Monitoring of Delamination Growth in CFRP Laminates using Chirped FBG Sensors

Chirped fiber Bragg grating (FBG) sensors are applied to the monitoring of the propagation direction and the size of a delamination in CFRP cross-ply laminates, since it has been found previously that the reflection spectrum from chirped FBG sensors is expressed as a function of their positions along the grating. The reflection spectra are measured at various delamination lengths under a four-point bending test. Two chirped FBG sensors are embedded in two opposite directions in order to investigate the relationship between the deformation of the spectrum and the propagation direction of the delamination. The spectrum changes sensitively depending on both the size and the propagation direction of the delamination. For confirmation of the measured results, the spectra are simulated using FEA computed strain distributions of the chirped FBG sensors. In the simulation, a birefringence effect of the optical fiber on the spectrum is observed in the case of chirped FBG sensors. The relationship between the measured spectral change and the delamination growth is illustrated by the calculated spectra, the strain distributions, and the properties of the chirped FBG sensors. As a result, it is found that not only the delamination size, but also the propagation direction of the delamination can be evaluated by using chirped FBG sensors.