Fatigue crack propagation monitoring of stainless steel using fiber Bragg grating ultrasoundsensors

Abstract

Fatigue crack propagation of stainless steel was monitored using an ultrasonic inspectionsystem consisting of a piezoelectric transmitter and a fiber Bragg grating (FBG) sensor.The fatigue crack developed perpendicularly to the line connecting the transmitter and thesensor, which were attached on the surface of the test specimen. The ultrasonic responsesignal was recorded at several fatigue crack lengths either in crack opening or in crackclosure. In order to examine the influence of ultrasonic waveform on response behavior, theultrasonic transmitter was driven by several kinds of excitation signal: a spikesignal and a sinusoidal toneburst signal, whose frequency ranged from 200 to800 kHz. The response signal delayed steadily with crack growth after the crackpassed through the line connecting the transmitter and the sensor, although thedelay in crack closure was smaller than that in crack opening. In crack closure, asignificant response delay with crack growth was observed in the latter response toultrasound excited by a 800 kHz toneburst signal, which was the highest frequencysignal applied in the present study. The FBG-based ultrasonic inspection systemproved to be effective for monitoring fatigue crack propagation even in crackclosure.