PSI - Issue 42

Denny Knabner et al. / Procedia Structural Integrity 42 (2022) 561–569 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The Böhme equivalent stress can be calculated with the profile of the stress components over time. The slip amplitude at each node is determined from the slip profiles and the mean values are determined from the contact pressure profiles. Equation (2) can be used to determine the fretting-fatigue strength for each node from the slip amplitude and the mean contact pressure. By comparing the Böhme equivalent stress and the fretting-fatigue strength, the local degree of utilization can be determined according to equation (3). = , (3) The calculated local degrees of utilization for the entire contact surface of the connecting rod are shown on the right in Figure 7. As can be seen, the target value of 1 (corresponding to the fatigue limit) is slightly exceeded. The maximum calculated degree of utilization is 1.257 , which means that the calculation error of this method is 25.7% . The location of the highest degree of utilization is also the calculated failure location. With an absolute error value of 0.5° , this is only slightly off the real mean value. However, it must be noted that the scattering band in the tests was ± 1° . The results show that the authors ’ approach has the potential to achieve the objectives stated at the beginning. There is nevertheless still room for improvement, especially in determining the degree of utilization. 6. Conclusion and outlook The authors ’ aim was to develop a calculation approach for fretting fatigue, which allows the strength of arbitrary components to be assessed even in the development phase. The approach is intended to make it possible to determine correctly both the failure location and the degree of utilization. Since the fretting-fatigue strength is strongly dependent on the parameters of slip amplitude and contact pressure, these had to be integrated into the calculation process. The fretting-fatigue strengths for various combinations of slip amplitude and contact pressure were therefore determined for one material pairing on a double-actuated, slip-controlled flat-pad test bench. This made it possible to derive an equation for this material pairing that describes the fretting-fatigue strength as a function of slip amplitude and contact pressure. Subsequently, the existing failure hypothesis according to Böhme was modified with regard to the findings. For this purpose, the original failure criterion, which represented the fixed value for the plain-fatigue strength, was replaced by the equation for the fretting-fatigue strength. With the advantage that the fretting-fatigue strength at a contact pressure of = 0 MPa gives the plain-fatigue strength, the new approach remains identical to the original for this special case. Experimental investigations were then carried out on a real-life connecting-rod geometry and both the fatigue limit and the failure location were determined. To validate the developed approach, the connecting-rod setup was simulated numerically. The coefficient of friction was iteratively adjusted by comparing the measured and simulated slip amplitudes at the failure location in the connecting rod. The validation showed that the degree of utilization can be determined with an error of 25.7%. The point with the highest degree of utilization was located at a distance of 0.5° from the experimental failure location. Whereby the scatter band in the tests was ±1° . The focus of further research is clearly on the analysis of the coefficient of friction and wear. It is known that the coefficient of friction develops under fretting conditions. In the simulations carried out here, however, the coefficient of friction was not predicted, but adjusted as described above until the slip conditions actually measured in the simulation were achieved. However, a change in the coefficient of friction has a strong influence on the resulting slip amplitude, as do changes in the interference fit due to wear. For an improvement of the calculation quality, further investigations on the mean stress sensitivity need to be carried out. In addition, it must be considered how the fretting-fatigue strength behaves when fretting and fatigue are not collinear. Finally, it should be pointed out that the developed equation of fretting-fatigue strength as a function of slip amplitude and contact pressure is only valid for the specific material pairing and mean stress and must be experimentally redetermined for any other.