Structural Integrity of Aged Hydroxyl-Terminated Polybutadiene Solid Rocket Propellant

Abstract

The effect of aging time on the mechanical properties and fracture parameters determined via linear elastic fracture mechanics and elasto-plastic fracture mechanics approaches of an ammonium perchlorate hydroxyl-terminated polybutadiene solid propellant extracted from a stored in-service missile was evaluated. Accelerated thermal aging was carried out in air at 60°C for 3, 6, and 9 months, equivalent to natural aging at 23°C for 4, 8 and 12.5 years, respectively. For fracture tests, the in situ video imaging and the electrical resistance techniques were used for crack length measurements and the blunting line determination. The energy release rate and the J integral at crack growth initiation were identical for all the cases, evidence of an elastic nonlinear behavior. The source of the nonlinearity was caused by blunting, voiding, coalescing, and growing. The failure process zone extended 200 μm. The thermal oxidative aging process did not produce relevant changes in either the mechanical behavior or the fracture characterization, probably because the material under study was already naturally degraded. The influence was mainly observed in the slope of the blunting line determined experimentally for the long-term aged samples, which was between twice and three times higher than the unaged and short-term aged specimens. The microstructural evolution during the fracture test showed that the intergranular fracture caused by nonadherent particles was more evident with aging time.