Fatigue behaviour of pseudo-ductile unidirectional thin-ply carbon/epoxy-glass/epoxy hybrid composites

This paper is the first detailed investigation of the fatigue behavior of pseudo-ductile unidirectional (UD)thin-ply interlayer hybrids made of thin-ply carbon/epoxy plies sandwiched between standard thickness glass/epoxy plies under two scenarios: without any initial damage (pristine hybrids)and after the introduction of damage in the laminates by loading past the pseudo-yield point (overloaded hybrids). The laminates were subjected to different percentages of the critical stress level at which multiple fragmentation of the carbon plies was established (knee-point stress). The stress levels for fatigue delamination initiation and growth were evaluated experimentally. Based on the experimental work, it was observed that (1)when pristine hybrid composites were fatigued well below the carbon failure strain, at a stress level of 80% of the knee-point stress, there is no stiffness reduction after a significant number of cycles (10⁵ cycles)(2)gradual stiffness reduction and very slow delamination growth was observed for pristine hybrid composites when fatigued at 90% of the knee-point stress, (3)when overloaded hybrid composites were fatigued at 90%, 80% and 70% of the knee-point stress, they did not fail immediately but delaminated slowly (4)the slow growth was due to the low energy release rate of the thin-ply hybrid composites (5)the strain energy release rate approach related to delamination rates provides a good way to characterize the fatigue damage accumulation of overloaded hybrid composites and as a basis to predict the fatigue life.