Fracture of geometrically scaled, notched three-point-bend bars of high strength steel

Abstract

The purpose of the experimental work reported in this paper was to provide data that may serve for the development of scaling rules for ductile fracture initiation at blunt notches. Fracture experiments were performed with three sizes of geometrically scaled notched bend specimens of high strength low hardening HY-130 steel using carefully scaled fixturing. Fracture initiation, defined as the appearance of the first sub-millimeter crack with a tensile opening, was reliably detected, using acoustic emission and direct visual inspection with a microscope. Comparison of the normalized load versus load-point displacement curves revealed a significant scale effect on the condition for fracture initiation, with large specimens fracturing at smaller normalized displacement than smaller specimens. The normalized displacement at fracture does not decrease in direct proportion to specimen size. Metallographic cross-sections of the specimens and fractographic observations revealed that at the microscale, fracture initiates by shear localization followed by nucleation and growth of voids under shear and tension in the localization zone. This mechanism profoundly modifies the stress and strain gradients at the notch root so that microstructural length scales become fracture controlling parameters. Both changes are probably responsible for the observed scaling behavior.