Stress scale factor and critical plane models under multiaxial proportional loading histories

It has been experimentally proven that the shear stress level needed to cause fatigue failure is lower than the axial one. This fact has led to consider a Stress Scale Factor (SSF) between shear and axial stress to reduce different applied stresses to the same shear stress space or principal stress space, consequently facilitating the yielding analysis or fatigue damage evaluations. Most of multiaxial fatigue models use an SSF, and materials can be classified as shear sensitive (low SSF values) or tensile sensitive (large SSF values), depending on the main fatigue microcrack initiation process under multiaxial loadings. The use of SSF is quite common in many multiaxial fatigue criteria based on the critical plane approach. Such criteria adopt a SSF value assumed constant for a given material, sometimes varying with the fatigue life (in cycles) but not with the SAR (Stress Amplitude Ratio), the stress amplitude level, or the loading path shape. In this work, in-phase proportional tension-torsion tests related to 42CrMo4 steel specimens for several values of SAR are presented. The SSF approach is then compared with critical-plane models, based on their predicted fatigue lives and the observed ones for the studied tension-torsion histories.