Damage development in carbon/epoxy laminates under quasi-static and dynamic loading

The contrasting characteristics of damage evolution have been examined in a multidirectional carbon/epoxy composite laminate (IM7/8551-7) subjected to both quasi-static and dynamic loading. Our experiments were performed on b end-test bars that were loaded either in 'supported' four-point bending or under 'unsupported' conditions with a Hopkinson pressure bar to induce dyna mic loading. We found differences in the damage that occurred in specimens loaded by the two techniques, in terms of the number of cracks and the leng th of the cracks. In the case of quasi-static loading, there were many matr ix cracks within individual plies and only a few delamination cracks betwee n plies; the maximum ratio of numbers of matrix to delamination cracks obse rved was 6:1. Despite their small number, the delamination cracks had a gre ater total length than the matrix cracks, and specimen failure occurred as a result of delamination crack propagation. During dynamic loading, the rat io between numbers of matrix and delamination cracks was 3:1, and in this c ase the ratio between the total crack lengths was unity. A quantitative ass essment of damage induced during quasi-static bending was made from specime n stiffness results. Using simple beam theory and knowing the location of t he damage. we correlated beam stiffness to the materials effective elastic modulus. We found that the composite's effective modulus decreased rapidly with small amounts of initial damage, but that subsequent increases in dama ge decreased the effective modulus at a much lower rate. (C) 1999 Elsevier Science Ltd. All rights reserved.