Effects of crystallographic orientation and strain-hardening on nanoindentation response of cyclotetramethylene tetranitramine (β-HMX) single crystals

Mechanical behavior of cyclotetramethylene tetranitramine (HMX) is complex and insufficiently understood, but it is imperative as it affects the initiation of the decomposition reaction though hot spots formation. We present a single-crystal plasticity model for b-HMX which accounts for pressure dependence of both the elastic constants and the yield surface. We evaluate the relative importance of pressure sensitivity of the elastic constants and plasticity, and of strain hardening, to the quasi-static mechanical behavior of HMX, in general, and to indentation, in particular. Further, we study the effect of the in-plane crystallographic orientations of the substrate with respect to the Berkovich indenter tip on the indentation force-displacement curve. We conclude that pressure sensitivity of plasticity makes the largest contribution to defining the response to indentation, followed by strain-hardening, sensitivity of which depends on hardening modulus, while the pressure sensitivity of elastic constants play a smaller role. Both the indentation plane normal orientation and the in-plane indenter tip rotation may influence properties determined from nanoindentation test (e. g. hardness).