A model of debonding instability for solid propellant rocket motor: Part I — Uniform longitudinal and transverse stress rate

Abstract

The interface between a soft and a hard material is vulnerable to debonding because of the prevailing high stress gradient that could be further aggravated under dynamic transient conditions. Such a situation is common in a solid-fuel rocket motor where unstable debonding could be triggered from the initiation of a macrocrack near the interface. The transition from a survival state to a failure state requires knowledge of how the nonlinear, dissipative and nonhomogeneous effects of the dissimilar material interface would interact with load. The solid-fuel rocket motor problem is modeled by a three-layered composite system made of steel, adhesive and rubber under plane extension. Assessed are the time dependent nonhomogeneous deformation and possible failure modes. Only initial properties of the materials were used to determine the evolution of nonequilibrium response. This is made possible by application of the isoenergy density theory that accounts for internal heat generation and energy dissipation effects. Results are presented in two parts. In Part 1, the applied stress rates are assumed to be 0.75 ksi/s in both the longitudinal and transverse direction while Part II assumes different stress rates in these two directions. At approximately one second after loading, a slanted but straight macrocrack of about 5 × 10 −3 in. is predicted to occur in the rubber next to the interface. This initial crack was found to become unstable at eight seconds and was estimated to be close to the adhesive/rubber interface over a length of 1.88 in. The onset of fracture depended directly on the load transient behavior.