Thermal shock failure of brittle materials

Changing the temperature in a specimen causes thermal strains. If these stresses are constrained - and to some extent this always happens - thermal stresses occur. Thermal cracks are created if these stresses reach the strength of the material. A very common and dangerous thermo-mechanical loading situation is a surface cooling of a specimen. In this loading case the constraint of the surface thermal strains is inherently made by the interior of the specimens and, in general, tensile stresses are generated in the cooled surface, They are counterbalanced by compressive stresses in the inner part of the specimen. Thermal cracks relax the constraint of the thermal strains and also the thermal stresses. Therefore thermal cracks often stop before they separate the specimen into two ore more parts. In contrast if the cracks are driven by external forces a complete fracture of the specimens occurs in general. In the first part of this paper the thermo-mechanical loading situation and the thermal shock behaviour of water quenched are analysed for two kinds of alumina, In the second part of the paper in materials with strongly temperature dependent thermo-clastic properties, some new and unexpected effects (e.g. a maximum tensile stress far away from the cooled surface) can be detected.