Assessing Fatigue in Materials with Small Defects: A New Multiaxial Model Based on Principal Stress Amplitudes

A new multiaxial fatigue model for materials containing small defects proposed by the authors is presented, which one can be considered as a modification of the SWT model. This new model relates the fatigue limit of the material obtained from the √area parameter with values associated with the principal stresses. These values are the amplitude of the principal stresses and the maximum principal stress observed. The amplitude value associated with the principal stresses is defined by using the Maximum Variance Method. This amplitude only can be easily obtained under uniaxial loading conditions, but its calculation for torsion, proportional and non-proportional multiaxial loadings is not trivial. Therefore, the calculation of the principal stress amplitude posed a challenge not yet addressed by other authors. The multiaxial fatigue model was evaluated with experimental data from AISI 4140 steel, with several different loading conditions, including uniaxial loading, combined loads in in-phase and out-of-phase configurations. In addition, two types of specimens were used: smooth cylindrical specimens and specimens with a surface micro hole. Comparing the experimental data with the prediction of the new model it was observed that the predictions are slightly conservative with average error not exceeding 6%.