Issue 37

N.R. Gates et alii, Frattura ed Integrità Strutturale, 37 (2016) 160-165; DOI: 10.3221/IGF-ESIS.37.22 160 Focussed on Multiaxial Fatigue and Fracture Interaction of shear and normal stresses in multiaxial fatigue damage analysis Nicholas R. Gates, Ali Fatemi Mechanical, Industrial and Manufacturing Engineering Department, The University of Toledo, 2801 W. Bancroft Street, Toledo, OH 43606, USA ngates@eng.utoledo.edu, afatemi@eng.utoledo.edu A BSTRACT . 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. K EYWORDS . Multiaxial Fatigue; Critical Plane; Fatemi-Socie; Mean Stress; Load Path Interaction. I NTRODUCTION lthough 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. 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 especially important. To this end, significant effort in the last few decades has been put into developing damage parameters which reflect the actual damage mechanisms of the fatigue failure process [1,2]. Chief among these are critical plane approaches, which build on the observation that fatigue cracks tend to initiate on preferred planes within a material. Critical plane approaches are typically based on the idea of crack initiation occurring on or around either the maximum principal plane or maximum shear plane. As a result, these approaches have the added benefit of being able to predict failure plane orientation, which is useful if a subsequent crack growth analysis is to be performed. A popular critical plane-based parameter for computing multiaxial fatigue damage in materials exhibiting shear failure mechanisms is the Fatemi-Socie (FS) parameter [3]. This parameter was formulated based on the idea that while alternating shear strain is the primary driving force behind fatigue crack initiation, the maximum normal stress on the A