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A processing approach has been identified and reduced to practice in which a residual stress profile can be designed such that cracks in a brittle material are arrested or grow in a stable manner. In the approach, cracks in the body encounter an increase in the magnitude of residual compression as the crack propagates. If correctly designed, the process increases strength and significantly decreases strength variability. This approach was demonstrated for a silicate glass, and multiple cracking was observed as a forewarning of the final failure. Normally, such glasses would fail catastrophically with the propagation of a dominant crack.
Brittle materials, such as ceramics and inorganic glasses, are sensitive to surface contact damage, which gives rise to flaws that reduce strength. Moreover, these materials usually fail in an unstable and catastrophic manner when subjected to applied mechanical and thermal stresses. For example, when most ceramics and glasses are tested in bending, uniaxial tension, or other types of tensile stress fields, a single flaw forms into a propagating crack that grows rapidly and in an unstable manner. The strength behavior is usually modeled with weakest link statistics, such as Weibull statistics, leading to a dependence of strength on specimen size (1). Extensive damage may also occur in a thermal shock type of loading, resulting in a multiplicity of cracks. In many cases, the crack also branches, forming splinters. This behavior leads to dangers when these materials fail, because there is often no forewarning and the splinters can cause harm. It would be beneficial to develop methods that could stabilize growth or arrest cracks in brittle materials.
Recently, it was shown that the microstructure of some brittle polycrystalline ceramics can be modified such that cracks encounter an increase in fracture resistance as the crack propagates ("rising R curve behavior"), usually by adding fibers, single-crystal whiskers, or transforming particles to the material (1, 2). Crack growth in these materials can be stabilized or cracks arrested even in destabilizing applied stress fields. (A destabilizing field can be defined as one in which the strain energy release rate increases with crack length.) This feature leads to flaw tolerance in the material, wherein the strength becomes a weak function of crack length. Such cases result in a reduction in strength...