13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Subsequently, cylindrical rods were eroded from the blade root by electro-discharge-machining and turned to the final specimen geometry with cylindrical gauge length and a diameter of 3 mm. Fig. 1shows the geometry for tests without mean stress (R = -1), Fig. 2 the specimen shape for tests with superimposed mean stresses (R > 0). Figure 1. Specimen geometry for R = -1 fatigue tests. Figure 2. Specimen geometry for R > 0 fatigue tests. Finally, the samples were ground with SiC abrasive paper and subsequently mechanically polished. The residual stresses in the polished state were determined to -34 ± 16 MPa by X-ray diffraction using the sin2ψ-method. Testing system is the Ultrasonic Fatigue Testing Equipment (UFTE) developed and manufactured by Boku Wien attached to an Instron 5967 load frame for fatigue tests with superimposed mean stresses. The experimental setup is similar to that described in [17]. For tests at R = -1, the specimens are clamped at one end to allow free oscillation. The UFTE system develops a standing wave during oscillation. Ultrasonic fatigue tests were performed in displacement control at constant-amplitude tension-compression loading in laboratory air at room temperature. Assuming linear-elastic behavior in the VHCF regime, strain gauges can be utilized to calibrate the system prior to the experiments by using the linear correlation between displacement and stress in the gauge section [18]. To limit self-heating of the specimen during loading, a load pulse / pause ratio of 80-200 ms / 800-1200 ms was applied together with forced air cooling of the specimen. To analyze the influence of high mean stresses, load ratios ranging from -1 up to 0.7 were applied. Additionally, load controlled fatigue tests were conducted using an Instron 1603 electromagnetic resonance testing machine at frequencies of about 100 Hz to investigate the frequency influence on fatigue life. An ultimate number of cycles of 2·109 was chosen for the tests conducted with the ultrasonic device and of 108 for the electromagnetic fatigue testing machine, respectively. After fatigue testing, fracture surfaces of failed specimens were observed by SEM, paying particular attention to classification of the fracture modes of surface and subsurface fracture at persistent slip bands and inclusions, respectively, and analyzing diameters as well as chemical composition of the inclusions.
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