## 4 Damage Detection Results by TD-BOCDA Sensor System  
Impact events of four different energy levels from 5 J of the lowest energy to 10, 15, and 20 J were applied to the composite cylinder with optical fiber embedment. These four impact events were repeated three times. Thus, a total of 12 impact events were applied to the cylinder. Even the largest value of 20 J caused an impact on the cylinder that was weak enough to cause BVID. Figure 3 shows that a drop weight impact machine was used for these impacts. The figure shows that the cylinder is firmly held by two external circular clamping devices in the machine, and the blue bar lying horizontally above these clamping devices contains an impactor. When the impactor hits the surface of the cylinder, the effect of each impact event is limited to a 12 cm section between the two circular clamps. The tip  
Figure 2: Experimental setup of the TD-BOCDA sensor measurement system with a sensing optical fiber. of the impact is hemispherical with a diameter and length of 12.7 and 25 mm, respectively. This blue bar with an impactor is controlled by a brake system to prevent multiple bounces after impact. The white circles mark the locations where the impacts are be applied. To measure the strain caused by the impact machine, the Brillouin frequencies of the optical fiber before and after impact should be compared. Therefore, first, before the impact events, the Brillouin frequency at each position of the composite cylinder was measured using the TD-BOCDA sensor system of 16 Gbps phase modulation. The frequency difference between the pump and the probe was scanned from 10.5 to 11 GHz. The inset in the top graph of Figure 4 shows the Brillouin frequency as the peak frequency for the highest SBS gain that was obtained at each fiber location. The Brillouin frequency is an intrinsic characteristic of the optical fiber that is determined by the manufacturing conditions and materials used. This intrinsic frequency changes with the stress applied to the material. Figure 4 shows that the changed frequency was measured throughout the whole fiber locations of 125 m. The top graph enlarges the dotted circle area on the bottom graph, which corresponds to one end of the cylinder at around 120 m. The inset in the top graph shows the Brillouin gain spectrum around this end, where the horizontal axis is the frequency difference between the pump and the probe, and the vertical axis is the Brillouin gain in arbitrary units. The peak frequencies were collected from the spectrum measured at each location, and Figure 4 plots them as the Brillouin frequency graph. These measured frequencies on the top graph are divided into five segments for explanation. These segments clearly show the conditions of the sensing optical fiber. Up to segment (1), the sensing fiber is embedded in the composite cylinder. The bottom graph shows that the frequency significantly fluctuates throughout the whole embedded region. This is because, during the filament winding process of embedding the optical fiber, the tension applied to the optical fiber was not uniform, so that the optical fiber was installed inside the cylinder under different tension states according to the location. The frequencies of segment (2) confirm this explanation, where the optical fiber exits the cylinder to connect to the BOCDA sensor system. There are no winding tensions in the fiber, so the frequencies in this segment show the uniform value of 10.7 GHz, as in the graph in the inset of the top graph. This segment is 70 cm, which exactly corresponds to the length of the optical fiber exposed outside the cylinder. Segment (3) shows the frequencies from another SSMF that is spliced to the optical fiber of the cylinder. This fiber in segment (3) is produced by a different company than that of the SSMF embedded in the cylinder, and so the Brillouin frequencies of the two SSMFs are quite different.