3D thermo-mechanical coupled phase-field fracture modeling of HMX single crystal with anisotropic thermal expansion

The development of cracks in the energetic material HMX during storage and transportation critically affects its ignition sensitivity and reduces safety and reliability. Nevertheless, the mechanism by which the anisotropic thermal expansion affects the initiation and propagation of cracks in HMX single crystals under thermal shock remains insufficiently elucidated. Here, we develop a 3D thermo-mechanical coupled phase-field fracture model to investigate how anisotropic thermal expansion, heat transfer coefficient, and specimen size affect crack morphology and volume fraction through temperature gradient loading induced by surface cooling. The results demonstrate that the model with anisotropic thermal expansion induces parallel cracks along the [100] crystal direction, which markedly differs from the three-dimensional intersecting crack morphology observed in that with isotropic thermal expansion. It also reveals that increasing the heat transfer coefficient aggravates thermal deformation mismatch, accelerates localized stress accumulation, shortens crack nucleation time, and leads to an expansion of the crack volume fraction. Furthermore, due to the significant anisotropy of HMX crystals, the crack evolution is highly correlated with the geometric characteristics: increasing the specimen size along the [010] orientation (the direction of maximum thermal expansion) leads to a notable increase in crack volume fraction. This work elucidates that the thermal expansion anisotropy of HMX crystals critically influences the correlation between thermal/geometric parameters and crack characteristics, offering new perspectives for controlling thermo-mechanical damage in HMX.