Hot target inspection using a welded fibre acoustic wave piezoelectric sensor and a laser-ultrasonic mirror scanner

Abstract

The direct attachment of piezoelectric transducers onto hot targets raises formidable challenges as piezoelectric transducers lose their piezoelectric characteristics at elevated temperatures or debond due to thermal expansion coefficient mismatches. We developed a welded fibre acoustic-wave PZT (FAWPZT) sensor to alleviate these temperature limitations. One end of the FAWPZT sensor, made from a stainless steel fibre, was welded onto a stainless steel target plate and the other end was bonded to a PZT sensor. An ultrasonic wave propagation imaging (UWPI) system consists of a laser mirror scanner and a Q-switched pulsed laser (QPL) acting as a non-contact ultrasonic generator was then used to scan a hot target surface with an artificial 2 mm-sized open crack. The result was presented in the form of an ultrasonic wave propagation movie. The damage was detected as a wavefield scattering from the damaged location and its size was evaluated from the plot of amplitude distribution along the propagating wavefront. Sensor performance was briefly discussed and the results confirmed that a FAWPZT sensor combined with a UWPI system has good potential for implementation in hot target integrated structural health management.