Void initiation,growth, and coalescence in ductile fracture of metals

Ductile fracture in metals and alloys occurs through the coalescence of voids in the necked region of the specimen. While considerable information exists on the propagation of cracks, the mechanism of their initiation is still unclear. This paper reports onin situ electron microscope investigations aimed at an elucidation of crack initiation and the enlargement of crack nuclei to final rupture. Single crystal ribbons of silver 0.5 − 7.0 μm thick were pulled to fracture inside of a high voltage electron microscope (HVEM). After considerable necking, cracks initiated at the edges; their propagation occurred by the formation of microcracks ahead of the macrocrack, followed by the growth of the microcracks and finally their coalescence. Thesein situ experiments were complemented by stress-strain data obtained from fractured austenitic 304 stainless steel foils; subsequent examination of fracture surfaces in a scanning electron microscope allowed the accurate measurement of intervoid spaclngs. Inter-particle spacings were determined by HVEM. It was found that the average void density is 100 times larger than the average particle density. The combination and analysis of all experimental data led to a detailed model of void initiation and growth, which is based on a dislocation-vacancy mechanism and crystal plasticity.