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Weibull Distribution-Fatigue S-N Curves

Carbon steel specimens tested at four different stress amplitudes with maximum stress of 346, 324, 302 and 270 MPa, respectively, has yielded the fatigue lives listed in the following table. Using the Weibull distribution derive the 90 and 99.9 % probability S-N curves and that at 90 % probability of survival with 90 % confidence P(90)C(90). Analyzed with the Weibull procedure, already applied in the previous problem, the four different stress amplimde yield the results shown in Fig. 4.25. Also shown are the best-fitting fines with the relative equations. It is worth noting how the Weibull slope m decreases as the maximum stress decreases passing from m — 5.16 at 346 MPa Xo m — 1.0 at 270 MPa. [Pg.235]

This actually indicates how the dispersion of experimental data varies as inverse function of stress level, very large for low stress and lower for higher stress level. This circumstance is clearly indicated by the probability distribution of Fig. 4.26. The following table lists the scale factor, the Weibull exponent m, [Pg.236]

The relative S-N curves are shown in Fig. 4.27. If the probability density p(N) were normally distributed to infer the 90 % probability of survival with 90 % confidence, P(90)C(90), it may be used the LCL approach of Sect. 2.9.4 using the Lieberman s k values of Table 4.6 [Pg.237]

With a sample size of 9 specimens is A = 2.133 therefore for the three stress levels likely to have a probability density P(N) that follows a normal distribution it is [Pg.237]


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