Microscopic mechanisms and modeling of nanopore collapse and growth during compression followed by tension loading

Previous research has predominantly focused on the evolution of damage related to voids, while the understanding of void evolution under tension, compression, and even cyclic loading remains limited. Using molecular dynamics (MD) simulations and copper as a representative ductile metal, this work systematically investigates the evolution of voids subjected to initial compression followed by tension. Uniaxial and triaxial loading conditions and incorporates crystallographic orientations were considered in current study. The results indicate that under uniaxial loading, a certain degree of compression causes the compaction of voids, leading to a stress concentration that hinders slip along dense planes, thereby enhancing the material's tensile strength. However, when localized plasticity accumulates beyond a certain threshold, it results in the nucleation of multiple new voids. Dislocations rapidly develop from the surfaces of these voids, ultimately leading to a reduction in strength. In the case of triaxial loading, the uniform loading conditions make plasticity more likely to propagate on the void surfaces. Beyond a certain threshold, the compaction of voids enhances tensile strength. Different local shear stresses and their interaction with the slip system normals under uniaxial loading in various directions result in different characteristics of dislocation evolution in terms of speed and morphology. Furthermore, a fitting formula for tensile strength-compression strain is established based on simulation results, the equation for uniaxial loading takes the form of an exponential function, while the equation for triaxial loading is a segmented power function, with the exponents of the two segments being reciprocals of each other. These findings contribute to a deeper understanding of the correlation between initial defects and fracture strength in real metals.