Retained Strength has been used to characterize the damage incurred by brittle materials when exposed to a thermal shock or to a sequence of thermal shocks. While it is a valid measure to qualitatively rank materials in a simplistic way, it cannot be used in any quantitative sense since it depends upon the specific thermal shock conditions and upon the type of test used to measure it. Given the current state of knowledge about fracture of brittle materials, retained strength should be considered as an artifact of a time when little was understood about fracture of brittle materials. When the residual strength of a ceramic is measured by mechanical tests subsequent to the exposure of a ceramic to a thermal shock, the isothermal test condtions must produce a stress field that is consistent with the stresses produced by the thermal shock. This paper describes the theoretical basis for retained strength and presents experimental data to show that retained strength measured from the isothermal tests cannot be used in general as an accurate measure of the thermal damage.

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