## 1 Introduction  
With a growing number of composite wind turbine blades now in service, blade maintenance is becoming a major issue [1]. As the service year increases, the hidden defects gradually show up [2, 3]. Being affected by environmental corrosion, manufacturing defects, insufficient design, aging of materials, and overload [4, 5], the hidden defects eventually appear as different failure modes [6] (such as fiber fracture, matrix failure, delamination, etc.) and damage types [7] (such as adhesive joint failure between skins, sandwich panel face/core debonding, gelcoat cracks, etc.). Through the process of assessing the extent of the damage, analyzing the damage stress state, designing a repair scheme, and timely maintenance, most of the structural damage of wind turbine blades in service (such as trailing edge cracking, leading edge corrosion, shell crack, etc.) can be effectively repaired [8]. However, when the damage occurs in the key load-carrying structures, such as the spar cap and root connection zone, the DNV GL (Det Norske Veritas Germanischer Lloyd) international certification standard for wind turbines clearly state it is necessary to construct a rational test model that can represent the real damage zone and preliminary repair scheme to evaluate the scheme before repair takes place [9]. Based on the model test results, the final repair scheme is developed. The spar cap is a pure unidirectional tape with a large aspect ratio structure. It is overlapped by a unidirectional cloth. The length is basically the same as the blade length, which is much larger than the width and thickness of the spar cap [10, 11, 12]. The thickness of the spar cap varies along the blade length; generally, the thickest zone (over almost 50 layers) is always from the maximum chord to 1/2 of the blade length, which is also the most important load-carrying zone of a spar cap. In this thick layer zone, it is easy to produce in-plane wrinkles and out-plane wrinkles during the blade manufacturing, which significantly reduces the local strength of the spar cap [13, 14]. Under the alternating action of the ultimate load and fatigue load, the coupling response of delamination and buckling may easily occur in this zone [15, 16, 17, 18, 19, 20]. Once the damage is macroscopic (Figure 1a), it may have already become very serious, and the repair area and the repair depth of the spar cap will be large (Figure 1b). Moreover, if the repair scheme is not rational and there is no effective health monitoring method after the repair, it will increase the operational risk. Under the conditions of strong wind speed and large changes in wind direction, it is easy to create new cracks near the repaired area (Figure 1c), which could cause serious accidents and economic losses (Figure 1d) [21]. Therefore, the damage of the spar cap (such as delamination or wrinkles) is very complicated and a difficult aspect of blade repair. According to the parent structure and patch configuration, composite material repair methods mainly include the patch and scarf methods [22], as shown in Figure 2a,b. The patch method fixes the external patch by bonding or co-curing outside the damage to restore the serviceability of the structure [23]. This method is simple and convenient, which is mostly used for structures with a thin plate thickness. The scarf method is an adhesively bonded repair method that requires the removal of the defects or damage zone and replacing them with new materials. There is no eccentric load in the scarf method, and the peeling stress in the adhesive layer is small, so the repair efficiency is high and the application range is wide [24]. It has many advantages for repairing surfaces with a large curvature or aerodynamic shape requirements [25, 26]. The scarf method is mainly divided into a tapered scarf and a stepped-lap scarf, as shown in Figure 2c,d. These two repair methods have completely different transmission paths and stress distributions due to the different configurations of the adhesive joint surface.