Sample Problems on S-N Curves

The starting point is the determination of points A = (lOOO ) e B = (A 5y) of Fig. 5.17. The resulting fatigue curve may be considered a mean fatigue curve obtained by smooth, mirror-polished specimens. This curve shall be modified to take into consideration all factors affecting fatigue strength such as load factor (see Sect. 3.6), surface finish (see Sect. 3.3), size effect (see Sect. 3.5) and, eventually, also the requested probability of survival (1 — P). 

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Always with the material of the previous sample problems, it has been run another experimental campaign to assess the fatigue limit of the carbon steel by the staircase test method. On the base of the S-N curves of Fig. 5.21 it has been chosen an initial stress amplimde of 225 MPa at a run out of 1,000,000 cycles and a step size d = 10 MPa. Twenty-six button hourglass specimens of 8 mm minimum diameter have been used aU having the same surface finish of the real work pieces to be built. The results of the testing campaign are shown in Fig. 5.24. Fourteen specimens failed and 12 run out at 1,000,000 cycles. Therefore, the low frequency event of run out has been selected for calculations. The characteristic quantities are iV = 12, A = 19 and B = 37 (see Eq. 5.37). 

This problem is circumvented by the ASP analyses of the sorption data as presented in Figure 2. The slopes of the rectilinear portions of the curves is directly related to the monolayer capacity and the respective volume capacity terms. There is a striking resemblance of the activated char data to the n, as, t, etc. comparative plots currently employed for discerning porosity for materials. These relative techniques measure sorption by a given sample with respect to that measured for a reference nonporous sample assumed to be of the same chemical composition. In this instance, adsorption on both of the unactivated char and the deactivated char is excellently defined in terms of the ASP parameters. If needed, either could serve as reference nonporous materials for comparative purposes. This would involve unwarranted operations, interpolation, and processing problems. The ASP plots could serve as reference isotherm(s) for analyses of the activated char. Alternatively, the direct analyses of the rectilinear trends permit one to come to the same conclusions. Why should we compare one unto the other when they both are valid in their own right ... 

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