Dynamic Detection of Microcracking by Acoustic Emission during Thermal Shock Fracture of Ceramicsand Analysis of Failure Mechanism

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Microfractureprocess in ceramics under thermal shock fracture was characterizedby acoustic emission technique. The specimen was alumina disc with 0.5 mm thicknessand 20 mm diameter. The disc specimen was heated to the designated temperature(410 - 460ﾟC) and the central part of disc was quenchedby means of contacting of metal rod. An AE transducer, in which amplifier is instrumented was attached on the opposite side of the metal rod and used for detectingthe 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 stressfield was verified. Macroscopicfracturebehavior was observed using a video systemand the initiation of macroscopicmaincrack 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 fractureprocess consists of microcrackinitiation immediately after contacting a metal bar, maincrack formation due to the coalescence of microcracks and the subsequentcrack propagation. Furthermore the microfracture process during plane bending tests of alumina disc specimens were also investigated by acoustic emission technique. Consequently, it was understood that the critical stress for maincrack formation during thermal shock was equivalentto that of mechanical loaded tests.