As the single unit capacity has been increased, the length of wind turbine blade is becoming longer, and the blade vibration fatigue damage caused by impact of wind turbines has become an important issue of wind turbine security. Therefore, modal analysis and study on the impact of crack on the natural frequency of the wind turbine blade are of great significance. The finite element software ANSYS was used to establish a finite element model of a 1.5MW composite wind turbine blade, with a structure of twisted variable cross-section and hollow core in the first place of this paper. Modal analysis of the model established in this paper showed that the blade vibrates in 3 different forms, they are flap within the rotating plane, flutter vibration perpendicularity to the rotating plane and torsional vibration around the blade shaft. Among all the orders, flap and flutter vibration are predominent in low modes, while torsional vibration appears only in high modes (above the fifth order). Then blade models with cracks in the root were established to analyze the regularity of the blade natural frequencies with the crack location, depth and the variation of the angle. The results showed that: as the location of the crack changed in wingspan direction, the change of frequencies showed two basic trends: one was declining gradually; the other was decreasing and then increasing before decreasing again, and the minimum the maximum value appeared at location around 32.5% and 87.5% of the blade root respectively. As crack depth increased gradually, the frequencies reduced continuously, and compared to crack location, influence of crack depth was more prominent. For slant crack, when the crack angle, that is the angle between the crack section chord line and the foliosine plane, increased, all orders of frequencies gradually increased, indicating that the influence of the crack on the blade stiffness decreases as the angle increases.

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