Abstract
Thickness transitions in load carrying elements lead to improved geometries and efficient material utilization. However, these transitions may introduce localized areas with high stress concentrations and may act as crack initiators that could potentially cause delamination and further catastrophic failure of an entire blade structure. The local strength degradation under an ultimate static loading, subsequent to several years of fatigue, is predicted for an offshore wind turbine blade. Fatigue failure indexes of different damage modes are calculated using a sub-modeling approach. Multi axial stresses are accounted for using a developed failure criterion with residual strengths instead of the virgin strengths. Damage initiation is predicted by including available Wohler curve data of E-Glass fabrics and epoxy matrix into multi-axial fatigue failure criteria. As a result of this study, proper knock-down factors for ply-drop effects in wind turbine blades under multi-axial static and fatigue loadings can be obtained.
Highlights
Damage tolerant design of composites attracts attention of industry, owing to significantly reducing the maintenance costs in remote structures
Damage initiation is predicted by including available Wohler curve data of E-Glass fabrics and epoxy matrix into multi-axial fatigue failure criteria
Ply-drop configurations have been modelled on two extreme stress concentration positions throughout the cap sections on pressure and suction sides of the reference wind turbine blade
Summary
Damage tolerant design of composites attracts attention of industry, owing to significantly reducing the maintenance costs in remote structures. Offshore wind turbine blades are designed to fulfil high endurance limits with minimum repair requirements+. Improving the effective life-time of the blades requires special considerations in areas, where continuous degradation of material characteristics due to high loading cycle is inevitable. Thickness transitions (ply-drops) are critical failure areas, because of local stress concentrations. Failure at ply-drops is largely dominated by interlaminar and transverse shear effects in the matrix [1, 2]. The process of failure can lead to ply delamination at relatively low applied strains under fatigue loading [3]
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