Fatigue crack propagation monitoring of stainless steel using fiber Bragg grating ultrasound sensors

Fatigue crack propagation of stainless steel was monitored using an ultrasonic inspection system consisting of a piezoelectric transmitter and a fiber Bragg grating (FBG) sensor. The fatigue crack developed perpendicularly to the line connecting the transmitter and the sensor, which were attached on the surface of the test specimen. The ultrasonic response signal was recorded at several fatigue crack lengths either in crack opening or in crack closure. In order to examine the influence of ultrasonic waveform on response behavior, the ultrasonic transmitter was driven by several kinds of excitation signal: a spike signal and a sinusoidal toneburst signal, whose frequency ranged from 200 to 800 kHz. The response signal delayed steadily with crack growth after the crack passed through the line connecting the transmitter and the sensor, although the delay in crack closure was smaller than that in crack opening. In crack closure, a significant response delay with crack growth was observed in the latter response to ultrasound excited by a 800 kHz toneburst signal, which was the highest frequency signal applied in the present study. The FBG-based ultrasonic inspection system proved to be effective for monitoring fatigue crack propagation even in crack closure.