THE THERMAL-SHOCK RESISTANCE OF SOLIDS

The thermal shock resistance of a brittle solid is analysed for an ort hotropic plate suddenly exposed to a convective medium of different te mperature. Two types of plate are considered: (i) a plate containing a distribution of flaws such as pores, for which a stress-based fractur e criterion is appropriate, and (ii) a plate containing a single domin ant crack aligned with the through-thickness direction, for which a cr itical stress intensity factor criterion is appropriate. First, the te mperature and stress histories in the plate are given for the full ran ge of Blot number. For the case of a cold shock, the stress held is te nsile near the surface of the plate and gives rise to a mode I stress intensity factor for a pre-existing crack at the surface of the plate. Alternatively; for the case of hot shock, the stress field is tensile at the centre of the plate and gives rise to a mode I stress intensit y factor for a pre-existing crack at the centre of the plate. Lower bo und solutions are obtained for the maximum thermal shock that the plat e can sustain without catastrophic failure according to the two distin ct criteria: (i) maximum local tensile stress equals the tensile stren gth of the solid, and (ii) maximum stress intensity factor for the pre -existing representative crack equals the fracture toughness of the so lid. Merit indices of material properties are deduced; and optimal mat erials are selected on the basis of these criteria, for the case of a high Blot number (high surface heat transfer) and a low Blot number (l ow surface heat transfer). The relative merit of candidate materials d epends upon the magnitude of the Blot number, and upon the choice of f ailure criterion. The effect of porosity on thermal shock resistance i s also explored: it is predicted that the presence of porosity is gene rally beneficial if the failure is dominated by a pre-existing crack. Finally, the analysis is used to develop merit indices for thermal fat igue. (C) 1998 Acta Metallurgica me. Published by Elsevier Science Ltd . All rights reserved.