13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- As a consequence, heat treatments [9,10] improve the microstructure of the type B (such as dislocation distribution and defect density, etc.), and accordingly enhance its mechanical properties. Figure.3 presents variations of the stress standard deviation of the two type steels. There is no obvious plastic deformation when the applied stress is lower. Accordingly the standard deviation is relatively small. If the stress is higher than the fatigue limit, it sets to increase sharply in the local zone due to stress concentration. The break point indicates that the mechanisms related to fatigue failure have changed into plastic effect. Local stress concentration, due to pores and impurities, governs the stress distribution. The damage status can be qualitatively identified by the standard deviation to avoid the sudden fracture. From Figure.3, the standard deviation of the type B is small relative to the type A since the grain refinement due to heat treatments improves the arrangement of the internal microstructure, which makes the type B take on better fatigue resistance. Figure.2 Relative temperature increment Figure.3 Standard deviation of stress Fatigue limit evaluation. From Figure.2 and 3, the fatigue limit of the type B should be higher than that of the type A. In this part, the intrinsic dissipation shown in Figure.4 was utilised to get their fatigue limits. The break points are in the range of 250MPa~350MPa. The intersection of the two straight lines denotes the corresponding fatigue limit [11]. Thus the fatigue limit of the type A is
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