Impact damage detection of stiffened composite panels by using embedded small-diameter optical fibers

It is well known that barely visible damage is often induced in composite structures subjected to out-of plane impact, and the mechanical properties of the composites decrease markedly. The stiffened composite panels, which are the representative structural elements of airplane, are characterized by different impact damage from that of the coupon level specimens. Therefore, the goal of this study is that small-diameter optical fiber sensors are applied in stiffened composite panels, and it is discussed about the possibility of the detection of impact damage in the structures by the sensors. In this study, both multi-mode optical fibers and fiber bragg grating (FBG) sensors are used for detecting impact load and impact damage in stiffened composite panels. The fibers have polyimide coating and about 40 mum in cladding diameter which will have no serious effect on mechanical properties of composites. Impact tests are performed using the stiffened composite panels with embedded optical fibers. The characteristics of impact damage are investigated. The impact load, the strain and the optical responses of the optical fibers are measured as a function of time. And we discuss the relationship among the optical responses, the impact load and the impact damage.