## 1 Introduction  
While composite blades are designed to have a design life of over 25 years, various damages have been reported in real composite blades. Among them, it is reported that the debonding damage in the spar cap-shear web joint is the most frequently occurring and dangerous damage mode. To solve these problems, a methodology has been developed to explain the initiation of the debonding effect as a source of major damage in blades and its growth due to loading in the adhesive joint area, and the corresponding mechanism for this methodology has been verified [1]. Wind turbine blades are designed by laminating unidirectional, biaxial, and triaxial composite materials in a hybrid form to improve structural performance and reduce weight. This improves the load resistance of the upper and lower skins directly under the load and the shear webs supporting them. This hybrid form utilizes composite materials with different characteristics to offset the shortcomings of each lamination composite material; therefore, the upper and lower layers vary in properties [2,3]. The mechanical properties evaluated for each composite material are considered in this type of design; however, the failure characteristics for the hybrid composite materials may differ from the performance of a single material [4]. In the existing research trend on the initiation and growth of debonding damage, [PERSON] et al. [5] conducted specimen-level tests, crack detection, and theoretical analyses for the adhesive joint of the trailing edge (TE). [PERSON] et al. [6] and [PERSON] et al. [7] proposed methods for detecting and testing the interlaminar debonding and delamination damage of wind turbine blades. [PERSON] et al. [8] conducted a debonding analysis on the T-joints of wind turbine blades, and [PERSON] et al. [9] developed a fracture mechanics approach for the failure of the joints of wind turbine blades. In this research, the test data for Mode II, that is, the sliding mode, were acquired, and the CZM method of the spar-web joint area was used to propose a methodology for the initiation and growth of the debonding damage in wind turbine blades. However, in addition to the spar cap-shear web area, the adhesive joint of blades encompasses the TE and leading edge (LE) joints where the suction and pressure sides are bonded. This TE/LE adhesive joint is treated as the most vulnerable structural part of wind turbine blades [10; 11; 12; 13]. Therefore, to study the initiation and growth of debonding damage to blade joints, it is crucial to acquire test data for Mode I (DCB), Mode II (ENF), and Mixed-mode (MMB) and verify the material properties. As debonding damage in the adhesive joint of blades initiates and grows with the externally applied load, the forces acting on the wind turbine must be reviewed. Particularly, if the wind blade is operated under various environmental conditions, the load applied to the blade leads to combined load conditions. These combined loads have a significant adverse effect on the structural integrity of blades compared to the simple load condition in which only a unidirectional load is applied [14]. Therefore, the debonding damage in the adhesive joints of wind blades should be analyzed under combined load conditions. In this study, a model was proposed for predicting the initiation/growth of debonding damage in the adhesive joint area of wind turbine composite blades. To this end, a blade model using the CZM method was constructed for the adhesive joint area, including the TE joint, which is the most vulnerable part of the NREL 5 MW-class composite blade. In addition, Mixed-mode tests and analyses were performed in accordance with ASTM D6671 [15] based on the fracture toughness calculated through the ASTM D5528 (Mode I) test [16] and D7905 (Mode II) tests [17; 18] in a previous study [19]. Moreover, the combined load calculated in the integrated load analysis [14] was applied to the blade model to simulate the effect of the combined load borne by the blade.