Effect of Loading Mode on Fibrous Crack Growth and Unstable Fracture Under Constant Loading

Abstract

Constant loading tests and monotonic loading tests have been carried out at two temperature levels, i. e. room temperature and -70°C at which unstable brittle fracture occurs after stable fibrous crack growth, by using double edge-cracked tensile specimens and 3-point bending specimen of 4 mm thickness extracted from mild steel. Fibrous crack growth, time-dependent deformation and unstable fracture transition behaviors have been observed. According to above results, consideration has been conducted on the effect of loading mode on fibrous crack growth and unstable fracture transition.Fibrous crack growth under constant loading is independent of kind of loading mode, i. e. tensile and bending. Though, near-tip strain distributions of tensile and bending specimens are different, equivalent plastic strain at 0.2 mm ahead of crack tip is the determined by the value of COD at the original crack tip. Fibrous crack growth and time-dependent deformation behaviors under constant loading at -70°C is the same as those at room temperature. Critical CODs, (δm) d (δu) d obtained by constant loading test are lower those obtained by monotonic loading, δm, δu. Particularly, (δu) d becomes about half of δu, and it is almost the same as the COD δi at fibrous crack initiation.