Application of chirped fiber Bragg grating sensors for damage identification in composites

Chirped fiber Bragg grating (FBG) sensors were applied for the identification of damages in carbon fiber reinforced plastic (CFRP) laminates since the reflection spectrum from a chirped FBG was expressed as a function of the position along the grating. First, for the identification of crack locations in the 90 degree(s) ply of cross-ply laminates, chirped FBG sensors, whose grating length and spectrum width were 50mm and about 5nm respectively, were embedded in CFRP cross-ply laminates, and the reflection spectra were measured after tensile loadings to the laminates. As a result, the spectra had dips corresponding to locations of transverse cracks. This change in the form of the spectrum was also confirmed by a theoretical calculation. Hence, the crack locations could be identified from the form of the spectrum. Secondly, this technique was applied to the identification of the delamination originating from a tip of a transverse crack in a cross-ply laminate. Since theoretical calculations indicated that the reflection spectrum changed its form corresponding to the progress of the delamination, it was also found that the chirped FBG sensor could be applied to the identification of the length and position of the delamination.