# Parametric study of lightning induced damage in carbon composite using X-ray phase contrast imaging

Laureen Guitard, Loic Toulemonde, Mouad Saliji, Olivier Ghibaudo, Jérôme Primot, Rafael Sousa Martins, Adrien Stolidi, Amélie Jarnac

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Laureen Guitard, Loic Toulemonde, Mouad Saliji, Olivier Ghibaudo, Jérôme Primot, et al. Parametric study of lightning induced damage in carbon composite using X-ray phase contrast imaging. Proceedings of the 10th International Symposium on Digital Industrial Radiology and Computed Tomography (DIR 2025), 1-3 July 2025 in Paris, France (85), COFREND; CEA/LIST; DGzfP, Jul 2025, Paris, France. cea-05197533

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# Parametric study of lightning induced damage in carbon composite using X-ray phase contrast imaging

Laureen GUITARD¹,², Loïc TOULEMONDE¹, Mouad SALIJI¹,³, Olivier GHIBAUDO¹, Jerôme PRIMOT², Rafael SOUSA MARTINS³, Adrien STOLIDI¹* and Amélie JARNAC³

¹Université Paris-Saclay, CEA, List, F91120 Palaiseau, France.

²DOTA, ONERA, Université Paris-Saclay, 91120 Palaiseau, France.

³DPHY, ONERA, Université Paris-Saclay, 91120 Palaiseau, France.

*Corresponding author, E-mail: adrien.stolidi@cea.fr

## Abstract

This study presents a parametric analysis of lightning-induced damage in Carbon Fibre Reinforced Polymers (CFRP) using X-ray Phase Contrast Imaging (XPCI) based on Interferometric technique (phase grating). Damage severity was evaluated at different levels of electrical energies deposited in the CFRP and cross-validated with other non-destructive techniques. The results demonstrate the potential of XPCI for precise characterization and improved damage modelling in aerospace applications.

**Keywords:** X-ray imaging, Phase-contrast imaging, CFRP, Lightning, Interferometry

## 1. Introduction

Carbon Fibre Reinforced Polymers (CFRPs) are increasingly employed in the aerospace industry due to their high strength-to-weight ratio. However, their low and anisotropic thermal and electrical conductivities make them vulnerable to damage from lightning strikes, which can cause localized heating, matrix evaporation, and internal defects such as delamination and fibre breakage [1]. Accurate characterization of this damage is crucial to understand the underlying mechanisms and improve damage prediction models [2-3]. While traditional non-destructive evaluation (NDE) methods such as ultrasonic testing [2] and thermography [3] provide insight into crack propagation and surface defects, they lack the spatial resolution and volumetric coverage required for comprehensive damage analysis.

X-ray Phase Contrast Imaging (XPCI) has emerged as a promising technique for high-resolution volumetric inspection of CFRPs, enabling the simultaneous measurement of attenuation and phase properties [4–6]. Recent studies have demonstrated the potential of grating-based XPCI, particularly the Multilateral Shearing Interferometry (MLSI) technique, to characterize materials with undetermined physical properties, such as lightning-damaged CFRPs [7]. This approach offers several advantages, including attenuation and phase decoupling with spatial resolution down to 5 µm, robust spatial filtering and intrinsic self-assessment tools for error detection. These features enable sensitivity to fibre damage, defect discernment, and directional analysis, providing complementary information to attenuation imaging to identify the nature and extent of lightning-induced damage.

Based on these findings, this proceeding presents a parametric study to investigate the effects of different levels of electrical energy injected into the CFRPs on their structural and physical properties. To this end, grating-based XPCI was used and compare to other NDE methods.

## 2. Materials and methods

### 2.1 ONERA's lightning bench and CFRP

We used in this work the ONERA's lightning test facility, named GRIFON [8]. This generator is designed as an approximate RLC circuit and can deliver a transient current derived from the standard current waveforms as defined in aeronautical recommendation documents [9]. To fully characterize CFRP damage
across various current levels, the generator has been set to deliver fractions of the transient D-component (100 kA peak), with waveforms of the same shape but ranging from 7 kA to 20 kA peak.
The samples subjected to the lightning waveform consist of a CFRP coupon drilled with two holes for the insertion of two metallic screws. One screw acts as the current entry point (injection) and the other as the current exit point for the lightning current (see Figure 1). We focus our attention on the current exit area (marked by the red dashed box). There, the screws are inserted into the hole with a clearance gap of 600 µm and tightened by a nut. A dielectric sheet (NOMEX T410 with a thickness of 250 µm) is inserted between the parts, for insulation, ensuring that the current flows only from the injection to the exit points through the CFRP. This configuration mimics aeronautic fastener.

<figure><img src="image_2.png" /><figcaption>Figure 1: CFRP tested sample (left), mounted on the lightning bench of ONERA (GRIFON) to be subjected to a transient electrical discharge (right)</figcaption></figure>

The CFRP coupons measure 50 × 20 × 4.1 mm³ and are constructed of 16 unidirectional pre-impregnated carbon-reinforced epoxy plies. Each ply utilizes T700 carbon fibres (Toray) within an epoxy thermosetting resin M21 (Hexcel). The layup structure features plies oriented at 45° and -45°, creating an unconventional arrangement where no specific ply is designated to carry current between the two fasteners. For three levels of current injected, one CFRP coupon is considered. The table above summarizes the peak current (in kA) injected into each sample and the electrical energy deposited into the CFRP, in J.

<table><thead><tr><th>Samples labels</th><th>peak<br>current<br>injected into the CFRP (kA)</th><th>Electrical energy deposited into the CFRP (J)</th></tr></thead><tbody><tr><td>A</td><td>7.65</td><td>282</td></tr><tr><td>B</td><td>11.96</td><td>474</td></tr><tr><td>C</td><td>17.50</td><td>1097</td></tr></tbody></table>

## 2.2 CEA's MLSI bench and other NDE methods

### 2.2.1 MLSI technique

The MLSI method involves using a single 2D checkerboard phase grating positioned between the source and the detector. This device creates an intensity pattern known as an interferogram, featuring maxima and minima. To effectively generate and sample the interferogram, the setup requires an X-ray source with a micrometre-sized spot and a high-resolution detector. First, the grating is imaged on its own to obtain a reference interferogram. The sample is then introduced into the field of view and a new image is acquired. The interferogram on the detector is modified by the presence of the sample. Due to the regular periodicity of the phase grating, all phase gradients can be extracted by operating in Fourier space, which also allows attenuation images, phase gradient images and phase image to be obtained. This approach offers a quick and efficient method. The images were acquired on the CEA's MLSI bench using a Hamamatsu microfocus tube with a tungsten anode, operating at 60 kV and 35 µA, with an 8 µm focal spot size. The phase grating which serves as a reference is a 2D checkerboard grid from MicroWorks, designed to impinge the wavefront intensity with a phase shift of [0; π] at 17.48 keV, achieved by a 3.49 µm gold layer on a polymer substrate. Among various available grids, we used the one with a periodicity of 20 µm and placed it 22 cm away from the source. The sample was positioned 14.5 cm away from the source, in front of the grid. A high-resolution Hamamatsu sCMOS detector with a 20 µm GadOx scintillator, 2048×2048 pixels, and a pixel size of 6.5 µm was located 47 cm away from the source. The divergent X-ray beam resulted in a magnification of 2.14 for the grid and 3.24 for the sample.
Measuring the phase $\varphi$ is particularly valuable because it is proportional to the material density as shown in [10]:

$$ \varphi = x \frac{2\pi\delta}{\lambda} \quad \text{with} \quad \delta(\lambda) = \frac{r_e\lambda^2}{2\pi} \sum_j n_j f_j^1(\lambda) \quad (1) $$

where $x$ is the sample thickness, $\lambda$ is the wavelength, $r_e$ is the classical electron radius, and $n_j$ and $f_j^1$ are the atomic density and the real component of the atomic scattering factor of the $j$-th element, respectively. For X-ray photon energies away from absorption edges, $f_j^1$ can be approximated by the atomic number $Z$. The term $\sum_j n_j f_j^1$ represents the electronic density $\rho e$, allowing Eq. (1) to be rewritten as:

$$ \delta(\lambda) = \frac{r_e\lambda^2}{2\pi} \rho_e = \frac{r_e\lambda^2 N_a \rho}{2\pi} \sum_j q_j \frac{Z_j}{A_j} \quad (2) $$

where $N_a$ is Avogadro's number, $\rho$ is the material density, $q$ is the mass fraction, and $A$ is the atomic mass.

### 2.2.2 Computed tomography

For this study, computed tomography of 901 projections were acquired with an angular increment of 0.4°. The equipment used includes a Viscom microfocus tube with a tungsten target and a Perkin Elmer flat panel detector featuring 1024×1024 pixels, each measuring 200 µm. The X-ray tube was operated at 70 kV and 150 µA, with an exposure time of 1 second per projection. The distance between the source and the sample was set at 160 cm, achieving a magnification of 1.7 to capture the entire sample dimensions of 50 × 30 × 4 mm³. The Feldkamp Davis Kress (FDK) algorithm was employed to reconstruct the volume image, enabling a high-resolution 3D analysis of the samples.

### 2.2.3 Thermography

Active infrared thermography was used in a complementary manner in this study. The excitation is provided by two halogen lamps, this heating mode is well suited for the detection of planar defects in CFRP materials such as delamination between layers. Each halogen lamp has an electrical power of 2 kW, the illumination beam is conical with a large aperture, and the tests were carried out in both transmission and reflection modes with a working distance of 50 cm between samples and lamps. The waveform of the lamp command signal is a step, with a short duration of 2.5 seconde, well adapted for CFRP high thermal diffusivity ($\alpha \sim 4.10^{-6} \text{ m}^2 \text{ s}^{-1}$ [11]). The infrared camera, “EDECAM” supplied by EDEVIS®, is composed of an uncooled µ-bolometer array of 640×480 pixels, pitch size of 17µm, the spectral range in LWIR 8-14 µm, and thermal sensitivity (NETD) of 40mK, the framerate can reach up to 120 image/seconde in full frame mode with rolling shutter refresh, and the integration time of 15.5 µs. The optics used for the lens have a focal length of 25 mm, a variable working distance between 25 cm – 2 m, a wide field of view of 24.5°×18.3°. For a working distance of 50 cm between camera and samples, the apparent spatial resolution is around 0.4 mm and the horizontal field of view is 27 cm which is wide enough to acquire the three samples simultaneously. From the acquired image sequence, the post-precessing is used to captured the phase-shifted thermal relaxation after step heating, based on Discret Fourier Transform (DFT).

### 2.2.4 X-ray fluorescence spectrometry

In this setup, the X-ray fluorescence (XRF) technique involves illuminating the CFRP specimen with an ionizing source, such as an X-ray tube, to induce X-ray fluorescence events. A detection system, like a solid-state detector, then collects the fluorescence lines, which are sorted by photon energies (Energy Dispersive-XRF). The energies of these lines are characteristic of the elements in the CFRP, enabling their identification. A Moxtek X-ray tube operating at 25 kV and 100 µA, equipped with a tungsten collimator of 100 µm in diameter, was used. Additionally, a Silicon Drift Detector (SDD) from PN-Detector, with an active volume of 30.0 mm² × 450 µm, was employed. Due to the single-pixel detector, a 2D image was
produced by scanning the CFRP specimens, leading to a spatial localization of the fluorescence source. The CFRP specimen was scanned every 500 µm, resulting in a 20×50 pixel image. The exposure time per pixel was 1 second to achieve a good compromise between scanning time and signal-to-noise ratio.

# 3. Results and discussions

## 3.1 Phase contrast analysis

Figure 2 presents the X-ray images of a CFRP area near the current injection point, after the screw has been disassembled. The images show the absorption (left), phase gradients (middle) and the phase image (right). Complementary information is revealed in each modality. The absorption image highlights local absorbing areas, while the phase image displays a more spread, non-homogeneous area (approximately 750 µm) near the hole. The phase gradient image emphasizes the orientation of the carbon fibres. The images are generally homogeneous, except near the hole, where the standard deviation of the gray values increases by a factor of two, indicating fibre disorientation.

<figure><img src="image_3.png" /><figcaption>Figure 2: X-ray images of a CFRP area near the current injection point: absorption (left), phase gradients (middle) and phase image (right)</figcaption></figure>

For a more quantitative analysis, a calibration object (a PMMA ball) was placed inside the CFRP hole. This well-known standard helps to calibrate the CFRP's phase values as described in Section 2 equation (1,2). Therefore, to quantify the damaged-induced phase change, MLSI measurements have been carried out on samples A, B, and C (see table in Section 2) before the controlled lightning strikes, to determine its average pristine phase value, $\varphi_0$. The phase variation is then calculated as follows:

$$ \Delta\varphi(x, y) = \frac{\varphi(x, y) - \varphi_0}{\varphi_0} \qquad (3) $$

Where $\varphi$ represents the phase value retrieved after the lightning strikes. The phase variation of the damaged CFRP was evaluated for the three injected currents. We observe that the phase variation increases with the injected current. According to Eq. 2, density and phase are proportional. Therefore, phase variations could be considered as variations in density under the assumption of a constant Z value. However, this assumption could be easily questioned, since the sample underwent material degradation (such as matrix evaporation). To interpret the change in phase variation, other NDE techniques have to be used.

## 3.2 Computed tomography and X-ray fluorescence spectrometry

Figure 4 presents three reconstructions of samples A, B, and C. Local inclusions can be observed, and a segmentation approach was carried directly in VGStudio Max to isolate and spatially identify the inclusions, as shown in Figure 3. First, a median filter was applied to the reconstructed 3D volume to reduce background noise and homogenize the grayscale distribution. Then, a density threshold-determined from the voxel attenuation histogram was used to differentiate the higher density inclusions from the lower
density surrounding matrix. As the injected current increases, the volume of the inclusions also increases. To identify the nature of these inclusions, X-ray fluorescence spectral imaging was performed on these samples. The method revealed that the inclusions are composed of titanium, the same material as the screws used. Therefore, computed tomography and X-ray fluorescence spectrometry enable us to identify and localize titanium inclusions, shedding new light on the phase variation measured by the MLSI method.

<figure><img src="image_4.png" /><figcaption>Figure 3: Tomography reconstruction and segmentation of the CFRP sample A (left) B (middle) and C (right)</figcaption></figure>

### 3.3 Thermography

Figure 4 shows the DFT phase image after halogen heating in reflection mode for samples A-B-C, on both front and rear face, without any spatial filtering. Our interpretation is that the elliptical-shaped patterns aligned along the direction of the folds and around the holes in the specimen are subsurface (near-surface) delaminations because they appear in advance of phase (white on the grayscale). Indeed, delamination induces an air gap whose associated thermal resistance causes back-diffusion of the axial heat flux. Conversely, a phase delay (which appears in black on the grayscale) is synonymous with an increase in the axial thermal diffusivity of the material, which can be induced by a local increase in density (mechanical compression) or by the introduction of thermally conductive foreign material such as metal (Titanium). The interpretations are however polluted by artifacts linked to the non-uniform optical absorption on the surface of the test samples with the presence of NOMEX residues stuck. NOMEX appears in black on the phase image. On the rear face of sample C, which was more impacted by lightning, we observe the presence of delamination at the bottom right of the hole, whereas delamination is not observed on the samples less impacted by lightning (A and B).

<figure><img src="image_5.png" /><figcaption>Figure 4: DFT phase image with halogen thermography in reflection mode, front face on the left and rear face on the right. The samples are aligned from left to right: #C-A-B for front face, and #B-A-C for the rear face. The corresponding photography on the bottom correspond, from left to right, to #B-A-C.</figcaption></figure>
## 4. Conclusions

This study presents a parametric analysis of lightning-induced damage in Carbon Fibre Reinforced Polymers (CFRP) using X-ray Phase Contrast Imaging (XPCI) with the MultiLateral Shearing Interferometry (MLSI) technique. The severity of the damage was evaluated and quantified by retrieving the pre- and post-lightning phase variations for various electrical energies deposited in the CFRP. We observed an increased phase value for all the damaged samples. Thanks to the use of complementary non-destructive techniques, the phase variation has been identified as a Titanium deposition into the CFRP. The results demonstrate the potential of XPCI for precise characterization and highlight the complementarity of other NDE methods, such as computed tomography, X-ray fluorescence spectrometry, and thermography, for improved damage modelling in aerospace applications. Furthermore, repeatability measurements are currently being analysed, and a publication detailing these findings is in preparation.

## Acknowledgements

The authors wish to thank the French Civil Aviation Authority (DGAC), France Relance and NextGenerationEU for their financial supports. The authors thank the Department of Materials and Structures (DMAS) from ONERA for the availability of the CFRP-test pieces. Part of this research is supported by the project ANR-23-CE42-0002 funded by the Agence Nationale de la Recherche.

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