Influence of residual-stress-induced R-curves on the fracture behavior of brittle materials

The fracture behavior of materials subjected to residual compressive surface stresses is examined. Compressive stresses are introduced on glass surfaces by the process of ion-exchange. Sharp diamond indentations are used to generate cracks on glass surfaces with and without compressive stresses. Various crack morphologies are observed during the loading and unloading indentation cycle for the unstressed surfaces. Thin layers of compressive stress significantly influence crack initiation. Cracking during the loading cycle is suppressed completely, and cracks during the unloading cycle are formed at a greater degree of unloading. The severity of all cracks on the stressed surfaces is diminished.; The strength of compressively stressed materials is investigated using in situ observations of the evolution to failure of indentation flaws. The surface trace and depth behavior of indentation cracks in stressed and unstressed glasses under applied loading are described. Indentation flaws on unstressed glass surfaces fail as radial cracks, while those on stressed surfaces propagate and fail as half-penny cracks. The difference in flaw morphology is responsible for low strength values obtained for some residually stressed materials.; A fundamental understanding of the influence of residual stresses on strength variability is desirable. An approach which relates scatter in strength to parameters governing the residual stress distribution is presented. This approach involves using fracture mechanics to extract strength distributions as a function of residual stress parameters. The strength distribution is then treated as a statistical parameter, and its first (mean) and second (variance) moments are used to evaluate the trends as a function of the residual stress distribution. The conditions for obtaining high strength and minimal strength variability for several stress profiles are identified. For uniform and linearly varying profiles, it is found that strengthening is generally achieved only at the expense of increased variability. For a stress profile which has its maxima away from the surface, stability in the crack behavior may be expected. Such profiles are studied in terms of R (or T)-Curve concepts, and it is found that for some cases, significant reduction in strength variability and an enhanced resistance to strength degradation due to contact induced flaws may be obtained.