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

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 fractu re initiation at blunt notches. Fracture experiments were performed with th ree sizes of geometrically scaled notched bend specimens of high strength l ow hardening HY-130 steel using carefully scaled fixturing. Fracture initia tion, defined as the appearance of the first sub-millimeter crack with a te nsile opening, was reliably detected, using acoustic emission and direct vi sual 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 small er normalized displacement than smaller specimens. The normalized displacem ent at fracture does not decrease in direct proportion to specimen size, Me tallographic 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 l ocalization zone. This mechanism profoundly modifies the stress and strain gradients at the notch root so that microstructural length scales become fr acture controlling parameters. Both changes are probably responsible for th e observed scaling behavior. (C) 1998 Published by Elsevier Science Ltd. Al l rights reserved.