Fiber-Optic Acoustic Emission Sensor based on a Chirped FBG Pair for Crack Detection in Aluminum Plate

Abstract

In this paper, a fiber-optic acoustic emission (AE) sensor system for the detection of AE signals generated from cracks within an aluminum plate is described. The sensor head consists of a pair of tandem chirped fiber Bragg gratings (CFBGs) that form a Fabry-Perot type interferometer (FPI). This CFBG-FPI features a series of resonant wavelength notches in the reflection spectrum, which are subject to the same wavelength shift as the sensor is stretched or compressed by AE signals. By locking a tunable laser to the slope of any individual notch, the AE induced high-frequency wavelength shift is converted into intensity variation. Using the sensor system, AE signals generated by three different types of sources, i.e., PZT actuator, pencil break, and cracks within aluminum plates, are detected and compared. Our experimental results suggest that cracks in the aluminum plates gave birth to broadband AE with peak intensity spanning over 100 kHz to 350 kHz.