A fiber Bragg grating interrogation system for health monitoring of metals and composites

This paper presents a novel Fiber-Bragg Grating interrogation system and its validation for detection of Lambwaves and acoustic emission events on both aluminum and composite substrates. The system utilizes a robust laser demodulation technique for FBG interrogation, based upon a simple laser wavelength tracking scheme. This technique enables detection of much higher frequency strains than previous FBG interrogation techniques, enabling the use of FBG sensors in acousto-ultrasonic structural health monitoring schemes such as Lamb-wave pitch-catch and acoustic emission detection in the presence of a quasistatic strain background. The principles of the FBG interrogation system are presented, including validation of the system for detection of ultrasonic Lamb waves, and results from a 4-point bending test of a braided composite tube wherein the FBG system was used to detect crack-growth induced AE events on the braided tube. The AE data agreed well with damage index values measured by a commercial acousto-ultrasonic system.