Fatigue damage accumulation in CFRP

A cyclic bending study using a stress ratio of 0.1 was conducted on unidirectional 45 degrees CFRP with a 60% nominal fibre volume fraction. This allowed the continuous matrix damage evolution to be followed without the influence of delamination. Damage was monitored by changes in the fatigue modulus, increase in crack density and amount of permanent. bending. It was found that their effects were complementary and that, in general, damage evolved in the same way as that observed in unidirectional tension-tension cyclic tests. All three monitoring techniques showed that damage accumulation was cycle dependent. Damage may be divided into two main stages, with rapid growth at the onset of cycling to about 10-20% life (Stage I), after which the damage increases slowly, but steadily, to failure (Stage II).