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

Abstract

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.

Characterization of a short fiber-reinforced adhesive designed for wind turbine rotor blades

“…On the contrary, a high porosity content decreases the strength substantially in the hand-mixed specimens, i.e., those specimens suffer from effects of defects. Another reason for the high tensile strength may be a very distinct fiber orientation in longitudinal direction in the machine-mixed specimens, which was identified by high-resolution micro-CT scanning [22]. In the hand-mixed specimens, the voids may have affected the fiber orientation.…”

“…For more details on the test setup, the S-N curve models, the experimental results, an engineering approach to very high cycle fatigue, the stiffness degradation model, a comparison with residual strength tests, and a run-out fatigue life estimation, the reader is referred to [21]. Results for multiaxial and non-proportional fatigue tests have recently been published [22].…”

“…However, available specimen designs from literature [16,17] had to be modified and the manufacturing processes had to be optimized in order to obtain reliable test results [19]. A very extensive test campaign was carried out including static and fatigue tests and both uni-and biaxial loadings to obtain static strengths and stiffnesses, fatigue strengths, S-N curves, and the fatigue-related stiffness degradation behavior [19,20,21,22]. An excerpt of the respective test results is presented in this paper.…”