Fracture characterization of rubbers by J integral testing

Unlike metals for which the fracture characterization methods have been standardized in the context of linear elastic and elastic plastic fracture mechanics theories, for polymeric materials the linear and especially nonlinear theories of viscoelastic fracture mechanics has not been completely developed due to complexities regarding the viscoelastic nature of these materials. For rubbers, even the rate independent theories based on nonlinear finite elasticity have not been widely used. In practice, most researchers make use of the same methods as applied for metals to rubbers. In this paper, the common methods of fracture characterization of rubbers based on J integral and the different challenges regarding them are reviewed. Specificly, the energy dissipation effects in regions far from the crack tip and the correction methods proposed to compensate for these effects are discussed. Performing fracture toughness tests on SENT specimens of a rubbery material based on polybutadiene, it is shown that the well-known multiple specimen method for determination of Jc has a strong sensitivity to experimental errors that exhibits itself as initial crack length dependence of Jc values and is just usefull when testing numerous specimens and removing the experimental errors. On the other hand, the locus method of dissipation correction, gives a single reliable Jc value using a fewer number of specimens and with a considerably lower sensitivity to the experimental errors. Also, using this method the specimen length dependence of Jc values reported in the literature is removed, and hence, it is possible to obtain a dimension independent Jc value.