Rotating Fatigue in transmission shafts and splines

The fatigue performance under service load of transmission shafts and spline couplings significantly influences their reliability. They are typically loaded by multiaxial stress states owing to the superposition of torsion loading and rotating bending, that make fatigue lifetime prediction challenging. This thesis was motivated by an industrial issue faced in practice reported by Dumarey Automotive Italia S.p.A., part of the international Dumarey Automotive Group, where commercial fatigue software Fe-Safe predicted anomalously low safety against fatigue in the middle of a spline groove. In order to decide whether such an anomaly is caused by a physical phenomenon or by numerical artefacts within invariant-based formulations (e.g., Von Mises), a postprocessing tool independent from the Fe-Safe tool was developed in Python. Stress tensors were extracted from Abaqus ODB files at three angular positions and rebuilt as complete cyclic stress histories by sinusoidal fitting. This allowed each stress tensor component to be decomposed into mean and alternating components, enabling the use of fatigue criteria. The simple uniaxial methods (Von Mises with Goodman fro mean stress correction) and the more advanced multiaxial approaches (Findley, Matake, Brown–Miller, Sines, Crossland) were both implemented and applied to some nodes of interest. Six important nodes were examined as follows: (i) tooth root, (ii) shaft fillet, (iii) groove centre at first; Then, in order to focus the attention on the specific area of the groove centre, another node in the groove centre and its two neighbours (iv–vi), which respectively experience tensile and compressive stresses around the groove, were added to this study. Even though all six nodes were examined systematically following the same procedure, particular emphasis was placed on the three nodes closer to the groove, where the anomaly was first noticed. The study is based on a confidential case study of Dumarey. Owing to proprietary data protection, the geometry is here examined in only a sub-region. In addition, a standard but simplified case study model was made during the course of the thesis, in accordance with the main geometrical and material properties of the original shaft, but left aside for future studies. In order to facilitate direct comparison between Fe-Safe and Python framework during test conditions, Dumarey also provided three representative loading blocks, condensed forms of the hundreds of cycles in real application. Their collective damage was summation according to Miner's rule so that the overall number of revolutions at the input shaft is equal to the real case study. The ultimate objective is two-fold: 1.??To compare Fe-Safe predicted mean and alternating stress components with the reconstructed ones through Python for each reduced load block, and 2.??To compare the fatigue safety factor obtained with Miner's rule, both in Fe-Safe and in Python, to verify if the suspected anomaly depends on a numerical issue in the commercial solver.