## 1 Introduction  
Wind energy is the fastest growing renewable energy resource [1]. Wind power could supply 12% of global electricity demand by 2020, according to a report by the European Wind Energy Association [2]. Wind blade manufacturing expenses account for about 20% of the total manufacturing cost, and the cost of running and maintenance account for 25%-30% of the cost of manufacturing. The quality and performance of the blade are determinants of long-term safe and stable operation of the wind turbine [3]. Most wind turbine blades are made of glass fiber reinforced polymer composite. The remaining air in the fiber layer might be trapped when the resin flows, which is easy to leave some air bubbles in the fiber layers [4,5]. The failure of structure with air bubble defect is a process of damage accumulation. Under the alternating wind load, plastic deformation of air bubble in blade occurs caused by the stress concentration, which might cause the nucleation and growth coalescence of micro-crack [6,7]. The surface work of air bubble is provided by the volumetric strain energy of matrix in a limited region around the bubble. The process of working A causes temperature rise [8]. During the process of air bubble defect evolves into macro crack, the stress at the defect tip gradually increases and both the temperature and the stress reach their maximums at this location. Therefore, the typical sign of the formation of early macro cracks can be recognized by studying the relationship between the temperature field and the stress field. Thus, it is important for the judgment on early damage of wind turbine blade to study the evolution of air bubble defect, which can provide a quantitative basis of early damage assessment and factory inspection of wind turbine blades. At present, the mechanics theory model simulation and experimental analysis have been applied to analyze the defect evolution of composites as the most common research techniques [9; 10]. The study on defect evolution is related to several mechanics theories, such as fracture mechanics, damage mechanics, and computational mechanics [11; 12]. As a bridge between the macro mechanics properties and the micro components, the meso-mechanical model of air bubble defect is important for the understanding of fatigue damage, because the quantitative relationship between the properties and geometry of micro components and the macro mechanics properties can be established by the meso-mechanical model [13; 14]. The meso mechanical model that is based on the failure analysis can establish the relationship between the meso field and macro field; however, the properties of the meso components are hard to obtain. Thus, the meso mechanical model is always established using FEA approach [15; 16]. The accuracy of meso mechanical model based on the finite element method is usually unsatisfactory, due to the inappropriate boundary condition and load [13]. The boundary condition and load/force are crucial, as they determine whether the model is consistent with the actual situation. The accuracy of mechanical model would be unsatisfactory if they are not appropriate [17]. Researchers have conducted a lot of fatigue tests and the scanning electron microscopy is used to observe the fracture surface of the composites in order to obtain the main failure mode of composites and summarize the evolution rules of mechanics properties based on the experimental data, as for the GFRP [18; 19; 20]. When compared to metal, it is much harder to research the fatigue of composites due to that the composites is anisotropic and the rules of defect evolution and fatigue damage mode are not unique [21; 22]. Besides, the current study methods are not suitable for analyzing the meso-defects in wind turbine blade, due to the limitations that are caused by the type of defect, the boundary conditions, and the structure of defect.