Study On The Fuzzy Fatigue Life Prediction Of Large Wind Turbine Blades

Fatigue failure is one of the main sources of wind turbine blade failure. Owing to the complex structure, severe working conditions, and complicated and unpredictable loads, sufficient fatigue strength of WTB is needed so as to meet the design life requirement. Limited by the budget of WTB production, blade thickness could not be very large. In order to reduce the cost, and also satisfy the design life command, the WTB fatigue should be reasonably analyzed, and the analysis result is regarded as an important basis for the WTB design, manufacture, optimization and reliability assessment.First, the loads of working wind turbine are analyzed, three kinds of loads that have the most powerful impact on the blade fatigue are aerodynamic loads, gravity loads and centrifugal loads, then, the theoretical basis of the traditional fatigue life prediction method is introduced, and the limitation of that method in fatigue life prediction is pointed out, finally, the fuzziness of the WTB fatigue life is analyzed, and the commonly used fuzzy membership functions are provided to lay a theoretical foundation for the WTB fuzzy damage estimation.In the software ANSYS, a finite element analysis software, the WTB model is established. The shell 99 unit is applied to establish model, layers of all blade parts including spar, web, root, leading edge and trailing edge are set by the way of defining and selecting unit real constant, the blade composite material properties such as density, Poisson’s ratio, elastic modulus and shear modulus are defined. Finally, the WTB model is meshed. As thus, the WTB finite element model is obtained. In this paper, the WTB modeling process, as a reference, is provided with ANSYS order flow.The most dangerous position of WTB is identified as the blade root. In the software GH Bladed, a professional software for analyzing the wind turbine performance, the load of airfoil section with 1.5m distance from the root section is simulated under the turbulent wind with different speed. In software ANSYS, The simulative loads are applied to the blade root model, of which the dynamic analysis is completed, and the Von Mises stress-time curve of the dangerous point in the blade root model is extracted. The stress data is statistically analyzed with the rain flow counting method to obtain the stress amplitude and cycle times.The Weibull distribution function is applied to calculate the annual wind distribution. Which is combined with the stress spectrum under turbulent wind at each speed to obtain the annual proportion of von mises stress at each level. Next, the fatigue life of the WTB under stress at all levels is solved by the S-N curve of the glass fiber reinforced plastic. Based on the above data, the fuzzy prediction method is applied to calculate the fatigue damage caused by the low amplitude loading, the normal fatigue damage after hardening effects and the fatigue damage caused by the load interaction effects, finally, the fatigue life of the WTB is obtained, and the estimated life is compared with that calculated based on the Miner’s rule.