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The scattering of acoustic waves by surface cracks is used in ceramics as both a method of nondestructive evaluation and a means of investigating the mechanics of failure from surface damage. Initially, experiments combining acoustic scattering, in situ optical observations, and fracture surface observations of controlled indentation flaws provide essential insight into the scattering process and the mechanics of failure. With more complex flaw configurations, such as machining damage, acoustic scattering measurements provide a unique method for examining the micromechanics of failure and thereby establishing a basis for strength prediction. The results indicate important differences between indentation flaws and ideal stress-free flaws, both in their response to applied loading and in their acoustic scattering characteristics. The differences are due to the influence of residual stresses associated with indentation flaws. Machining-induced cracks behave similarly to indentation cracks. A basis for failure prediction from acoustic scattering measurements can be established for indentation cracks and machining cracks but not for ideal stress-free flaws.
failure, strength, machining, scratching, indentation, residual stress, nondestructive testing, acoustic scattering, fractography, structural reliability, brittle materials
Research engineer, Structural Ceramics Group, Rockwell International Science Center, Thousand Oaks, Calif.