Directional effects in asymmetric fully plastic crack growth

Abstract

Asymmetrically cracked specimens fail with considerably less ductility than symmetrically cracked ones. Indeed, welds, shoulders or other asymmetries may eliminate one of the shear bands and thus result in crack propagation through predamaged material instead of the relatively unstrained region between two plastic shear zones of the symmetric case. An incremental approach is presented for predicting the direction of the growing crack and the crack growth conditions (far field displacement, strain, triaxiality). The formulation is based on strain increments following a power law relation and on the hole growth fracture criterion of McClintock, Kaplan and Berg [5]. At each step several sites are considered ahead of the crack and the damage due to crack initiation and prior growth is calculated. The crack is assumed to advance to the direction that requires the minimum far field displacement to reach critical damage. The predicted displacement to crack initiation is found to be of the order of the critical strain times the mean inclusion spacing. Results for two strain hardening exponents n = case1 3 and n = case1 13 and several initial crack-shear band angles are presented. In general the crack does not progress along the shear band but at an angle of 23°–32° giving a higher triaxiality. Strain hardening affects the rate of crack advance per unit displacement and the critical growth strain as well as the final crack orientation. The overall computer program provides a quick and direct approach that enables estimating the failure conditions of asymmetrically cracked structures from material data.