Mixed-mode crack propagation tests of composite bonded joints using a dual-actuator load frame – Constant and variable GII/GI conditions

The increasing use of high-performance composite materials in primary structures is boosting the interest in structural bonding since it allows for higher mechanical performances under static and fatigue loading conditions. The design of highly efficient bonded structures, however, requires the capability of being able to correctly characterize the bonded joints which usually present multiple failure mechanisms. In this work, crack propagation experiments are performed under mixed mode conditions on composite bonded specimens using a dual actuators loading frame. Three different test protocols are developed and implemented in order to investigate the debonding failure envelope in the GI-GII space. At first, a constant displacement rate is imposed on both the actuators. According to the simple beam theory analysis, large mode mixity ratio variation is expected during the test under such loading condition. As a result, large variation of fracture energy is measured during the test, and crack path bifurcations are observed. Then, two different strategies are proposed to keep the mode mixity ratio constant during the test. When keeping GII/GI ratio constant during the test, stable fracture energy values are recorded all along the crack propagation path with identical fractured surface aspect.