Shoukroun, Dana; (2021) Multimodal Imaging for Characterisation and Testing of Composite Materials. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Carbon fibre reinforced polymers (CFRP) are widely used across several industries, including aerospace, as they are lightweight and offer superior mechanical properties. Barely Visible Impact Damage (BVID), including cracks, delaminations, fibre debonding, as well as manufacturing defects such as porosity, are detrimental to CFRP structural integrity and detection of such faults is important. Different non-destructive evaluation (NDE) methods exist, including ultrasound, X-ray computed tomography (X-ray CT), infrared, and liquid penetrant testing. Edge Illumination X-ray Phase Contrast imaging (EI XPCi) was benchmarked as a viable NDE method for damage detection in CFRP, as it offers additional information through multimodal imaging. With the acquisition of at least three images, EI XPCi allows for the retrieval of the attenuation, differential phase, and dark field signals, using a pair of apertured masks. EI XPCi CT was compared with ultrasonic immersion C-scan imaging and high-resolution X-ray CT for the detection of severe impact damage in a composite plate (visible indent damage on surface of plate and protrusion on the back). The full extent and scale of the different defects were observed in the phase-based signals to a better standard than ultrasonic immersion imaging, with observations confirmed using high resolution X-ray CT. Planar EI XPCi was then compared to contrast agent X-ray imaging and ultrasonic immersion C-scan imaging on a different, less damaged specimen (only small crack visible on surface), showing that planar EI XPCi can detect a network of cracks across the specimen and overcame some of the limitations of contrast agent X-ray imaging. However, in the planar imaging, delamination damage was only detected by the ultrasonic measurement, showing the necessity of using both ultrasonic imaging and EI XPCi for a complete understanding of the damage in the plate. EI XPCi was used for the quantification of porosity for woven composite plates with varying porosity (0.7% to 10.7%), compared to ultrasonic through transmission imaging and destructive matrix digestion. The introduction of the standard deviation of the differential phase (STDP) showed excellent correlation with the porosity calculated from matrix digestion. The STDP signal quantifies the variation of the distribution of inhomogeneities for features of a scale equal to or above the system resolution (in this case, 12µm along the direction of phase sensitivity), which was advantageous for the investigated set of specimens with larger porosity.

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