13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Moreover, the maximum density value for each group of specimens increases with the delay of surface rolling time. For S0.0 specimens, the mean value of maximum density is 662 mm-2, while for S0.7 specimens, surface rolling time, f, is postponed to 0.7, and the density increases to 941 mm-2. So postponed rolling time will lead to strengthened SFC collective effect, so that local micro-structure conditions get more and more advantageous to DSC growth. (a) (b) (c) (d) (e) Figure 4. Effective short crack density for each group of LZ50 axle steel specimens repair times for specimens are (a) f=0.0, (b) f=0.3, (c) f=0.5, (d) f=0.6, and (e) f=0.7 3.3. Comparison of Fatigue Life Fatigue life is the most direct indicator to reflect surface rolling effect. Table 5 gives the cyclic numbers before rolling, after rolling, and in total. To calculate the prolonged-life rate (defined as the ratio of total cyclic number to standard life), an average fatigue life of 137705 cycles, which was obtained from previous unrolled specimens fatigue tests[7], is cited as the standard life. It can be seen from the table that surface turning and rolling extends the fatigue life greatly than that of
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