Interaction of shear and normal stresses in multiaxial fatigue damage analysis

Nicholas R Gates,Ali Fatemi

doi:10.3221/igf-esis.37.22

Abstract

Due to the abundance of engineering components subjected to complex multiaxial loading histories, being able to accurately estimate fatigue damage under multiaxial stress states is a fundamental step in many fatigue life analyses. In this respect, the Fatemi-Socie (FS) critical plane damage parameter has been shown to provide excellent fatigue life correlations for a variety of materials and loading conditions. In this parameter shear strain amplitude has a primary influence on fatigue damage and the maximum normal stress on the maximum shear plane has a secondary, but important, influence. In this parameter, the maximum normal stress is normalized by the material yield strength in order to preserve the unitless feature of strain. However, in examining some literature data it was found that in certain situations the FS parameter can result in better fatigue life predictions if the maximum normal stress is normalized by shear stress range instead. These data include uniaxial loadings with large tensile mean stress, and some non-proportional axial-torsion load paths with different normal-shear stress interactions. This modification to the FS parameter was investigated by using fatigue data from literature for 7075-T651 aluminum alloy, as well as additional data from 2024-T3 aluminum alloy fatigue tests performed in this study.

Highlights

There are many different steps in the fatigue life estimation process, relating the variation of stresses and strains to the fatigue damage that occurs within a material is the most fundamental part of any fatigue life analysis
A factor of ±2.5 error is still very reasonable for multiaxial fatigue life predictions, these results suggest that there is room for some improvement in fatigue damage calculation with respect to the quantification of normal-shear stress/strain interaction
Considering the load path dependence of fatigue damage, the second part of this study focuses on a limited number of fatigue tests designed to differentiate between the original FS parameter and the modified parameter developed in the first part of the study

Summary

Introduction

There are many different steps in the fatigue life estimation process, relating the variation of stresses and strains to the fatigue damage that occurs within a material is the most fundamental part of any fatigue life analysis. The FS parameter, based on either shear or uniaxial fatigue properties, has been shown to correlate experimental and predicted fatigue lives well for a variety of materials and loading conditions. In addition to mean stress effects, Shamsaei [6] performed fatigue tests on 1050 steel for different multiaxial loading paths which produced the same fatigue damage predictions based on the FS parameter.

Results

Conclusion