13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- al. [13] studied the gigacycle fatigue behavior of titanium alloy Ti-6Al-4V using ultrasonic fatigue at 20 kHz. Yin et al. [14] analyzed the fatigue failure mechanism of welded joint by SEM observation, and discussed the effect of ultrasonic peening treatment on the fatigue performance. In this paper, a special designed specimen was fatigued in very high cycle fatigue regime to investigate the fatigue performance of base metal, heat affected zone and fusion zone for steel Q345. A high-precision infrared video camera was used to measure the variation of surface temperature. The natural frequency changes of specimen were monitored to discuss the fatigue crack propagation lives of FZ and HAZ. 2. Material and experimental method Table 1 Chemical composition of the base metal and solder (wt.%) Material C Si Mn S Alu P Mo Fe Q345q(Base metal) 0.15 0.38 1.36 0.00003 --- 0.00012 0.00002 Rest E45(Solder) --- 0861 2.236 --- 0.648 --- 1.569 Rest Table 2 Mechanical properties of Q345 steel E /GPa /MPa UTS/MPa /g.cm-3 A/% HV30 201 420 570 7.85 13 136 The material used in this study was a widely used low alloy steel, Q345, with yield strength of 420MPa. It’s chemical composition and mechanical properties are presented in Table 1 and Table 2 respectively. With good tensile strength, plasticity, weld ability and low temperature deformability, Q345 steel has a widespread application in automobile, watercraft, pressure vessel etc. Fig.1 Specimens of ultrasonic fatigue test; (a) Parent specimen; (b) Stress field of welded joint specimen in mode analysis; (c) Dimensions of welded joint specimen; (d) schematic diagram of FZ and HAZ specimens The dimensions of hourglass shape specimen for base metal are indicated in Fig2a.Welded joint specimen was machined from butt-welding plates with a thickness of 10mm. In ultrasonic fatigue test, the maximum stress always located in the middle of specimen. To ensure that the fracture section of HAZ and FZ specimen located in respective area, the center line of welding area
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