## 2 Experimental and Numerical Failure Analysis of Adhesive Joints
### Experimental Test Procedure  
The fatigue crack growth behavior in wind turbine blade adhesive joints is influenced by changes in fracture energy rate and crack growth rate [25]. Quantitative analysis at the specimen level is essential to understand the crack growth behavior and failure process. For analyzing crack growth characteristics under fatigue loading in mode I, paralleling static test conditions, test parameters were selected in line with ASTM E647 [26]. The DCB specimen, used in interlaminar fracture toughness tests, was employed. Fatigue loading was applied at 1 Hz using an Instron fatigue tester, as illustrated in Figure 2. To facilitate low-load fatigue crack growth testing, a 5 kN capacity load cell was integrated with the tester. A data acquisition (DAQ) device sets the crack measurement intervals and synchronizes measurement times. Real-time crack length analysis and measurement were performed using Mercury software v2.9, a 75 mm lens, and a 5 MPx camera [27]. The test results were evaluated through the relationship between crack growth rate and fracture energy rate across three regions. For the analytical model, the material constant was derived based on the correlation between fracture mechanics parameters--energy release rate and fatigue crack growth rate.  
The specimen for testing failure to determine the interlaminar fracture toughness of the laminated biaxial/triaxial hybrid composite was designed and fabricated similarly, as  
Figure 2: Experimental equipment of fatigue crack growth test.  
shown in Figure 3, using a mode I DCB specimen. The specimen, composed of a 2-axis glass fiber reinforced composite material, measured 175 mm \(\times\) 25 mm, featuring 3 plies with a total thickness of 1.68 mm (0.56 mm/ply). Additionally, a 3-axis composite material was laminated with 2 plies, totaling 1.82 mm in thickness (0.91 mm/ply). A 0.2 mm thick Teflon film, inserted between the biaxial and triaxial laminations, initiated cracks while adhesive securing the remaining parts. The initial crack length (\(a_{0}\)) was set to 50 mm. A self-manufactured aluminum hinge was attached to facilitate connection with the load cell of the tester during the experiment.  
Test conditions were selected to be displacement-controlled, aligning with the methodology of the Wind Turbine Materials and Constructions Research Center [25]. The evaluation was based on the minimum-to-maximum displacement ratio, setting the maximum displacement (\(\delta_{max}\)) at 18 mm, a critical value \(\delta_{Cmax}\) determined from the static test's critical fracture toughness. Minimum displacements (\(\delta_{min}\)) were established at 1.8 mm, 5.4 mm, and 9 mm under three reference load conditions (R values) of 0.1, 0.3, and 0.5 to examine the impact of loading displacement on crack growth characteristics. Accordingly, crack growth rate was analyzed with respect to cycle count (N), crack growth length (a), N-fracture energy rate (G), and the _da/dN-G_ relationship across three representative regions. Notably, the crack growth rate is significantly affected by the loading force. Under varying load conditions, the fracture energy rate either accelerates or decelerates crack propagation compared to a constant loading scenario, influenced by the sequence of loading [27].