## 1 Introduction
Thus, a new theoretical analysis method needs to be proposed to study the evolution of air bubble defect according to the feature of air bubble and the ply configuration of wind turbine blade. A study method that is based on the rheological theory is proposed in this paper in order to solve the current problems of study on defect evolution and damage properties of the wind turbine blade. The whole GFRP composite is regarded as a set and the bubble is regarded as a null set. The volumetric strain energy is changed under the tensile load, which causes a temperature jump at the location of defect. The temperature jump can be used to represent the process of evolution [23]. The air bubbles are randomly distributed in wind turbine blade composites with different types and size scales. This disorder effect can be strongly amplified under tensile load. At the same time, the mechanical characteristics will dramatically change and \"evolution cause mutation\" is claimed to occur. Temperature is an effective parameter for representing the structure state. Thus, the temperature detection of the evolution process of bubble defects is also one of the best monitoring and diagnostic methods. A lot of achievements are reported regarding the study of defect evolution based on the rheological theory, such as the study on oil shale and polypropylene [24; 25]. The analysis method that is based on temperature jump is feasible to represent the health condition of material structure. A non-destructive test technology is needed to measure the temperature. The infrared thermal imaging technology can capture thermal radiation signal at a particular wave band and convert it into temperature, and then provide a visible thermal image of the target. This technology is always used to recognize the type, distribution, and geometry of the defect. The researchers obtain their research results by processing the thermal image or extracting the feature of temperature-time curve [26; 27; 28]. However, in this paper, the research object is the quantitative relationship between temperature and stress, which can be used to identify the critical moment when the bubble evolves into crack. Thus, the infrared thermal imaging technology is just a tool for obtaining the temperature and testifying the accuracy of the relation equation. An infrared thermal camera can record the temperature jump then the rules of evolution of grown-in defect in wind turbine blade under external load can be obtained. One of the practical missions of studying the evolution of bubble defects in wind turbine blade is to ascertain how the evolution of bubble defects inside the material affects the macroscopic mechanical properties of the material [25; 26; 27; 28; 29]. Besides, the ply configuration is the basis of structural design and it greatly affects the success or failure of the structural design. The thermal dissipation of structure and the temperature distribution are also affected by the ply configuration. Therefore, in this paper, two defective specimens with different lay-up angles are manufactured for tensile fracture test to analyze the influence of bubble defects on the mechanical properties and establish a quantitative relation between the temperature distribution and the defect, and then obtain an effective and accurate criterion of damage assessment of wind turbine blade. In this paper, a function of temperature jump during the process of evolution was constructed to establish the relation between the temperature and the strength factor of critical stress at the critical moment when the air bubble evolves into crack. At the critical moment, the air bubble defect evolved into crack, so it is feasible to apply a calculation formula of stress field of macro crack to obtain a functional relation equation between the critical temperature rise and the critical stress. The equation can be used to recognize the temperature variation of wind turbine blade and judge the conditions of defect. The experiment verified the analytic solution.