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.

Non-proportionality analysis of multiaxial fatigue stress histories in trailing edge adhesive joints of wind turbine rotor blades

“…This approach translates the stress tensor to the scalar-valued global equivalent stress, which is easy to handle in a classical rainflow counting scheme (Endo et al, 1974;Downing and Socie, 1982;Rychlik, 1987;Lee and Tjhung, 2012). However, the transition from an engineering stress space to an equivalent stress space must thoroughly consider potential tension-compression asymmetries (which are present in epoxy-based adhesives), and the method is only valid for proportional stress histories, see e. g. Kuhn et al (2023). In this case, the major principal stress direction is not changing with time, but the amplitude may vary (Stephens et al, 2001).…”

“…Consequently, the orientation of the major principal stress changes as well, which, by definition (e.g., Socie and Marquis, 2000), results in non-proportional stress time series. These cannot be captured by global equivalent stress approaches (Fatemi and Shamsaei, 2011;Kuhn et al, 2023).…”