13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- Fig.1 Sample size Instron8872 hydraulic servo fatigue testing machine was used. Samples with different rotation speed and welding speed were tested in three different R ratio: R=0.1, R=0.3 and R=0.6. ΔP was set a constant value 6.3 KN. 2.2 Results In Fig.2, crack growth rates under three different R ratio fatigue loads were shown. For three different welding parameters, crack growth rates at R=0.1, 0.3 were shown in Fig. 3. The experimental data from all specimens was compared in Fig.2 and Fig.3, for samples with crack, da/dN versus ΔKapplied. Even though had different rotation speed, welding speed and R ratio, the crack growth rate of these samples tended to be the same tendency. Previous studies [9] indicated that the fatigue life was based on the R ratio, which shown the stress level played an important role in crack growing. However, the fatigue life of the three kinds of samples certificated that even in different rotation speed and welding speed, the microstructure was the primary factor controlling the crack propagation for cracks grow in the welded nugget. (a) 400/100 (b) 800/400 Fig.2 Crack growth rates at R=0.1, 0.3, 0.6
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