1

Use of micro-focus X-ray system and pulse thermography to

measure porosity content into aerospace composite parts

Pierre Servais1, Christophe Greffe2, Alice Wallon3, Ezio Gandin

1 Director MPP Sprl (NDT service), Belgium

2 Technical director, XRIS Sprl (Manufacturer Digital X-ray system), Belgium

3 R&I Engineer: Non-Destructive Testing, Solvay SA, Belgium

4 R&I Principal Specialist: Polymers and Composites, Solvay SA, Belgium

Abstract

A major problem for aerospace composite parts is to size and measure the porosity content. Ultrasonic is

used for many years but relies on the back wall echo amplitude to determine the porosity content of the

defect. As production parts have now more and more complex geometries with internal ribs and locally

high curvature, it is here proposed to use the latest state-of-the-art X-ray digital systems using high

resolution Flat Panel combined with a micro-focus X-ray tube.

This new digital technique is more and more portable, also wireless and can be combined with robots to be

able to X-ray zones in less than 20 seconds for large area. Future artificial intelligence could recombine

the different X-ray images to provide a full mapping of the part and assess directly the porosity level.

New design of Inspection Reference Panel suitable for porosities are now used to build up a first

correlation between the ADU gray level of different porous zones using CNR as given by ASTM E2737.

Porosity is usually very difficult to measure for chamfer zone or joggle or curved areas for the historic

ultrasonic Testing method. Digital X-ray is a very good tool to visualize and analyze the porous areas

based on quantitative measurement of radiographic attenuation which was not possible for the old silver

film technique.

This paper will study different artificial and natural porous zones inspected by ultrasounds (UT) and

compared to digital X-ray (RT) and pulsed thermography (IRT). A combination of both digital methods

(RT and IRT) will be presented as a future alternative to ultrasounds as X-ray is highly sensitive to any

defect perpendicular to plies orientation and Thermography to any air gap parallel to the plies. Combining

both images (RT and IRT) will be a very clever way to inspect composite parts. These digital methods are

very fast and don’t need use of water as ultrasonic testing. Then NDT Data Fusion algorithms used in

Artificial Intelligence can be used.

Keywords: composite testing, porosity evaluation, thermography, phased array ultrasonic, digital X-ray, multimethod

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1. Introduction

In the last years, the use of continuous Carbon Fiber Reinforced Plastics (CFRP) has seen an important

growth in the transportation sector, and especially in the aircraft industry, due to the necessity to reduce

weight. Additionally to its lightweight, CFRP has excellent mechanical properties, including a very high

modulus of elasticity and strength. Moreover, it has far superior fatigue properties and corrosion resistance

than metals, when combined with suitable resins, making it as an ideal material for several industries. Its

use is nevertheless still limited due to the cost, although in recent years, it has become more affordable (1)

(2).

The difficulty with CFRP materials is the manufacturing process. It is quasi-impossible to manufacture a

specimen made of CFRP without defects as fiber breakage, porosity or cracks. Process conditions have to

be adjusted to reduce as much as possible these defects in the structure, especially porosity. Above a

critical level of porosity, the mechanical properties of the material drop. It has become thus mandatory to

develop Non-Destructive Testing (NDT) to quantify the local porosity level in CFRP structure after the

manufacturing process.

For many years, Ultrasonic Testing (UT) is the leading technique for porosity detection (3) (4) (5). It has

been considered as reliable and is widely used in the aircraft industries. However other NDT techniques

have recently emerged to detect porosity. The first one is the Infrared Testing (IRT), which knows today

an increasing interest (6). Lots of methods are set up to fix two major issues: first, the difficulty to obtain

quantitative results; second, the dependence of the results to the heating conditions and the surface

properties. The second technique is the Radiographic Testing (RT), which requires more complex set-up

because the inspection has to be done in a closed lead-shielded cabin. Nevertheless, the accuracy and

sensitivity of digital RT is so high that this method should be further investigated.

The first part of this study will compare the performances of UT, IRT and RT to detect porosity in a

continuous CFR epoxy. The selected specimen is a porosity caliber in agreement with the aeronautic

standards. Qualitative and quantitative results are discussed.

3

2. Experimental investigation

2.1. Description of the specimen

The present work focus on a composite laminate of 4.4 mm thickness made of epoxy and unidirectional

continuous carbon fibers. The specimen contains five square zones (6mm X 6mm) in which multiple holes

at random depth were drilled. The aim of these zones is to imitate the effect of porosity according to UT

aeronautic standards. The porosity levels of the caliber range from 1% to 3%. Figure 1 shows a 3D

representation and the main dimensions of this caliber. These volume porosity levels are located exactly in

the interesting range for aerospace as usually critical parts requires maximum 1.5% Vol porosity and less

critical composite parts to 2.5% Vol porosity.

Fig 1a: Representation and dimensions of the porosity caliber

Holes depths are 25, 50 and 75% of the mean plate thickness and are randomly distributed. The holes

diameter is 1mm. From the left to the right the porosity levels are 1%, 2%, 3%, 1.5% and 2.5%.

Multimethod NDT evaluation of a porosity caliber provides first information on the equipment

performances and limitations for porosity detection. This study will first compare 5 academic porous

zones from 3 NDT methods and finally the same way of calculation will be applied to natural porous

zones from 6 different porous zones. The purpose of this study is to test the ability of using latest digital

NDT ( Xray and Infrared testing) to size and evaluate quantitatively porosity levels instead of the standard

ultrasonic testing.

Fig 1b: original drawing for the five zones of porosity simulation

3x3 5x4 5x5 4x4

5x4

+4x3

4

2.2. Description of NDT equipment

2.2.1. Infrared testing (IRT)

Infrared testing equipment is based on a calibrated FLIR camera A8200sc which is a high resolution and

high sensitivity MWIR cooled camera that produces low-noise HD thermal images. The maximum frame

rate is about 50Hz. The main technical specifications are given in table 1.

Camera FLIR A8200sc

Temperature range for the tests -20 to +50°C

Thermal sensitivity / Noise Equivalent Temperature Difference (NETD) 18mK at 25°C

Measurement accuracy ± 1°C

Detector 1024 x 1024 UFPA

Spectral range 3-5µm

Image frequency 30 Hz

Resolution (pixel size) 0.23mm

Detector pitch 18µm

Table 1: Technical specifications of the infrared camera FLIR A8200sc used in the experiments

Fig 2: Infrared Testing setup used to inspect porosity panel with two techniques

5

2.2.2. Ultrasonic testing (UT)

Ultrasonic tests are performed with a SOFRANEL VEO+ flaw detector with a 5 MHz 16:128PR phased

array unit equipped also with a conventional UT channel for pulse-echo, pitch-catch or TOFD inspection.

Phased-array elements can be individually or by group activated and the resulting data can be combined to

form a visual image representing a slice through the material. The main technical specifications are given

table 2.

Ultrasonic device VEO+ 16:128PR

Emitter types Phased-array, conventional and TOFD channels

Scans combination Until 6

Tension -50 to 130V

Impulsion duration 25ns to 1000ns

Resolution 2.5ns

Bandwidth 0.2 to 23MHz

PRF Until 50 000Hz

Focal laws number Until 1024

Table 2: Technical specifications of the VEO+ device used in the experiments

Tests were performed in an ultrasonic water tank. It allows controlling composite 2D & 3D parts in

immersion with 5-axis computer-controlled unit for the displacement of the probe. The tank contains a

mirror plate of one-meter square on which the specimens are placed.

Fig 3: Ultrasonic Testing Immersion Tank with phased array transducer

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2.2.3. Radiographic testing (RT)

Innovative Xray equipment was designed and made by X-Ray Imaging Solutions (Belgium) for a

specific request of 1 m² thin composite plate inspection inside a shielded cabinet. A robot is used

for part positioning and a stitching software provides till 100 cm x 100 cm reconstruction for big

part inspection. In combination with microfocus tube, and geometric magnification, resolution

from 139 microns down to 20 microns is possible in 2D imaging mode. Computed Tomography

(CT) technique is also available to analyze further the in depth imaging of each porous zone

(voxel size between 20 and 80 micron depending on the magnification).

Flat Panel Detector Dereo HE 4343

Image size 430 * 430 mm

Pixel resolution 139 microns

Nb of pixels per image 9 millions pixels

Xray generator focus variable focal spot from 5 to 50 μm

Maximum kV Monobloc 150kV

Positioning Kuka Robot

Table 3: Technical specifications of digital Xray system used

Fig 4: Digital Xray system with robotic manipulator and microfocus tube

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2.3. NDT inspection

2.3.1. Infrared inspection

Tests are carried out according to two different setups in order to compare results between infrared

inspection in reflection and infrared inspection in transmission modes. Reflection experiment was

conducted with a short thermal excitation (about 10ms) of two flashes of 6kJ each and the cooling step is

recorded by the IR camera. This infrared technique is called pulsed thermography (PT). Compared to

other techniques, pulsed thermography is fast and straightforward. Although heat diffusion is a complex

problem, relationship between defect depth and time is simplified in this application as IRT is only used to

size the surface and quantify the amplitude of each porous zone and not to determine their depth.

Transmission experiment was conducted with a longer thermal excitation of 10s with a halogen lamp of

1000W. Heating and cooling cycles are recorded: this technique is called square pulsed thermography

(SPT). SPT is generally processed to improve defect visibility. Complete details of the setups are given

table 4.

Thermal excitation Flash Thermal excitation Halogen

Power 2 x 6kJ Power 1000W during 10s

Acquisition mode Reflection Acquisition mode Transmission

Lens 25mm Lens 50mm

Specimen-camera distance 0.28m Specimen-camera distance 0.28m

Frequency 30Hz Frequency 20Hz

Acquisition time 30s Acquisition time 80s Table 4: Technical details of setups used in IRT( reflection (left) and in transmission technique (right)

Fig 5: Infrared image evolution with Td (temperature of porosity defect) and Sa for Sane Area (defect free zone)

8

2.3.2. Ultrasonic inspection

Ultrasonic experiments were carried out in the water tank with a phased-array probe of 128 elements and

a frequency of 5MHz. The main advantage of immersion is to keep a perfect coupling for the propagation

of ultrasounds in the specimen and to provide a cleaner signal. The probe is guided by a motorized system

which ensures its straight movement at a constant speed and along two axes. Complete details of the

setups are given table 5.

Voltage 50 V

Probe type Phased-array

Frequency 5MHz

Elements 128

Impulsion type Linear

Sabot No

Control type Immersion

OL speed 3136 m/s

Gain 4.4 dB

Values range 0 to 7 mm

Scan step 1 mm

Encoder 1 axis of 250mm Table 5: Technical details of setups used in the ultrasonic experiments

Fig 6: A-Scan of porous zone 2 (on top) and B-Scan of the 5 zones

9

2.3.3. Radiographic inspection

Two Xray techniques were used to study the porosity panel, one simple Xray shot of 30 seconds at 150 kV

using the small focus source of 0.05 mm (microfocus Xray tube) and shot distance at 700 mm (from focus

source to panel distance). On the 2D Xray image obtained in 30 seconds, each zone of porosity can be

analyzed by zooming in the image.

Fig 7: Porous Zone 4 zoom 2D Xray view and line profile across 4 porosities with distance and amplitude values

10

2.4. Post-processing

2.4.1. IRT: Principal Component Thermography (PCT)

Principal component thermography (PCT) is a direct application of the statistical model called Principal

Component Analysis (PCA). The principle is to regroup pixels with a similar thermal behavior and

forming clusters from a linear combination of variables. In fact, PCT decomposes data into a set of

orthogonal statistical functions obtained through Singular Value Decomposition (SVD). The number of

variables is reduced by keeping the most amount of relevant information possible and by keeping useful

information about the defects.

2.4.2. UT: signal to noise ratio dimensionless calculation

Measured signal in ultrasonic testing is defined by its amplitude on the detector screen. The amplitude of

an echo is described by how high it is relative to the height of the screen and is expressed in percentage

Full Screen Height (% FSH). To calculate Contrast to Noise Ratio (CNR) for UT, the amplitude of each

echo is compared to the amplitude of the residual noise: 𝐶𝑁𝑅 = 20𝑙𝑜𝑔 𝐴𝑠𝑖𝑔𝑛𝑎𝑙𝐴𝑛𝑜𝑖𝑠𝑒 (1)

Where the amplitudes are expressed in % FSH.

To estimate the amplitude of the noise, sane areas of the porosity caliber have been considered. Several

amplitudes have been measured and the average value has been calculated at about 5% FSH. For the rest

of the study this value is chosen as the noise amplitude for the CNR calculation.

2.4.3. RT: Post processing of digital images

After usual calibration in gain and offset, the detector has accumulated 30 images (frequency = 1 Hz) to

provide a 14 bits DICOM image with high resolution. High-pass filter was used to eliminate the influence

of beam spread variation in the image as CNR is strongly influenced by the variation of signal around

each porous zone. This way brings the graylevel in the defect free area near a constant value which

corresponds to full thickness without porosity. Also as Flat Panel has a very high resolution, a gaussian

spatial filter is used to denoise the image and gives a mean value of each porous zone increasing the CNR

as the noise decreases.

11

3. Results and discussion

3.1. Qualitative evaluation

Figure 9 presents the resulting images of the multimethod inspection of the porosity caliber. PCT

treatment has been applied for both IRT images and high-pass filter has been applied to filter X-Ray

image. Each porous zone is encircled.

UT inspection shows all the porosity levels, even the last one appearing on the right in green. Red color

represents the maximal amplitude, white color the signal loss and green color represents an attenuation of

50% - or 6dB – of the signal amplitude. So, a loss of signal is obtained for porosity level bigger than 1%;

and for 1% an attenuation of 50% is observed so at the known limit of 6dB for ultrasonic testing.

IRT inspections detect both very well the four bigger porosity levels. The last level of 1% is just guessed

on the pictures. It can be established that the porosity detectability limit of the equipment is about 1%. In

addition to porosity levels, IRT in reflection technique reveals clearly fibers orientation inside the

composite.

X-Ray 2D inspection presents the best definition and detectability of the porosity: it is possible to

distinguish the holes of the caliber that compose the porous zone. The five areas are clearly detected and

CT technique (voxel size of 80 micron) gives a great view in depth for each porous zone (Figure 8).

Fig 8: CT Scan showing 3D view for two porous zones and cut view obtained from the CT showing 4 porosities

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(a)

(b)

(c)

(d)

Fig 9: Results of the multimethod inspection. (a) Ultrasonic inspection. (b) Infrared inspection in transmission. (c)

Infrared inspection in reflection. (d) X-Ray inspection 2D image of the 5 zones

13

3.2. Quantitative evaluation

The next step of the analysis consists in a quantitative approach. This approach avoids subjective

evaluation of the final images. In order to evaluate the results, the CNR metric is used to quantitatively

assess the signal-to-noise-ratio of the porosity zones. CNR is calculated from two regions: a porous region

called the signal and a sane region called the noise. The signal is defined by the region that encloses pixels

in the images where the porosity appears. The noise is defined by the region close to the porosity but

without containing it. This approach is described in ASTM E2737 §3.2

The evolution of CNR values for each technique is plotted in Figure 9. This quantitative analysis leads to

the following conclusions:

i. Infrared active thermography in transmission presents a better CNR than infrared active

thermography in reflection. This means that the detectability is also better. Indeed, in reflection

the signal is noisier due to the surface emissivity variation, whereas transmission analysis

averages the thermal signal over all the material thickness.

ii. X-Ray testing is much more sensitive than other techniques. Indeed, X-Ray is sensitive to each

pixel, which means sensitive for each little hole or air bubble. The sensitivity of well calibrated

digital X-Ray set-up is about 1% of the thickness which is commonly rechecked during initial

qualification of Xray system using IQI (Image Quality Indicator). Considering the thickness of

the plate at 4.4mm, X-Ray detects a hole of 45 microns thickness. This explains why each

porosity level is perfectly observed with this technique.

iii. The aeronautic standards as Nadcap AC7114/10 consider CNR values below 2.5 as non-relevant

and non-significant. All calculated CNR for the lowest level of porosity – 1% - were below this

quantification limit. This porosity zone would be the limit of acceptability. This point shows how

the quantitative analysis is essential: the porosity zone of 1% was well observed by X-Ray and

detected with IRT and UT. With the only qualitative inspection, this area can be considered as

relevant and dubious zone, whereas CNR evaluation shows that the zone is acceptable.

iv. A similar inflection point is observed for X-Ray and IRT in transmission curves. This is explained

by the fact that both techniques are influenced by the back side of the material where the porous

simulations made by drilling are reaching the bottom of the panel during the manufacturing

process, which creates a higher volume of air removed when the energy (heat or Xray beam) is

coming from the rear of the panel. However, the caliber is mainly made to compare UT and IRT

in reflection – techniques that can be executed only on one side. IRT has also a limited depth of

penetration which is smaller than the part thickness so best way should be to inspect both sides of

part when its thickness is more than 4 mm.

14

Figure 9: Evolution of the CNR for each NDT techniques as a function of the porosity level

4. Illustration of natural porosity on aerospace composite parts

4.1. Natural porosity panels and Ultrasonic Testing

A kit of Boeing porosity panels 16 plies was tested also using the different NDT methods. The porosity

range of 0% to 8% was used. Same thickness of 4.4 mm and same number of plies 16 (Carbon) for all the

panels.

Figure 10: 6 panels tested first by Ultrasonic Testing (immersion tank) – 5 MHz – 128 elements phased array

0

10

20

30

40

50

60

70

80

90

0 0,5 1 1,5 2 2,5 3 3,5

CN

R v

alu

e

Porosity level (%)

CNR IRT trans CNR IRT ref CNR UT CNR RX

15

When plotting the UT attenuation results, for high porosity levels a limit is obtained in dB which is close

to 30dB. It means that ultrasonic quantification is not always linear to the porosity levels and it depends

also on the technique (immersion or A-Scan contact pulse echo). The attenuation results are plotted

against a linear regression line for C-Scan and A-Scan techniques.

Fig 11: C-Scan UT results for the 6 porosity levels (top) and A-Scan results (below)

16

4.2. Natural porosity panels and Infrared Testing

The Boeing porosity panels were tested by infrared testing using reflection technique and PCT

post processing. A color code was used to emphasize the zone reaching 2.5% porosity as above

this value, the aerospace composite parts are rejected. Taking the temperature amplitude on each

panel, a plot can be done showing the linear link between porosity level and temperature signal.

Fig 12: infrared image showing the 6 porosity panels (3D view)

Fig 13: Temperature signal Versus porosity levels

17

4.3. Natural porosity panels and Radiographic Testing

Boeing porosity panels were shot using radiographic technique and post processing described in

section 2. Each panel is individual and is a cut zone of constant porosity level, so the noise

content as described in CNR was used to plot each %Vol Porosity. Std Dev value represents the

standard deviation around the same average graylevel value of 23000 ADU (arbitrary Digital

Unit) which is constant for all 6 panels. The standard deviation of graylevel around the average

(Noise) is a perfect way to quantify the porosity level percentage as R² = 0.99 for the linear

regression.

Fig 14: Xray image of 6 porous panels (top) and Xray Std deviation Vs porosity levels (down)

y = 9,4931x + 35,667

R² = 0,9903

0

20

40

60

80

100

120

140

0 2 4 6 8 10

Xray Standard Deviation

18

Using digital Xray, other advanced filter can be used also to show an image of same mean

graylevel but where each porosity level is shown in 3D embossing mode. 0% porosity level is on

top left and 8% porosity level is on bottom right (same way as figure 14).

Fig 15: Embossing filter used to show each porous panel in 3D

19

5. Summary and conclusion

As shown during this study, new digital NDT methods as infrared and radiographic testing can be

used now to inspect composite parts looking for porosity.

Firstly, we compared results obtained with standard ultrasonic methods and latest digital methods

based on academic artificial porous zones machined in an epoxy CF thermoset and densities for

small zones of 6x6 mm).

Secondly, we showed that natural porous panels could also be inspected using the same

technique. Color code or 3D filter can be used during initial evaluation of composite testing but

also the amplitude of the signal for IRT or RT methods can be used in a linear model to evaluate

the porosity level of the composite. For each porous zone, the best contrast and sensitivity is

obtained using digital high resolution and microfocus Xray system.

The main advantage of IRT and RT digital methods is that it does not require any couplant fluid

and it does not requires to be exactly at 90° from the surface as it is for ultrasonic transducer. As

more and more robots are scanning complex geometry for NDT, the latest digital methods as

infrared testing or digital radiography sensor combined with robot will supersede the old

ultrasonic method also for porosity detection.

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