Application of small-diameter FBG sensors for detection of damages in composites

Small-diameter fiber Bragg grating (FBG) sensors have been developed by Hitachi Cable Ltd. and the authors. Since the outside diameter of polyimide coating is 52 μm, embedding of the sensors into carbon fiber reinforced plastic (CFRP) composite prepregs of 125 μm in thickness does not deteriorate the mechanical properties of the composite laminates. In this research, the small-diameter FBG sensor was applied for the detection of transverse cracks in CFRP composites. The FBG sensor was embedded in 0° ply of a CFRP cross-ply laminate. The reflection spectra from the FBG sensor were measured at various tensile stresses. As a result, the reflection spectrum became broad and had some peaks with an increase of the transverse crack density in the 90° ply. In order to confirm that the change in the spectrum was caused by transverse cracks, the spectra were calculated theoretically. The calculated result reproduced the change in the measured spectrum very well. These results show that the small-diameter FBG sensors have a potential to detect the occurrence of transverse cracks through the change in the form of the reflection spectrum, and to evaluate the transverse crack density quantitatively by the spectrum width.