Study on Acoustic Emission of Crack Propagation in Piezo Ceramic Materials under Thermal Stress (1)

PZT piezoceramic has advantages that other materials such as metal do not have excellent high temperature resistance, wear resistance and corrosion resistance, but the disadvantage of poor thermal shock resistance of piezo ceramic materials has greatly limited by its function. Piezo ceramic materials are damaged due to cracking and fracture caused by thermal shock, which is caused by the growth and expansion of microcracks due to thermal stress. However, it is technically difficult to study the crack growth of piezo ceramic materials under thermal stress. At present, there are few reports about this, especially the dynamics of crack growth in piezoceramic materials under the direct monitoring of thermal stress have not been seen by process coverage.

Acoustic emission detection technology provides a new method for the study of the aboving topics. Acoustic emission refers to the phenomenon that elastic waves are generated due to the release of strain energy when plastic deformation or crack formation and expansion occur under the influence of external force or internal stress. The oscillating signal of an acoustic emission event is a damped sine wave, Vo is the initial output voltage of the transducer; β is the attenuation constant; t is the width of the acoustic emission event. The number of pulses η generated by an acoustic emission event issued inside the material is the number of peaks in a range exceeding Vt, but lower than Vo of the transducer. For an actual test condition, ω, β, and Vt are constant, so η generated by an acoustic emission event issued inside the material reflects the magnitude of △ E released during the material's stress relaxation process.

For brittle piezo ceramic materials, the acoustic emission intensity during crack propagation has a high amplitude and is easy to distinguish from noise, so it is very effective to study the fracture process of piezo ceramic materials by acoustic emission. Used acoustic emission technology in the SENB bending test of thermally is damaged by piezo ceramic materials. This work directly applies the acoustic emission technology in the thermal cycle to accurately determine the dynamic process of crack growth and propagation of PZT material piezoelectric strip under thermal stress.

The experiment uses alumina-mullite piezo ceramic material rich in A1₂O₃ (≈77% by mass). The specimen is a long rod-shaped cylinder of Φ20 m m x 230 mm, and both ends are ground and polished. The middle part of the sample was placed in a heating electric furnace, and one end was coated with vacuum grease to connect with the acoustic emission transducer and fixed with a clamp. The heating and cooling rate of the electric furnace is controlled at 5 ° C / min. The AE-400B four-channel acoustic emission instrument was used to detect the crack propagation signal of the sample under thermal stress. The gain of the preamplifier was 40dB, the dynamic range of the main amplifier was 60dB, and the bandwidth was 40 ～ 4. 0 0 Hz, the test process is shown in Figure 2.

The quench test is performed as follows: Put the Φ20mm x 130mm sample in the furnace, raising the temperature to a predetermine temperature and keep it at o.hs, and then put it into a 20 ℃ oil bath container equipped with an acoustic emission transducer. Acoustic emission signal of crack growth during the sample cooling. The height difference when the piezo disk piezoelectric transducers are dropped into the oil bath was 30 cm. The strength test uses the three-point bending method with a span of 120mm and a loading rate of 05.mm/min.