PSI - Issue 4

Pavel Hutař et al. / Procedia Structural Integrity 4 (2017) 42 – 47 Author name / Structural Integrity Procedia 00 (2017) 000 – 000

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5

Then the stress ratio is defined as:

min ( ) ( ) K a R K a  . max

(3)

Based on Eq. (3) the operation stress ratio for considered axle was determined in the range from R = -2.5 (for extreme values of residual stresses) to R = 0.

Fig. 5. Experimentally obtained v-K curves of EA4T steel for stress ratios R = -2, -1, -0.5 and 0.1 with NASGRO fit: a) v-  K expression, b) v-K max expression. The fatigue crack propagation rates were determined experimentally, see Pokorný et al. (2016b) for details of experiments. The measured v-K curves (curves describing dependence of the crack propagation rate v = da/dN on the stress intensity factor K values) were obtained for stress ratios R = -2, -1, -0.5 and 0.1 which fit well the range of stress asymmetries obtained based on Eq. (4).

Table 1. Estimated values of RFL (fatigue crack growth from initial crack length a 0 up to final one) [thousands of km] peak of axial residual stress initial crack length a 0 2 mm 3 mm 5 mm 0 97 39 18 -10 204 63 23 -20 628 107 31 -30 2 943 218 41 -40 15 717 682 55 -50 85 230 3 249 75 -60 inf 17 326 107

Measured v-K data are shown in Fig. 5. It is evident that the experimental curves fit each other well in the case of v-K max expression. Based on this fact, the fatigue crack propagation rate can be sufficiently described just by one v-K max curve. The v-K max interpretation of the fatigue data corresponds well with NASGRO relationship in K max expression, see Pokorný et al. (2016b). Therefore, NASGRO relationship can be expressed in the simplified form:   max, max max 1 p n th K da C K dN K         , (4) where C , n and p are material constants, K max is maximal value of the stress intensity factor in load cycle, K max,th is