13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- coincides with that of specimen length for FZ, while a distance of 4mm was designed between the center lines of welding area and HAZ specimen (Fig.1d). The radius of rounded roof for welded joint specimen was reduced to 2mm and other dimensions was determined by the method introduced in literature [15]. FEM results (shown in Fig.1b) showed that specially designed specimen was better to control the range of high stress area by restricting it at the root of arc during the test process. Fatigue test was performed on the USF-2000 ultrasonic fatigue test system (Fig.2) up to a limit of 109 load cycles under the conditions of stress ratio R=-1, at room temperature and compressed air cooling. The varying test resonance frequency of each specimen was measured by the computer control system. Moreover, an infrared thermometer was used to monitor the variations of temperature on the surface of specimen during test. Fig.2 Ultrasonic fatigue test system with an infrared thermometer 3. Results and discussion 3.1 Summarizes of S-N curves Fig.3 S-N fatigue curves of very high cycle fatigue test Fig.3 presents the S-N curves for the three groups of specimens tested at room temperature in ultrasonic testing. Run out specimens over 109 cycles are represented by arrows. It was observed that that fatigue strength of HAZ and FZ was markedly lower than that of BM, with a percentage of 51.4% and 42.8% respectively at 109 cycles. Due to the local high temperature cycle in welding
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