Chirped fiber Bragg (FBG) sensors were applied for the detection of delamination in carbon fiber reinforced plastics (CFRP) cross-ply laminates. Reflection spectra from the embedded chirped FBG sensor were measured at various delamination lengths under static four-point bending test. The sensor were embedded into two different positions for the investigation of the relation between the spectrum and the direction of delamination propagation. The spectrum changed sensitively depending on both the delamination length and the direction of the delamination propagation. For confirmation of the measured results, the spectrum was simulated considering the strain distribution in the chirped FBG sensor theoretically. The change in the form of the measured spectrum was consistent with that of the calculated spectrum. Moreover, the area ratio of the two peaks in the spectrum and the spectrum width were proposed as effective indicators for the identification of the local delamination.
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.
This paper presents delamination monitoring techniques in composite laminates using small-diameter fiber Bragg grating (FBG) sensors that was developed for embedding in the laminates without inducing any structural defects. First, this FBG sensor was embedded in carbon fiber reinforced plastic (CFRP) cross-ply laminates. The reflection spectra from the FBG sensor were measured at various delamination sizes through a static four-point bending test. From experimental results, the spectrum was found to have two peaks due to the initiation of the delamination. The intensity ratio of these two peaks changed as the delamination size increased. In order to confirm the deformation of the measured spectra, the spectra were calculated theoretically. The calculated result reproduced the change in the measured spectrum very well. Thus, we proposed the intensity ratio of the two peaks as an effective indicator for evaluation of the delamination size. Secondly, detection of the edge delamination in quasi-isotropic CFRP laminates was conducted using the FBG sensor. The reflection spectra were measured at various delamination sizes through fatigue test. The form of the spectrum changed sensitively by growth of the edge delamination. Hence the small-diameter FBG sensor could also be applied to detect the edge delamination in quasi-isotropic composite laminates under cyclic loading.