Fretting Fatigue

The local tribological parameters slip amplitude and pressure are decisive for the evaluation of the fretting fatigue. Since these cannot be determined analytically for every application, dimensioning with nominal stress-based approaches such as DIN 743 or FKM guideline is not possible. However, considering the aspects of lightweight construction and the economic use of material resources, a dimensioning of the components adequate to the real degree of utilisation is required. The present project tries to meet this demand by developing a local calculation concept for the evaluation of the fretting fatigue, which takes into account the real tribological load condition. Within the scope of the planned project, fatigue strength tests of fretting-corrosive damaged key and polygon connections will be evaluated. For this purpose, the test series will be numerically simulated. The determined local tribological stress conditions are reproduced on the laboratory model and the slip-specific uniaxial comparative fatigue strengths are determined by simulation. By comparing the local equivalent stress of the WNV with the corresponding slip-specific equivalent fretting fatigue strength (both determined with the same equivalent stress hypothesis), the local degree of utilisation is obtained. By validating different hypotheses, the most accurate one can be found, which shows the smallest deviation from the degree of utilisation over all test series. The development of a generally valid and standardisable calculation concept requires comprehensive experimental validation. For SMEs, these can only be financed to a limited extent. Therefore, the project and its transfer of results make it possible to make the results accessible to SMEs as well. The resulting concept will be processed and documented in a guideline in such a way that access via established FE programmes is also possible for SMEs.

Based on the generally valid calculation method for steel-steel contacts subjected to fretting fatigue stress developed in the preliminary project, the calculation method is to be extended in the project to the material groups of cast iron and aluminium. This is primarily due to the planned integration of the calculation method into the FKM guideline (Analytical strength assessment), which is a widely used instrument for the evaluation of complex component geometries across all industries. The FKM guideline already combines the strength assessment of steel, cast iron and aluminium components on free surfaces (plain fatigue). This calculation approach, which covers all material groups, is to be retained for components subjected to fretting fatigue and thus requires the extension of the calculation procedure described above. The phenomenon of fretting fatigue can be traced back to a tribological stress that is present in addition to the stress-mechanical load. Using the double-actuated fretting pad test rig (laboratory model) of the research centre, an influence analysis of the tribological parameters on the fretting fatigue strength was first carried out for the materials EN-GJS-700 and EN AW-7075 T651 and thus the most critical parameter combination of slip amplitude and contact pressure for cast iron and aluminium materials was determined. Based on this, the minimum fretting fatigue strengths of the materials EN-GJS-700, EN-GJS-400 and E723 (EN-GJL-300) as well as EN AW-7075 T651, EN AW-6082 T651 and EN AW-5083 were determined on the laboratory model. With the help of numerical methods, fretting factors were derived which make it possible to take tribological damage into account and which thus form the basis of the calculation method developed. The determined fretting factors as well as the entire calculation method were verified on connecting rods. Thus, a validated calculation method is finally available to the user, which enables the strength verification for components made of steel, cast iron and aluminium that are at risk of fretting fatigue, based on the known FKM guideline.

The goal of the research project has been achieved

FVV-Abschlussbericht Heft-Nr. 1329

The research project was carried out in the framework of the industrial collective research programme (IGF no. 20916 BR). It was supported by the Federal Ministry for Economic Affairs and Climate Action (BMWK) through the AiF (German Federation of Industrial Research Associations eV) based on a decision taken by the German Bundestag and the FVV (funding no. 601393).