Issue 41

K. Slámečka et alii, Frattura ed Integrità Strutturale, 41 (2017) 123-128; DOI: 10.3221/IGF-ESIS.47.17 124 experimental data for the two loading channels and formulating the equivalent loading based on a single constant parameter. T HE NEW S-N CURVE AND THE BENCHMARK CRITERIA n the following, the combined cyclic loading is considered to be proportional, i.e., it consists of well-defined loading cycles and no additional non-proportionality effects, need to be taken into account. Furthermore, we assume that the relationship between loading and fatigue life, N f , over the studied life range is reasonably linear in log-log coordinates. This relationship is expressed in terms of stresses using the formalism in ASTM E 739 standard [5]. For symmetric loading, the new equivalent S-N curve ( the middle-curve criterion ) is constructed as   2 2 f σ,τ σ,τ eq,a σ,τ σ,τ a 0 a 1 log log log 2 N A m A m k         , (1) where A  and m  are the intercept and the (negative) slope and  eq,a is the amplitude of the equivalent stress expressed as a quadratic combination of bending and torsion amplitudes,  a and  a . The constant k 0 which describes the relation between bending and torsion loading is obtained as 2 a0 0 2 a0 k    , (2) where  a0 and  a0 are bending and torsion fatigue strengths corresponding to fatigue life N f = N f0 being in the middle between the axial and torsion midpoints (log N f0 = ½ (log f 0 a N + log f 0 t N )), see Fig. 1, where superscripts a and t reference, respectively, to the axial and torsion S-N curves and f 0 a N and f 0 t N correspond to the middles of the curves. The middle curve intersects the axial S-N curve at  a0 , N f0 and bisects the angle between the axial and torsion S-N curves. Its intercept and slope can be obtained from the intercepts and the slopes of bending ( A  , m  ) and torsion ( A  , m  ) curves: σ τ σ,τ arctan arctan tan 2 m m m         , (3)   σ,τ f 0 σ,τ 0a σ σ σ,τ 0a log log log A N m A m m        . (4) Figure 1 : Definition of N f0 , σ a0 , and τ a0 using data of S355J2G3 alloy steel reported by Karolczuk et al. [4,5]. I