## 3 Experimental Results and Discussing
### Experimental Results
#### 3.3.2 The Results of Infrared Thermography  
Plastic deformation is a main factor leading to the accumulation of fatigue damage, which can cause irreversible change of meso-structure. Most of plastic deformation energy is given off as heat, which can reflect the energy transition indirectly when the fatigue damage accumulates. Thus, the evolution of defects can be regarded as the change process of temperature field, which can be recorded by an infrared thermal camera. The bubble was fabricated with the gel ball. The location of the bubble was consistent with the position to place gel ball. In this paper, two thermal pictures were taken for every specimen at typical moments, such as the moment when the air bubble appears in a thermal image and TCI (The Time to Crack Initiation). Another two thermal pictures were taken during the evolution process. As shown in Figure 4, during the whole test process in 1350 s, the highest temperature of the whole specimen was always on the air bubble. The temperature rose quickly from 0 s to 822 s, due to the thermal dissipation that is caused by the rapid transfer of stress. Figure 3: (a) Load vs. time graph of the tensile test. (b) Stress vs. time graph of the tensile test. As shown in Figure 3(a), the temperature rose from 28.6 \({}^{\circ}\)C (i.e., the initial temperature) to 29.1 \({}^{\circ}\)C in 300 s, due to the release of energy accumulated in air bubble. The temperature variation mainly appeared in a circular area with a radius of 2 cm from the centre of the bubble. The bubble image was blurry, due to the bubble being located near the edge of the specimen. However, it was clear that the temperature in the centre of air bubble was always the highest, due to the stress disturbance with localized features. The stress region that was transmitted through this centre point was broader. Since the ply angles of A1 was [45]/[45]s, the bridging between fibers was good, and the stress was not completely concentrated on a certain fiber, but was dispersed to the surroundings. In Figure 3(b),c, the temperature of air bubble kept rising. In Figure 3(d), a large amount of energy accumulated during the entire evolution process was violently released due to the sudden rupture of the bubble. The bubble radius increased from 2 mm to 3.5 mm and the temperature jump occurred. The measured temperature variation was approximately 4.4 \({}^{\circ}\)C at the critical moment when the air bubble evolved into macro-cracks. The specimen was in a failure state, and the 45\({}^{\circ}\) fibers were all pulled out from the matrix. The tremendous changes of the friction caused a tremendous change of the temperature. The fibers at the rupture of the bubble were distributed at 45\({}^{\circ}\), which indicated that macro crack was formed at this time. In Figure 4(a), at the initial stage of test, the overall temperature difference of the specimen A2 was not obvious. The highest temperature value was approximately 0.2 \({}^{\circ}\)C higher than the initial temperature. The profile of bubble was blurry. As the bubble was extremely small, the temperature variation of the bubble was not obvious, the specimen was a uniform temperature distribution, and the difference between the maximum temperature and minimum temperature was only 0.6 \({}^{\circ}\)C.  
As shown in Figure 4(b), the profile of bubble was clearer, due to the internal temperature variation that was caused by the accumulation of energy. As the radius of the bubble increased, the internal stress was uniformly transmitted to the fibers. Some fibers broke at this moment, and the stress was continuously transmitted to the outside. The radius of air bubble and the number of broken fibers continuously increased. The localized micro plastic deformation caused by the high stress concentration was accumulated continuously. In Figure 4(c), the profile of bubble was clear and the temperature distribution was obviously uneven.