Predicting the Crack Growth Behavior in a Filled Elastomer

In this study, single-edge cracked uniaxial specimens with an initial crack length of 0.1 in or 0.3 in and wedge-shaped sheet specimens with an initial crack length of 0.3 in were tested at a constant displacement rate of 50 in/min under 1000 psi confining pressure. The specimens were made of a highly filled polymeric material, containing 86% by weight of hard particles embedded in a rubbery matrix. The uniaxial crack growth data were used to develop a crack growth model, relating crack growth rate da/dt and Mode I stress intensity factor KI. The developed crack growth model was used to predict the crack growth behavior in the wedge-shaped specimen. The results of the analysis indicated that the predicted crack growth rate compared well with the experiment. Introduction In designing a structural component, a thorough knowledge of the material properties of the material and the pertinent failure criterion for a specific failure mode is required. During past years, the fracture mechanics approach has been used frequently as a failure criterion for high strength materials. According to the fracture mechanics approach, fracture occurs when the stress intensity factor attains the critical value, which is a material property to define the onset of brittle, or unstable, fracture of the material. This fracture initiation criterion implies that a structure will fail as soon as a crack is initiated. It is based on the assumption that a crack, once it is initiated, will propagate at a very high speed and the structure will fail immediately. However, under certain conditions, subcritical cracks in a structure can slowly extend and result in a time-dependent fracture process as well as fracture stress. Therefore, under this condition, a structure’s useful life will be governed by the subcritical crack growth in the material. Thus, in an attempt to predict the ultimate service life of a structure, the failure criterion should include the crack growth aspect of the subcritical crack growth, and a detailed knowledge of the characteristics of the crack growth behavior in the material is required. During the past years, the fracture behavior of particulate composites has been investigated experimentally (1-4).The basic approach is to determine the kinetics of the crack growth in terms of the relationship between the crack growth rate da/dt and the Mode I stress intensity factor KI. Experimental data indicate that power law relationships exist between da/dt and KI. This experimental finding supports the theories developed by Knauss (5) and Schapery (6) in their studies of crack growth behavior in linear viscoelastic materials. It is known that classic fracture mechanics principles, especially linear elastic fracture mechanics including small scale yielding, are well established for single phase materials. However, experimental data indicate that linear fracture mechanics theories are applied to particulate composites with varying degrees of success.