PSI - Issue 68

Martin Nesládek et al. / Procedia Structural Integrity 68 (2025) 527–533 Martin Nesládek et al. / Structural Integrity Procedia 00 (2025) 000–000

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reduces the number of specimens required to find the fatigue limit. Usually 1 to 2 samples are sufficient to describe the change in the trend, which greatly favors this method over conventional ones. Several approaches have been published for evaluating the fatigue limit σ !" from the SH curve. The Risitano and La Rosa approach (Fig. 2b) is generally more conservative than the Luong method (Fig. 2c). These methods are based on graphical evaluation and there is no clear recommendation how many points to take for fitting each line. This drawback is eliminated by a novel method that applies an unbiased approach based on the linear-exponential behavior of the SH effect (Matušů et al., 2024), which is schematically shown in Fig. 2d. In this paper, the thermographic method is used to experimentally estimate the fatigue limit under fretting fatigue conditions for 42CrMo4+QT high-strength steel in two different fretting pad configurations. These estimates are compared to values obtained from conventional S-N curve tests. Additionally, the temperature directly associated with friction during fretting is analyzed numerically and subtracted from the temperature observed through thermal measurements. This process aims to obtain SH data that are free from any potential bias introduced by friction in the contact area. The results and insights are discussed in the concluding sections of the paper. 2. Experimental work Test specimens and pads were manufactured from the same batch of 42CrMo4+QT steel as those used in the previous study by (Nesládek et al., 2024) that provides detailed characterization of this material. Specimens have square cross-section, fretting pads are of the two types – first type has single flat contact surface while the second is of the bridge type having two flat contact surfaces (Fig. 3). This configuration was designed specifically to achieve wide range of contact slips and the resulting specimen temperatures. A free proving ring fretting setup was employed to carry out both the S-N curve and SH fretting fatigue tests – Fig. 4a. Cyclic axial load with asymmetry = −1 and frequency = 125 − 130 Hz was applied. Constant normal load at contact was 9.0 kN. The resulting S-N curves and fatigue limits derived from them are plotted in Fig. 4b. The fatigue curves are represented by the Kohout-Věchet equation in the following form: # = . + + 3 $ , (1) where A, B, C, β are regression parameters.

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Fig. 3. (a) Geometry of test specimens. (b) Geometry of fretting pads with single contact surface. (c) Geometry of bridge fretting pads.