Micromechanics of coalescence—I. Synergistic effects of elasticity, plastic yielding and multi-size-scale voids

Abstract

Void growth and coalescence under physical states similar to those found in highly stressed regions ahead of a crack is investigated. The analysis introduces a representative material volume containing several large voids and a population of microvoids present from the very beginning, all of which are modeled as discrete entities. Plastic yielding has pervaded the material volume of interest. The underlying micromechanics of final rupture is dominated by a succession of rapidly growing microvoids. This involves the synergistic interaction between elasticity associated with high stress triaxiality, stiffness softening caused by plastic yielding and a rich supply of length scales arising from voids of vastly different sizes. A primary feature of the coalescence phase is an unstable deformation mode whereby a minute, benign void rapidly enlarges reaching a size set by the characteristic length of the locally elevated stress field. The process begins with a large void growing in concert with the plastic strain. Simultaneously, a local zone of high stress concentration emanates from the large void and spreads across the material raising the stresses at nearby microvoids. As a result, the hydrostatic stress surrounding one or more microvoids is raised to a level that activates an unstable deformation mode in which the stored elastic energy drives the plastic expansion of the microvoid. Although the overall stress decreases rapidly, small zones of high stress concentration are generated near growing voids—causing even smaller nearby microvoids to grow rapidly. This process continues until the submicron ligament fails by microcleavage or by shearing along crystallographic planes. Plastic yielding plays a crucial role in the above process by lowering the stress level required for the unstable-like growth mode of microvoids. The process outlined above appears to be the main operative mechanism in several observed failure modes in metal alloys. The morphologies of fracture surfaces dominated by flat dimpled rupture and voidsheet formation can be elucidated by the present work. For high-strength metals, our studies suggest that microvoid cavitation and link-up will increasingly dominate the failure process resulting in a brittle-like ductile rupture mode in which very little energy is expended.