ABSTRACT

The suggestion has been put forward that unstable growth of microcracks that are thermomechanically-produced below the melt surface of burning crystals could produce a sudden increase in the pressure-dependent burn rate and hence transition to unstable combustion behavior. Here, such behavior is proposed to occur at relatively low pressures for two reasons: (1), the presence of a melt layer over the crystal surface lowers the required crack surface energy for thermal cracking to the liquid-solid interfacial value; and, (2) the presence of the network of thermally-produced, micrometer-sized, cracks reduces appreciably the pressure needed for unstable crack growth in accordance with fracture mechanics predictions. Evidence is shown of such microcracking below a liquid surface layer produced at localized hot spots on the surface of RDX (cyclotrimethylenetrinitramine) crystals with an incident laser beam directed at a grazing angle to the crystal surface. A comparison of surface and interfacial energy values determined for RDX, HMX (cyclotetramethylenetetranitramine) and PETN (pentaerythritol tetranitrate) crystals shows substantial reduction of the surface energy requirement. Then, fracturing pressures are estimated from indentation fracture mechanics measurements. The fracture pressure estimates are comparable to burn rate pressures for HMX materials where sudden pressure exponent increases are shown to occur, in one case, at lower pressures for higher porosity material and, in another case, for larger sized crystals.