## 1 Introduction  
At present, wind turbines are under continuous development for large-scale deployment and oceanization. Blade lengths of more than 100 m and turbines of more than 12 MW were available on the market in 2018 [1]. However, wind turbine blades had increased failure rates or decreased reliability as simple designs with small rotor diameters progressed to more advanced technologies with bigger rotor spans [2]. Factors such as cost and reliability should be considered in the design of longer and lighter blades. First, the lengthening of blades increases the cost of the wind turbine itself. In addition, the cost of blade replacement and the shutdown caused by blade damage also increase. In 2013, [PERSON] [3]reported that the downtime caused by blade damage accounted for 8-20% of the total downtime, second only to the downtime caused by gearbox problems. About 2% of the blades need to be replaced in their first 10 years of lifetime. In 2013, [PERSON] [4] investigated 81 blades of 100 kW wind turbines and 18 blades of 300 kW turbines, with the working life ranging between \(6.5\times 107\) and \(1.1\times 108\) cycles, and proposed that the geometrical form and the manufacturing technique made the trailing edge of the wind turbine blade more susceptible to damage. In 2015, [PERSON] [5] announced that all rotor blades at the Horns Rev2 wind farm in Denmark would be replaced. Most of the observed damage was related to leading- and trailing-edge issues. Consequently, the understanding of the structural behavior of blade trailing edge, especially the damage of trailing edge under fatigue load, needs to be improved. Research on structural problems can improve blade design and service life, thus providing a great cost-saving potential for blade manufacturers and operators. However, not many previous studies were devoted to the study of the adhesive joint failure of wind turbine rotor blades. Glass/Carbon fiber-reinforced plastics have been widely used in the manufacture of large wind turbine blades due to its lightweight and high performance [6; 7]. For most wind turbines, the composite blades are usually manufactured in parts and then bonded with adhesives in the mold (Figure 1). This bonding is of two main types. One of them is T-joint bonding for web and shell. The failure of T-joints has been explored theoretically and experimentally from different points of view by many investigators [8; 9; 10; 11; 12; 13; 14; 15]. [PERSON] [16] studied the damage of T-joints using finite element simulation of a realistic part of the blade structure. The results of fatigue simulation were compared with the experimental results of a previous study [17]. Another type of bonding is the lap bonding used to bind the upper and lower shells in the leading and trailing edges, as shown in Figure 1. The maintenance inspection of wind turbine blades showed that the local debonding of the blade trailing-edge shell is a common failure mode among other types of damages [18]. The cause of trailing edge failure in wind turbine blades is complex, and detailed information on this is limited. [PERSON] [19] explored a new blade simulation method, in which the shell lamination of the blade was simulated by the shell element, while the adhesive of the trailing edge was simulated by the solid element; the shell and solid elements were connected by a multi-point constraint (MPC). [PERSON] and [PERSON] [20] studied a comprehensive method by considering both linear and nonlinear effects and their interaction in the model and test structure. Figure 1: Schematic diagram of blade bonding: (**a**) blade mold in open state; (**b**) blade mold in a closed state; the parts of the blade in the mold were bonded; (**c**) schematic diagram of a blade section, and places of bonding are shown in the-red dotted box. Many achievements have been made in the research of adhesive damage analysis. In 2011, [PERSON] et al. [21] studied the static and fatigue crack growth of thick adhesive joints when used to join fiberglass laminates.