Evaluation of crack tip shielding using a photoelastic model

The debate over the existence and mechanisms of crack closure and shielding phenomena has been fuelled by the difficulty in evaluating the stresses involved in driving crack growth during fatigue. Polycarbonate exhibits plastic deformation as well as being a transparent birefringent material, so that it has been successfully used to create a photoelastic model of crack tip and crack wake plasticity. In previous studies, Westergaard or Muskhelishvili-type series have been used to describe the elastic stress field around the crack tip and these analytical descriptions of the stress were fitted to the photoelastic data based on the MPODM. In this new work, the description of the elastic stress field has been extended to model the crack wake and terms have been included to account for contact forces in the wake and interfacial shear between the plastic wake and surrounding elastic material. This new description allows the retarding effect of the plastic shielding to be characterized by a retardation intensity factor and the interfacial shear stress by a shear intensity factor as well as the usual opening mode displacement and the T-stress. The results from fatigue cracks in a CT specimen demonstrate the effect of the development of crack tip shielding generated by plasticity and allow a better understanding to be gained of the mechanisms underlying plasticity induced crack shielding.