13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- unrolled specimens. The prolonged-life rate increases from 4.49 to 6.41 with the advance of surface rolling time. Theoretically, average fatigue life for S0.0 specimens to S0.7 specimens should be close to each other due to the same test stress amplitude. However, the choice of surface rolling time makes great difference in life extension effect. For example, the average life of initial rolled specimens (S0.0) is about 1.69 times the cyclic number after rolling of S0.7 specimens. The later surface treatment is performed, the slighter the life extension effect may be. Possible reasons are: Fatigue damage cumulated in material before surface turning and rolling can not be eliminated entirely. This residual damage gets more and more serious with delayed rolling time. Secondly, to remove existed surface cracks as thoroughly as possible, cutting thickness has to be increased with the postponement of rolling time. Thus, diameter and effective section of specimen are also reduced more seriously. Assume the same short cracks initiate in two specimens, local stress level for specimen with smaller diameter is relatively higher. Table 5. Mean value of fatigue life and corresponding prolonged-life rate for each group of specimens Repair time Cyclic number before repair, cyc Cyclic number after repair, cyc Total cyclic number, cyc Prolonged-life rate f=0.0 -- 882562 882562 6.41 f=0.3 41311 720580 761891 5.53 f=0.5 68852 627380 696232 5.06 f=0.6 82623 567379 650002 4.88 f=0.7 96393 522247 618640 4.49 4. Conclusions (1) Ferrite grain boundary and rich pearlite banded structure are two primary micro-structural barriers to short fatigue crack growth for LZ50 axle steel. No matter surface rolling or not, dominant short crack growth data of all specimens exhibits deceleration twice in MSC stage due to above micro-structural obstacles. Growth rate of specimens after surface rolling is significantly slower than that of specimens before rolling. (2) Effective short crack density for all five groups of specimens indicates the same overall trend, i.e., first rises then falls. Density after specimen rolling is much less. Surface rolling effect restrains the initiation of collective short cracks. However, this effect is weakened with the delay of rolling time. (3) Surface turning and rolling to specimens prolongs the fatigue life greatly compared to the life of unrolled specimens. It is because turning cuts off possible existed short cracks in material surface and rolling improve the fatigue performance of new surface. But postponed implementation of surface rolling make the accumulated fatigue damage can not be eliminated thoroughly. In addition, increase of cutting thickness also increase the actual local stress. In conclusion, for unrolled specimens, the earlier the surface treatment is performed, the better the collective effect of short cracks can be restrained as well as the longer the fatigue life of the material will be. Acknowledgements Present research is supported by Natural Nature Science Foundation of China (No. 51205326), Fundamental Research Funds for the Central Universities (No. SWJTU11CX075), and Opening
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