L ucas Carneiro Araujo

University of Brasília, Brasilia, Brazil



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  parameter with values associated with the principal stresses generated by the loads. These values are the amplitude of principal stresses and the maximum principal stress observed. For the new model, an 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, because not only the value but also the direction of the principal stresses varies instant to instant. 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, such as traction-compression, torsion and combined loadings with different ratio between shear and normal stresses amplitudes, in-phase and 90° out of phase. In addition, specimens were tested in two conditions: (i) smooth specimens, considering the effect of non-metallic inclusions, and (ii) specimens where a superficial micro hole with a straight bottom was machined with 550 μm in diameter and depth. 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 15%.


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