13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- damage depending on the height of the maximum load amplitudes. 2. Material and experimental procedure 2.1. Material The experimental investigations are performed with specimens with a minimum diameter of 4 mm shown in Fig. 1. All specimens were machined from a 20 mm round bar of the quenched and tempered high-strength steel 34CrNiMo6. The chemical composition of the steel is summarized in Table 1 and the static material parameters are shown in Table 2. Table 1. Chemical composition of the investigated high-strength steel (in wt %) C Si Mn Cr Mo Ni 0.34 0.3 0.5 1.5 0.2 1.5 Table 2. Mechanical Properties UTS [MPa] YS [MPa] Young´s modulus [GPa] Breaking elongation [%] 1200 1000 210 9 An average Vickers hardness of the material was 350 HV. The surface of the specimen has been emery-polished after machining. Figure 1. Specimen 2.2. Ultrasonic fatigue testing system In service, components and structures are rather subjected to variable amplitude loadings (VAL) than to constant amplitude loadings (CAL). Fatigue tests with variable amplitude loadings or load time histories in the regime of 109 cycles or even higher are too time consuming and expensive using conventional testing systems. Therefore, an ultrasonic fatigue testing system of the BOKU
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