Localization and macroscopic instability in nanoporous metals
Ductile fracture generally relates to microscopic voiding and to strain localization in metallic materials. When the void size is reduced to the nanoscale, size effects often lead to a different macroscopic plastic behavior from that established for the same material with larger voids. For example, irradiation of metallic materials can generate a large number of voids at the nanoscale, leading to complex deformation behaviors. The present work advances the understanding of strain localization in nanoporous metallic materials, connecting both the microscopic (nano-) and macroscopic scales. To explore the physical mechanisms at the nanoscale, molecular dynamics (MD) simulations were here carried out, capturing multiple nanovoids explicitly. Then, a homogenized continuum theory based in Gurson’s constitutive framework is proposed, which enables us to explore how localized behavior at the macroscopic scale evolves. The homogenized model incorporates the surface tension associated with nanosized void. The importance of this surface tension is illustrated by several parametric studies on the conditions of localization, when a specimen is subjected to uniaxial tension. Our parametric studies show that for smaller nanovoid sizes, and for a hardening matrix material, shear localization onset is delayed. Our proposed homogenization model was then used to predict localization behavior captured by our MD simulation. The yield stress and the localization strain predicted by our continuum model are in general agreement with the trends obtained by MD simulation. Moreover, based on our present study, experimental results of shear failure strain vs. dose of irradiation for several metals could be qualitatively explained rather successfully. Our model can therefore help shed light on prolonging the operation limits and the lifetime of irradiated metallic materials under complex loading conditions.