13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- 3.4. Effects of loading type and residual stress on the fatigue property Fatigue limits of the respective series of T-386 in both loading types are depicted in Fig.13, where values in the left hand side of each couple of the results are higher than the values in the right hand side. This aspect has been often reported for many kinds of metallic materials by many researchers. Thus, it was reconfirmed that the fatigue limit in rotating bending is a little higher than that in axial loading. It is another finding that the fatigue limit for T-386EP specimen is clearly lower than that for T-386G and T-386SP specimens. This can be attributed to the fact that the compressive residual stress is fully released by the electrochemical polishing. Figure 14 represents the difference of the crack initiation depth between both loading types for same fatigue test results as in Fig.13. The depth of crack initiation site for T-386EP specimen is almost same between both loading types, but the depth in axial loading is significantly deeper than that in rotating bending. Figure 13. Difference of fatigue limits Figure 14. Difference of crack initiation depths The fact that a significant difference of the crack initiation depth took place depending on the loading types can be well explained as follows; The stress distribution across the cross section in rotating bending has a significant gradient, while the stress distribution is uniform in axial loading as illustrated in Fig.15. In the case of rotating bending, the stress distribution has a steep slope and, therefore, the stress in the core portion is markedly lower than the outer layer. The crack is supposed to occur at higher stress region, but strength of the surface layer is improved by some kinds of surface treatments. In such a circumstance, the fatigue crack can occur at a definite Figure 15. Schematics of stress distributions across the cross section in both loading types
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