Evaluating probabilistic fatigue life of trailing-edge adhesive joints in wind turbine blades with emphasis on stress concentration in bi-material corners

Two major types of stresses affect the crack initiation in the trailing-edge (TE) adhesive joint of a rotor blade: thermal residual stresses that develop during manufacture, and mechanical stresses due to operating loads. Although giving consideration solely to the longitudinal stress component, which contributes mainly to the fatigue, is state-of-the-art, the current design guideline encourages designers to also take account of other stress components, i.e., peel and shear stresses. Hence, this research investigates the impact of the multi-axial stress state due to cyclic loading on the tunneling crack initiation in the TE adhesive joint in order to develop an engineering approach for predicting the number of load cycles toward crack initiation. To this end, a two-dimensional finite element model of the TE adhesive joint to approximate the asymptotic stress field at the bi-material corner is introduced. The model takes account of the adhesive layer’s free-edge geometry, as well as the boundary conditions, and the multi-axial thermal and mechanical internal loads in a blade. The validity of the approach was proved in a cyclic full-scale test through the good agreement between the prediction of the approach parametrized with simulated test loads and observations on crack initiation made during the cyclic test. The thermal residual stress at the inner adhesive edge dominated the crack initiation.