Strain hardening in molecular crystal cyclotetramethylene-tetranitramine (β-HMX): a theoretical evaluation

Abstract

The molecular crystal cyclotetramethylene-tetranitramine (β-HMX) is a broadly used energetic material. Its plastic deformation is important when describing the detonation behavior. This work aims to clarify the importance of strain hardening for the plastic deformation of this crystal. To this end, we use a line tension model to evaluate the strength of junctions formed by dislocations moving in different slip systems. We evaluate analytically the contribution to the flow stress of repulsive interactions between dislocations. Further, we test using atomistic models and confirm the conjecture that neutral core–core interactions of crossing dislocations do not contribute to the flow stress. This information is used to define the hardening matrix which can be further used in continuum crystal plasticity models. We conclude that strain hardening is weak at all realistic dislocation densities, and leads to a modest increase of the flow stress above the critical resolved shear stress corresponding to the vanishing dislocation density limit. A procedure is provided which allows extrapolating these results from ambient conditions to pressures and temperatures relevant for shock loading.