Simulating the Mechanically Induced Reaction of Energetic Materials

Abstract

Munition articles may be sensitive to case-crushing loading. This is a potential safety hazard since it may lead to accidental detonation. A better understanding of the mechanism causing the munition articles to react can help improve safety. A finite element model combined with post-processing is used to model a Steven test. This thesis specifically looks at the absence or occurrence of a self-sustaining reaction in energetic material PBX-9501, a polymer bonded HMX based explosive. A reference paper (Gruau et al., 2009) was used to model the behaviour and validate some of the results.
The PBX is predicted to react if it has satisfied a certain reaction criterion based on the hot spot theory of ignition. This theory poses that known chemical properties of the material, the plastic shear strain rate and the hydrostatic pressure can predict the occurrence of a reaction. A finite element model was used to find the plastic strain rate and hydrostatic pressure in the energetic material subjected to a case-crushing loading. Next, the reaction criterion is computed in a separate script. The energetic material was modelled as a homogeneous continuum with a Drucker-Prager yield criterion. This material model did not include any post-yielding behaviour, like strain hardening.
First, a finite element model was constructed that was used as the baseline model throughout the thesis. The combined FEM and post-processing model was shown to be more sensitive than the model in the reference paper. The combined model also predicted a reaction at a lower velocity, which, according to tests in the reference paper, should have not produced a reaction.
The baseline model, and consequent models in the parameter study, also showed excessive deformation. This can be attributed to the use of a simpler Drucker-Prager material model for the PBX as opposed to a more complex material model. The absence of any hardening behaviour, and the resulting deformation, also contributed to the instability of the finite element model.
A parameter study was conducted by adapting the baseline model. Varying the velocity of the projectile was the most impactful change.
In conclusion: the developed model gives a first indication of parameters that contribute to the sensitivity of certain munition articles to case crushing. However, the material model used should be improved to create a more stable finite element model. An improved material model could also better capture the mechanical behaviour of the complex energetic material. It is recommended that future research should focus on this improvement.