Inspection of fiber waviness in carbon fiber laminates by talbot-lau x-ray grating interferometry

In this work, we explore the possibilities of Talbot-Lau grating interferometer (TLGI) X-ray computed tomography (XCT) and radiographic testing (RT), to investigate and define fiber waviness in carbon fiber reinforced polymers (CFRP). The additional X-ray contrast mechanisms gained by TLGI provide high sensitivity to inhomogeneities and fiber misalignment in laminate CFRP. Particularly, dark-field contrast (DFC) caused by small angle scattering of X-rays allows the investigation of relatively large specimens, since the scattered signal provides information on a sub-pixel level. We investigated an aerospace grade vacuum infusion CFRP specimen by TLGI-XCT to visualize and quantify fiber waviness using DFC imaging. However, because of long TLGI-XCT scanning durations, that are often a multitude higher than standard XCT in combination with very limited specimen dimensions, this method is mostly not applicable at industrial standards. Therefore, the reduction to single radiographic images for the inspection of out-of-plane fiber waviness is a potential way to tackle both of these limitations at once. By applying an increasing tilt angle to the specimen, fiber waviness will cause a distinct peak in the scatter signal once fiber bundles are aligned horizontally and thus parallel to the optical axis of the X-ray tube. We show that by exploiting this characteristic behavior information about out-of-plane fiber waviness can be extracted from radiographic TLGI images.