Experimental compressive damage analysis on holed laminates under dynamic loadings using in-situ MHz synchrotron X-ray phase contrast imaging

This study highlights the importance of investigating the behaviour of the inner layers of carbon fibre-reinforced polymer (CFRP) laminates under dynamic compression, using Synchrotron-based techniques to understand damage initiation and failure propagation at high strain rates. Open and filled hole specimens with three different CFRP architectures were tested using the Split Hopkinson Pressure Bar (SHPB). Digital image correlation (DIC) and Synchrotron-based X-ray MHz radiography were performed simultaneously with ultra high-speed cameras (two Shimadzu HPV-X2 for X-ray images and one Photron SAZ for visible light). This allowed the first observation of matching outer and inner failures with surface strain fields at a micrometer level. Results showed that, in certain conditions, failure initiated earlier in the inner layers, leading to through-thickness propagation. This correlation between strength and internal damage initiation might be overlooked if only outer visible light images are used. The proposed methodology, which emphasised post-processing of X-ray images, can serve as a baseline for future research on the behaviour of inner layers of composite materials under high strain rates. Additionally, analysis of different CFRP architectures revealed their influence on unloading time after initial failure.