PSI - Issue 51

Michael Horvath et al. / Procedia Structural Integrity 51 (2023) 95–101 M. Horvath et al./ Structural Integrity Procedia 00 (2022) 000–000

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2. Experimental investigations In order to establish experimental datasets for calibration, and validation, of the implemented methodology, high cycle fatigue experiments are conducted on a RUMUL ® Testronic 150 kN resonance test rig. Notched and unnotched specimens made of high-strength cast steel alloy G12MnMo7-4+QT are utilized and depicted in Fig. 1. Regarding the notched specimens, both geometries feature a notch depth of 3 mm and a notch radius of 0.1 mm .

Fig. 1. Specimen geometries utilized for high-cycle fatigue experiments.

Each specimen geometry is investigated at load ratios of R = -1, R = 0 and R = 0.5; summing up 176 specimens. After testing, the experimental results are statistically evaluated in order to derive generalized S-N curves according to Haibach (Haibach and Matschke 1982). The evaluated dataset consists of standardized slopes and knee points for each individual specimen geometry, which are assumed to be independent of the load ratio R . As the investigated specimens are made from cast steel which is affected by manufacturing process-based imperfections, the evaluated S N curves feature a second slope in the high-cycle regime as recommended (Sonsino 2007). In order to extend the methodology to arbitrarily shaped casting imperfections, high-cycle fatigue tests utilizing defect-afflicted specimens made of G12MnMo7-4+QT are conducted on a SincoTec ® MOT600kN resonance test rig. The specimens are investigated at load ratios of R = -1 and R = 0 and feature a specific casting geometry, which provokes the formation of bulk imperfections (Schuscha et al. 2019b). In order to characterize the geometry of the bulk imperfections, X-ray examinations of each specimen in two perpendicular planes parallel to the specimen axis are carried out prior to fatigue testing of the respective specimen. 3. Application study of the TCD for fatigue assessment in the medium-cycle regime The TCD-framework is applied to evaluate the generalized S-N curves. 3.1. Calibration procedure Calibration is carried out according to (Susmel and Taylor 2007) using the S-N curves for a probability of survival of P S = 50% of plain, unnotched as well as notched specimens with a notch opening angle of 2  = 45°. For each investigated load ratio, the material dependent parameters A and B are derived in order to establish the relationship L M ( N f ). Due to the second slope of the evaluated S-N curves, the course of L M ( N f ) shows three distinctive regions as exemplarily depicted in Fig. 2 for a load ratio of R = -1. It should be noted, that the depicted stress ranges are normalized in regard to the plain material fatigue strength at an alternating load ratio. Hence, the presented work introduces a dependency of the parameters A and B upon the number of cycles to failure N f to take the second slope found in S-N curves of imperfected materials into account. 3.2. Validation procedure After initial calibration of the TCD-dataset, validation is conducted utilizing the generalized S-N curves of the notched specimens featuring a notch opening angle of 2  = 135°. Fig. 3 depicts the results for the investigated load ratios following Susmel procedure (Susmel and Taylor 2007).