This residual strain field will be perturbed by the microscopic damages brought about by an impact. Residual strain changes are reflected as a shift in the peak wavelength of the FBG spectrum. The second technique monitors the appearance and growth of delamination . In this method, the FBG sensors must be deployed on the delamination interface. As impact caused a delamination to extend across part of the FBG, strain on the debonded section of the sensor is largely relieved while the bonded portion is still under strain. This is reflected as a shifting and chirping of the FBG spectrum. A third technique used the FBG as an ultrasonic wave receiver. Ultrasonic waves are usually generated by a piezo-actuator. On encountering a delamination, the properties of the wave changes and is picked up by the FBG [9�C11].
FBG has also been shown to be able to monitor the progressive damage due to repeated impacts  and compression after impact . Similar study on the monitoring of damage development during post-impact fatigue is still lacking. The latter is deemed important as an impact-damaged composite structure often needs to face cyclic service loading in practical applications. In the current work, we investigated the feasibility of employing FBG sensor to monitor defect growth during post-impact fatigue loading.2.?Material and MethodsWhen a broadband light is coupled into an optical fiber with a uniform Bragg grating, a single peak with wavelength �� satisfying the Bragg diffraction criterion will be reflected:��=2ne��(1)where ne is the effective refractive index and �� is the periodicity of the grating.
When either or both of the ne and �� change, the center wavelength of the reflected spectrum shifts. �� will be changed if the FBG is subjected to a deformation. Such deformation may be caused by mechanical or thermal strains. ne will be affected by variation in temperature and the triaxial stress state acting on the fiber. If the uniformity of the grating period is perturbed, the single peak reflected spectrum will broaden or chirped. In general, the reflected wavelength will shift by ~1 pm under a strain of 1 �̦�. or a temperature change of 0.1 ��C. If temperature variation is negligible, then the change in the spectrum basically reflects a change in the stress/strain status along the FBG.
Quasi-isotropic laminates with GSK-3 T300/3501 Graphite/Epoxy prepreg stacked in the sequence [0/45/90/?45]s were cut into specimen coupons (200 �� 25.4 �� 1 mm). FBG sensors were embedded right under the two outer 0�� laminae along the axial loading direction as shown in Figure 1a. Each of the fibers was offset by 3 mm from the centerline of the specimen. The fibers were led into the specimen coupon at one side and terminate inside the specimen at some distance short of the gripping position at the other side, as shown in Figure 1b.