Mode-II total fatigue life model for unidirectional IM7/8552 carbon/epoxy composite laminate

This paper presents a method of developing Total Fatigue Life Model (TFLM) for a unidirectional composite laminate with a delamination subjected to mode-II stress state. The model includes three domains of delamination growth rates: the middle Paris domain and the two end, onset and fracture, domains. The constants in the equation were determined from fracture and fatigue test data of End Notched Flexure (ENF) specimens for IM7/8552 carbon/epoxy composite laminate. The mode-II fracture toughness, GIIc was determined from fracture test and it was found to be 0.978 kJ/m2. The fatigue crack growth rate data was generated for different initial GIIimax values by load controlled constant amplitude cyclic load fatigue tests for a stress ratio (R) of 0.1. The fatigue threshold energy release rate, GIIth based on 1% compliance change for one million cycles (N) was found to be 0.167GIIc. The generated fatigue test results da/dN versus GIImax/GIIc was found to follow the equation 0.8(GIImax/GIIc)5.8 ((1 − (GIIth/GIImax)12)/(1 − (GIImax/GIIc)3)) for IM7/8552 carbon/epoxy composite laminate. The Paris law exponent (β) agreed well with the values reported in the literature. The test data also demonstrated that a single fatigue test at GIIimax = 0.3GIIc, would be sufficient to generate the complete da/dN growth rate data instead of multiple specimen tests at different values of GIIimax.