentration in this
area originates an increase of the delamination area of around 260 mm2.
5. This area presents signal characteristics similar to those of area number 2, with the sound beam
suffering a strong attenuation as well. As the overall thickness of this area remains the same as
well, once more it can be assumed that this attenuation originates from a lack of resin in the
prepreg. Again, this statement is validated recurring to visual inspection of the panel outer
layers. No increasing of this area is identified after the fatigue testing of the panel.
6. The final defect identified in the panel is an adhesive debonding between an omega stringer
and the flat panel. It can be seen in Figure 4.16: it has the shape of a triangle. This defect is
manually fabricated by the author, to test if the PAUT could detect a debonding. From the
analysis of the bonded sections after the fatigue cycles, it is concluded that no debondings are
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present, but as the author wanted to take account of a defect of this type in the range of
detectable defects, it is decided to artificially fabricate one. This debonding has about 55 mm2.
Figure 4.16 – Debonding defect
To summarize, the capability of the PAUT system to successfully detect a wide range of defects is
proved. The results acquired here are thus capable of being transferred to an in-situ inspection of an
aircraft, both during common maintenance routines or, with more detail, during an airplane complete
overhaul.
4.7.2 Omega stringers
The next and final phase of this work, is the study of the reinforcing omega stringers. The chosen stringer
is the one subjected to impacts number 1 and 2, with 13 J and 10 J, respectively. These impacts are
done on the top section of the stringer.
The several stringer sections where the PAUT acquisitions are performed are indicated in Figure 4.17.
The following acquisitions outline only the results for planar and convex sections: since the probe
apertures used when inspecting the convex and concave sections are not the same; because it is not
possible to vary a probe’s aperture during the acquisition; and also due to the fact that the impact tests
are done on the top of the stringer (green and blue areas).
Figure 4.17 – Colour highlights for the several sections of the omega stringer
It is essential to stress the relevance of using the SAUL adaptive algorithm in the inspection of this
complex geometry. Next, in Figure 4.18 can be seen the differences between two partial C-Scans, one
performed without the use of the adaptive algorithm, on the left, and the other with SAUL engaged, on
the right.
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Figure 4.18 – Stringer inspection performed without and with the SAUL algorithm
Analysing the results, it can be said that the signal-to-noise ratio when using the adaptive SAUL
algorithm is higher. For the convex section, when the algorithm is not used, the signal strength drops
considerably, with only some of the elements being able to detect the returning echo: the black areas
represent areas where no echo exists, when SAUL is not used. When engaged, it becomes possible to
obtain results with an improved quality.
In Figure 4.19 the complete acquisition for the omega stringer is presented. To complete this inspection,
it is necessary to programme nine different probe positions and inclinations in the immersion tank’s axis,
to assure the probe is facing the several areas to be inspected. However, due to the use of the adaptive
algorithm, this positioning is does not have to possess an extremely high level of precision, which would
increase the overall inspection time: as the algorithm also adapts to geometry deviations, positioning
requirements are reduced.
Figure 4.19 – PAUT results for the omega stringer. On the top, amplitude results; below, time of flight results.
Once more, the top image concerns the amplitude measurements for the backwall echo, while the
bottom image is the thickness measurement, obtained from the calculated Cl.
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Two defects can be clearly observed in the image, inside the yellow frames. These defects, in the form
of near-surface delaminations, are the result of the impact tests. The analysis above is performed after
the fatigue loading is complete; as could be expected, the larger defect (on the left, with 11,3 x 24,6
mm), is originated from the 13 J impact, while the defect on the right (with 10,5 x 22,2 mm) exists due
to the 10 J impact. Moreover, the delaminations propagate along the plies, and are not located only on
the top of the stringer, but on the top edges.
The black lines in the image, as the one inside the blue frame, are originated by the electric cables that
power the strain gauges, and that do not allow the passage of sound into the composite.
In the image it is also possible to see some apparent undulations in the backwall echo, mainly on the
right. After some time analysing the obtained data and observing the part, the conclusion is that this
unevenness in the backwall echo actually originates in the manufacturing process: as the stringers are
manufactured in an open mould, using vacuum bagging, the folds that appear in the plastic bag when
the air is removed actually appear in the backwall echo results, once more showing the level of detail
that is possible to obtain when using PAUT.
4.8 Chapter summary
Along this chapter, the author discusses the work performed during the physical inspection of the several
CFRP components.
The chapter starts with an overview of the immersion inspection system existent in ISQ, of both the
hardware and software used. Then, before the final acquisitions are presented, some important
parameters of the inspection setup are tested and explained.
The data for the PAUT for the CFRP calibration specimen and flat fatigue panel are then presented and
discussed. Afterwards, the author reveals an acquisition made with the SAUL adaptive algorithm for the
omega stringer.
This chapter includes the most important results of the work developed by the author in ISQ. Via the
use of the available test pieces and inspection tools, it is proved that ultrasonic testing is a reliable and
accurate method of evaluating the CFRP components used in the aerospace industry.
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5 Conclusions
The use of CFRP composites in critical areas of an airplane poses an added responsibility to airline
companies. Not only an inhomogeneous, anisotropic material is a challenge to produce and repair, but
also the NDT methods and techniques that must be used for the evaluation and testing of these
components are more complex, when compared, for example, with the more common inspection
methods of metal parts. From the range of available testing methods, ultrasonic testing continues to play
a major role in evaluating a component’s integrity and in assuring the safety of structures subject, for
example, to fatigue loads and/or impacts. Only via the monitoring of the structural integrity of airplane
components, can major accidents be avoided and operating costs reduced.
This work intends to respond to the above challenge, via the development of ultrasonic inspection
procedures that can be used to examine aerospace components in situ. The success of this work
enables ISQ to assure its clients that the more detailed and time consuming testing operations,
performed in ISQ’s premises, can be directly transposed to a faster inspection performed either during
the manufacturing and assembly stages, or during regular maintenance and overhauling routines.
Nowadays, software such as CIVA is a valuable tool, since it can be used to reduce both time and total
inspection cost. Such a tool, when used by an engineer to prepare a NDT inspection, allows the user to
decide which probes to use, parameters to apply and also to understand the type of results that are
expected, based on components with embedded characteristic defects. Using CIVA, the author selected
four phased array ultrasonic probes to use in CFRP components, both with planar or complex
geometries. The selected probes are two 64 element, 2-D matrix probes, with frequencies of 3,5 and 5
MHz, and two linear 32 element probe, with 4 and 10 MHz. These choices are validated following a
series of tasks:
1. Characterisation of the CFRP components’ shape and material;
2. Understanding of the homogenization algorithms used by CIVA, to guarantee the calculations
performed by the software are correct;
3. Insertion of the probe characteristics, including signal;
4. Select computation parameters, based on the compromise calculation time vs result quality;
5. Testing of focal laws and inspection techniques, both without and with defects, during several
iterations, with the aim of defining the correct setups to be used in the physical inspection of
CFRP components.
After satisfactory results are obtained from the CIVA software, the author uses the numerically controlled
immersion tank developed in ISQ to perform the inspection of three CFRP components. A number of
acquisitions are performed, using the MultiX hardware and Multi2000 software. The high attenuation of
the CFRP, both due to sound scattering in the fibre and viscoelastic losses in the epoxy matrix, poses
an added challenge to the PAUT. The most critical scenario encountered, occurs when using the 10
MHz probe to inspect the 11,4 mm section of the CFRP test specimen: for this setup, if a null delay law
is used, the overall attenuation of the backwall echo amounts to more than 40 dB. Using a multi-point
focalisation technique, combined with an electronic scanning, the difference in amplitude between the
- 78 -
interface and backwall echo is reduced. For the planar components, several defects are characterized
successfully: delaminations, debondings and lacks of resin. Deviations as small as 0,1 mm are
measured regarding the defects’ size, emphasising the system’s precision.
However, for the complex geometry of an omega-shaped reinforcing stringer, also made out of CFRP,
not even using focalization techniques enables the successful detection of defects created by impact
loads. For those geometries, only when using a self-adaptive algorithm (SAUL), the embedded defects
are detected and characterized. Hence, it is proved the validity of the use of this algorithm, and its
applicability in the NDT of the complex geometry parts, common in the aerospace industry.
Concluding, this work has allowed the author to acquire and develop new knowledge in phased array
ultrasonic testing of composites. The objectives the author proposed himself to achieve, defined both
initially and during the work development stage, are complete, and can now be used by IST to continue
the academic work related to carbon fibre composite component inspection. As a final point, it can also
be said that ISQ now has the ability to supply the industrial aerospace market with the PAUT and SAUL
methodologies validated in this work, with a commercial solution that works, and can satisfy its clients’
NDT needs.
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6 Future Work
In this last chapter are given some suggestions of likely future studies that can be performed, aiming to
strengthen the understanding of the physical mechanisms involved in the PAUT of CFRP components:
1. Study a broader range of testing specimens: ideally, components having different Vf should be
analysed, to define an improved model for the attenuation in these composites;
2. Further to the point above, specimens with different stacking sequences (for example,
[0°/90°]ns and quasi isotropic laminates) should be analysed, to confirm if the attenuation
obtained in CIVA successfully matches the real results;
3. A better understating of the noise existent in the acquisitions is required. Attention should be
given to the noise inputs in CIVA, to have a more accurate definition of the expected signal-to-
noise ratio. Specifically, the noise obtained in a physical acquisition should be inserted in CIVA;
4. Compare the obtained stiffness matrix inputs for the transversely isotropic case with real test
data;
5. Analyse the orthotropic case with a greater set of material specifications, to better define the
homogenization model. For the used material and stacking sequence, no direct comparison with
real test data can be done. However, by doing some tests with different stacking sequences
(other than those from the existent CFRP components), the author discovered that when a
cross-ply [0°/90°]ns stacking sequence is used, the obtained data is closer to real test data. So,
it is important to better characterize what happens when modifying, for example, the
unidirectional lamina orientation. What is the evolution of Ei, νij and Gij for the complete
laminate?
6. For different probe frequencies, define the attenuation in function of the water path;
7. Understand if the SAUL algorithm can be used to inspect CFRP components produced by
vacuum bagging, using as the entry surface the irregular face. This can prove useful for
components where the face that is in contact with the mould during manufacturing is not
accessible;
8. Introduce defects with various sizes in the composite-adhesive interface, and evaluate these
debonds, as well as the minimum size of defects that is possible to detect.
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References
[1] Reinforced Plastics magazine, May/June 2013, Elsevier Ltd, pp. 28.
[2] Hellard, G. (2008), Composites in Airbus - A Long Story of Innovations and Experiences, Global
Investor Forum, pp. 6.
[3] Altairenlighten (2014), http://altairenlighten.com/2012/12/the-a350-xwb-prepares-for-static-
testing/. Accessed on December 2014.
[4] Nondestructive Testing Market by Technology (Visual, Magnetic Particle, Electromagnetic,
Liquid Penetrant, Radiography, Ultrasonic) & Application (Aerospace, Defense, Automotive, Oil
& Gas, Infrastructure, Power Generation) - Global Trends & Forecasts to 2014 – 2020, April
2014.
[5] Instituto de Soldadura e Qualidade (2014), http://www.isq.pt/homepage.aspx. Accessed on
December 2014.
[6] Quadro Estratégico de Referência Nacional, http://www.pofc.qren.pt/areas-do-
compete/incentivos-as-empresas/projectos-aprovados-
qren/page/8?area=2&instrumento=d32&search=y. Accessed on December 2014.
[7] Amorim, J. (2014), Sistema de Inspecção de ultra-sons por Phased Array para Materiais
Compósitos Aplicados à Indústria Aeronáutica, MSc Thesis in Mechanical Engineering, Instituto
Superior Técnico, Universidade de Lisboa.
[8] Optimal Structural Solutions, Lda., http://www.optimal.pt/Site/Home.html. Accessed on
December 2014.
[9] Edison, T. (1880), U.S. Patent 223898.
[10] Bacon, R. and Tang, M.M. (1964), Carbonization of Cellulose Fibers I, Carbon, Vol 2, pp. 211.
[11] ASM International Handbook Committee (2001). ASM Handbook: Composites. 9th Edition ed.,
ASM International, pp. 94.
[12] American Chemical Society (2003), High Performance Carbon Fibers, pp. 2.
[13] Tenney, D. R. et al., NASA Composite Materials Development: Lessons Learned and Future
Challenges, NASA; http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090037429.pdf.
Accessed on December 2014.
[14] Yutaka A. (2008), Pitch-Based Carbon Fiber with Low Modulus and High Heat Conduction, July
2008, Nippon Steel Technical Report No. 84, pp. 15.
[15] INVENT GmbH, http://www.invent-gmbh.de/uploads/pics/STANT-Sandwich_01.jpg. Accessed
on December 2014.
[16] The American Society for Nondestructive Testing, Inc.,
https://www.asnt.org/MinorSiteSections/AboutASNT/Intro-to-NDT.aspx. Accessed on
December 2014
[17] Ship, P. L. (1943), "Wreck of the SS John P Gaines," 6 December 1943. http://www.armed-
guard.com/gaines.html. Accessed on December 2014
- 81 -
[18] Wright, D. (2005), "Fracture- some maritime examples," Mechanical Engineering Department,
University of Western Australia., May 2005. Available:
http://school.mech.uwa.edu.au/~dwright/DANotes/fracture/maritime/maritime.html. Accessed
on December 2014.
[19] Shigley, J., Mischke, C., and Budynas, R. (2004), Mechanical Engineering Design. 7th Edition.
New York, NY: McGraw-Hill.
[20] Official Website for the City of Brockton. http://www.brockton.ma.us/docs/fire/the-grover-
disaster.pdf?sfvrsn=8. Accessed on December 2014.
[21] Purch, http://www.space.com/19436-columbia-disaster.html. Accessed on December 2014.
[22] “No Left Turns: United Airlines Flight 232 Crash”, NASA, https://sma.nasa.gov/docs/default-
source/safety-messages/safetymessage-2008-08-01-unitedairlinesflight232crash-
vits.pdf?sfvrsn=4. Accessed on December 2014.
[23] Sioux City Journal, http://siouxcityjournal.com/special-section/visitors-guide/former-reporter-
recalls-shooting-crash-of-united-flight/article_a2a810e0-a794-525b-b841-3bd52f009dab.html.
Accessed on December 2014.
[24] Mix, P. E. (2005), Introduction to Nondestructive Testing, A Training Guide, 2nd Edition.
[25] Campbell, F. C. (2010), Structural Composite Materials, ASM International.
[26] Naplocha, K. and Kaczmar, J. W. (2012), Wear Behavior of Fiber-Reinforced Aluminum Alloy
Composites, Wrocław University of Technology, Poland.
[27] Gibson, R. F. (2007), Principles of Composite Material Mechanics, 2nd Edition, McGraw-Hill.
[28] U.S. Department of Transportation, Federal Aviation Administration (2012), Advanced
composite materials.
https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_airframe_handbook
/media/ama_Ch07.pdf, pp. 7. Accessed on December 2014.
[29] Gurit, Guide to Composites, http://www.gurit.com/guide-to-composites.aspx. Accessed on
December 2014,
[30] Jones, R. M. (1975), Mechanics of Composite Materials, 2nd Edition, Taylor & Francis.
[31] Reddy, J. (2004), Mechanics of Laminated Composite Plates and Shells – Theory and Analysis.
2nd Edition. Florida: CRC Press.
[32] eFunda, Inc.,
http://www.efunda.com/formulae/solid_mechanics/mat_mechanics/hooke_iso_transverse.cfm.
Accessed on December 2014.
[33] Roesler et al. (2007), Mechanical Behaviour of Engineering Materials, Metals, Ceramics,
Polymers, and Composites, Springer-Verlag Berlin Heidelberg, pp. 54.
[34] University of Cambridge, http://www.doitpoms.ac.uk/tlplib/bones/derivation_mixture_rules.php.
Accessed on December 2014.
[35] Messiry, M. E. (2012), Theoretical analysis of natural fiber volume fraction of reinforced
composites, Elsevier Ltd.
- 82 -
[36] WikiPedia, http://en.wikipedia.org/wiki/Rule_of_mixtures. Accessed on December 2014.
[37] Richardson, M. O. W., Wisheart, M. J. (1996), Review of low-velocity impact properties of
composite materials. Composites Part A: Applied Science and Manufacturing, pp. 1123-1131.
[38] Appleby-Thomas, G. J. et al. (2011), On the response of two commercially-important CFRP
structures to multiple ice impacts. Composite Structures, vol. 93, pp. 2619-2627.
[39] Department of Defense – United States of America (2002), Composite Materials Handbook
Volume 3. Polymer Matrix Composites Materials Usage, Design and Analysis.
[40] MDPI AG, http://www.mdpi.com/materials/materials-07-
00413/article_deploy/html/images/materials-07-00413f2-1024.png. Accessed on December
2014.
[41] Wikipedia, http://en.wikipedia.org/wiki/Hearing_range. Accessed on December 2014.
[42] ASM International (1992), Nondestructive Evaluation and Quality Control, pp. 490.
[43] Dr. Rakesh Kapoor,
http://www.rakeshkapoor.us/ClassNotes/HTMLFiles/ForceAndMotion_40.gif. Accessed on
December 2014.
[44] Ihara, I. (2008), Ultrasonic Sensing: Fundamentals and Its Applications to Nondestructive
Evaluation, pp. 287-305.
[45] White, F. (2003), Fluid Mechanics, 5th Edition, McGraw-Hill.
[46] StudyBlue, https://s3.amazonaws.com/classconnection/320/flashcards/4877320/jpg/picture12-
1492BA21B24085F574A.jpg. Accessed on December 2014.
[47] Lamb, H. (2004), Dynamical Theory of Sound (Dover Phoenix Editions). Dover Publications.
[48] Luo, W., Rose, J. L. (2003), Lamb waves thickness measurement potencial with agle beam and
normal beam excitation. Penn State Department of Engineering Science and Mechanics.
[49] Pinto, F., Barata, J. e Barros, P. (1992), Ensaios Não Destrutivos, Lisboa: ISQ.
[50] Ensminger, D. Bond, L. J. (2011) Ultrasonics: Fundamentals, Technologies, and Applications,
3rd Edition.
[51] Wikipedia, http://en.wikipedia.org/wiki/Inverse-square_law. Accessed on December 2014
[52] Princeton University, https://www.princeton.edu/~achaney/tmve/wiki100k/docs/Signal-to-
noise_ratio.html. Accessed on December 2014.
[53] Wikipedia,
http://upload.wikimedia.org/wikipedia/commons/thumb/a/a2/Damped_sinewave.svg/800px-
Damped_sinewave.svg.png. Accessed on December 2014.
[54] Krautkramer, J., Krautkramer, H. (1990), Ultrasonic testing of materials. Springer.
[55] Beijing Ultrasonic, http://www.bjultrasonic.com/ultrasonic-technical-info/how-ultrasonic-
transducers-work. Accessed on December 2014.
[56] Olympus Corporation,http://www.olympus-ims.com/en/ndt-
tutorials/transducers/characteristics/, Accessed on December 2014.
[57] Olympus NDT (2007), Advances in Phased Array Ultrasonic Technology Applications.
- 83 -
[58] Subramanian, A. (2013), Damage detection and characterization of fiber-reinforced composites
using ultrasonics. Graduate Thesis and Dissertations. Paper 13169.
[59] Imasonic SAS, http://www.imasonic.com/Industry/PA_principle.php. Accessed on December
2014.
[60] Mahaut, S. et al. (1998), An adaptive system for advanced NDT applications using phased
arrays. Elsevier Science B.V..
[61] Robert, S. et al. (2012), Real Time Nondestrutive Testing of Composite Aeronautical Structures
With a Self-Adadptive Ultrasonic Technique, France.
[62] Brassard, M. et al. (2011) Self-Adaptive Robotic Phased Array Inspection System for Small
Composite Parts of Various Geometries.
[63] Hopkins, D. L. et al. (2012), Surface-adaptive ultrasound (SAUL) for phased array inspection of
composite specimens with curved edges and complex geometry, QNDE.
[64] Framezelle, G., Dainelli, P. (2013),Surface Adaptive Ultrasound, Aerodatings 1st of October
2013
[65] Hillger, W., Pfeiffer, U. (2006), NDT and SHM of Carbon Composites using Acoustic Ultrasonic
Methods, DLR Braunschweig, Aerospace Texting Expo 2006.
[66] Summerscales, J. (2014), Non-Destructive Testing of Fibre-Reinforced Plastics Composites,
School of Engineering University of Plymouth, https://www.plymouth.ac.uk/staff/john-
summerscales. Accessed on December 2014.
[67] Kapadia, A. Non-destructive testing of composite materials, TWI Ltd,
http://compositesuk.co.uk/system/files/documents/ndtofcomposites.pdf. Accessed on
December 2014.
[68] Busse, G. (2011), Microwave NDT System for Industrial Composite Applications,
http://www.ndt.net/article/CompNDT2011/papers/8_Meier.pdf. Accessed on December 2014.
[69] Ospald, F. et al. (2014), Aeronautics composite material inspection with a terahertz time-domain
spectroscopy system. Optical Engineering, Society of Photo-optical Instrumentation Engineers
pp. 1-14.
[70] Vaara, P., Leinonen J. (2012), Kemi-Tornio University of Applied Sciences, http://theseus32-
kk.lib.helsinki.fi/bitstream/handle/10024/54515/vaara%20leinonen%20B%208%202012.pdf?s
equence=1. Accessed on December 2014.
[71] Quinten, C. (2013), New NDT technology for fast inspection of composite parts:
Vibrothermography,
https://www.b2match.eu/system/aerodatings2013/files/COMPOSITE_INTEGRITY.pdf?138124
7516. Accessed on December 2014.
[72] Gong, J. et al. (2013), Investigation of carbon fiber reinforced polymer (CFRP) sheet with
subsurface defects inspection using thermal-wave radar imaging (TWRI) based on the multi-
transform technique, School of Mechatronics Engineering, Harbin Institute of Technology, P.R.
China.
[73] EXTENDE (2015), http://www.extende.com. Accessed in March 2015.
- 84 -
[74] Lonné, S. et al. (2004), Modeling of ultrasonic attenuation in unidirectional fiber reinforced
composites combining multiple-scattering and viscoelastic losses, Review of Progress in
Quantitative Nondestructive Evaluation Vol. 23
[75] Deydier, S. et al. (2005), Ultrasonic field computation into multi-layered composite materials
using a homogenization method based on ray theory, Review of Progress in Quantitative
Nondestructive Evaluation.
[76] Imasonic SAS (2015), http://www.imasonic.com/. Accessed in March 2015.
[77] M2M (2015), http://www.m2m-ndt.com/index.htm. Accessed in March 2015.
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Appendix A: Maple code
A.1 Single Ply Composite
>
>
>
>
>
>
>
>
>
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>
A.2 Multiple ply composite
>
>
>
>
>
>
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Appendix B: Immersion tank dimensions
- 88 -
Appendix C: MultiX datasheet
