Durability analysis and structural health management of smart composite structures using small-diameter fiber optic sensors

Abstract

The authors and Hitachi Cable, Ltd. have recently developed small-diameter optical fiber sensors in order to embed the sensors inside a lamina of composite laminates without strength reduction. The outside diameters of the cladding and polyimide coating are 40 and 52 μm, respectively. This paper presents a summary of applications of the small-diameter optical fiber sensors to damage monitoring in composite structures. First, small-diameter fiber Bragg grating (FBG) sensors were applied to detect transverse cracks and delamination in CFRP cross-ply laminates using the changes in the form of the reflection spectrum. The same monitoring technique was extended to detect both free-edge and impact-induced delaminations in CFRP quasi-isotropic laminates. The sensors also detected the debonding of CFRP repair patches from an aluminum structure. Small-diameter FBG sensors were also successfully used to monitor the thermal residual stress during the fabrication process. Then, the multi-mode damage pattern of a notched cross-ply laminates was identified as an inverse problem using the reflection spectrum as an objective function. Furthermore, applications of small-diameter optical fibers and FBG sensors were demonstrated in a stiffened CFRP fuselage structure of 1.5 m in diameter and 3 m in length. The small-diameter optical fiber sensors were successfully embedded into the upper panel of the fuselage, and the real-time detection system could well detect impact locations and impact-induced damages.