A study of the pore size effect on the formation of hot spot in a 1,3,5,7-tetranitro-1,3,5,7-tetrazocane crystal under a low-to-medium shock velocity

It is rather difficult to observe, record and analyze the pore-collapse-induced initiation of a 1,3,5,7-tetranitro-1,3,5,7-tetrazocane or HMX crystal via experiment because it is an extreme phenomenon involving multiphysics (such as shock, phase change of HMX) and chemical reactions (thermal decomposition of HMX) occurring in microseconds or even nanoseconds. Herein, a mechanical-thermal-chemical coupled finite element model is proposed to simulate the pore collapse in a HMX crystal under a shock loading at velocities under 500 m/s. A viscoplastic constitutive model and a Birch-Murnaghan equation of state (EoS) are employed to predict the mechanical response of the HMX crystal. The melting of the HMX crystal is also considered. A multi-step thermal-decomposition model is employed to simulate the chemical reaction of the HMX. Firstly, numerical simulations were implemented for a few plane shock experiments and it is demonstrated that the material model of HMX have reasonable accuracy. Then, the pore collapse in a HMX crystal under a shock at 500 m/s was simulated. It is shown that severe plastic deformation and high-velocity impact of HMX jet both contribute to the temperature rise of local materials. Finally, the influence of pore size on the calculated maximum temperature of the HMX crystal was simulated under various shock velocities ranging from 100 m/s to 500 m/s. It seems that there exists an "optimal" pore size for the HMX crystal to obtain the highest temperature under a shock, which is explained with simulation results.