13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- Figure 7. Crack growth curve in comparison to experimentally evaluated crack growth rates with variable amplitude loading tests (FELIX) Using the Palmgren-Miner’s rule a fatigue life Nf,calc with an assumed damage sum of 1 can be calculated for the investigated variable amplitude loadings (see chap. 2.3). Beside the original Palmgren-Miner (PM) approach, the modified and elementary Palmgren-Miner approach as well as the approach by Liu/Zenner have been taken into account in order to consider amplitude levels beneath the fatigue strength. The calculated fatigue lifes Nf,calc of the two-step loadings are compared with the appropriate experimental mean fatigue lifes Nf,exp (Fig. 8). Moreover, in Fig. 8 the scatter bands Nf,calc/Nf,exp for 10% and 90% probability of survival based on the CA tests are given. 1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09 Nf,calc (log) Nf,exp (log) PM-original PM-elementar PM-modified Liu-Zenner 1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09 Nf,exp(log) PM-original PM-elementar PM-modified Liu-Zenner Rbl = 1 Rbl = 10 Rbl = 100 Rbl = 1 Rbl = 10 Rbl = 100 a) b) Figure 8. Comparison of two-step loading tests a)⎯Sa = 584 MPa (1.1·σD) and b)⎯Sa = 637 MPa (1.2·σD) with calculated lifetimes using Palmgren-Miner’s rule with a damage sum of 1
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