Damage evolution in a [±45]2S CFRP laminate under block loading conditions

Cyclic tests were conducted on [±45]2S angle ply carbon–epoxy specimens using stress ratios, R (minimum/maximum stress) of 0.1 and −1.0. Damage was monitored by measuring progressive strain changes in the loading direction. The fatigue damage parameter was found to satisfactorily describe the evolution of damage throughout life, facilitating fatigue life prediction. Two distinct stages of damage evolution were identified. In Stage I, the fatigue damage parameter and the density of matrix micro-cracking rapidly increased to a level dependent upon the stress (Characteristic Damage State). This was followed by Stage II which was a long period (∼90% life) of gradual increase in damage, involving crack coalescence, debonding and delamination. On subjecting the specimens to two step block loading tests, synergistic interaction occurred whereby the total fatigue life was greater than that predicted by the summation of the individual blocks of cycles. The effect of crack density and crack closure appeared to play important roles in extending the fatigue life. For the low to high stress level block transition, more cycles were required to reach the Characteristic Damage State, whereas for the high to low sequence, the presence of a large number of cracks and matrix debris within them resulted in closure at the lower stress. Again, the number of cycles to failure increased.