Stiffness degradation and fatigue life prediction of adhesively-bonded joints for fiber-reinforced polymer composites

Adhesively-bonded joints, including double- and stepped-lap joints (DLJs and SLJs), were experimentally investigated under cyclic tensile loading. The joints were composed of pultruded GFRP laminates and epoxy adhesive. A critical stiffness was found for DLJs and a critical elongation for SLJs at which failure occurs independently of load level. Based on measured uniform stiffness degradation during the fatigue life of DLJs, a linear model of stiffness degradation was established. For SLJs, after crack initiation, stiffness decreased at a higher rate during the initial and final stages of crack propagation. A non-linear sigmoid model was established. Results of both models compared well to experimental results and enabled F–N curves suitable for design to be established.