## 3 Results and Discussion
### Discussion of FE Analysis  
#### 3.2.1 Mode I Test  
In Figure 12a, we can observe the crack growth behavior characterized by a maximum load of 45 N and a minimum load of 5 N. Furthermore, as we adjust the displacement conditions to 0.3 and 0.5, we note that the minimum load increases to 9 N (Figure 12b) and 14 N (Figure 12c), respectively. This shift in behavior is consistent with loading in the high displacement range. However, it is important to acknowledge that our test data were collected solely at the point of maximum displacement, necessitating confirmation through subsequent analysis. Additionally, we conducted reliability verification, which exhibited behavior similar to the test results, albeit with generally lower load levels. This discrepancy is attributed to the lower critical load for fatigue crack growth tests, influenced by both the dynamic load of the testing apparatus and environmental factors. Importantly, these variations have minimal impact on the properties of the blade unit. The material characteristics within the three regions analyzed at the specimen unit are closely linked to the slope, which is evident in the fatigue crack growth test data. Consequently, our analysis is grounded in the critical interlaminar fracture toughness values derived from mode I, mode II, and mixed mode tests conducted under static conditions. These values are then integrated into the XFEM within our finite element model. Furthermore, following the insights of [PERSON] et al. [31], fatigue crack growth characteristics typically align with the conditions \(G_{th}/G_{c}=0.01\) and \(G_{pl}/G_{c}=0.85\). Notably, the employed \(G_{pl}\) in our study exceeds the \(G_{c}\), suggesting the rapid occurrence of unstable crack growth behavior. To establish a stabilized material factor parameter, we selected a value of 0.95 kJ/m\({}^{2}\) to ensure compliance with \(G_{pl}/G_{c}=0.85\). Additionally, to meet the requirements of \(G_{th}/G_{c}=0.01\), it is imperative to extend the testing cycle to minimize crack occurrences as much as possible. To achieve this, we selected a value of 0.011 kJ/m\({}^{2}\) in accordance with the criteria proposed by [PERSON] et al. Based on this selection, we conducted an analysis of stress propagation geometry and changes in principal stress within the glass fiber for \(R=0.1\), 0.3, and 0.5, as depicted in Figures 13-15. Specifically, our observations indicated a behavior similar to the evolution of mode I. In Figure 12, at \(R=0.1\), we noted a maximum in-plane principal stress of 147.3 MPa occurring within the first 11,274 cycles, with an average Von Mises adhesive stress of 144.3 MPa. After 159,020 cycles, the principal stress reduced to 101.7 MPa, and the adhesive's average stress decreased to 92.64 MPa, signifying a gradual stress reduction. Similarly, at \(R=0.3\), the principal stress was 119.6 MPa with an average adhesive stress of 108.9 MPa within 11,284 cycles, indicating lower stress levels compared to \(R=0.1\). Consequently, we observed that the crack growth rate was influenced by \(R\), with the crack growth behavior slowing down as stress gradually decreased. Notably, there  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{Test} & \multirow{2}{*}{\(R\)} & \multicolumn{4}{c}{Fracture Energy Rate [ KJ/m\({}^{2}\)]} \\ \cline{3-6}  & & \((\delta_{min}/\delta_{max})\) & \(G_{c}\) & \(G_{pl}\) & \(G_{th}\) \\ \hline
1 & 0.1 & 1.12 & 1.18 & 0.12 \\ \hline
2 & 0.3 & 1.12 & 1.13 & 0.13 \\ \hline
3 & 0.5 & 1.12 & 1.14 &  
was a notable concentration of stress at the initial crack, followed by a sharp reduction in stress after approximately 100,000 cycles. Our analysis demonstrated that the analytical model exhibits a similar phenomenon when the fatigue crack growth rate per cycle (\(da/dN\)) analyzed in the previous test approaches the range of \(10^{-4}\) mm/cycle to \(10^{-5}\) mm/cycle. As a result, we validated the reliability of XFEM-VCCT in simulating crack growth behavior when the initial crack occurs in the adhesive joint within the wind blade unit model.