## 3 Results and Discussion
### Analysis of Stress Characteristics
#### 3.2.2 Analysis of Stress in the Wingspan Direction  
As shown in Figure 13, the radius of the blade is \(R\), the chord length of the aerofoil section is \(C\), and the distance from the center of rotation to a certain aerofoil is \(r_{i}\). The dimensionless chord length positions of \(x/C\) = 0.10, 0.25, 0.30, and 0.90 are the leading-edge position of the blade, the position of the aerodynamic center line, the position of the highest  
Figure 11: Stress distributions of wind turbine blades on the windward side of the wind turbine (unit: MPa). Figure 12: Stress distributions of wind turbine blades on the leeward side of the wind turbine (unit: MPa). point of the blade back [46], and the position of the trailing edge of the blade, respectively, where \(x\) is the distance from the aerofoil section to the leading-edge point. Figure 14 shows the 3D stress surfaces of a blade leading edge, aerodynamic center line, the highest point on the blade back, and the blade trailing edge along the spanwise stress with time and yaw angle in one cycle. Near the root of the blade, obvious fluctuations first decreased, then increased, then decreased, then increased, then increased. With changes in the yaw angle, the stress in the blade tip area's outward direction did not change noticeably. From the middle to the tip, the force on the blade rapidly dropped after initially increasing and then decreasing along the spanwise direction. The shape of the blade was distorted as the cross-sectional dimensions expanded outward with different twist degrees along the spanwise direction, and the aerofoil's interface size and stiffness steadily decreased along the spanwise direction. Figure 15 shows curves of the stress change along the spanwise direction under \(\lambda=5.0\). In one deflection cycle of the wind turbine, the stress-concentration region of the three blades was near the middle part of the blades at \(r/R\in(0.38,\,0.59)\). Under the influence of a yaw state, the stress on a blade from the root to the middle fluctuated markedly with increases and decreases in the yaw angle. In contrast to the stress near the tip, which was less impacted by the yaw condition, the stress varied slightly from the middle to the tip with the yaw angle. At the position of the blade's maximum chord length \(r/R=0.18\), the stress on the aerodynamic center line was not substantially different from that on the middle position of the blade, while the stresses on the other three positions were all lower than that on the middle position. The stress at the blade's windward surface trailing edge fluctuated with the yaw angle, while that at the other three changed only slightly. The greatest stress values of the three blades were determined from the location of the aerodynamic center line by examining the scale stress value of the time-domain diagram of the stress surface. Therefore, the position of the maximum stress value in the middle of the aerodynamic center line (\(r/R=0.49\)) was selected and the stress changes of the three blades within one cycle were analyzed, as shown in Figure 14. As shown in Figure 16, during a yaw cycle, the stress of the three blades showed a cosine fluctuation, while the stress value of blade 3 was greater during negative yaw, which increased by 2.22%, and the minimum value of blade 1 occurred at a negative yaw of \(\theta=-30^{\circ}\), which decreased by 13.85%. The stress in the wind turbine blades demonstrated a cosine distribution with regard to yaw fluctuation, as per the analysis above. In a single yaw period, blade 1 showed a large range of stress fluctuations during negative yaw angle deflection, with the minimum stress occurring at a yaw angle of \(-30^{\circ}\). Blade 2 showed a gentle fluctuation in overall stress, and blade 3 had greater stress fluctuations at positive yaw angles, with the minimum occurring at a positive yaw of \(\theta=+30^{\circ}\).