Fracture mechanical characterization of thermal damage of ceramics by means of laser irradiation

By suitably scanning a circular sample with a beam from a 1 kWNd-YAG laser, steep temperature gradients and high stresses can be generated. Ceramics with outstanding high temperature strength have been tested, such as silicon nitride. Thermal shock figuresof-merit have been calculated from the measured temperature field at the instant of fracture. By means of repeated thermal loading, the number of cycles to failure and the parameter of subcritical crack growth have been determined for unnotched samples. In addition to elastic effects, damage due to tensile stresses arising after stress relaxation by creep has been demonstrated.