Predicting the damage/failure transition in polymer-bonded explosives

Abstract
Polymer-bonded explosives are composite materials that suffer mechanical degradation when exposed to mechanical or thermal stimulus. This degradation takes the form of debonding of particles from binder and the formation of cracks. Continuum damage mechanics descriptions in constitutive models use a single damage parameter to predict mechanical properties but do not distinguish between differing microstructural features that can be called damage. The sensitiveness of damaged polymer-bonded explosives is higher and this is ascribed to cracks in the material allowing hot gas access to increased internal surface area and thereby increased burning rates. Prediction of this increased sensitiveness in hazard simulations requires some distinction between damage that will and will not lead to increased burning rates. To this end we distinguish between damage as debonding at the scale of the microstructure and failure as the formation of cracks at scales larger than the microstructure. Percolation theory is explored as a means to a simple damage measure with physical meaning; measurements of existing damage levels and the rate of damage evolution using that measure can be used directly in numerical simulations and allow models to be validated. The concept is extended to prediction of gross fracture and to fragment size distributions.