Cohesive zone modelling of hydrogen environmentally assisted cracking for double cantilever beam samples of 7xxx aluminium alloys

Abstract

Double cantilever beam (DCB) samples are typically used to estimate the crack growth rates during hydrogen environmentally assisted cracking (HEAC) of 7xxx aluminium alloys. In this investigation, a cohesive zone model was developed to simulate the HEAC behaviour of 7xxx alloys during DCB tests. A coupled mass-diffusion and stress analysis was performed in Abaqus to elucidate key parameters affecting crack growth rates via sensitivity analysis. The threshold stress intensity for cracking was mainly dependent on hydrogen solubility, while the stage II crack growth rate was influenced by the mass-transfer coefficient and diffusivity. Model parameters were fitted to match experimental results of AA7449-T7651 at different temperatures, analyzing hydrogen distribution within the samples. Results indicated that hydrogen concentration at external and fractured surfaces remained at saturation levels throughout the tests. The postulated crack propagation mechanism involves enhanced hydrogen diffusion near the crack tip due to a high hydrogen concentration gradient, crack growth driven by hydrogen accumulation, and enhanced hydrogen ingress at newly formed surfaces.