On the mode II fatigue threshold analysis of composite bonded joints using the End-Notched Flexure test

This work describes experimental and numerical procedures for determining the mode II fatigue threshold of carbon-epoxy bonded joints using the End-Notched Flexure (ENF) test. Experimental tests were carried out considering load control with the aim of defining the coefficients of the modified Paris law and the mode II strain energy release rate (SERR) at threshold, representative of this series of tests. These parameters were input in a finite element analysis including a cohesive zone model (CZM) appropriate for high-cycle fatigue loading. Equivalent crack length data reduction methods have been used to obtain the evolution of SERR along the entire extension of crack propagation. The numerical analysis was extended to account for crack propagation beyond the specimen mid-span in order to capture the fatigue threshold. It was verified that the input modified Paris law is well reproduced for crack propagating before and beyond the specimen mid-span. Furthermore, the input SERR threshold is also well captured at the end of the simulation, since the crack growth rate asymptotically decreases to zero. Several aspects are analysed and discussed in detail to explain the observed behaviour. It was concluded that the ENF test under load control is suitable for determining the mode II SERR at the threshold for composite bonded joints subjected to high-cycle fatigue.