13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- The machining technology for load relieving groove of RD2 axle is listed in Table 3 in detail. The purpose of finish turning is to remove possible surface cracks, while surface rolling is to strengthen the surface of load relieving groove. Surface treatment parameters of present research are also listed in Table 3. It can be seen that the simulation technology meets the maintenance requirements of real RD2 axle. After rolling, surface Vickers hardness of specimens increases from 201.68 HV0.1 to 222.90 HV0.1. Meanwhile, absolute value of axial and circumferential compressive stress for specimens after surface rolling is about 170 MPa and 101 MPa higher than that before rolling[8]. Table 3. Practical repair procedure for RD2 axle and simulated surface treatment procedure for specimen Procedure Indicator In practice In simulation Equipment Numerical control machine Numerical control machine Rotate speed of axle ≥350 r/min =350r/min Cutting thickness ≤0.2 mm ≤0.15 mm Finish turning Feeding speed 40~70 mm/min =50 mm/min Equipment Numerical control machine Numerical control machine Rotate speed of axle 330~400 r/min =350 r/min Rolling time 1 1 Surface rolling Diameter deformation ≤0.02 mm ≤0.02 mm 2.3. Replication Tests Replication technique is a widely applied fatigue test method in short crack research field. Test is interrupted at given time intervals (depending on the number of cycles), then replicate specimen surface with softened acetyl cellulose films, and finally these films can be dried and preserved for subsequent observation[9]. Present tests were performed under a stress-controlled sine wave mode on Rumul 250 kN high frequency fatigue test machine. The symmetrical cyclic stress amplitude was 230 MPa. To study the relationship between cracks and micro structures, specimen surfaces were etched by 4% nitric acid alcohol and the metallographic structure was exposed. Firstly, all specimens without surface rolling were tested according to replication technique. Secondly, when cyclic loading number met predetermined cyclic number for surface treatment, test was suspended and specimens were dismounted. Thirdly, above specimens were turned and surface rolled complying with Table 3. Finally, processed specimens were mounted on test machine again and still tested at 230 MPa by replication method to final failure. Two things are important to note: (1) Number of loading cycles according to surface rolling time. Previous test result has shown that the average fatigue life of LZ50 axle steel specimens without surface rolling is 137705 cycles[7]. To investigate the influence of rolling time, surface treatment time for five groups of specimens was determined according to this life. That is, surface rolling was applied when life fraction, f, was 0.0, 0.3, 0.5, 0.6 and 0.7, respectively. To facilitate the presentation, specimens were indexed according to their surface rolling time, i.e., S0.0, S0.3, S0.5, S0.6, and S0.7 specimens. (2) Test stress amplitude after surface rolling. After turning and surface rolling, axle diameter will be slightly smaller than before, which will lead to higher local stress level even at same service loading condition. However, the purpose of present research is to investigate the influence of surface treatment time and maintenance technology on short fatigue crack behavior for LZ50 axle steel. Increase of stress amplitude caused by size decrease at changeless test load can make subsequent analysis more complex. Therefore in present study, test load was recalculated based on actual specimen diameter after surface rolling, so that replication test could be finished still at 230 MPa. Number of effective specimens for S0.0 to S0.7 is 6, 6, 7, 7, and 7, respectively. After tests, dried replication films, which had been flattened with two glass slides, were observed using an
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