Modern wind turbines, which are usually made of composite materials, are fatigue critical structures that are subjected to variable multi-axial fatigue loading. Therefore, they should be designed as safely as necessary to withstand the fatigue loads over the designed life time. Path-Dependent Maximum Range (PDMR) is a multi-axial fatigue life assessment tool developed by Battelle researchers. PDMR has been successfully applied to fatigue analysis of isotropic structures under general variable amplitude, multi-axial fatigue loading histories. The effectiveness of the PDMR method has been validated by its ability to correlate a large amount of fatigue data available in the literature. For uniaxial loading data, PDMR gives exactly the same results as ASTM standard Rainflow cycle counting method. In this paper, the PDMR method is extended to composite materials, such as glass fiber reinforced plastics (GFRP) and carbon fiber reinforced plastics (CFRP). The proposed multi-axial fatigue damage model effectively correlates fatigue lives of unidirectional composites for various off-axis ply angles under cyclic tensile loading. With this extended capability, the PDMR can now be used to assess the multi-axial fatigue life of composite structures used in wind energy industry.

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