Study on Acoustic Emission of Crack Propagation in Piezo Ceramic Materials under Thermal Stress (2)

Acoustic emission characteristics of crack propagation when temperature sensor slowly changes.

When it is heated to different maximum temperatures T max, and then slowly cooled, the acoustic emission characteristics of the microcrack propagation process are shown in figure 4. When Tmax <50 0 ℃, the detected acoustic emission signal has a peak in the temperature range of 1 80 ~ 2700 ℃, it is indicating that the growth and expansion of microcracks are mainly concentrated around 200 ℃, and thus in this temperature range ,Arousing rich acoustic emission signals, When Tmax = 80 ℃, the acoustic emission signal obviously moved to the high temperature region, and the peak value of the acoustic emission count rate appeared in the temperature range of 500 ~ 600 ℃, indicating that the growth and expansion of microcracks were mainly concentrated at 500-600. ℃. It can also be seen from Figure 4 that the larger the Tmax, the stronger the acoustic emission signal.

When the sample of low frequency piezoelectric strip is slowly cooled, microcracks are mainly caused by the thermal stress caused by the differences in the thermal expansion coefficients of the various phases in the porcelain billet. X-ray diffraction and HF method were used to quantitatively analyze the piezo ceramic crystal composition and glass phase content of the sample. The results showed that the piezo ceramic crystal contained about 3.5% of the quartz crystal phase (see the table on the next page). The crystal phase of the quartz crystal phase is transformed at 5 70 ℃ and 1800-1270 ℃, respectively. Therefore, the thermal expansion coefficient of the quartz crystal phase will change greatly around these two temperatures, which will cause the thermal stress . The peak of the acoustic emission signal shown in Figure 4 corresponds to these two temperature ranges of quartz crystal transformation, which indicates that in the piezo crystal transformation temperature range of quartz, the thermal stress around the quartz particles will develop to cause a large amount of cracks, which stimulate a rich acoustic emission signal. The acoustic emission curve fully reflects the dynamic process of microcrack formation in the sample under thermal stress. When the temperature is raised to different Tmax, the micro-cracks generated during the cooling process of the porcelain billet will be healed to different degrees. The higher the Tmax, the greater the degree of micro-crack healing. When is cooling, the micro-cracks are formed again. The more energy is released, so the acoustic emission signal of the sample during cooling increases with increasing Tmax.

4 Conclusion
The acoustic emission characteristics of ceramic materials piezo disc transducer under thermal stress reflect the process of crack propagation and propagation inside the material:

(1) The formation and growth of emblem cracks in corundum-mullite ceramic materials under thermal stress mainly occur during the cooling process, and the peak value of the acoustic emission count rate during the cooling process is about 400 times that during the heating process.

(2) When the grain size decreases, the propagation and propagation of emblem cracks in ceramic materials subjected to thermal stress is gradually suppressed to a smaller range.

(3) Under quenching conditions, the acoustic emission characteristics of the steady-state expansion and instability propagation of the emblem crack caused by thermal stress are consistent with the strength change trend of the sample under thermal shock.