The effect of load phase angle on wind turbine blade fatigue damage

This paper examines the importance of load phase angle variations on fatigue damage and evaluates the potential effects of varying the load phase angle during dual-axis constant amplitude fatigue testing. The scope of this paper is limited to results from simulated wind and dynamic loads. The operating loads on a generic three bladed upwind 1.5 MW wind turbine blade were analyzed over a range of operating conditions, and an aggregate probability distribution for the actual phase angles between the peak in-plane (lead-lag) and peak out-of-plane (flap) loads was determined. Using a finite element model (FEM) of the 1.5 MW blade and Miner's Rule [Miner A., 1945, "Cumulative Damage in Fatigue," Trans. ASME, 67], the accumulated theoretical fatigue damage (based on axial strains) resulting from a fatigue test with variable phase angles using the aggregate distribution was compared to the damage resulting from a fatigue test with a constant phase angle. The FEM nodal damage distribution at specific blade cross sections were compared for the constant and variable phase angle cases. Single-node stress concentrations were distributed arbitrarily around one cross section to simulate material defects in a blade undergoing testing. Results show that the variable phase angle case results in higher damage on the critical nodes. In addition, the probability of discovering a material defect during a test was substantially increased when variable phase loading was used. The effect of phase angle sequence on the damage accumulation was also considered. For this analysis, the finite element results were processed using a nonlinear damage accumulation model. Results show that the sequence of the phase angle can have a large effect on the fatigue damage, and multiple, shorter length sequences produce higher damage than a single, long term sequence.