A numerical study on damage characteristics in composite tapered laminates under cyclic loading with different stress ratios

The composite fan blade requires increased thickness near the root and often is designed with tapered sections to carry the load, but they can introduce local material and geometric discontinuities, resulting in large interlayer stresses and also being the site of damage initiation. Fatigue damage mechanisms of modestly tapered laminates, designed by composite laminate design approach, were investigated by high-fidelity finite element modelling and a constitutive equation within the cyclic cohesive interface model approach. Static damage and weak-link index assessment were proposed to predict the location of fatigue damage based on the composite constant life diagram model. The findings show the sites of maximum damage index can be recognized as the ply interface of ply to ply in continuous cohesive element plies, and as the location of resin pocket in dropped cohesive element plies. The maximum scaling factor is only the direction of S13 with different steady stress, which indicates interlaminar shear stress along the longitudinal orientations reaches its fatigue failure first, and also explains the reason why interlaminar failure is easier to break. The high-cycles fatigue site of damage initiation is different with different stress ratios by the weak-link index assessment approach, and it is different from the site of static damage. Fatigue crack growth rates are also quite sensitive at the first natural frequency under cyclic loading and the weak-link zone appears in a crack growth area.