Multiscale and Multimodal Approaches for Threedimensional Materials Characterisation of Fibre Reinforced Polymers by Means of X-ray based NDT Methods

Non-destructive testing (NDT) and three-dimensional materials characterisation of fibre reinforced polymers using X-ray based methods can be carried out at different length scales and by using different modalities. This work gives an overview of different X-ray based NDT methods and their characteristics. Multiscale X-ray computed tomography (XCT) usually includes scanning an entire part at lower resolution – governed primarily by specimen diameter. Subsequently, a smaller sample is cut out of the respective specimen and scanned at a higher resolution. Accordingly, in this work typical XCT resolutions ranging from (135 µm)³ voxel size down to (250 nm)³ are presented. Using different XCT modes such as region of interest scans or laminography (XCL) modes this multiscale approach is also possible without destroying or cutting the sample in smaller pieces. However, some limitations in image quality and sample geometry have to be considered. We show that cracks with a width between 122 and 56 µm can be clearly seen at a relatively low resolution of (135 µm)³ voxel size in one example of a larger carbon fibre reinforced polymer (CFRP) sample from the aeronautic industry. With XCL voxel sizes down to (0.75 µm)³ can be reached, showing clear structures in the range of 16 µm. Main disadvantage of XCL is that only a view layers and not the full 3D-microstructure can be represented. Using an XCT resolution in the range of (2 µm)³ voxel size for CFRPs may lead to misinterpretation in relation to porosity because of propagation-based phase contrast effects. High-resolution region of interest XCT scans at (250 nm)³ voxel size show that epoxy-rich areas between individual C-fibres smaller than 6 µm are leading to relatively dark grey values, easily misinterpreted as voids. Multimodal XCT data was generated using a Talbot-Lau Grating Interferometer (TLGI) XCT to obtain modalities such as dark-field contrast and differential phase contrast in addition to standard attenuation contrast. In one example it is shown that metal artefacts in CFRP issued by a Cu-mesh can be significantly reduced by TLGIXCT. This provides improved image quality and the possibility to segment voids close to metallic components. For easier interpretation and a better understanding of material features the open source software open_iA was used with new implemented visualisation approaches for multimodal and multiscale data-visualisation.