Nanoindentation in cyclotetramethylene tetranitramine (β-HMX) single crystals: the effect of pressure-sensitivity

Cyclotetramethylene tetranitramine (HMX) is a molecular crystal used as explosive in a broad range of civilian and military application. Its mechanical behavior is important as it relates to the initiation of the decomposition reaction though the formation of hot spots. However, the mechanics of the HMX crystal is complex and insufficiently understood. In particular, the pressure sensitivity of the mechanical behavior was largely ignored in the past, despite indications that material properties are strongly dependent on pressure. In this work we develop a pressure-sensitive crystal plasticity model for single crystals of β-HMX which accounts for the dependence on pressure of both the elastic constants and the yield surface, and calibrate it based on results from molecular simulations reported in the literature. The model is implemented in a finite element solver and is used to represent nanoindentation in β-HMX single crystals using a Berkovich tip. It is observed that accounting for pressure sensitivity is required if predictions close to experimental indentation results are to be obtained. The pressure sensitivity of the yield surface has a larger impact on predictions than that of the elastic constants. While these effects are demonstrated here in the context of quasi-static indentation, it is suggested that they become essential under shock loading conditions when stress states with large pressure components result. (paper)