13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Specimens were machined into the shape and dimensions indicated in Fig.3. Figure 3(a) indicates the shape and dimensions of SUP7 specimen for rotating bending. Notch radius of the specimen was 7 mm and the diameter of the critical section was 3mm. The stress concentration factor of this specimen was Kt =1.06 as given by Peterson’s handbook. The shape and dimensions of SWOSC-V specimen are quite same as SUP7 except for D=8mm. On the other hand, Fig.3(b) indicates the shape and dimensions of specimen for axial loading. Its notch radius was 15mm and the diameter of the critical section was designed to be 3mm similar to the rotating bending specimen. The stress concentration factor of this specimen was Kt =1.04 from the above handbook. Round notch surface of the specimen was finished by polishing with emery paper of #2000 after the grinding process. Of course, T-386EP series and T-386SP series were tested without such a polishing by emery paper. In order to characterize the microstructure, the micro-Vickers hardness was measured and the microstructure was observed at cross-section using an optical microscope and a scanning electron microscope (SEM). The distribution of the residual stress was measured at the transverse section of the smallest diameter of the specimen using the X-ray diffraction (XRD). The residual stress distribution in the direction of inside was also measured after electrochemically removing the local surface area. 2.2. Fatigue tests Photographs of the fatigue testing machines used in this study are shown in Fig.4. The left hand machine was used in rotating bending, whereas the right hand machine was in axial loading. Each of these machines can perform fatigue tests for four specimens simultaneously. The former machine has two spindles driven by an electric motor via a flat belt and each spindle has specimen grips at both ends [5]. The rotating speed of the spindles is 52.5 Hz (3150 rpm), and the stress ratio is given as 1 R . The latter machine has four actuators driven by hydraulic power, and the loading frequency is 80Hz. Fatigue tests were carried out at a definite stress ratio of 1 R . All the fatigue tests were performed in room atmosphere without any control of temperature and the moisture. After fatigue tests, fracture surfaces were observed by means of SEM and were analyzed by an energy dispersive X-ray spectrometer (EDS). (a) Rotating bending (b) Axial loading Figure 4. Multi-type fatigue testing machines used in this study
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