Prediction of stable crack growth in surface cracked plate of Aluminum 7050 alloy

Abstract

To simulate stable crack growth, three-dimensional finite element analysis using the constant Crack Tip Opening Angle (CTOA) fracture criterion was performed for a thin plate made of Aluminum 7050 alloy. The critical CTOA value was experimentally obtained by the Rubber Impression Method, which directly measures the three-dimensional crack profiles by inserting the gel-state silicon rubber into the crack, and taking out the solid-state rubber later. From the microscopic observation from the broken specimen, it was found that, as the crack extends, the amount of crack growth near the free surface is more than that in the depth/thickness direction, which creates the special tunneling, e.g., canoe-shaped crack extension. For the numerical simulation, the surface-cracked plate was analyzed by quasi-static elastic-plastic finite strain analysis with the node release and the reloading technique. Consistent with experimental observations, numerical simulation with constant CTOA fracture criteria produced tunneling of a surface crack, but the shape of the crack front deviated from the experimental crack front as the free surface was approached. To address the local crack tip constraint effect on the stable crack growth, various fracture parameters - crack tip triaxiality, equivalent plastic strain, and void growth ratio as the crack extends – were also investigated.