Ultrasonic Assessment of Microcrack Damage in Ceramics
The inherent brittleness of ceramics often results in catastrophic failure due to microcrack damage caused by thermal treatment or mechanical loading. Extensive theoretical and experimental studies have been performed to analyze microcrack damage in ceramics caused by thermal shock [1–7]. Hasselman [1,2] proposed a simple model describing the strength behavior of ceramic materials as a function of thermal shock temperature difference ΔT. The important characteristic parameter in this model is the critical temperature difference, ΔT c . For thermal shock temperature differences less than ΔT c (stage I, Fig. 1) ceramics retain their strength. Thermal shocks with temperature differences equal to ΔT c (stage II) are characterized by unstable crack propagation and instantaneous decreases in strength. Above ΔT c is a plateau of constant strength (stage III), where cracks are subcritical and gradual decrease in strength is observed at higher thermal shock temperatures (stage IV). As shown experimentally [3,6], the actual behavior depends on the composition and the microstructure of the material.