HOT TARGET INSPECTION BASED ON FIBER ACOUSTIC WAVE PZT SENSOR AND Q-SWITCHED PULSED LASER

초록

Direct attachment of piezoelectric transducers on hot targets raise formidable challenges as piezoelectric materials will lose their piezoelectric characteristic at elevated temperature due to Curie temperature limitation. In this paper, an attempt was made to build an integrated probe for hot target which enables to alleviate innovatively the Curie temperature limitation. In addition, the problem of sensor debonding due to the mismatch of coefficient of thermal expansion between the target, sensor and bonding material was experimentally illustrated. A fiber acoustic wave PZT (FAWPZT) sensor was then used to solve the debonding problem and Curie temperature limitation. The FAWPZT sensor was made from a stainless steel fiber, with one end welded on the hot target and the other end bonded with a PZT sensor. An ultrasonic propagation imaging (UPI) system consisted of a Q-switched Nd-YAG pulsed laser (QPL) with 20 Hz-repetition rate and a motorized tilting mirror system (TMS) for rapid scanning of target was successfully applied on a stainless steel target plate at a temperature of about 150oC. Although the target temperature stood at 150oC, the temperature of the PZT part in the FAWPZT remained almost room temperature. The UPI system scanned a area of interest on the target enclosed with an artificial 2 mm-open crack. The result was presented in the form of ultrasound wave propagation movie visible as a concentric wavefield emerging from the sensing point. The damage was detected and visible as scattered wave emerging from the location of damage. Overall, the result confirmed that the FAWPZT sensor combined with a QPL is an economic technology with good potential for implementation in hot target integrated structural health monitoring.