| Axial components with interference fit are widely used to transfer power and motion due to the advantages of compact structure, better centering and lower impact. However, a great deal industrial application shows that the fatigue strength of shaft is decreased largely by the fretting damage between the interference fit interfaces, which induces a huge of economic losses and casualties. Presently, most of researches focus on tension-compression fretting fatigue in the field of fretting fatigue. However, the axial components are borne rotary bending fatigue loads whose fatigue damages are the results of the combination of bending fretting fatigue and tension-torsion fretting fatigue. The research reports on rotary bending fretting fatigue (RBFF) are very rare. Thus, the study on RBFF for the interference fit structure of shafts is not only a significant to understand the mechanisms of RBFF, but also can provide some efficient guidance on design of anti-fretting fatigue damage in actual engineering.In order to reveal the phenomenon and essence of fretting damage of axial components, the actual components of railway wheel-axle and electric motor axis/pinion shaft were used to do failure analysis in detail by using advanced micro analysis methods of optical microscope, scanning electrical microscope, electron energy dispersive spectrum, X-ray photo electron spectrum, Leeb hardness tester and3D profilometer. The results indicated that the fracture phenomena of interference fit positions of typical axial components are the result of RBFF.The main obtained research results are as follows:(I) Study on RBFF damage mechanisms of LZ50railway axle steelThe experimental results of railway wheel-axle in small scale reproduced the RBFF damage of the real wheel seat of axle in service.Based on a number of tests, the S-N curve of small scale samples of LZ50railway axle steel has been established, and the result shows the fatigue life presented a strong non-monotonic behavior with the change of bending fatigue load.The micro-analysis reveals that the outer damage band (close to the sideof bending load end) was dominated by fretting wear. However, in the inner damage band with a narrow width, fretting wear is slight, and the main damage exhibited cracking of fretting fatigue.The result of FEM simulation indicates that the relative slip amplitude in the outer damage band was about three times of the inner one and this value was dependent upon the imposed fatigue load.The fracture surface of small scale samples showed the profile of multi-source and step-like feature.The research found that the RBFF life decreased with the increase of test speed, however the damage mechanisms unchanged.lt has been found that with the change of contact stress (z.e.the interference fit stress), the RBFF life exhibited a concave zone in dramatically drop-down, and namely this contact stress region corresponded to the mixed regime of fretting running behavior. Thus, it indicated that the engineering application must avoid this zone/regime.The FEM analysis testified that the Ruiz parameter can be used to forecast the behavior of RBFF life varied with the contact stress.(Ⅱ) Study on RBFF damage mechanisms of30CrNiMo8steelBased on the small scale RBFF railway wheel-axle test rig, the S-N curve of RBFF of30CrNiMo8steel was established in this research. The result shows that the RBFF life of30CrNiMo8steel was sharply dropped to the RBFF fatigue limit of about90MPa. The fretting damage phenomena of30CrNiMo8steel were similar to the LZ50steel, however, its life was more sensitive to the fretting damage. Under almost all imposed fatigue load levels, the fretting cracks can be observed in all damage bands, which presented much stronger dependence of fatigue load.(Ⅲ) Basic characteristics of RBFF damage mechanisms of axial componentsBased on the results of failure analysis of typical axial components and the RBFF experimental research for the two steels mentioned above, the basic features of RBFF damage mechanisms were summarized as follows:(1) The distribution of RBFF logarithmic life of axial components does not obey the traditional Basquin function but shows an intensive dependence on fatigue loads.(2) Fretting damage occurred at the contact edge area of the interference fitted zone and the most serious wear location is located at a certain distance apart the contact edge. The width of fretting wear band is dependent upon the level of fatigue load. The main wear mechanisms of samples surface are abrasive wear, oxide wear and delamination. (3) The fracture surface shows the multi-source and step-like profile, which caused by the action of multi axial stress. The fretting fatigue cracks initiated at the sub-surface, some defects such as caves have been observed in the crack initiation area. The micro-cracks are probably induced by the conjunction of the caves. The propagation of fretting fatigue crack shows three stages:a) The stage control by contact stress--the crack propagates inclined to the normal of surface with an small angle (about20°～40°). And the inclined crack connects with the surface wear oblique cracks, b) The stage control by the combination of contact stress and fatigue stress--the effect of contact stress decreased with the increase of depth and the inclined crack gradually transforms to the normal of surface, c) The stage control by fatigue stress--, the crack propagates normal to the contact surface until to final fracture as same as the plain fatigue process.(4) The competition mechanism exists between the local wear and fatigue during the RBFF damage process. At the outer damage region, the stress, strain and slip amplitude are higher, and the damage is very serious mainly with local wear. And the stress concentration is effectively released by the particle detachment and the third-body formation after local wear. Thus, the fatigue crack initiation is restrained. However, in the inner side, the wear is slighter and the damage dominates by local fatigue, where the fatigue crack is easy to initiate and propagate.(5) The RBFF damage strongly depends upon the material properties, i.e. the higher strength of materials, the higher factor of life reduction. At meanwhile, the RBFF damage also shows dependence upon the running speed and fatigue load. |
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