Fatigue properties of a structural rotor blade adhesive under axial and torsional loading

Axial and torsional fatigue tests at different stress ratios were performed on a structural adhesive designed for wind turbine rotor blades. By employing previously optimized specimens, fatigue properties were recorded without influences of manufacturing-induced defects such as pores. The Stussi S-N model was an excellent fit to the data and was combined with a Haibach extension line to account for uncertainties in the gigacycle fatigue regime. A comparison of the results with hand-mixed specimens revealed significant and load level-dependent differences, indicating that manufacturing safety factors should be applied to the slope of the S-N curve. The experiments were accompanied by stiffness degradation measurements, which enabled an analysis of Young's and shear modulus degradation interactions. The degradation was modeled using power law fits, which incorporated load level-dependent fitting parameters to allow for a full description of the stiffness reduction and a prediction of the residual fatigue life of run-out specimens.