PSI - Issue 42

Jan Klusák et al. / Procedia Structural Integrity 42 (2022) 1369–1375 Jan Klusák et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Experimental campaign Testing of steels characterized above was performed up to very high cycle fatigue (VHCF) region. The desired number of cycles to fracture N f up to 10 10 was achieved through high-frequency loading. The ultrasonic frequency device was used to generate longitudinal vibrations of specimens of the tested material. Loading was realized in fully reversed push-pull mode (mean stress is zero, and the stress ratio R = -1). The specimen must be adjusted to have its intrinsic frequency of longitudinal vibrations close to 20 kHz. Adjustment of testing samples is based on knowledge of dynamic modulus of elasticity E d and mass density  of the material. For E d = 199 GPa and  = 7890 kg/m 3 . The geometry of the testing specimens was calculated as shown in Fig. 3. The minimum diameter of the sample is 3 mm. Loading is realized through the excitation of vibrations entered in micrometers. Relation between the vibration amplitude u a and the amplitude of stress  a is expressed by means of the stress factor S f , where  a = u a  S f . The stress factor determined by harmonic analysis was S f = 26.3 MPa/  m. After machining, the test samples were polished electrolytically to get fine surface without defects and traces of machining. The electrolyte for polishing was composed of 93.9 % of C 2 H 5 OH, 1.4 % of HNO 3 , and 4.7 % of HClO 4 . High frequency loading of austenitic steels performed at the resonant frequency of 20 kHz leads to high heat generation. For this reason, the samples must be efficiently cooled. In our case, cooling by deionized water in a closed circuit was used, see Fig. 4.

Fig. 3. Shape and dimensions of the test samples.

Fig. 4. Water cooling of the specimen during ultrasonic test.