X-ray imaging looks inside lightning damage to carbon composites

Increasingly, the aerospace industry is turning to strong, lightweight carbon-fiber-reinforced polymer composites (CFRPs). These low-absorption materials, with their low thermal and electrical conductivity, can suffer delamination, fiber breakage, and other damage when struck by lightning. CEA-List is working with ONERA to develop new imaging methods to get a more complete picture of lightning damage. The partners are leveraging a technique called phase-contrast X-ray imaging, which provides additional information to round out conventional X-ray absorption and is particularly well suited to the inspection of composites. Recent research at CEA-List and ONERA, including a PhD dissertation by Laureen Guitard, resulted in a robust quantitative method for characterizing carbon fiber reinforced polymer (CFRP) materials damaged by lightning.

 

 

A transient electrical discharge created by a current representative of lightning test standards was applied to a CFRP sample (Figure 1), sized in the laboratory. After impact, an image of the sample was obtained using multilateral shift interferometry (Figure 2), a technique co-developed by ONERA and CEA-List on an X-ray imaging bench.

 

 

Figure 3 shows the absorption (µ), phase gradient (dΦ), and phase (Φ) images of the area of the sample near the point of impact. The absorption image shows locally absorbing areas, while the phase image shows a wider, heterogeneous area (around 750 µm) around the impact point. The phase gradient images illustrate the direction of the carbon fibers (±45°). When taken together, the images are homogeneous overall, except near the impact point, where the standard deviation of the gray values doubles, indicating a disorientation of the fibers.

 

Then, to provide a more precise measurement of the damage, the phase values were correlated to relative density variations using a standard object. Figure 4 shows a map of density variations in percentage terms: White areas indicate unchanged density, blue areas lower density, and red areas higher density. By converting the phase data into density variations, the extent of damage can be quantified at different points in the material. At the same time, new data is produced for the numerical lightning-material interaction models developed at ONERA.

 

The technology in use

Aeronautics, composites

 

Projects

The research is ongoing in two areas: use cases, with a parametric study of lightning strikes on carbon composites; and methodology, with a focus on:

1. the transition from 2D to 3D imaging, the topic of a new ONERA/ CEA-List PhD dissertation;
2. the development of a digital phase contrast imaging model (Carnot OSIRIS);
3. the design of methods for fast transient phenomena like lightning strikes (ANR DyXPLAY, PTC MOVIE).

Major partnerships

Collaboration ONERA/CEA-List: PREPEND-X (Research platform in phase imaging dedicated to non-destructive testing by X-ray)

 

Flagship publication

“Robust quantitative X-ray phase diagnostic for carbon composite characterisation in the context of lightning induced risk”
Laureen Guitard, Adrien Stolidi, Georges Giakoumakis, Rafael Sousa Martins, Jérôme Primot & Amelie Jarnac Scientific Reports volume 14, Article number:21803 (2024)