A constitutive model incorporating the phase transition effects for metals under hypervelocity impact

Phase transition, as an important phenomenon in hypervelocity impact (HVI) events, greatly affects the formation of debris clouds, but has been less studied due to its complexity. The objective of this work is to develop a constitutive model incorporating the phase transition effects, to be applied to the study of the phase transition problems in HVI. On the foundation of Wu's multiphase equation of state (EOS), the phase transition effects are introduced into the deviatoric response and fracture response of metals, to model the constitutive equations. Subsequently, by the provided iterative algorithm, the novel constitutive model was embedded into the AUTODYN-SPH hydrocode for HVI simulation. Afterward, two HVI experiments, where Al2024 spherical projectiles impacted Al2024 bumper plates at 4.9 km/s and 7.9 km/s, were conducted to verify the accuracy of this model. The results showed that the simulations can well capture the morphological features of the debris clouds in the experiments, and the features of the debris clouds in the simulations can be well correlated with the features in the damage patterns on the witness plates in the experiments. Meanwhile, the simulations revealed that the formation of the double-layer structure of the debris cloud at a high impact velocity was induced by the phase transition. Finally, by simulation, the phase and failure evolutions of materials at different impact velocities were displayed, both are associated with the propagation of stress waves, and it can be found that the phase transitions mainly occur during the release process at 7.9 km/s, but during the shock process at 12 km/s. Besides, as the impact velocity increases, the failure mechanism of the material is from a combination of shear fracture and tensile fracture to one dominated by tensile fracture. And, the occurrence of the phase transitions is accompanied by a sudden drop in the tensile fracture threshold, making the projectile more susceptible to being completely smashed.