## 3 Experimental Results and Discussing
### Experimental Results
#### 3.3.2 The Results of Infrared Thermography
As shown in Figure 7d, the temperature in the middle section of the specimen, which was 1.6 \({}^{\circ}\)C higher than that at the edge of the specimen, rose rapidly. None of the fibers in other parts fractured and the field of the temperature rise was relatively concentrated. The effect of internal fiber interactions was not obvious. The temperature variation appeared in the fiber direction of 0\({}^{\circ}\) direction, but stopped in the fiber direction of 0.2\({}^{\circ}\), which manifested in the stress distributed along this fiber being restrained by the 90\({}^{\circ}\) fiber. The stress cannot continue to transfer along this direction and it was concentrated on the fiber. The temperature jump did not occur, which showed that the [0/90]\({}_{\text{s}}\) layer played a critical role in determining the stability of the wind turbine blade structure. In Figures 4-7, the principle of thermal energy dissipation and stress transfer caused by internal friction in the fiber structure were reflected by the series of infrared thermal images. The whole process of plastic deformation of bubble defects could be observed from the infrared thermal images under cyclic loading, even though the ply angles of specimens were different. Among all of the specimens with different ply angles, the friction phenomenon was more obvious and the stress transfer was faster when the ply angles of specimen were [45; 46; 47; 48]. The probability of sudden failure was also higher. The temperature jump did not appear in specimens without the bubble defect. Thus, it is feasible that the infrared thermal imaging can be used to detect the blade defects in a wind turbine. It can not only detect the type and position of defects, but also record the serial thermal images of failure precursor. The defects in the wind turbine blade can be efficiently and quickly detected by this method and the blade can be repaired before it fails. As shown in Figure 3, among all of the specimens, the stress of A2 was largest. The stress variation of specimen A2 was obvious, especially in the later stage of loading. It was obvious that the air bubbles affected the overall structure of specimen. The stress of specimen A4 was generally lower than that of the other three specimens, which indicated that [0/90]\({}_{\text{s}}\) was the most appropriate ply configuration for structural stability. The images that were captured by the infrared camera were actually infrared thermal image of temperature field. By analyzing the infrared thermal image, it can be seen that the stress change and the rules of temperature field distribution were proportional to time. Within 0-500 s, no damage occurred in any specimens and the temperature smoothly increased. Within 500-800 s, the evolve process started and the profile of bubble became clear. After 800 s, the stress of all specimens substantially increased, and the infrared thermal image also showed a sharp increase in temperature. Thus, no matter whatply configuration was, no matter whether a defect existed, there was always a moment when most of the fibers were fractured and large-scale fiber misalignment occurred. At that moment, the thermal dissipation of structure increased suddenly. This phenomenon was caused by the internal friction. The temperature went higher and the deformation of structure became clearer on the infrared thermal image. Finally, the meso-defect evolved into crack. According to Figure 3 and the thermal images, at the moment when the temperature rose fastest, the stress also arrived at the peak value. If the stress at this moment was substituted into equation 39, the critical temperature increase can be obtained, which was consistent with the temperature rise in the infrared thermal image.