13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- initiation. The second part has high fracture times and describe the volume fatigue crack initiation. The equation (19,20) allows as to propose the analytical representation for S-N curve of material. To obtain the upper bound for life time we can take into account the stress amplitude, only. Figure 3. S-N curve for aluminum alloy 2024-T3. Failure caused by surface crack initiation (1), failure caused by under surface crack initiation (2). In the case of proposed approximation of equilibrium solution of (2) the critical time in the equation (19) can be estimated as follows 0 b 0 m b sur m f dp p p n 1 h p t . (21) To describe the full S-N curve we have to use several equation (19) with different values h ,n which could be connected with initial heterogeneity of materials. The equation (21) gives the traditional representation of S-N curve in Basquin form ln N a bln f , (22) where 0 b 0 sur m m dp n p p 1 h p a , is loading frequency. 5. Conclusion Analysis of experimental data devoted to the study of microcracks kinetics in the bulk and surface layer of the metals allowed us to choice of scale level for description of fracture of metallic materials - the level of submicrocracks with the size of 0.1-0.3 microns. To describe the geometry of these defects, as well as their effect on the mechanical material properties were used dislocation theory. The high initial concentration of these defects in metals and their initiation during initial stage of deformation process allows us to propose the importance of the consideration of their evolution
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