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EVITA Project: Comparison Between Traditional
Non-Destructive Techniques and Phase Contrast X-Ray
Imaging Applied to Aerospace Carbon Fibre
Reinforced Polymer
Matthieu Gresil
1
&Vincent Revol
2
&
Konstantinos Kitsianos
3
&Georges Kanderakis
4
&
Ilias Koulalis
4
&Marc-Olivier Sauer
3
&Hervé Trétout
5
&
Ana-Maria Madrigal
2
Received: 8 September 2016 /Accepted: 3 October 2016 / Published online: 15 October 2016
#The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract The EU-project EVITA (Non-Destructive EValuation, Inspection and Testing of
Primary Aeronautical Composite Structures Using Phase Contrast X-Ray Imaging) aims at
bringing Grating-based Phase Contrast X-ray imaging technology to Non-Destructive Evalu-
ation and Inspection of advanced primary and/or complex aerospace composite structures.
Grating-based Phase Contrast X-Ray Imaging is based on the so-called Talbot-Lau interfer-
ometer, which is made of the combination of a standard X-ray apparatus with three transmis-
sion gratings as documented in the literature. This paper presents a comparison of two
traditional non-destructive techniques (NDT): ultrasonic through transmission (immersed
and water jet) and ultrasonic phased-array pulse echo, with the developed phase contrast X-
Ray Imaging applied to advanced aerospace carbon fibre reinforced polymer. Typical defects
produced during manufacture is examined as part of the testing and validation procedure. The
following defects have been identified as being those most likely to be detected more
effectively by the Grating-based Phase Contrast X-Ray Imaging process than other state of
the art industrial NDT techniques: porosity, foreign objects, cracks, resin rich, cut fibres, and
wavy fibres. The introduction of this innovative methodology is expected to provide the
aeronautical industry with a reliable and detailed insight of the integrity of thin and thick
Appl Compos Mater (2017) 24:513–524
DOI 10.1007/s10443-016-9540-1
*Matthieu Gresil
matthieu.gresil@manchester.ac.uk
1
i-Composites Lab, School of Materials, University of Manchester, Manchester, UK
2
Centre Suisse d’Électronique et Microtechnique - CSEM, Neuchatel, Switzerland
3
GMI Aero, Paris, France
4
National Technical University of Athens, Athens, Greece
5
DASSAULT Aviation, Argenteuil, France
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compositestructures as well as of complex geometry ones, such as integrated closed boxes and
sandwiches.
Keywords X-Ray imaging .Phase contrast .Porosity.Cracks .Image fusion
1 Introduction
Grating-based Phase Contrast X-Ray Imaging is based on the so-called Talbot-Lau
interferometer, which is made of the combination of a standard X-ray apparatus with
three transmission gratings as documented in the literature [1–4]. The method derives its
potential from the fact that three different contrast mechanisms are combined in a single
measurement. Indeed, not only the conventional absorption image can be extracted but
also the refraction image (also called differential phase contrast image) and the scattering
image (also called dark field image), which are related respectively to the refraction of the
X-ray beam inside the sample and to the ultra-small angle scattering caused by its
microstructure. Preliminary studies have shown that the scattering image provides a
powerful tool to detect any change in the arrangement of the fibres due to the presence
of defects such as porosity, fiber waviness, micro-cracks or resin rich/resin poor areas
[5–10].
Within the project EVITA (www.evita-project.eu), the requirements and needs of
the aeronautics industry in terms of the non-destructive inspection of thick and thin
composite components were collected and analysed. From there, a customized dem-
onstrator was designed and realized in order to benchmark this novel technology
against the following non-destructive inspection (NDI) techniques: water jet ultrasonic,
phased array ultrasonic, thermography and computed tomography.
2 Imaging Principle
At low energies, the interaction of X-rays with matter can be described by a complex refractive
index [11,12]:
n¼1−δ−iβð1Þ
Where (1 −δ)andβare the real and the imaginary part of the complex refraction index,
respectively. The imaginary part βdescribes the attenuation of the X-rays in matter and is
related to the linear absorption coefficient μ
1
(E) at energy Eby:
μ1EðÞ¼
4πβ EðÞ
λð2Þ
Where λis the wavelength of the X-rays. In conventional X-ray testing, the attenuation of
an X-ray beam by the sample under test is measured and is given by T(E)=exp(−μ
1
(E)×t)for
an homogeneous sample of thickness t. Recently, a technique called phase contrast X-ray
imaging was invented, which allows for the measurement of changes of the phase front of the
X-ray waves as shown in Fig. 1.
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The phase shift of the X-ray wave inside the sample is given by:
ϕEðÞ¼
2π
λδEðÞtð3Þ
Since the refraction coefficient can be up to three orders of magnitude large than the
attenuation coefficient, a better contrast can be achieved between similar materials (e.g. two
different polymers). This is especially true for light materials, such as biological soft tissues or
plastics, and this relative increases with the X-ray photon energy.
The phase contrast X-ray imaging system developed here is based on the Talbot-Lau
grating interferometer [4]. The grating interferometer consists of a conventional X-ray source,
a commercial X-ray detector and three X-ray gratings G0, G1, and G2 as shown in Fig. 2a.The
phase sensitive part consists of the beam-splitter grating G1 and the analyser grating G2. G1 is
a phase grating, which induces periodical phase modulation on the X-ray wave front and acts
as a beam-splitter. The divided beams then interfere downstream of G1 and result in an
intensity pattern I
fr
in the plane G2. G2 is an absorbing mask with a periodicity matching that
of the interference fringes. Depending on the position of G2 relative to the interference pattern,
a varying amount of radiation is transmitted and recorder by the X-ray detector. Thus, for a
given position of G2, any change in the intensity modulations (for example, of the mean,
amplitude, or position) locally induces a change of the intensity I
pix
recorded by the pixel (r,c)
of the X-ray detector, where r and c are integers indicating the pixel position (row and column).
For polychromatic radiation, the intensity I
pix
can be expressed as
Ipix r;c
ðÞ
¼Q0r;c
ðÞ
1þνr;c
ðÞ
⋅sin ψ1r;c
ðÞðÞ½
ð4Þ
Where Q
0
is the mean intensity (averaged over ψ
1
∈[0, 2π] ) received by the detector pixel,
which depends on the X-ray flux and the absorption of the gratings. νis the visibility of the
interference, which ranges from 0 to 1 indicates the quality of the interference fringes. ν=0
means that there is no interference at all, while ν= 1 is the ideal case (only achievable for
perfect gratings and monochromatic radiation) The phase coefficient ψ
1
is related to the
relative position of G2 with respect to the interference. For example, it is equal to 0 when
Fig. 1 In conventional X-ray imaging, the attenuation of the X-ray beam due to its absorption in the sample is
measured and is related to the attenuation coefficient β. On the contrary, in phase contrast X-ray imaging,
changes in the phase front of the beam are measured which are linked to the refraction coefficient δ
Appl Compos Mater (2017) 24:513–524 515
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the interference maxima are aligned with the absorbing part of G2. Inversely, ψ
1
amounts to
π/2 when the maxima are aligned with the transmitting part of G2.
In the presence of a sample, the measured pixel intensity will be changed due to three
different effects, as shown in Fig. 2b. For convenience, the subscript Br^and Bs^to refer to the
measurement done without sample (Breference^) and with sample are used, respectively.
First, the attenuation of the X-ray beam due to photoelectric absorption and Compton
scattering at large angles leads to a decrease of the mean intensity Q
0
of the interference
fringes. Secondly, the beam is refracted by an angle αand this results in a local displacement
(along the y-direction) of the fringes and thus a change of the phase coefficient ψ
1
.Finally,in
the presence of inhomogeneity (porosity, cracks, etc.) at the microscopic level (0.1 to 50 μm),
the beam is scattered at small angles and a loss of visibility νof the interference fringes ensues.
The different contributions can be decoupled using different imaging modes and thus three
images can be reconstructed, namely the absorption image T, the differential phase contrast
image DP, and the scattering image V:
Tr;cðÞ¼
Qs
0r;cðÞ
Qr
0r;cðÞ
;DP r;cðÞ¼
ψs
1r;cðÞ−ψr
1r;cðÞ
2π;Vr;cðÞ¼
νsr;cðÞ
νrr;cðÞ ð5Þ
For example, Fig. 3shows the results of the reconstruction of the three images T,DP,andV
for a carbon fibre composite sample.
It is important to note that both the refraction angle and the small angle scattering are
measured only along the transverse direction, perpendicular to the gratings’lines. This means
that anisotropic defects, such as cracks running along the other direction, parallel to the
gratings’lines may not be seen in the images. In order to solve this problem and see defects
Fig. 2 a Schematic of the grating-based Phase Contrast X-Ray Imaging system and bcross sectional view of the
interactions of the X-ray beam with the sample
Fig. 3 Example of aabsorption; bdifferential phase contrast and cscattering images obtained for a curved fibre
reinforced polymer
516 Appl Compos Mater (2017) 24:513–524
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in all directions, the gratings can be rotated by 90° and another image can be acquired, which
will be sensitive in the perpendicular direction.
Actually, the anisotropic sensitivity of the grating interferometer can even be used to
extract directional information on the sample, such as the number of plies orientated along
a given direction. Flaws such as wavy fibres are then easily detectable using this
directional sensitivity [9].
In order to use this advantage, the grating interferometer must be designed to allow the
rotation of the ensemble (G1+ G2) independently of the detector, in such a way that images of
the sample can be acquired for different directions of the sensitivity. More details can be found
in the literature [6,7].
3Experiments
3.1 Grating-Based Phase Contrast X-Ray Imaging (XPCI)
A schematic view of the grating-based Phase Contrast X-Ray Imaging system is shown in
Fig. 2a. It consists of a standard high power X-ray source (Varian HPW-160-11) and pixelated
detector array (Dexela DEX2315) coupled to three gratings forming an X-ray interferometer.
The principle of the X-ray grating interferometer is explained in details in the literature [1–3,
13]. A collimator made out of lead isused to control the illuminated area while a shutter allows
to block the X-ray beam during the idle time of the detector. In the present configuration, the
sample can be moved independently using an XY gantry (IAI axis).
The gratings were manufactured at CSEM using MEMS fabrication processes on Silicon
wafers of diameter 150 mm, which allows to achieve a grating size of 100 ×100 mm on a
single wafer. The parameters of the demonstrator are summarized in Table 1.
Three images are obtained using the EVITA demonstrator, namely the absorption, refrac-
tion and scattering images. The absorption image corresponds to the conventional X-ray
image, except that the blurring effect due to the Compton scattering is suppressed in the
direction perpendicular to the grating lines. The absorption image is related to the attenuation
coefficient of the material and its thickness.
The refraction image is proportional to the refraction angle measured pixel-wise by the
interferometer. The refraction angle is linked to the derivative of the phase shift in the
direction perpendicular to the grating lines (direction y in Fig. 2a). In contrast to the
absorption image, the refraction image is thus related to the refraction coefficient as well as
to the thickness of the sample [4].
Tabl e 1 Key parameters of the
EVITA demonstrator Parameters
Source acceleration voltage 40–70 kV
Maximal sample thickness (CFRP) 50 mm
System length (source to detector) 1.45 m
Measurement area (stitching mode) 1× 0.5 m
2
Image size (single field) 7 × 7 cm
2
Effective pixel size 50–60 μm
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Finally, the scattering image is related to the ultra-small angle X-ray scattering of the beam
inside the sample. It has been shown that the USAXS can be expressed in terms of variations
of the electronic density of the material at the microscopic level [14]. The scattering image is
thus a perfect tool to probe the microscopic texture of composite materials and detect porosity,
cracks and variations of the fibre density or orientation [15].
The images were reconstructed using the phase stepping method [13], where the phase
stepping was achieved by translation of the grating G2. The X-ray tube source was set to the
small focal spot (0.4 ×0.4 mm
2
) with the acceleration voltage at 60 kVp and the anode current
at 10 mA. 19 phase steps were acquired over 4 periods with an individual exposure time of
750 ms. The measurement was repeated 4 times and averaged. The measurement time for a
single field inclusive reconstruction amounts to about 60s.
3.2 Materials
The samples were made out of Epoxy-carbon prepreg (HexPly 914C-T300H(6 K)-5-34 %).
Two thickness composite materials were made, 4 and 20 mm. The results presented here were
obtained with a 32 and 160 plies quasi-isotropic lay-up (+45°/90°/-45°/0°)4 s and (+45°/90°/-
45°/0°)40s, respectively. In order to test the capability of the PCI technique, manufacturing
routes were developed which produced controlled defect in a repeatable manner. Table 2
shows the range of defect reference samples investigated in this publication.
The results obtained with the EVITA demonstrator were benchmarked against four state-of-
the-art NDI methods: Ultrasonic through transmission (UTT) (i.e. water jet and immersed),
phased array ultrasonic (PA), and conventional X-Ray computed tomography (X-Ray CT).
In our preliminary work [9], the EVITA demonstrator was benchmarked on two different
types of artificial flaws: fiber cut and out-of-plane wrinkle. These two flaws were detected by
the EVITA demonstrator with a comparably fast exposure time. No other NDI method used for
the benchmarking was able to detect both flaws.
4 Results and Discussions
4.1 Porosity Detection
Figures 4,5,and6show the results of the comparison for the first samples, where different
porosity contents were introduced during the manufacturing process. The porosity can be
detected in the scattering image using the EVITA demonstrator as well as with the UTT and PA
systems. All other benchmarking methods (i.e. IR-thermography and X-Ray CT) were not able
Tab l e 2 Composite sample list
Sample ID Defects Materials Comments
137186-14 porosity <2 % Monolithic, Epoxy-carbon prepreg layup thick (20 mm)
137186-16 porosity 2 to 5 % Monolithic, Epoxy-carbon prepreg layup thick (20 mm)
137186-18 porosity> 5 % Monolithic, Epoxy-carbon prepreg layup thick (20 mm)
137186-2 cracks Monolithic, Epoxy-carbon prepreg layup thin (4 mm)
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to detect the defects. The same colour bar on the scattering image is used for all the samples
with porosity. Moreover, Moiré artefacts are observed in the absorption and refraction image.
It was observed that the scattering signal is increasing with the porosity level. Moreover, in
the zoom-in of the refraction image, elongated voids can be observed. In order to evaluate
quantitatively the relationship, the average and standard deviation of the absorption and
scattering images were calculated over a region of 100 × 100 pixels (about 6 × 6 mm). The
averages of the absorption and scattering images are plotted in Fig 7as a function of the
porosity level, where the standard deviation is represented as the error bar. The signal intensity
Fig. 4 Sample with porosity <2 %. The images obtained by UTT and PA are shown in (a), (b) and (c). The
absorption, refraction and scattering images obtained with the EVITA demonstrator are shown in (d)
Fig. 5 Sample with porosity 2 to 5 %. The images obtained by UTTand PA are shown in (a), (b)and(c). The
absorption, refraction and scattering images obtained with the EVITA demonstrator are shown in (d)
Appl Compos Mater (2017) 24:513–524 519
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of the scattering increase linearly with the level of porosity which is not the case with the
absorption. With higher magnification, the EVITA system is able to identify the porosity
distribution and size. To obtain the information through the thickness, this system has to be
coupled with the computed tomography technique.
Fig. 6 Sample with porosity >5 %. The images obtained by UTT and PA are shown in (a), (b) and (c). The
absorption, refraction and scattering images obtained with the EVITA demonstrator are shown in (d)
Fig 7 Plots of the average absorption and scattering signal as a function of the porosity for the 20 mm thick
samples. The error bar represents the standard deviation over the region of interest
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4.2 Cracks Detection
Micro-cracks are usually following the orientation of the fibre layup. For this reason, they
display usually a clear orientation (typically 0° / 90° / +45° / -45°). A special geometrical
filter was developed to enhance the crack signal. The following procedure is implemented:
(i) remove low frequency from the image; (ii) filter the image suing ellipsoidal Gauss
kernels with angle scanning between 0 and 180° (1° step); (iii) combine the filtered images
by normalising according to the filter response at each angle. An edge along the filter
direction will have a greater response than all other edges in the image. And the edges
which are not in the direction of the filter will be smoothed out providing a low response
close to the average. Figure 8shows the results of the geometrical filter applied to the
scattering images of samples with micro-cracks. This is clear that this geometrical filter
strongly enhances the detectability and visibility of the elongated micro-cracks. These
measurements demonstrate the ability of the EVITA system to detect micro-cracks in
composite components. Moreover, the crack shape and density can be obtained.
Fig. 8 (left) Scattering image of a sample with micro-cracks and (right) corresponding image after application of
the geometrical filter
Fig. 9 All three images deliver complementary information, which is combined for defect detection and
discrimination
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4.3 Image fusion
The EVITA phase contrast imaging system aims at detecting and discriminating different
defects. As each of the imaging modes (absorption, differential phase contrast (refraction) and
small angle scattering) has different sensitivity to different types of defects, the image
processing module (IPM) allows to process and combine the different images (Fig. 9). It is
thus envisaged that a drop down menu system indicating the defect types under examination in
the image processing software will allow the correct combination of images to be obtained
which give the best chance of detecting the particular defect type.
The image fusion algorithm aims at visualising both the filtered structure and the back-
ground scattering image (unfiltered) at the same time. In the present case, we decided to
display the scattering image as grey levels and to overlap the filtered refraction image with
yellow colour. Figure 10 displays an example of the image fusion for composite samples with
different levels of porosity while Fig. 11 shows the example for a composite sample with
micro-cracks.
Fig. 10 Example of image fusion of the scattering image with the filtered refraction image for composite
samples with different level of porosity 1, 3.5 and 7 %
Fig. 11 Example of image fusion of the scattering image with the filtered scattering image for a composite
sample with micro-cracks
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5 Conclusion
The EVITA demonstrator was presented and its performance was illustrated using two
coupon samples with two types of artificial flaws: porosity and cracks. The flaws could
be detected and quantified by the EVITA demonstrator with a comparably fast exposure
time. The EVITA demonstrator was benchmarked against other state-of-the-art NDI
methods. While phased array ultrasonic and through transmission ultrasonic were also
able to detect the different porosity level, the ability of the EVITA demonstrator to
quantify the level of porosity seems to be promising and will be further investigated in
future research. Furthermore, while the detection of cracks can be readily achieved using
the EVITA demonstrator and a geometrical filter, no other NDI method used for the
benchmarking was able to detect the cracks induced in the structure. In combination with
a relative fast exposure time (comparable to phased array ultrasonic), the EVITA demon-
strator demonstrated unique features, which can benefit to the non-destructive inspection
of lightweight materials such as composites.
The introduction of this innovative methodology is expected to provide the aero-
nautical industry with a reliable and detailed insight of the integrity of thin and thick
composite structures as well as of complex geometry ones, such as integrated closed
boxes and sandwiches. By increasing the level of detectability of defects in composite
structures, as well as by detecting defects invisible to standard industrial non-
destructive testing methodologies, the novel method will play a major role during
the whole life cycle of composite components, reducing their inspection cost and
increasing their reliability.
6 Acknowledgments
The authors acknowledge financial support from the European Union’s Seventh Framework
Programme for research technological development and demonstration under grant agreement
n°314735.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International
License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and repro-
duction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were made.
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