Mechanisms of plastic deformation in molecular crystal cyclotetramethylene-tetranitramine (β-HMX)

Khan, Mohammad Jane Alam
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Picu, Catalin R.
Blanchet, Thierry A.
Maniatty, Antoinette M.
Shi, Yunfeng
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Mechanical engineering
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Shear localization happens when dislocation motion cannot accommodate the high strain rates imposed in the crystal. Shear localization is observed to be more frequent in β-HMX at higher pressures when dislocation motion is hindered by steric shielding. This study evaluates the shear band viscosity as a function of the applied pressure, temperature, and shear strain rate using atomistic models of the HMX crystal. It is observed that the viscosity of a fully formed shear band decreases as a power function of the strain rate (shear thinning) and decreases linearly with increasing temperature. The non-Arrhenius dependence of the viscosity on temperature is due to the high strain rates involved and the high non-equilibrium conditions in which the band operates. Pressure increases the viscosity of the fully formed band significantly. The band behavior is observed to be independent of the crystallographic orientations. 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 previously developed for shear bands in metallic glasses. The stress required to nucleate a shear band from a straight pre-existing dislocation is also reported as a function of the applied pressure, temperature, and shear strain rate.
December 2020
School of Engineering
Dept. of Mechanical, Aerospace, and Nuclear Engineering
Rensselaer Polytechnic Institute, Troy, NY
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