org/licenses/by/3.0/).AbstractIt has been shown that impact damage to composite materials can be revealed by embedded Fiber Bragg Gratings (FBG) as a broadening and splitting of the latter’s characteristic narrow peak reflected spectrum. The current work further subjected the impact damaged composite to cyclic loading and found that the FBG spectrum gradually submerged into a rise of background intensity as internal damages progressed. By skipping the impact, directing the impact to positions away from the FBG and examining the extracted fibers, we concluded that the above change is not a result of deterioration/damage of the sensor. It is caused solely by the damages initiated in the composite by the impact and aggravated by fatigue loading.

Evolution of the grating spectrum may therefore be used to monitor qualitatively the development of the incurred damages.Keywords: fatigue, impact damage, polymer-matrix composites, health monitoring, fiber Bragg grating1.?IntroductionComposite structures such as aerospace vehicles and wind turbine blades are susceptible to impact damage caused by careless handling during manufacturing and maintenance and by foreign object impacts such as bird-strikes and hailstorms during service. Such damages involve a number of different failure mechanisms such as delamination, debonding, fiber breakage and matrix cracking [1]. The damages caused are often insidious without leaving marked evidence on the impact-suffering surface [2]. Although small in extent, these microscopic damages can lead to deterioration of mechanical properties [3�C5].

In particular, on subsequent cyclic service loading, these microstructural defects may grow and eventually lead to catastrophic failures. Current non-destructive examination techniques are only sensitive to some of these failure mechanisms and are responsive only when the defects reach a certain size. It is often impracticable to use Batimastat these techniques for close monitoring of the development of these defects. Recently, there are general interests in the development of integrating fiber optic sensors into composite structures for structural integrity monitoring. Optical fiber has a small diameter, long fatigue life and may be embedded inside a composite material and literally come into contact or at least into very close proximity of the internal defects.

Fiber Bragg Grating (FBG) is one of such sensor elements that has been shown to be able to detect impact damages in composites [5�C11] and monitor impact event occurrence [12�C14].The output from an FBG is the wavelength shifting and shape changing of its characteristic reflected spectrum. There is no simple correlation between the spectrum changes and the damage mechanism involved. The techniques employed for FBG sensor to detect impact damages fall into three categories. The first technique makes use of residual strain changes [5�C7].