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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therefore proposes a two-step calculation procedure using Ruiz ’s approach to indicate the likely failure location and then the critical-plane method according to Findley [24] to check the strength at this location. However, for the aforementioned reasons, the authors’ aim is to develop a new approach that combines both aspects and consists of a tribological adaptation of an existing failure hypothesis with a good reputation. The fretting-fatigue strength is considered as a two-dimensional parameter from the variables slip amplitude and contact pressure. 2. Materials Historically, research at the au thors’ institute has focused on the strength analysis of shaft – hub connections (e.g., [25 – 27]). Fretting fatigue is a major cause of failure in these connections, as also shown in the international literature [28 – 30]. Shaft – hub connections are often steel – steel connections, which is why this work is limited to this group of materials. Due to its tribological aspect, fretting-fatigue strength is a system variable that is influenced by the base-body and counterbody materials. Therefore, the materials 34CrNiMo6+QT as the base-body material and C45+N as the counterbody material, on which the investigations in this work are carried out, are specified. The base-body material is defined in this paper as the one to which the cyclical load is applied. The bar stock for each material was ordered from a single batch. Table 1 shows the material properties determined. The material was characterized with respect to the static properties of yield strength ( ) and ultimate tensile strength ( ) as well as Young ’ s modulus ( ) . Due to the cyclic loading in the fretting-fatigue experiments, the base-body material was additionally characterized according to the dynamic properties fatigue limit in fully reversed axial loading ( −1 ) , and the fatigue limit in repeated axial loading ( 0 ) . Table 1: Properties of the materials used Material Usage (MPa) (MPa) −1 (MPa) 0 (MPa) (MPa) 34CrNiMo6+QT Base-body 1092 1172 521 387 206.7 C45+N Counterbody 307 560 − − 206.2 3. Experiments In real-life applications, the microslip movements that cause fretting fatigue occur due to the elastic deformation of the components or vibrations in operation. Since real components can have an arbitrary shape, the strains and thus the slips change from location to location in the contact surface of the components. However, as shown in Figure 1a, the slip amplitude has a major impact on fretting-fatigue strength, so this also changes locally. The connecting-rod geometry used in this work represents such a real-life component connection with interdependent, superimposed parameters. It is intended to serve as a validation object for the failure hypothesis developed. However, in order to determine the necessary relationships and thus derive the required formulas, it is necessary to perform a separate analysis of the parameters. For this purpose, experiments are performed on a double-actuated, slip-controlled flat pad test bench. The experiments performed on both setups are presented in the following sections. 3.1. Connecting-rod experiments For the tests on the connecting-rod setup, the connecting-rod base bodies are shrink-fitted with bushings as counterbodies (Figure 2a). By measuring the connecting-rod specimens using a coordinate measuring machine and then grinding the bushings to exact oversize, an interference fit with a nominal contact pressure of = 10 MPa is achieved (calculated using German standard DIN 7190). The connecting rod ─ bushing assemblies are then tested in a hydraulic pulsator test bench. The exact specimen geometries and the test principle have already been explained in detail in [31] and correspond to these. The test series was carried out according to the stair-case method of Hück [32] to determine the fatigue limit. The load cycle limit was = 10,000,000 . Table 2 shows the results of these fatigue tests.