Issue 35

X. Liu et alii, Frattura ed Integrità Strutturale, 35 (2016) 88-97; DOI: 10.3221/IGF-ESIS.35.11 90 Figure 1: Microstructure photo of tested material by SEM. Testing methods An MTS 810 system was used to conduct the tensile test with cylindrical specimens of 6 mm in diameter at a strain rate of 10 -4 s -1 . Five tensile specimens were tested, and the average value of the tensile strength was 1896 MPa. The micro- hardness measurement was performed on three samples by a micro-hardness tester at a load of 50 g with the load holding time of 15 s. Twenty points were tested in each sample, and the average value of micro-hardness was 760 kgf/mm 2 . Fatigue test was conducted by using an ultrasonic fatigue test machine at a resonance frequency of 20 kHz at room temperature, and air-cooling was used to cool the specimens during ultrasonic fatigue test. The ultrasonic fatigue machine was hung in a tensile machine. The amplitude stress was loaded by the ultrasonic fatigue machine, and the mean stress was superimposed by the tensile machine. Stress ratios of -1, -0.5, 0.1 and 0.3 were chosen. The geometries of the specimens are shown in Fig. 2. Before fatigue test, the round notch surface of each specimen was ground and polished to eliminate machine scratches. The fracture surfaces of the failed specimens were observed by using a field-emission type scanning electron microscope (SEM). Moreover, Focused Ion Beam (FIB) technique was used to prepare the Transmission Electron Microscopy (TEM) samples of crack initiation region. Then, the samples were observed via TEM. Figure 2: Shape and dimension (in mm) of specimens. (a) For testing without mean stress ( R = ‒ 1), and (b) For testing with superimposed mean stress ( R ≠ ‒ 1). E XPERIMENTAL RESULTS AND ANALYSES S-N curves ig. 3 presents the S - N curves for the tests at the four stress ratios. It is seen from Fig. 3 that the tendency of the S- N curves from low-cycle fatigue to VHCF is similar for all the four stress ratios. But the stress ratio has a substantial effect on the fatigue strength, which decreases with the increase of stress ratio. Characterization of crack initiation region Fig. 4 presents the typical morphology of the fracture surfaces for the specimens at the stress ratio of ‒ 1. As shown in Fig. 4, the fracture surface of specimens is divided into the following four types. Type I: Crack initiated from surface without F (a) (b)