ORAL/POSTER REFERENCE: EFFECT OF FIBER COATINGS ON DETECTION OF TRANSVERSE CRACKS IN CFRP COMPOSITES USING FBG SENSORS Y. Okabe, N. Tanaka, T. Mizutani, S. Yashiro, and N. Takeda Graduate School of Frontier Sciences, The University of Tokyo c/o Takeda Lab., Komaba Open Laboratories, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, JAPAN ABSTRACT FBG sensors have been applied to the detection of transverse cracks in CFRP cross-ply laminates using the sensitivity of the sensors to non-uniform strain distribution. However, there is the possibility that the resin coating of the FBG sensors will relax the non-uniform strain distribution and deteriorate the sensitivity of the sensors. Hence, in this study, the strain transfer from the 0° ply to the core of the optical fiber was calculated by a theory developed from the shear-lag model. As a result, it was found that the variation of axial strain in the core of a polyimide-coated FBG sensor was smaller than that of an uncoated FBG sensor. Nevertheless, the reflection spectra of the coated and uncoated FBG sensors calculated from the strain distributions were almost the same. This result shows that the coating of the optical fiber does not deteriorate the sensitivity of the FBG sensor for the detection of transverse cracks, and the coated FBG sensors can be used without removal of the fiber coatings. KEYWORDS Fiber Bragg grating sensor, CFRP, transverse crack, strain distribution, resin coating, shear-lag model. INTRODUCTION The authors have applied FBG sensors to the detection of transverse cracks in CFRP cross-ply laminates [1]. When transverse cracks appear in 90° ply under tensile loading, the longitudinal strain distribution in 0° ply becomes non-uniform. Since an FBG sensor is embedded in 0° ply, the reflection spectrum of the FBG sensor is distorted by the non-uniform strain distribution. Thus, the occurrence of transverse cracks can be detected from the change in the form of the reflection spectrum. With regard to the application of this technique for practical use, the optical fibers without resin coatings have problems in handling for embedding, and the durability of the fibers is lower than that of coated optical fibers. Hence optical fibers coated with polyimide should be used. However, the coating relaxes the non-uniform strain distribution transferred from the 0° ply to the optical fiber, because the stiffness of the coating is much smaller than that of the glass optical fiber. This effect may decrease the sensitivity of the FBG sensor. Hence, in this research, the strain transfer from the 0° ply to the core of the optical fiber was calculated by a theory developed from the shear-lag model, and the effect of the fiber coating on detection of transverse cracks was
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