Fatigue Damage in Carbon Fibre Epoxy Composite Under Variable Loading Conditions

Abstract

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. Two stages of damage evolution were identified. In Stage I, the fatigue damage parameter and the density of matrix microcracking rapidly increased to a level dependent upon the stress. This was followed by Stage II, a long period (90% life) of gradual increase in damage, involving crack coalescence, debonding and delamination. On subjecting the specimens to variable amplitude loading tests, the total fatigue life was greater than that of the constant amplitude tests. Changes from low to high stress levels required more cycles to complete Stage I. For the high to low stress changes, the presence of large numbers of cracks and matrix debris within them resulted in closure, with a consequent increase in Stage II life.