13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Figure 2 shows shape and dimensions of specimens. Fatigue strength was investigated by using plain specimens (Fig. 2a). To localize crack initiation and to better observe, partially notched specimens (Fig. 2a) were employed. Specimens were machined after solution treatment, and then aged at the different conditions displayed in Fig. 1. Prior to fatigue testing, specimens were electropolished to remove work affected layer and to better observe. Fatigue tests were carried out using a Ono-type rotating bending machine with a capacity of 15 N·m, operating at about 50Hz in the relative humidity (RH) of 25%, 45%, 65%, 85% and 95%, respectively. The humidity was controlled in the range of RH±5% while the temperature without control fell in the range of 298±3K. Surface observation and crack length measurement were conducted using plastic replication technique. Crack length, a, was defined along circumferential direction vertical to stress axis. Fracture morphology was analyzed under scanning electron microscope (SEM). 3. Results and discussion 3.1 Effect of humidity on fatigue properties Figure 3 shows S-N curves of Steels A and C in different humidity, respectively. Fatigue strength decreases significantly with increasing humidity, irrespective of the kind of steels. (a) (b) Figure 2. Shape and dimensions of (a) plain and (b) partially notched specimens Figure 3. Variation of fatigue strength with humidity in Steels A (left) and C (right) 10 5 10 6 10 7 0 100 200 300 400 500 600 700 800 Steel A RH25% RH45% RH65% RH85% RH95% Stress amplitude, σ a MPa Number of cycles to failure, N f cycle 10 5 10 6 10 7 0 100 200 300 400 500 600 700 800 Steel C RH25% RH45% RH65% RH85% RH95% Stress amplitude, σ a MPa Number of cycles to failure, N f cycle
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