Instrumented indentation testing provides the means to measure many mechanical properties and characteristics of materials. One such mechanical property that can be ascertained from indentation testing is a material's yield stress, the stress corresponding to the onset of permanent deformation. In brittle materials such as ceramics and glasses, traditional testing methods fall short of precisely establishing the yield stress. A new technique to estimate the yield stress is under development and described here. It utilizes the sensing and interpreting of the initiation of a residual surface impression through the change of the instantaneous contact-stiffness~3/load (S~3/P) as measured by load and depth-sensing indentation with spherical indenters. Several brittle materials (borosilicate, soda-lime silicate, and bulk metallic glass) are evaluated, and the test method and manner of interpreting the yield-like response through S~3/P are described.
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