In-situ investigation of dynamic failure in [05/903]5 CFRP beams under quasi-static and low-velocity impact loadings

In this paper, experimental work to investigate the failure sequence in cross-ply CFRP beams under quasi-static and low-velocity impact (LVI) loadings is presented. [0(5)/90(3)](s) CFRP beams with thick clustered plies are investigated for clear observation of damage mechanisms. A non-standard LVI test setup is designed and built in order to conduct line impact experiments and to allow in-situ observation of damage evolution using a high-speed camera. Full-field strain measurement on the visible edge prior to initial failure is performed with digital image correlation (DIC) analysis. In both static and dynamic loading cases, failure sequence is demonstrated to initiate in the middle 90 degrees plies as a diagonal matrix crack leading to delaminations at the upper and lower 0/90 interfaces. Major diagonal matrix crack formation is shown to occur consistently at a local transverse shear strain peak in conjunction with a tensile transverse normal strain. Two additional distinct types of matrix damage, namely hairline cracks and micro-matrix cracks, are discovered in post-mortem examination. Delamination propagation in a composite beam subjected to line loading is monitored using high-speed photography at half-a-million frames-per-second for the first time. It is observed that, in both static and impact cases, delaminations are not only dynamic, but they also reach the same speed plateau of 850 - 900 m/s. We suggest that the experimental crack tip speed data can be used as a benchmark to fine-tune interlaminar damage models. (C) 2021 Elsevier Ltd. All rights reserved.