Microfracture process in ceramics under thermal shock fracture characterized by acoustic emission

Microfracture process in ceramics under thermal shock fracture was characterized by acoustic emission technique. The specimen was alumina disk with 0.5mm thickness and 20 mm diameter. The disk specimen was heated to the required temperature (410 - 470 °C) and the central part of disk was quenched by means of contacting a metal rod. An AE transducer, in which amplifier is instrumented, was attached on the opposite side of the metal rod and used for detecting the AE signals due to microcrackings during thermal shock fracture with excellent sensitivity. The temperature distribution on the specimen was measured by the spot-type infrared thermometer and then the 2-dimensional thermal stress field was calculated analytically The thermal stress field was also analyzed by FEM calculation and the experimentally estimated stress field was verified. Macroscopic fracture behavior was observed using a video system and the initiation of macroscopic maincrack was observed before maximum stress. On the other hand, the remarkable increase of AE signals was detected when the maincrack was initiated. From these results, it was understood that the thermal shock fracture process consists of microcrack initiation immediately after contacting a metal rod, maincrack formation due to the coalescence of microcracks and the subsequent crack propagation. Furthermore, the microfracture process during bending tests of alumina specimens were also investigated by acoustic emission technique. Consequently, it was understood that the critical stress for maincrack formation during thermal shock was equivalent to that of mechanical loaded tests.