Evolution of pore ensemble in solid and molten aluminum under dynamic tensile fracture: Molecular dynamics simulations and mechanical models

Construction of the mechanical model for dynamic tensile (spall) fracture is one of the cornerstone problems in the mechanical description of the material behavior under dynamic loading. The spall fracture takes place for both solid and liquid substances, typically referred as cavitations in the latter case. We perform larger-scale MD simulation of the uniform triaxial stretching of molten and solid Al with the strain rate of 3/ns at different temperatures. Formation of multiple pores under used conditions of loading allows us to obtain detailed information about the evolution of pore ensemble at dynamic tensile fracture. This information can be used for verification of the mechanical models of dynamic tensile fracture. We formulate the mechanical models for the pore ensemble evolution in both liquid and elastic-plastic mater. The common part of the model describes the random nucleation of pores in the sample bulk. The size variation part of the model for liquid uses the Rayleigh-Plesset equation. The corresponding part of the model for elastic-plastic solid takes into account the dislocation activity in the pore vicinity. The detailed comparison of the model predictions with the MD data shows their adequacy in description of both the pore ensemble evolution and the general behavior of system parameters for room and elevated temperatures. The collapse of small pores after the relaxation of pressure from strong negative to close to zero values takes place for both molten and solid Al. In the case of melt, the larger and larger pores become collapsing with time, and the number of survived pores tends to one. In the case of solid, the elastic resistance of the surrounding material suppresses the collapse of the middle-sized pores, and only small pores disappear after the pressure relaxation. This feature does not allow one to use simple viscous growth models for pores in plastic solids. MD simulations show the nucleation of secondary voids in the vicinity of the primary nucleated pores and a more ordered defect structure around the pores in the case of low temperature of 100 K. These features cannot be addressed in the frames of the present mechanical model, which needs further development for description of the fracture at low temperatures.