Non-destructive evaluation of defects in polymer matrix composites for aerospace applications using x-ray Talbot-Lau interferometry and micro CT

One of the main challenges to accelerate the acceptance and use of advanced materials in the ESA, NASA, and commercial space applications is to establish a broadly accepted materials and process quality system, including adequate non-destructive testing (NDT) procedures. However, to profoundly exploit the advantages of advanced manufacturing for space applications, and to ensure highly reliable parts, new approaches to both manufacturing and NDT techniques are needed. NDT procedures must be able to track unique features such as small scale and deeply enclosed porosity, complex part geometry, and subtle internal features. Such NDT procedures have to be applicable to various products and processes while being flexible to fulfil materials, design, and test standards encountered throughout the components´ life cycle. In this contribution we employ advanced X-ray imaging technologies, e.g. high resolution X-ray computed tomography (XCT) and grating interferometer X-ray computed tomography (TLGI-XCT) to investigate porosity and crack propagation in carbon fiber reinforced polymers (CFRP). In contrast to XCT, TLGI-XCT provides three complementary characteristics in a single scan of the specimen: a) the attenuation contrast, b) the differential phase contrast (DPC), and c) the dark-field contrast (DFC). Using two different TLGI systems, we visualize crack-like defects in CFRP laminates that were subjected to impact forces up to 9 Joules and subsequent bending tests. Using DFC, we are able to detect cracks in samples that were subjected to low impact forces whereas these defects are merely detectable using AC. Specimens were scanned at isometric voxel sizes between 4.8 µm and 22.8 µm. TLGI-XCT results are compared to ultrasonic examinations and high resolution XCT scans. Due to the fact that DFC delivers morphological information in the sub-pixel regime depending on the local scattering power, dark field imaging delivers information that may otherwise be inaccessible using conventional XCT. Using a Talbot-Lau XCT we show that dark field images yield a high contrast and a strong signal at interfaces, in particular for matrix cracks and pores.