Fretting Fatigue of Shape Optimised Polygon-Shaft-Hub Connections
Author: Großmann, Christoph
Supervisor: Blessing, L.
Institution: Technische Universität Berlin
The application of the current polygon standard ranges for power transmission with shaft-hub connections is mainly impeded by the shortcomings of nominal size-specific profile shapes. Within the scope of a collaborative research project optimised profile shapes were developed which are intended to build up future standard ranges providing geometrical similarity with improved transmission behaviour. The experimental part of the research project, which is reported in this dissertation, focussed on the influences of the shape design on the fretting fatigue behaviour because of its decisive role for the load carrying capacity of polygon con- nections. The studies revealed that the profile shape is responsible for a specific stress gra- dient at the initial crack location and therefore determines the fatigue strength of the connec- tion. High cycle fatigue tests on polygon connections with optimised profile shapes showed improved load carrying capacities in comparison to the currently standardised profile shapes. Besides the development of optimised profile shapes different dimensioning methods were examined to derive a reliable prediction method for the fatigue strength of polygon connec- tions. A comparison between the experimentally obtained fatigue strengths of P4C- connections and spline connections showed the difficult interpretation of the widely-used fatigue stress concentration factor because the specific reference diameter used to compute the nominal stress for the fatigue stress concentration factor does not correlate to the fatigue strength. Analyses of the application of unified scatter bands to describe the general fatigue strength of polygon connections showed that the use of these bands is limited to only few parameter variations even at the same profile type and therefore not suitable for a wider us- age. Alternatively two local stress methods were examined which considered the local stress and strain state directly at the designated crack location. The methods were assessed using FEA and the experimentally obtained fretting fatigue strength of selected P3G polygon test blocks. The first approach was a strain based life prediction method which is based on a classic analytical linkage of crack initiation and crack propagation. The life estimations apply- ing this method showed only limited success because of its large dependency on the chosen material parameters. However the application of the second method, a short-crack growth approach, showed a very precise agreement with the experiments. The approach uses the KITAGAWA-TAKAHASHI diagram to assess the propagation behaviour of a short crack. The comparison between a material-specific threshold stress and a load-specific equivalent stress allows the prediction of whether a short crack arrests or propagates to failure. This short-crack growth approach seems promising as a future dimensioning method for the fret- ting fatigue strength of polygon connections.