Three-dimensional mechanical–thermal–chemical coupled mesoscopic simulation of the collapse of an air bubble in an HMX crystal

Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is extensively employed in military weapons as the main constituent of HMX-based polymer-bonded explosives (PBXs). It is known that the safety of PBXs is closely related to their microstructures. Discontinuities in PBXs, such as micron-sized pores, air bubbles, and interfaces between the explosive crystal and polymer bonds, may transform into hot spots when subjected to impact. Herein, a three-dimensional mechanical-thermal-chemical coupled mesoscopic model is proposed to study the collapse of an air bubble in an HMX crystal under impact. A viscoplastic constitutive model and the Birch-Murnaghan equation of state are employed for the HMX crystal to predict its mechanical response. Thermal decomposition of HMX is taken into account by using multistep thermal decomposition equations. The viscoplastic model yields results that reasonably agree with data obtained in the plane shock experiment. The influence of the edge length of the mesh elements on simulation results is analyzed. Then more simulations are conducted for studying the feasibility of using the viscoplastic model for different orientations of the HMX lattice. Afterwards, the coupled model is applied to study the collapse of an air-bubble/pore in the HMX crystal for different impact velocities.