Sensitivity Criterion and Mechanical Properties Prediction of HMX and RDX Crystals at Different Temperatures

In order to explore and compare the progressive changes of structure, energetic and mechanical properties of RDX(cyclotrimethylene trinitramine) and HMX(cyclotetramethylene tetranitramine) crystals with temperature, molecular dynamics simulation was conducted for their suitable models containing the same number of atoms. COMPASS force field was applied in the simulation in NPT ensemble. Five different temperatures, i.e., 195, 245, 295, 345 and 395 K, were considered. The simulation results show that the maximum bond length(L-max) of their N-N trigger bond increases, the interaction energy(EN-N) between two N atoms of the N-N trigger bond decreases and the cohesive energy density (CED) decreases as the temperature increases. These results agree with the experimental fact that both RDX and HMX become more sensitive as the temperature increases. Moreover, the L-max value of RDX is larger than that of HMX, and the values of EN-N and CED of RDX are lower than those of HMX at all temperatures. These also agree with the fact that RDX is more sensitive than HMX. It is thus suggested that the L-max EN-N and CED indeed can be used as theoretical criteria to predict the relative degree of heat and impact sensitivity of high energy materials under certain conditions. The mechanical properties were obtained with static analysis method. The results show that the engineering modulus such as tensile modulus, bulk modulus and shear modulus decrease with the increasing temperature, which agrees with the experimental results.