In the present research, fiber Bragg grating (FBG) sensors were applied for the detection of transverse cracks, which cause strain distribution within the gage length, in carbon fiber reinforced plastic (CFRP) cross-ply laminates. An uncoated FBG sensor was embedded in 0 • ply on the border of 90 • ply in 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. After the crack density was saturated, the spectrum became narrow and had one large peak again. 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 occurrence of transverse cracks can be detected from the change in the form of the reflection spectrum, and that the spectrum width at the half maximum is a good indicator for the quantitative evaluation of the transverse crack density in real time.

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This study proposes a new approach to monitoring the damage process in holed CFRP laminates using an embedded chirped fiber Bragg grating (FBG) sensor. To this end, we experimentally and numerically investigated the damage process and the damage-induced changes in the spectrum shape. It was experimentally confirmed that multiple types of damage (e.g., splits, transverse cracks and delamination) appeared near a hole, and that the spectrum shape of the embedded chirped FBG sensor changed as the damage extended. Our proposed simulation for the reflection spectrum considering the damage agreed with the experiments. Furthermore, this study investigated the effect of each damage pattern on the changes in the spectrum shape. Finally, based on these discussions, we present simple damage identifications with the embedded chirped FBG for the holed CFRP laminates under completely unloaded conditions.