Elastic Energy at Fracture and Surface Energy as Design Criteria for Thermal Shock

The physical properties which affect the propagation of cracks in specimens fractured by thermal shock are discussed. The driving force for crack propagation is provided by the elastic energy stored at fracture. The mechanism of energy dissipation which will tend to arrest the propagating cracks is provided by the “effective surface energy” required to produce the newly formed crack surfaces. An expression is derived applicable to a body of spherical shape for the mean area traversed by cracks nucleated by thermal shock. Three numerical examples are given for materials with widely different physical properties, and their fracture behavior is predicted. Good agreement with experiment was obtained. Thermal shock damage resistance parameters suitable for the relative comparison of the “degree of damage” to be expected in materials fractured by thermal shock are proposed. The criteria for a low degree of damage are high values of Young's modulus of elasticity, Poisson's ratio, and effective surface energy and low values of strength. Recommendations are made for the selection of materials for severe thermal shock, where the best materials available are known to fail.