Dynamic Tensile Fracture of Iron: Molecular Dynamics Simulations and Micromechanical Model Based on Dislocation Plasticity

Molecular dynamics (MD) shows a difference in the mechanisms of fracture of iron under uniaxial and isotropic tension. Uniaxial tension leads to the formation of a complex dislocation structure in the bulk of material, which reduces the pore formation threshold and spall strength. Increase in temperature up to about 900 K suppresses increase in dislocation density during uniaxial tension, which leads to increase in the spall strength to the level close to the case of hydrostatic (isotropic) tension. This temperature-induced increase in dynamic tensile strength is rather unusual. Preliminary compression of material mimicking the shock compression prior to tension creates a defect structure, which reduces the spall strength during the following tension. The MD results of isotropic tension are used to model the material fracture dynamics on continuum level. The Bayesian approach to model calibration is used to identify the parameters of the fracture model. The parameterized fracture model describes well the MD results in the considered temperature range 300-1100 K. The fracture model with additional accounting of void initiation on initial defects of material is used to describe the experimental observations of the spall strength of iron over a wide range of strain rates. The simulation results for undeformed iron are in good agreement with the experimental values of the spall strength at strain rates greater than 4 x 105 s1 and give overestimated values at lower strain rates. The results of calculations for precompressed iron describe well the spall strength over the entire range of strain rates indicating that accounting for the defect structure created by the shock compression prior to tension is critical for moderate strain rates.