Comparison between SSF and Critical-Plane models to predict fatigue lives under multiaxial proportional load histories

Manuel De Freitas,Jaime Tupiassú Pinho De Castro,Marco Antonio Meggiolaro,Luis Reis

doi:10.3221/igf-esis.38.16

Abstract

Materials can be classified as shear or tensile sensitive, depending on the main fatigue microcrack initiation process under multiaxial loadings. The nature of the initiating microcrack can be evaluated from a stress scale factor (SSF), which usually multiplies the hydrostatic or the normal stress term from the adopted multiaxial fatigue damage parameter. Low SSF values are associated with a shear-sensitive material, while a large SSF indicates that a tensile-based multiaxial fatigue damage model should be used instead. For tension-torsion histories, a recent published approach combines the shear and normal stress amplitudes using a SSF polynomial function that depends on the stress amplitude ratio (SAR) between the shear and the normal components. Alternatively, critical-plane models calculate damage on the plane where damage is maximized, adopting a SSF value that is assumed constant for a given material, sometimes varying with the fatigue life (in cycles), but not with the SAR, the stress amplitude level, or the loading path shape. In this work, in-phase proportional tension-torsion tests in 42CrMo4 steel specimens for several values of the SAR are presented. The SSF approach is then compared with critical-plane models, based on their predicted fatigue lives and the observed values for these tension-torsion histories. KEYWORDS. Multiaxial fatigue life prediction; Critical-plane approach; Polynomial stress scale factor approach.

Highlights

I nitiating microcracks under multiaxial fatigue loadings can be sub-divided into shear or tensile types [1]
Low stress scale factor (SSF) values are associated with a shear-sensitive material, while a large SSF indicates that a tensile-based multiaxial fatigue damage model should be used instead
For tension-torsion histories, a recent published approach combines the shear and normal stress amplitudes using a SSF polynomial function that depends on the stress amplitude ratio (SAR) between the shear and the normal components

Summary

INTRODUCTION

I nitiating microcracks under multiaxial fatigue loadings can be sub-divided into shear or tensile types [1]. Other materials may initiate fatigue cracks on planes of maximum tensile strain or stress ranges, e.g. 304 stainless steel under certain load histories and cast irons [2]. In this case, even if the microcrack nucleates in shear, its so-called initiation life (which always includes some microcrack propagation) is controlled by its growth in a direction perpendicular to the maximum principal stress or strain. Models based on the critical-plane approach calculate multiaxial fatigue damage on the plane where it is maximized (not on the plane where the load is applied), while adopting a SSF value that is assumed constant for a given material, sometimes varying with the fatigue life (in cycles), but not with the SAR, stress amplitude level, or loading path shape. The SSF and two critical-plane approaches are compared, based on their predicted fatigue lives and the experimentally measured ones

SSF EQUIVALENT SHEAR STRESS APPROACH

CRITICAL PLANE APPROACH

RESULTS AND DISCUSSIONS

CONCLUSIONS