Residual stress effects in sharp contact cracking

A study is made of residual stress effects in the mechanics of median fracture in sharp indenter contact. Starting with a simplistic treatment of the elastic-plastic indentation field, the problem is conveniently resolved into two separable parts, involving reversible (elastic) and irreversible (residual) components. The assumption of geometrical similarity in the residual field about the deformation zone, later backed up by stress birefringence measurements, leads to a stress intensity factor for median crack propagation containing the elastic and residual parts as the sum of two terms. The resulting formulation for equilibrium fracture shows some differences in the crack response during the loading and unloading half-cycles. By imposing certain stress states on the specimen surface during indentation the residual component of the field may actually cause the median crack to continue in downward extension as the indenter is withdrawn, a response which is especially amenable to experimental investigation. Direct observations of median crack evolution in soda-lime glass confirm this and other essential predictions of the fracture mechanics theory. The contribution of the residual component to the crack growth is found to be by no means secondary in importance to that of the elastic component.