NTUA_Theodorakeas_byTK

Thermal NDT & E of CompositesThermal NDT & E of Composites

P. Theodorakeas

NTUA, Materials Science & Engineering Section, School of Chemical Engineering,

Zografou Campus, 15780 Athens, Greece.

OUTLINEOUTLINE

1. IR Thermography

2. Active Thermography

a. Energy Sources

b. Experimental Techniques

c. Data Processing

3. Transient Thermography NDT Techniques

a. Thermal Modelling Parameters

b. Defect Assessment of Composites

c. Thermography vs NIR Imaging

4. Final Remarks

5. Conclusions

IR THERMOGRAPHYIR THERMOGRAPHY

Thermal Properties

Infrared Thermography

Optical Properties

Microstructure

Chemical Composition

Surface (i.e. roughness)

Endogenous Parameters Exogenous Parameters

(Environment)

Passive Approach

IR camera

IR radiationIR radiation

to PCPCPC

IR THERMOGRAPHYIR THERMOGRAPHY

Active Approach

LightLight

defect

Heat conductionHeat conduction

IR radiationIR radiationLamps

IR camera

to PCPCPC

A wide variety of energy sources are available and can be divided in:

optical, if the energy is delivered to the surface by means of optical

devices such as photographic flashes (for heat pulsed stimulation) or

halogen lamps (for periodic heating)

EENERGY NERGY SSOURCESOURCES

or mechanical, if the energy is injected into the specimen by means

of mechanical oscillations, e.g. with a sonic or ultrasonic transducer.

Optical excitation stimulates the defects externally, i.e. the energy is

delivered to the surface of the specimen where the light is

transformed into heat.

Thermal waves propagate by conduction through the specimen until

they reach a discontinuity that act as a resistance reflecting the

thermal waves back to the surface.

Mechanical excitation on the other hand, heats up the defects

internally, i.e. mechanical oscillations injected to the specimen travel

in all directions dissipating their energy at the discontinuities in the

EENERGY NERGY SSOURCESOURCES

in all directions dissipating their energy at the discontinuities in the

form of heat, which travels to the surface by conduction.

There are three classical active thermographic techniques based on

these two excitation modes: lock-in thermography and pulsed

thermography, which are optical techniques applied externally; and

vibrothermography, which uses sonic or ultrasonic waves (pulsed or

amplitude modulated) to excite surface or internal features.

EEXPERIMENTAL XPERIMENTAL TTECHNIQUESECHNIQUES

Active Active

thermographythermography

Optical

Other forms of active excitation

e.g.

-Hot or cold water/air (external)

-Thermo-elastic heating

(external)

-etc.

Electromagnetic

Passive Passive


Infrared Infrared


approaches

excitation

Mechanical Optical

(external)

Lock-in or

modulated

thermography

Pulsed

thermography

Other optical configurations

e.g.

-frequency modulated,

-step heating,

-etc.

Electromagnetic

(external/

internal)

Pulsed eddy

current stimulated

thermography

Induction

lock-in

thermography

Mechanical

(internal)

Lock-in

vibrothermography

Burst

vibrothermography

Other mechanical configurations

e.g.

-frictional heating (external),

-etc.

techniquestechniques techniques


• There are different techniques depending on the stimulation

source, basically: pulsed or modulated.

• The term step heating is also found in literature, referring to a long

pulse excitation, and line excitation can be employed as well, i.e.

the camera and excitation source moving while the specimen

remains static or vice versa.

• For instance, pulsed thermography (PT) and lock-in (or modulated)• For instance, pulsed thermography (PT) and lock-in (or modulated)

thermography (LT) are generally used when working with optical

stimulation.

• Data obtained by optical stimulation is commonly represented as

thermograms, i.e. a map of the thermal patterns on the specimen

surface, although other representations have been proposed as

well, such as maxigrams (maps of maximum thermal contrast),

timegrams (maps of the time of maximum thermal contrast), and

diffusivity maps.


� Electromagnetic excitation is achieved by inducing Eddy currents

through electromagnetic coils and it is commonly referred as:

� Thermo-inductive thermography

� Induction thermography or

� Eddy current thermography (ECT).

� As is the case for optical and ultrasound excitation, both pulsed and

lock-in configurations can be used.

DDATA ATA PPROCESSINGROCESSING

� Data obtained by optical stimulation in either PT or LT, is

processed by the fast Fourier transform (FFT), which is commonly

refer as pulsed phase thermography (PPT) in the case of pulsed

thermographic data; and phase angle thermography or phase

sensitive thermography in the case of modulated data.

� There are many other advanced processing techniques developed

to improve the PT transient signal.

� Thermographic signal reconstruction (TSR) is one of such

techniques. It allows reducing the amount of data, to de-noise the

signal and to further process synthetic data using first and second

time derivative images as well as the FFT, which considerably

improve the signal-to-noise ratio.

� There are many other processing techniques available. These

processing techniques can be applied to any thermographic

regardless of the energy source used for stimulation.

TTHERMAL HERMAL MMODELLINGODELLING

ThermoCalc-3D software was selected.

It was used for calculating 3D (three-dimensional) temperature

distributions in thermally isotropic and/or anisotropic solids of

various layers that contained subsurface defects.

The solid body was modelled in Cartesian coordinates and it was

possible to solve a heat conduction problem by means of an implicit

finite-element numerical scheme.

The specimens were heated uniformly using a square pulse in an

attempt to match most pulsed thermal non-destructive techniques.


The thermal properties of the specimens and defects were specified

separately in all three spatial directions (i.e. modelling a fully

anisotropic material).

The thermo-physical properties and various heating time parameters

were considered for the models.

Thermal images, spatial profile and thermal contrast curve of composite sample

containing a delamination.


Initial detectability of defect - delamination in relation to time and depth.

DDEFECT EFECT AASSESSMENT OF SSESSMENT OF CCOMPOSITESOMPOSITES

Optical pulsed

thermography

Optical lock-in

thermography

Paint detached

from the surface

Vibrothermography Eddy current thermography

Real crushed core produced

during VT inspection


Defect

number

Thickness,

t [mm]

Lateral

size,

D [mm] Between plies

Depth,

z [mm]

Ratio

D/z

1 0.16 3 1 and 2 0.25 12

2 0.16 5 2 and 3 0.5 10

3 0.16 7 3 and 4 0.75 9.3

4 0.16 10 4 and 5 1 10

5 0.16 15 5 and 6 1.25 12

6 0.16 5 6 and 7 1.5 3.3

7 0.16 7 7 and 8 1.75 4

8 0.16 10 8 and 9 2 5

9 0.16 15 9 and 10 2.25 6.7

10 0.16 3 9 and 10 2.25 1.3

11 0.33 7 1 and 2 0.25 28

Zone I

Defect distribution in Zone I and Zone II

11 0.33 7 1 and 2 0.25 28

12 0.33 10 2 and 3 0.5 20

13 0.33 15 3 and 4 0.75 20

14 0.33 3 4 and 5 1 3

15 0.33 5 5 and 6 1.25 4

16 0.33 10 6 and 7 1.5 6.7

17 0.33 15 7 and 8 1.75 8.6

18 0.33 3 8 and 9 2 1.5

19 0.33 5 9 and 10 2.25 2.2

20 0.33 7 9 and 10 2.25 3.1

21 0.16 15 adhesive and core 2.5 6

22 0.16 7 adhesive and core 2.5 2.8

23 0.16 3 adhesive and core 2.5 1.2

24 0.16 15

face sheet and

adhesive 2.5 6

25 0.16 7

face sheet and

adhesive 2.5 2.8

26 0.16 3

face sheet and

adhesive 2.5 1.2

Zone I

Zone II


Optical pulsed thermography Optical lock-in thermography

Best defect

contrast once the

proper frequency

is selected

Easiest to

perform, best

overall results.

Only node failure

defects were not

detected.

3 nodes5 nodes 10 nodes

Vibrothermography Eddy current thermographyNot inspected

Not detected

Only technique able

to detect node

failure, detects all

defect types with low

resolution IR camera

Fast, to

perform, most

difficult to

implement in

practice


Q

Q

2nd derivative (5th order)

t=1.8 s

Phasegram

f=0.2 Hz

Holes:

φ = 4 mm

L = 7 cm

z= 3 mm

L = 6 cm

z= 6 mm

L = 7 cm

z= 9 mm

L = 7 cm

z= 3 mm

Mivim.gel.ulaval.ca - v1.7.5Mivim.gel.ulaval.ca - v1.7.5Mivim.gel.ulaval.ca - v1.7.5Mivim.gel.ulaval.ca - v1.7.5 Mivim.gel.ulaval.ca - v1.7.5

Not

detected Not

detected

Mivim.gel.ulaval.ca - v1.7.5Mivim.gel.ulaval.ca - v1.7.5Mivim.gel.ulaval.ca - v1.7.5Mivim.gel.ulaval.ca - v1.7.5 Mivim.gel.ulaval.ca - v1.7.5

z= 9 mm

L = 6 cm

z= 12 mm

L = 6 cm

z= 6 mm

L = 7 cm

z= 9 mm

L = 6 cm

z= 12 mm

Holes:

φ = 2 mm

Hole:

φ = 8 mm, L = 8 cm, z= 9 mm


glass fibre reinforced laminate carbon fibre reinforced laminate

A carbon and a glass fibre plate (30 cm x 30 cm) with different types

of fabricated defects were investigated.

A NIR camera (0.9-1.7 µµµµm, 640x512 pixel resolution) was used for NIR

vision testing, and an IR camera (3-5 µµµµm, 320x256 pixel resolution)

for the IR vision inspection.

TTHERMOGRAPHY VS HERMOGRAPHY VS NNIR IR IIMAGINGMAGING

D1

D2

D3

D4

I3

I2

I1

C1

C2

C3

B1

B2

B3

O50 mm

O

O

O

50 mm

300 mm

300

mm

D: Delamination

D1: 1 mm x 1 mm

D2: 2.5 mm x 2.5 mm

D3: 5 mm x 5 mm

D4: 10 mm x 10 mm

I: Impact

I1: Load 1

I2: Load 2

I3 :Load 3

C: Countersink

B: Burned drill hole

O: Other defects

NIR vision recovers the reflected or transmitted (non-thermal)

radiation from or through the specimen in the near portion of the

infrared spectrum (0.9-2.5 µµµµm).

This technique, commonly referred as reflectography (in reflection

mode), is extensively employed in the examination of artworks where

underdrawings (opaque to NIR radiation) can be detected through the


underdrawings (opaque to NIR radiation) can be detected through the

painting layers (semi-transparent to NIR radiation) providing

information about the integrity of the piece, intentional and

unintentional alterations and artists' motifs.

Nevertheless, to our knowledge, NIR vision has seldom been exploited

for the assessment of industrial parts.

CFRP back side PPT in reflection


PPT phasegram,

f=0.03 Hz (painted)

CFRP NIR camera

Back side

CFRP front side TSR in reflection


2nd derivative (4th degree),

t=15.5 s

2nd derivative (4th degree),

t=1.3 s

CFRP front side PPT in transmission


CFRP NIR camera

Front side

PPT phasegram,

f=0.01 Hz

GFRP – NIR camera, front side in transmission


Without painting With painting

GFRP: NIR vs PPT in transmission


NIR camera PPT phasegram,

f=0.12 Hz (painted)

From the NIR images, at least three of the delaminations ("D" defects)

can be clearly identified when assessing the GFRP sample.

Evidences of the relative loading differences impact defects (type "I")

can also be noticed.


The countersink defects (type "C") and the burned drill holes ("B" type

defects) of different sizes can be perfectly seen (holes), although no

apparent differences between them can be made.

Further testing with increased spatial resolution would be required

for this manner.

No good for CFRP samples.

For the IR thermography testing the GFRP front surface was black-

painted.

Some of the defects can be located, although in general defect

visibility is lesser than with NIR vision.


For instance, One of the type "O" defects cannot be detected by IR

thermography but it is seeing by NIR vision.

These results demonstrate that NIR vision could be an interesting

approach for the assessment of glass fibre components, whereas for

CFRP is the other way around.

FFINAL INAL RREMARKSEMARKS

Pulsed Thermography is a common method for detecting defects in materials

and components.

The flash lamp(s) can thermally excite the surface at a relative short pulse

(a few milliseconds). It is thus a prompt investigation thermography

approach.

Thermal properties of materials have a major effect on the exact procedure

to be used.

Composites and/or plastics have relatively low thermal conductivity values

and mainly for this reason the use of relatively low frame rate is usually

required for the acquisition of the thermograms.

Pulsed Thermography was decided to be used due to it’s non-destructive

nature and since it is capable of detecting buried flaws.

Trials were carried out using this technique combined with signal processing

have been successful in characterising flaws in the thinner sections, including

some of the most subtle impact damage.

FFINAL INAL RREMARKSEMARKS

CCONCLUSIONSONCLUSIONS

• IR Thermography approaches can be applied to different

materials, according to the case - application.

• For composites assessment, we need to consider:

� Direction of fibers

� Thickness

� Material� Material

� Emissivity – Transmission

� Geometry

• Usually, prompt & reliable results can be obtained in the

inspection of relatively thin sections of GRP composites

using NIR Thermography, whilst for CRP and honeycomb

composites other thermography approaches are employed

(i.e. PT, PPT, LT, VT).

Thank You For Your AttentionThank You For Your AttentionThank You For Your AttentionThank You For Your Attention

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