Structural Study and Parametric Analysis on Fatigue Damage of a Composite Rotor Blade

Fatigue damage calculations are conducted as state-of-art design process for wind turbines. Fatigue analysis is performed on spar caps and trailing edge tape (UD material) considering uni-axial stress state. The main objective of the research project is to analyze the effect of various parameters like yaw errors, wind turbine design load cases and blade mass imbalances on fatigue damage of blade. Also, an overall structural study of the blade has been performed based on the guidelines of International Electro-technical Commission (IEC) 61400-1 Ed. 3 to understand the structural robustness of blade in terms of stiffness and strength. Loads based on an onshore site have been generated using aero-(servo)-elastic code, Fatigue Aerodynamics Structural and Turbulence (FAST). Based on these loads and predominantly linear Finite Element Model (FEM) calculations on a blade model in ANSYS (except for flapwise loading on spar caps), stress time series are calculated. Finally, using these stress time series, the fatigue damage at different blade sections are calculated in ’Octave’ environment using rain-flow cycle counting method and Miner’s rule. Results show that blade is stiff enough from blade-tower interference and resonance per- spective, but buckling is observed in the trailing edge close to the tip, with flapwise buckling mode coming out as the most critical one. Fatigue is not a structural issue based on state-of- art calculations and design load cases, even in the critical transition region (from circular to DU airfoil along the span of the blade). It was found that positive yaw errors are more detri- mental to fatigue life than negative ones for an anti-clockwise rotating turbine. Trailing edge tape is more affected by blade mass imbalance while spar caps are sensitive to changes in yaw errors in terms of fatigue life. Power production design load case is completely dominating fatigue life when compared to start up and shutdown load cases. All the results that have been presented here are based on a particular wind site conditions, a specific blade and a wind turbine model. The idea has been to give a qualitative and relative overview of different trends in the structural analysis, rather than coming to a conclusion in an absolute sense. Due to the presence of considerable cone angle, shaft tilt and rotor overhang, blade-tower interference is not an issue for the analyzed blade design. It should be, however, considered to increase the trailing edge tape length towards the tip region to prevent buckling issues. Also, extra core materials could be provided (thus increasing bending stiffness and thickness) to prevent buckling problems. Operation of turbine under other significant loading conditions like icing and heavy leading edge erosion should be incorporated to study fatigue in more detail. Fatigue life should be evaluated and validated with models other than Miner’s sum (for example, residual strength degradation model) along with full blade test (for an existent blade) for precise results. An aero-elastic stability analysis of full wind turbine model should be performed to understand the dynamics of whole system on blades. Finally, fatigue damage in bond lines between different components of blade is also a critical consideration for future work.Sustainable Process and Energy TechnologyProcess and EnergyMechanical, Maritime and Materials Engineerin