Detection of transverse cracks in CFRP composites using embedded fiber Bragg grating sensors

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 degrees ply an the border of 90 degrees 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 degrees 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 goad indicator for the quantitative evaluation of the transverse crack density in real time.