When polycrystalline energetic materials are shocked, recompression and rarefaction waves form inside the material due to crystal anisotropy. These waves interact with one another and with material defects (e.g., grain boundaries and cracks), localizing temperature in regions known as hotspots. Hotspots increase the ignition sensitivity of energetic materials. Several ignition mechanisms have been proposed, including void collapse, plastic heating, friction between surfaces, adiabatic shear during failure, and heating at crack tips. However, despite the development of new experimental techniques, identifying the relative importance of these mechanisms remains challenging due to their short duration and small length scales. Furthermore, many of these mechanisms interact with one another, making it difficult to quantify their individual effects. In this work, we studied the relative importance of crystalline anisotropic plasticity and fracture on the formation of hotspots in polycrystalline HMX impacted at weak shock strengths. We use a continuum model for large deformations that includes an equation of state, single crystal plasticity, fracture evolution, and heat transport. The results indicate that although crystal anisotropy creates heterogeneous temperature fields, the temperature increase due to solely plastic dissipation is not enough to form critical hotspots. In contrast, when cracks nucleate, frictional heating at crack surfaces becomes the dominant hotspot formation mechanism for the shock conditions studied here, with impact velocities of 0.1 km/s and 0.4 km/s. In addition, crystal anisotropy affects temperature dissipation by creating more hotspots at grain boundaries due to the impedance mismatch between grains in the polycrystal.
2025-04-09 09:00:00
