Development of small-diameter optical fiber sensors for damage detection in composite laminates

Optical fiber sensor systems have been recently used for health monitoring of fiber reinforced plastic (FRP) composite laminate structures. However, the diameter of optical fibers for optical communication is normally 125 mu m, which is similar to the thickness of one ply in FRP composite laminates. In order to embed optical fibers within one ply of FRP without introducing any defects in laminates, small-diameter optical fibers are necessary. The present paper describes development of small-diameter optical fiber sensors for damage detection in composite laminates. The coating diameter of the prototype small-diameter optical fiber is 52 mu m, while the diameters of the cladding and the core are 40 mu m and 8.5 mu m, respectively. FBG (Fiber Bragg Grating) sensors were also fabricated with these fibers. Gratings were inscribed into the optical fiber by ultra-violet irradiation method. The wavelength shift and the response by strain of FBG itself were measured. The strain response, when small-diameter optical fiber and FBG sensors were embedded in composite laminates, were evaluated for practical application to detect the damage evolution.