The establishment of transient thermal stresses during quenching brittle materials involves a number of parameters, such as Young's modulus E, Poisson's ratio v, coefficient of thermal expansion a, thermal conductivity of the material k, the size and shape of the sample, the quenching temperature difference AT, and the coefficient of heat transfer in the quenching medium h. the damage resulting from a critical thermal shock is governed by the toughness, the statistical distribution of initial surface flaws and thus by the rupture stress [1,2,3,4,5]. The evaluation of the thermal shock behaviour of brittle materials passes thus through a thorough investigation of all these parameters and properties, and a lot of thermal shock resistance parameters have been proposed [6]. The controlled flaw technique [7] made use of indentation crack systems from which the residual stresses had been removed in order to determine more accurately the maximum value of the thermal transient stress [8][9]. However, as formerly [10], only couples of critical values of quenching temperature difference and thermal stress could be scarcely determined. More recent work made use of the preliminary stage of stable extension of the radial cracks, as it is allowed if the residual contact stress remain [11,12,13]. A qualitative assessment of thermal shock resistances or severities ofquenching became thus possible [14,15].It is shown here that, additionally to the preliminary determination of the material's toughness and the quantification of the maximum value of the thermal transient stress [16], the combination of toughness and these thermal stresses yields descriptions of a new thermal shock resistance parameter for temperature differences AT lower than the critical one ATC. The derivations will be verified on various microstructures of high-temperature superconducting ceramics, "YBaCuO", and on alumina.
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