New approach to evaluate non-proportionality in multiaxial loading conditions

Abstract

The shear stress amplitude is one of the most important parameters in the formulation of many multiaxial fatigue models proposed in literature. The shear stress amplitude is usually evaluated by the longest projection or the minimum circumscribed circle approach in a shear stress space. Further, the von Mises and Tresca stress spaces are the most used ones under multiaxial loading conditions, where the damage ratio between axial and shear damages is a constant value: 0.577 for von Mises and 0.5 for Tresca. However, the damage scale in each stress space's axes may vary significantly depending on materials' type. Therefore, evaluating proportional and non-proportional damages using conventional stress spaces (von Mises or Tresca) may lead to erroneous interpretations. In this work, systematic fatigue experiments under proportional and non-proportional loading conditions are presented for three structural steels: Ck45, AISI303 and the 42CrMo4. To quantify the relative damage between proportional and non-proportional loading paths, a material's non-proportional sensitivity parameter (Y parameter), determined based on materials experimental tests, is proposed. From this research, it can be drawn that the appropriate axial/shear damage relation should be used to estimate multiaxial fatigue life.