Issue 50

J. Papuga et alii, Frattura ed Integrità Strutturale, 50 (2019) 163-183; DOI: 10.3221/IGF-ESIS.50.15

has any impact on the fatigue life, which is manifested in their criterion by comparable stress amplitudes on appropriate load channels for in-phase and out-of-phase loadings. Their assumption is contradicted by a substantial part of the experimental data that they use as a proof. If the data presented by Nishihara and Kawamoto, and by Lempp, show the intrinsic phase shift effect, it projects itself in the values of the error indexes for this criterion. However, a careful check of the results for the whole data set casts doubts on usefulness of these two data sets for quantitatively showing the effect of the phase shift.

Figure 1 : Examples of Lempp’s [17] experimental data.

Similar data sets when processed in the equivalent stress amplitude by Pejkowski [20] lead him to the conclusion that out of-phase loading causes a significant reduction in the fatigue limit. This conclusion is driven by the use of the equivalent stress that Pejkowski proposes. Other equivalent stresses can be proposed to evaluate the phase shift effect. Technically, any multiaxial high-cycle fatigue strength estimation criterion is designed to compute this equivalent stress, which is then compared with the uniaxial fatigue strength for the same lifetime. The correct solution can only be found by evaluating experimental data on various load combinations, including in-phase and out-of-phase data. To avoid any other potential effects that could affect the quantification, only load cases with the same stress ratio (the ratio of the normal stress amplitude to the shear stress amplitude) should be compared. The admissible stress amplitudes for each load case then clearly show whether the fatigue strength rises or falls when a comparison is made between in-phase and out-of-phase load combinations. This paper addresses the question of the existence of the phase shift effect. The authors have made every effort to build a credible database of multiaxial experiments by collecting data from the available literature and by performing their own experiments. This database was to be used to verify the prediction quality of selected criteria in conditions of multiaxial fatigue with the phase shift effect. To demonstrate only this effect, without mixing it with other effects, the experimental

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