Residual stress effects in sharp contact cracking

A detailed strength analysis for brittle surfaces containing dominant flaws produced under elastic-plastic indentation loading is presented. The condition for failure is formulated in terms of stress intensity factors representing driving forces associated with applied tension and residual indentation fields. Incorporation of the second of these components depresses the equilibrium applied stress-crack size function; this depression is accentuated at small crack size, such that the function passes through a maximum. Depending on the relative intensity of the residual indentation field, the starting size of the median cracks, as determined from Part 1 of this study, may lie on either side of this maximum: large cracks, i.e. those starting beyond the maximum, fail spontaneously from an unstable branch of the applied stress curve; small cracks undergo precursor stable growth to a critical depth at the stress maximum before failing. Observations of median crack growth in annealed and tempered soda-lime glass discs taken to failure in biaxial flexure confirm the existence of an energy barrier to crack instability. The important implications of these manifestations of the residual indentation field in predicting strength degradation characteristics for prospective adverse contact conditions are discussed for test pieces subjected to various imposed surface stress states.