SEM investigations of slip-line fields around blunted cracks under mixed-mode conditions

Abstract

The slip-line field around the tips of cracks was studied using extensive experimental evidence with electron scanning microscopy. Thin and thick tension specimens made of polycarbonate were edge-cracked ensuring predominant conditions of either plane stress or plane strain. The evolution of blunting and of the creation of the respective slip-line field in the intermediate phase between blunting and initiation of slow propagation of the crack were studied under a continuously increasing loading mode. It was shown that blunting depends on the type of stress state (plane stress or plane strain), on the mode of loading (mode I, mixed mode), and on the ductility of the material. Three different types of blunting were detected, the already known round-nosed and flat-nosed modes and, furthermore, an intermediate type of cornered and rounded-off front of the crack. The characteristic properties of these types are presented. The slip-line fields accompanying the phenomenon of blunting and indicating the initiation and evolution of the plastic enclaves were detected in all types of loading and specimens using the brittle-coating technique and its variations. It was shown that not only the plastic enclaves developed during this step of loading but also the significant elastic components of stresses influence the mode of development of the respective slip-line yielding a combined type corresponding in between the Rice-Johnson field for plane strain and the Theocaris field for plane stress.