13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- The tests on cyclic tension of 30CrMnSiNi2 steel specimens have shown that surface nanostructuring by Zr+ ions gives rise to double-or-triple time increase of fatigue durability. In doing so nucleation of fatigue crack in specimens with the surface modification happens later, while rate of its growth is noticeably slower in contrast with non-treated specimens. 2.5. Test on cyclic alternating bending According to the testingdata the fatigue life-time of specimens under cyclic alternating bending is increased due to nanostructuring of a surface layer by~ 2 times. The series of optical images were used for plotting the graphs to characterize dependence the crack length versus the number of loading cycles. At cyclic bending the main crack originates nearly at the same time for both types of specimens but has significant differences in thepropagation rate. Crack growth rate forthe untreated specimen was 0.05 µm/cycle while for the specimen with modified surface layer it made 0.024 µm/cycle. Thus, it is shown that nanostructuring of surface layer by ion beam to slow the rate of fatiguecrack growth by about 2 times. 2.5.1. Optical observation of strain induced relief Images of surfaces of both type specimens after the different number of cycles before fracture to cause distinction in the formation of strain induced relief are given in fig. 4. One can see formation of manifested thin folds (fig. 4,a) located close to the fracture area (region of maximum curvature) in the specimen without treatment which shows an intensive deformation development in this area. Most likely their presence contributes to a rapid propagation of fatigue crack in this specimen. Reason of formation of these folds is apparently related to the testing scheme that governs periodical application of tensile and compressive stresses. Mechanism of material deformation for this case is similar to the corrugation formation which is formed as a result of greater deformation in the subsurface layer. In the nanostructured layer the number of such folds is slightly visible and deformation relief is smoother and changeable more lightly (fig. 4, b). The analysis of surface roughness of failured specimens in the fracture area was carried out with a help of optical interferential profilometer. According to obtained values of roughness parameter Ra the surface roughness of the specimen without treatment is more than 1.5 times higher in comparison with one for the specimen with nanostructured surface layer. During the test, in the specimen without the treatment clearly pronounced microcracks are formed along the grain boundaries and the main crackis more clearly manifested and propagates just along the microcracks. The specimen after the treatment has the number and size of the microcracks noticeably smaller and the main crack develops to a less extent. More detailed view of the main crack for specimen without treatment testifies for the fact that it is developed intensively not only on the surface, but into the bulk of the specimen as well. The main crack in the nanostructured specimen is less pronounced that could indicate that it develops mainly in the modified surface layer which hinders its spreading into the bulk of the material. This may cause a reduction in growth rate atmain fatigue crack propagation.
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