Mechanical Characterisation and Cohesive Law Calibration for a Nitrocellulose Based-Cyclotetramethylene Tetranitramine (HMX) Polymer Bonded Explosive

Background Mechanical characterisation of polymer bonded explosives (PBXs) is crucial for their safe handling during storage and transportation. At temperatures higher than the binder's glass transition temperature, fracture is caused predominantly by interface debonding between the binder and explosive crystals. Interfacial friction between debonded crystals can lead to accidental detonation of the PBX material, even under a very small external load. Cohesive zone laws can describe this interfacial debonding. Objective This study aims to experimentally calibrate the interfacial cohesive zone parameters of a nitrocellulose based-cyclotetramethylene tetranitramine (HMX) PBX, a particulate composite with an 88% volume fraction of crystals. Methods Compact tension fracture tests, coupled with Digital Image Correlation (DIC) were used to capture the strain fields around the crack tip. The experimental data were used in conjunction with an extended Mori-Tanaka method considering the effect of interfacial debonding. Results The cohesive zone parameters were successfully calibrated and were found to be crosshead rate independent. The values of the critical traction sigma(max)(int) and interfacial energy release rate, gamma(if), dropped significantly with increasing temperature. The experimental method followed in this study is generic, and it can be employed to extract the cohesive zone parameters characterising the interface behaviour between the filler and matrix in other particulate filled, polymer composite materials. Conclusions Cohesive zone properties can be experimentally determined to provide inputs in micromechanical simulations linking the microstructure of the PBX composite to its macroscopic response as well as enabling the estimation of hot spot formation at debonded crystal interfaces.