Analysis of the geometry dependence of fracture toughness at cracking initiation by comparison of circumferentially cracked round bars and SENB testson Copper

In the first part of the paper, the use of circumferentially cracked round bars (CRB geometry) for characterizing fracture toughness of a ductile mate rial, namely copper, is assessed experimentally through a comparison with t he single edge notched bend (SENB) geometry. The J(R) curve method with mul tiple-specimens was applied, but, as unstable cracking appeared very early in the CRB specimen, an engineering definition of fracture toughness was no t pertinent. Unloaded specimens were analyzed metallographically to determi ne the CTOD at physical cracking initiation. The fracture toughness measure d using the CRB geometry was 50% larger than using the SENB geometry. The s econd part of the paper aims at justifying this difference of fracture toug hness at cracking initiation. Finite element simulations revealed a slightl y higher constraint in the SENB specimens. The main difference between the two specimen geometries lies in a 50% larger extension of the finite strain zone with respect to the CTOD in the case of the SENB specimens. Based on the observation that, in the studied material, the critical CTOD is one ord er of magnitude larger than the void spacing, we conclude that the geometry dependence of the fracture toughness is caused by the difference in the fi nite strain zone extension rather than by a stress triaxiality effect.