Highly-sensitive Fiber-optic Ultrasonic Sensing Systems for Damage Monitoring in CFRP Laminates

Abstract

Optical fiber sensors (OFSs), especially fiber Bragg gratings (FBG), have been applied to ultrasonic structural health monitoring (SHM) of composite structures. However, because of their low sensitivity, they have not been able to detect acoustic emission (AE) signals. Recently, the authors developed novel OFS systems by introducing phase-shifted FBGs (PS-FBGs). The systems can achieve the compatibility of high sensitivity and broadband performance owing to the characteristic structure of PS-FBGs. First, we developed a PS-FBG balanced sensing system. This system succeeded in detection of small AE signals under tensile test of CFRP laminates. The results indicated the Kaiser Effect and were able to discriminate different damage types. Moreover, AE measurement was conducted under a three-point bending test. The positions of transverse cracks could be identified through signal processing. Next, erbium fiber ring laser (EFRL) sensing system with inbuilt PS-FBG was developed. This system was found to have high robustness due to its self-adjustment function for the environmental disturbances. In addition, this system achieved much higher sensitivity than the traditional piezoelectric ceramic sensors and ultrabroadband performance of up to 7 MHz. These systems have large potential to realize the ultrasonic SHM of composites