Shear localization in molecular crystal cyclotetramethylene- tetranitramine (beta-HMX): Constitutive behavior of the shear band
M Khan and CR Picu, JOURNAL OF APPLIED PHYSICS, 128, 105902 (2020).
The energetic molecular crystal cyclotetramethylene-tetranitramine (HMX) is used in plastic bonded explosives, and reaction initiation and detonation are usually triggered by plastic deformation. However, the mechanism of plastic deformation in beta -HMX, which is the HMX phase stable in ambient conditions, is still a matter of debate. A recent observation that pressure developing under shock conditions inhibits dislocation activity, leaves shear localization as the main deformation mechanism in this crystal at high pressures and strain rates. In this work, the steady state shear band viscosity is evaluated as a function of the applied pressure, temperature, and shear strain rate using atomistic models of the HMX crystal. The viscosity of a fully formed shear band decreases as a power function of the strain rate and decreases linearly with increasing temperature, demonstrating shear thinning and non-Arrhenius behavior. The viscosity increases with increasing pressure. The fully formed band behavior is independent of the crystallographic orientation. It is shown that viscosity can be expressed exclusively in terms of the density of the non-crystalline material in the band, and hence the results can be explained in terms of the excess free volume theory developed for shear bands in other material systems, e.g., metallic glasses. The stress required to nucleate a shear band from a straight pre-existing dislocation is reported as a function of the applied pressure, temperature, and strain rate.
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