Ductile Fracture under Multiaxial Stress States between Pairs of Holes

摘要

As a means of examining ductile fracture under multiaxial stress and strain states, the deformation and fracture between pairs of holes has been studied in a 7075 Al alloy. The test technique, which is unique, utilizies tensile samples each with a pair of holes through the thickness. The holes are sufficiently close (two hole diameters apart) so as to concentrate slip between them, and the local states of stress and strain can be controlled by the orientation of the holes to the stress axis. The influence of hole orientation on the strain at fracture has been determined in the 7075 Al alloy in the T6 as well as TO tempers. Fracture occurs by flow localization between the holes an very little hole growth occurs, especially in the T6 condition. Fractography indicates a transition in fracture appearance with hole orientation with dimpled fracture predominating, but the degree of shear fracture increases as the pair of holes become more inclined to the stress axis. A modification of the Bridgeman analysis is used as a description of the approximate state of stress between the holes as a function of the hole orientation. The results indicated that the criterion for ductile fracture depends on both the state of stress and the strain state, but the functional dependence could not be determined. The results are also discussed as a crude model for void coalescence in ductile fracture. (Author)