Hybrid guided wave based SHM system for composite structures for impact and delamination detection combining fiber Bragg grating sensing and piezoelectric patches

Structural Health Monitoring deals mainly with structures instrumented by secondary bonded or embedded sensors. Sensors, acting passively or actively as both signal generators and receivers, are able to “listen” to any event happening in the structure (passive SHM) and to “interrogate” the structure to check its “health status” (active SHM). Structures embedded with sensors appear promising for reducing the maintenance costs and the weight of aerospace composite structures, without any reduction of the safety level required. Among many actuators/sensors technologies under investigation for active SHM systems, the combination of piezoelectric patches employed as guided wave exciters or impact sensors and optical fiber Bragg gratings (FBG) as stress wave detectors look promising for their distributed sensing capability as well as weight reduction compromise in a so-called “hybrid structural component”. FBGs have been employed only recently as stress ultrasonic wave sensors due to the reduced number of high-frequency optical interrogators available. One such device, a multi-channel fiber optic acoustic emission (FAESense™) system developed by Redondo Optics, has been employed by the authors for this purpose. Hybrid SHM systems employing FBGs as sensor arrays could provide more distributed data about the local integrity of the structure with less weight addition compared to other sensor types. Typical diameter of fiber optics could allow the embedding of sensor arrays within the composite laminate. Finally, FBGS can provide simultaneously high frequency data characterizing guided wave propagation as well as low frequency local deformations permitting an SHM approach combining global and local impact and damage detection. Intent of this paper is to summarize the first experience gained by the authors in developing SHM systems for composite plate-like hybrid structures for impact detection.