Issue 41

K. Slámečka et alii, Frattura ed Integrità Strutturale, 41 (2017) 123-128; DOI: 10.3221/IGF-ESIS.47.17 127 The results reveal that the middle-curve criterion provides, in general, the best estimates of fatigue life. The Gough- Pollard criterion also yields very reasonable predictions but it is computationally more complicated since it requires an iterative solution. As expected, the von Mises criterion provides good results for materials with the k 0 -value close to 3 but much worse estimates for S355J2WP steel with k 0 = 4.29, for which the Tresca criterion is more suitable. The latter criterion is, however, generally inaccurate and nonconservative. Figure 3 : Box chart of prediction differences: MC – middle curve, VM – von Mises, TR – Tresca, GP – Gough-Pollard. Material MC VM TR GP 2017A-T4 1.4 1.5 -1.2 1.4 S355J2WP 1.2 -3.8 1.1 1.1 S355J2G3 1.5 1.6 -1.9 1.5 Inc713LC -1.3 1.0 -2.5 -1.4 30CrNiMo8 1.5 3.0 -78 1.7 Table 3 : Average band factors. C ONCLUSIONS AND PROSPECTS his paper introduces a simple, computationally non-intensive empirical method, termed as the middle-curve criterion, to be used as a fast and efficient tool for the initial estimate of life of structural component subjected to combined bending and torsion proportional loading. The criterion requires one constant material parameter, k 0 , to be determined from experiments based on a suitably defined reference number of cycles to failure N f0 . The new referential S-N curve is obtained as the curve that intersects the axial S-N curve at N f0 and bisects the angle between the axial and torsion curves, thus reflecting the fact that the axial and torsion curves are generally divergent. The von Mises and Tresca criteria are special cases of the middle-curve criterion which is equal to the Gough-Pollard criterion for materials with parallel S-N curves. A comparison of all these criteria was made by using experimental data on five diverse metallic materials and the analysis clearly demonstrated advantages of the new criterion over the studied classical criteria. Therefore, the middle-curve criterion is very useful for a preliminary estimation of life of rotational structural parts under mixed axial and torsion loading. Despite the formulas presented in this paper do not account for the mean stress effect, it can be easily included by using experimental data obtained from non-zero mean stress tests. Transformation of the data based on the Goodman diagram or other similar methods [10] is also possible. Furthermore, our unpublished data show that the precision of prediction can be increased by using referential S-N curve obtained by fitting of joined axial data and torsion data expressed in terms of equivalent stress   1/2 2 2 a 0 a k    . The non-proportionality of loading can also be taken into account by a suitable definition of a dependence of k 0 on the phase shift. T