Yoshinori Kurosawa

Polyimide-coated small-diameter optical fiber sensors for embedding in composite laminate structures

We have studied health-monitoring systems using embedded optical fiber sensors for damage detection in FRP (Fiber Reinforced Plastics) composite laminates. For sensing of strain or temperature changes in these composite materials, we developed the FBG (Fiber Bragg Grating) sensors using the polyimide-coated small-diameter optical fiber with cladding diameter of 40 micrometers , which can be embedded in FRP composite laminates without inducing any structural defects. The FBG was subjected to high-temperature heat treatment at 300 degree(s)C in order to suppress the optical characteristics change of the FBG over a long period of time. Even after such high temperature treatment, the FBG retained sufficient mechanical strength and reflectivity. We embedded the FBG into FRP composite laminates and measured the optical characteristics against temperature and tensile strain. These experimental results show this type of the embedded FBG is very promising for practical health-monitoring systems. In addition, we have investigated various multipoint measuring systems and propose the time division multiplexing system using these FBG sensors.

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 μm, 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 μm, while the diameters of the cladding and the core are 40 μm and 8.5 μm, 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.

Improved theoretical estimation of mechanical reliability of optical fibers

We propose an enhanced theory that can be commonly applied to fiber reliability evaluations under various stress and failure probability levels, under prolonged stress and in stressed fiber distribution conditions. In the theory, the inert strength distribution of fibers is composed of two types of Weibull distributions with different slope parameters. We included proof-test conditions that adhere to Mitsunagas theory in the calculations. New parameters that express the high-strength distribution can be obtained from the tensile strength data using a theoretical equation. We analyzed the static fatigue characteristics under uniform bending and two-point bending using this theory. The experimental results agreed well with the calculated results for bent fibers. We believe that the theory will play a vital role in the design of optical fiber cords for sharp-bending uses and in the minimization of various optical packages.