Effects of non-proportionality and tension-compression asymmetry on the fatigue life prediction of equivalent stress criteria

Biaxial tension/compression-torsion fatigue tests with varying levels of non-proportionality were performed employing a structural adhesive designed for wind turbine rotor blades. The cycles to failure were found to be independent of the level of non-proportionality. It is demonstrated that numerical fatigue life predictions via rainflow-counted equivalent stress histories are not able to replicate these experimental observations and overestimate the fatigue life up to a hundredfold. The tension-compression asymmetry of the adhesive resulted in significant damage prediction differences depending on the stress space representation of the Haigh diagram. If not properly taken care of, the asymmetry will also lead to non-conservative results. While demonstrated with a short fiber-reinforced adhesive, the results can be transferred to other materials.