## 2 The Fabrication and the Damage Mapping Method of a Composite Cylinder with an Optical Fiber  
A composite cylinder with an embedded optical fiber was fabricated through a filament winding process using T700 carbon fiber tape; Figure 1 shows its schematic. The lay-up configuration of this cylinder enlarged as the cross section in the figure is [90\({}^{\circ}\)\({}_{1}\)/OF/90\({}^{\circ}\)\({}_{1}\)/\(\pm\)20\({}^{\circ}\)\({}_{1}\)/90\({}^{\circ}\)\({}_{3}\)/\(\pm\)20\({}^{\circ}\)\({}_{1}\)/90\({}^{\circ}\)\({}_{3}\)/\(\pm\)20\({}^{\circ}\)\({}_{2}\)/EPDM]\({}_{\rm T}\). EPDM is ethylene propylene diene monomer, a kind of rubber pad that is used to cover the mandel of the cylinder. The OF (optical fiber), which was purchased from Fibrecore Co in UK, in the carbon fiberlayers is the sensing optical fiber embedded in this cylinder. All of the 15 carbon fiber layers were fabricated in the CFRP (Carbon Fiber Reinforced Plastic) part. The cross angled lay-up of \(\pm\)20\({}^{\circ}\) was fabricated to have the longitudinal stiffness of the cylinder, and the hoop winding of 90\({}^{\circ}\) was wound for the hoop directional stiffness of the cylinder. This fiber has the same structure and materials as standard single-mode optical fiber (SSMF) for telecommunication, except that, instead of an acrylate coating, it has a polyimide coating. This OF was placed between 90\({}^{\circ}\) hoop carbon layers, and processed to move longitudinally at the pitch of 12 mm in one rotation in the filament winding process. The thickness of the total layers was 4.6 mm. During the remainder of the fabrication process, both ends of the optical fiber were handled with care so as not to break, leading to the outside of the cylinder. They were then connected to the TD-BOCDA sensor system. The figure showsthat the diameter and length of the cylinder were 251.6 and 1920 mm, respectively. The length of the optical fiber for one turn around the cylinder was 786 mm. This value was slightly smaller than the outer circumference of the cylinder, 790 mm, so it agrees well with the position of the optical fiber in the multilayer composite structure in Figure 1. The longitudinal distance of the cylinder in which the optical fiber was embedded was 1.8 m, which is slightly shorter than the cylinder length of 1.92 m. The overall number of winding cycles was 160 turns. Therefore, the total optical fiber length in the cylinder was approximately (786 mm \(\times\) 160)/1000 \(\cong\) 125 m. After the cylinder fabrication with SSMF embedment, the total insertion loss of the optical fiber was measured as less than 1 dB, including the loss of fiber connectors for the measurement.  
When the cylinder was impacted from the outside, the deformation of the composite material, albeit in small amounts, was transferred to the imbedded optical fiber along the circumferential and longitudinal directions of the cylinder. In general, the impacted composite cylinder recovers its initial shape. However, if the cylinder sustains damage to the material under impact, then some amount of strain remains at the impact position in the form of matrix cracks and delamination. Because the residual deformation of the composite material varies around the position of the optical fiber, the residual strain along the optical fiber also depends on the position after impact. Based on the fact that the proportional constant between the Brillouin frequency of the used optical fiber and the residual strain is 0.049 MHz/\(\mu\varepsilon\) for the SSMF, this residual strain can be quantified by measuring the Brillouin frequency of the optical fiber [6; 11]. Thus, it is possible to shape the cylinder damage by mapping the Brillouin frequency measured in this manner according to the position of the optical fiber. The spatial resolution with the Brillouin frequency measurement is decided by the bandwidth of the optical signal modulation, and this determines the sharpness of the image in the circumferential direction of the cylinder.