Micromechanics Investigation of Wind Turbine Blade Fatigue Behavior Considering Multi-Axial Loading

The effects of complex stress states are not typically taken into account for determining the fatigue life of thin-walled structures made of composite materials, such as wind turbine blades. Fatigue life predictions, as per state-of-the-art standard design codes, account only for normal stresses, neglecting the contribution of shear stresses and the fact that the failure in the spar-cap region of a wind turbine blade is dominated by axial load. This is due to either misconceptions, a lack of experimental data and theoretical models suitable for design work. Many mechanical tests are required to fully characterize the composite laminates made of various materials and having different layup sequences under many load combinations of in-plane stress tensor components. A micromechanics approach called the Micro Level Approach (MLA) was developed to reduce the number of tests by using fatigue properties of the constituents, i.e. the fiber, the matrix and the interface. This fatigue life prediction methodology of composite laminates can handle multi-axial loading. The MLA is composed of three sub models to account for the stress in the constituents. For the matrix, the equivalent stress model, which is generally used for isotropic materials, was employed to take care of multi-axial fatigue loading. For the fiber, a maximum stress model considering only stress along the fiber direction was used. A critical plane model is introduced for the interface of the fiber and matrix. The modified Goodman approach is then utilized to take into account the mean stress effects. In order to validate the proposed methodology, fatigue tests on three different Glass Fiber Reinforced Plastic (GFRP) laminates, unidirectional transverse (UDT) laminate, bi-axial (BX) laminate, and tri-axial (TX) laminate were performed. The predictions from MLA are compared with the experimental data and it is found that the predictions are in good agreement with the test data. A comprehensive example is also presented for the case of wind turbine rotor blade composite laminates. An initial estimate of the effect of neglecting shear stresses in fatigue life calculations is provided based on the MLA predictions. It was concluded that the life predictions could be reduced by 20 times with respect to situations where only the in-plane axial normal stress component was used in calculations.

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