DUCTILE FRACTURE BEFORE LOCALIZED NECKING IN A STRIP UNDER TENSION

Unexpected cracking with a 22-degrees opening angle grew out of a rough-sheared edge before appreciable necking in an 0.79 by 37 mm mild steel strip, which normally fractures after diffuse and then localized oblique necking. From the crack tip, two shear bands formed at 55-degrees to the load direction, consistent with isotropic plane stress characteristics (53-degrees was predicted from anisotropy, but necking in thin strips occurred at 67-degrees). Photomicrographs showed that the 22-degrees crack growth occurred by first tunnelling at mid-thickness, and then spreading along through-thickness shear planes. Springback on unloading caused a 0.038 mm crack closure and local buckling. This form of cracking illustrates a size effect in fracture under macroscopically plane stress. It also gives an example of a local mechanism triggering a fracture mode that can require more total work than an alternative. Analysis of isotropic localized necking shows the equivalent strain at fracture in thin strips to be uniquely related to the Reduction in Squared Thickness (RST). With smooth edges, width and thickness strains before and during necking differed by factors of 1.4 and 1.7; such measures of anisotropy should be routinely found and reported for strips.