24th International Colloquium Tribology - January 2024 143 Simulation of the Local CoF Development in Dynamically Loaded Contact Surfaces (Fretting) Silvano Oehme 1* , Alexander Hasse 1 1 University of Technology, Chemnitz, GER * Corresponding author: E-mail silvano-giuseppe.oehme@mb.tu-chemnitz.de 1. Introduction In dynamically loaded component contacts microslip movements result in local change of the coefficient of friction (CoF development), which needs to be known in terms of appropriate component design. Due to lack of experimental accessibility to the contact surface, the resulting CoF distribution has to be obtained by simulation. This paper presents an approach to simulate the local CoF development in dynamically loaded component contacts, in dependence of the dissipated friction work. 2. Simulation approach The simulation procedure requires a CoF-friction-workcurve as input data, which shows the friction-work-dependend local CoF development. It has to be obtained experimentally. The Institute’s fretting test rig uses standardised model specimens with flat annular contact surfaces [1]. Defined dynamic contact loads (pressure p F and slip amplitude s a ) lead to an oscillating friction moment, from which the CoF and friction work W fric are calculated. Fig.-1: Exp. obtaining of local CoF development The FE-model containing the component contact then calculates the dissipated friction work at each node and assigns the resulting CoF in an iterative calculation loop (see fig.-2) until the final contact condition is reached. Fig.-2: Iterative calculation loop The dissipated cyclic friction work W fric,cyc is calculated for a determined load cycle (starting with N-=-1). The load cycle numbers relating to the subsequent calculation points are calculated using cycle jump technique [2]. This contains load cycle jumps, that may vary in dependence on particular gradient in tribological stress distribution and predefined critical change in resulting CoF Δµ th . A small value for Δµ th leads to small size of the cycle jumps between calculation points and, therefore, result accuracy and computation time increase, vice versa (see fig.-3). The value for Δµ th has to be chosen in a way that accurate results can be achieved in reasonable computation time. Fig.-3: Cycle jump technique, effect of step size Due to the modular structure of the simulation tool, the module can be connected to any FESystem by adapting only the FESystem-dependent modules. 3. Study on effects of CoF distribution on calculation of machine component contacts 3.1 Performed simulations Simulations were performed in two steps at a model of a pressfit-connection under bending load (fig.-4). Fig.-4: Pressfitconnection under bending load In the first step the simulation method was applied to the model to obtain the CoF distribution in the final state. In the second step the exact same load as in the first simulation was applied to final state of the connection to obtain stress and slip distribution in contact. These were compared with calculation results where CoF had been set constant. 3.2 Results 3.2.1 CoF development Fig.- 5 shows the CoF development of the simulated model. The CoF increases locally starting from a base level of 144 24th International Colloquium Tribology - January 2024 Simulation of the Local CoF Development in Dynamically Loaded Contact Surfaces (Fretting) µ 0 -=-0.2. The CoF has the biggest increase at the hub edge, where slip amplitude and contact pressure and, therefore, the friction work are at their maximum. As a result, the final CoF distribution establishes as shown in Fig.-6. Due to discretization of the geometry and load cycles (fig.-3) chattering effects can be observed in CoF-development, that lead to an alternating increase in CoF of adjacent contact nodes. Fig.-5: CoF distribution and development in contact Fig.-6: Final CoF distribution (green) in contact The impact of including the final CoF distribution in the calculation on result variables are presented in the following section. 3.2.2 Effects on Stress and Slip Fig.- 7 and Fig.- 8 illustrate the results of stress and slip in for distributed CoF and constant CoF throughout the contact surface. Fig.-7: Slip distribution in contact with distributed CoF (green) compared to results with constant CoF The slip distribution in contact (fig.-7) differs considerably when taking CoF distribution into account. The slip value at the edge of the hub as well as the depth into the contact are higher compared to results with constant CoF of µ fin -=-0.8. Fig.-8: Stress distribution in contact with distributed CoF (green) compared to results with constant CoF In contrast CoF distribution has less impact on the stress distribution as shown in fig.-8. A summary of the identified effects of CoF distribution on the considered result variables for the considered shaft-hub-connection is shown in Tab.-1, effects on stiffness and torque transmission added (not presented in particular in this paper). Tab.-1: study summary - influence of CoF distribution on calculation results of a shaft-hub-connection (from green - high influence to grey - low influence) Slip Stress Stiffness Torque transmission When calculating dynamically loaded contact surfaces of a specific application, taking CoF distribution into account can increase the result accuracy of the required result variables considerably. Conclusion A simulation method for calculating CoF development has been introduced which enables to identify the impact of local CoF distribution on calculation results. The simulation method was applied to specific models of shaft-hub-joints. Thus the capability of the simulation method was shown and suggestions for appropriate simulation settings were given. The impact of CoF distribution on slip amplitude and shear stress (commonly considered surface damage parameters) compared to a constant CoF was shown. Using the simulation method, CoF development, resulting from component geometry, load case and assembling conditions (e.g. interference), can be taken into account when calculating component contacts for fretting wear, fretting fatigue and crack initiation location more exactly. References [1] Leidich E, Maiwald A, Vidner J. A proposal for a fretting wear criterion for coated systems with complete contact based on accumulated friction energy density, Wear 297 (2013), S. 903-910. [2] Titscher T, Unger JF. Efficient higher-order cycle jump integration of a continuum fatigue damage model. Int J-Fatigue 141 (2020).