of transverse cracks. The agreement of the spectra between coated and uncoated FBG sensors were confirmed also in the cases that the crack density was very small or saturated, from theoretical calculations. CONCLUSIONS In this research, Duck’s method was modified to be applied to multiple cylinder models and calculate the strain transfer from a surrounding material to the core of an optical fiber more accurately. From the analysis, it was found that the polyimide coating of an optical fiber relaxed the non-uniform strain distribution caused by the occurrence of transverse cracks in CFRP laminates. However, the reflection spectrum of a polyimide-coated FBG sensor, which was calculated from the relaxed strain distribution, was almost the same as that of an uncoated FBG sensor. Thus, the polyimide-coated FBG sensor can be applied to the detection of transverse cracks without removal of the fiber coating. ACKNOWLEDGEMENTS This research was conducted as a part of the ‘R&D for Smart Materials Structure System’ project within the Academic Institutions Centered Program supported by NEDO (New Energy and Industrial Technology Development Organization), Japan. REFERENCES 1. Okabe, Y., Yashiro, S., Kosaka, T., and Takeda, N. (2000) Smart Mater. Struct. 9, 832. 2. McCartney, L.N. (1992) J. Mech. Phys. Solids 40, 27. 3. Duck, G. and LeBlanc, M. (2000) Smart Mater. Struct. 9, 492.
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