## 3 Results  
### Fracture Toughness for Mode I, Mode II, and Mixed-Mode Tests  
Through the Mode I test, the load value was obtained according to the displacement at the loading point, as shown in Figure 7. For the overall crack growth behavior encompassing this load and the critical crack point, the fracture toughness was calculated based \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **Load Case** & & **Fx [kN]** & **Fy [kN]** & **Fz [kN]** & **Mx [kNm]** & **My [kNm]** & **Mz [kNm]** \\ \hline \multirow{2}{*}{Fx} & **Max** & **dlc1.2k4** & 542.2 & \(-154.0\) & \(857.0\) & \(13,664\) & \(2784.7\) & \(-385.1\) \\ \cline{2-8}  & **Min** & **dlc6.4b1** & \(-345.5\) & \(-51.5\) & \(-125.8\) & \(-7659.2\) & \(1339.5\) & \(127.3\) \\ \hline \multirow{2}{*}{Fy} & **Max** & **dlc1.2k5** & \(138.0\) & \(346.1\) & \(894.8\) & \(4482.3\) & \(-8479.7\) & \(-200.6\) \\ \cline{2-8}  & **Min** & **dlc1.2k5** & \(193.6\) & \(-270.5\) & \(826.3\) & \(1962.2\) & \(7784.2\) & \(-147.6\) \\ \hline \multirow{2}{*}{Fz} & **Max** & **dlc1.2k5** & \(112.6\) & \(64.5\) & \(1207.9\) & \(1622.7\) & \(-1579.8\) & \(-193.3\) \\ \cline{2-8}  & **Min** & **dlc6.4b3** & \(-137.7\) & \(-6.10\) & \(-236.5\) & \(-4069.6\) & \(684.0\) & \(80.0\) \\ \hline \multirow{2}{*}{Mx} & **Max** & **dlc1.2f4** & \(499.9\) & \(-46.6\) & \(614.9\) & \(18,051\) & \(635.5\) & \(-414.9\) \\ \cline{2-8}  & **Min** & **dlc6.4b3** & \(-297.5\) & \(-59.2\) & \(-203.7\) & \(-8005.4\) & \(2009.3\) & \(151.5\) \\ \hline \multirow{2}{*}{My} & **Max** & **dlc1.2k5** & \(193.6\) & \(-270.5\) & \(826.3\) & \(1962.2\) & \(7784.2\) & \(-147.6\) \\ \cline{2-8}  & **Min** & **dlc1.2k5** & \(138.0\) & \(346.1\) & \(894.8\) & \(4482.3\) & \(-8479.7\) & \(-200.6\) \\ \hline \multirow{2}{*}{Mz} & **Max** & **dlc6.4b5** & \(-211.7\) & \(-39.5\) & \(-233.5\) & \(-6822.9\) & \(2050.6\) & \(193.1\) \\ \cline{2-8}  & **Min** & **dlc1.2f3** & \(535.6\) & \(-204.3\) & \(838.8\) & \(17440\) & \(4034.7\) & \(-472.5\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Ultimate loads at the blade root [14]. Figure 6: Boundary and load conditions in finite element analysis. on Equation (3). Particularly in the crack length range of 50-53 mm, because the test is in the pre-crack (preliminary crack) state of producing a uniform initial crack tip, nonlinearity in the fracture toughness occurs with the crack growth length as the load increases. \[G_{IC}=\frac{3P\delta}{2 ba} \tag{3}\]  
where \(P\) is the load, \(\delta\) is the displacement, \(b\) is the width of specimens, \(a\) is the crack length, and \(G_{IC}\) is the critical fracture toughness of Mode I. The results of the Mode II test based on ASTM D7905 are shown in Figure 8, and it was confirmed that the cracks grew from a load of 400 N on average. The fracture toughness for Mode II was obtained based on Equation (4). \[G_{IIC}=\frac{9a^{2}P\delta}{2b(2L^{3}+3a^{3})} \tag{4}\]  
Figure 8: Fracture toughness test results of Mode II: (a) \(P-\delta\) curve and (b) \(G-a\) curve. Figure 7: Fracture toughness test results of Mode I: (a) \(P-\delta\) curve and (b) \(G-a\) curve. In addition, the deviation of the fracture toughness in Mode II was confirmed to be greater than that of Mode I. This phenomenon is attributed to the fiber bridging effect, which causes a high resistance to crack growth on the surface of the specimen. Furthermore, after the initial crack growth, the load and fracture toughness continued to increase at the crack length of 45 mm; this is believed to be the reason for the drastically slower crack growth rate as the crack growth point passed through the loading point, which reduced the shear directional load relative to the applied load. The Mixed-mode test and fracture toughness calculation were performed in accordance with ASTM D6671. To select the location of the load for the Mixed-mode calculation, the non-dimensional crack length correction material constant was calculated using Equations (5)-(7) based on the elastic modulus of the material.