Abstract
The present study consists of a theoretical and experimental investigation of the effect of axial mean stresses on the high cycle fatigue behaviour of DIN 34CrNiMo6 high strength steel in quenched and tempered conditions. The axial S-N curves under 4 different stresses ratios were obtained. Experimental results show that increasing the value of the tension mean stresses gradually reduces the axial stress amplitude the material can withstand without failure. Moreover, the compressive mean stresses show a beneficial effect in terms of the axial fatigue strength, resulting in a non-symmetrical Haigh diagram. A historic review of the axial mean stress effect is presented, showing the shape of the Haigh diagrams for ductile metals and presenting the most-known empirical and physical theories. The results for this steel are compared with the physical theories of Findley based on the critical plane; the Froustey’s and Marin’s methods, based on energetic theories; and the Crossland invariants method based on the Gough’s theory of fatigue damage. Taking into account the experimental results, a physical fatigue function based on energetic considerations is proposed. Its application to the fatigue case with mean stresses can be interpreted in terms of a balance of elastic energies of distortion and volume change. Macro-analyses of specimen fracture appearance were conducted in order to obtain the fracture characteristics for different mean stress values.
Highlights
The objective of this study is to determine the mean stress effect on the fatigue behaviour of quenched and tempered DIN 34CrNiMo6 steel with tensile strength σuts = 1210 MPa and tensile yield strength σyp = 1084 MPa, in order to obtain a theoretical model based on physical principles which could be used to model its behaviour under any combination of mean and variable stresses
The effect of the mean stresses on the fatigue strength is measurable for the studied range of fatigue life, between 2 × 104 and 2 × 106 cycles
In order to model the fatigue behaviour with mean stresses, the empirical methods, namely the Gerber [18], Goodman [23], Morrow [24] and Dietmann [28] lines are the most used in the engineering practice
Summary
The influence of the mean stress on the fatigue limit was already described by Wöhler in. 1870 [1] as one of the main influential factors in the fatigue strength, decreasing the value of the fatigue limit with the increase in mean tension. The relative importance of axial mean stresses has been especially pointed out in the exhaustive comparative multiaxial methods performed by Papuga in 2011 [2], which concluded that the ability to correctly collect the effect of mean axial stresses is the most determining factor in the prediction. Practically all of the multiaxial methods take into account the influence of axial mean stresses, the way in which they treat the effect of the same can differ to a great extent
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