Multiaxial variable amplitude fatigue life analysis using the critical plane approach, Part I: Un-notched specimen experiments and life estimations

Abstract

Despite its importance in many engineering applications, fatigue life assessment is still not well understood under multiaxial variable amplitude loading conditions. Although critical plane-based analysis approaches have been shown to work well for constant amplitude multiaxial loading, results for variable amplitude life estimations have been mixed. This is likely due to the consideration of additional factors in a variable amplitude analysis, which can include load history effects, varying degrees of loading non-proportionality, changes in mean stress, cycle counting, and fatigue damage accumulation. In order to better understand the impact that each of these factors can have on life estimation accuracy, this study compares experimental results for both un-notched and notched specimens of 2024-T3 aluminum alloy to variable amplitude life estimations computed using a critical plane-based analysis approach. A modified version of the Fatemi-Socie parameter was used to calculate fatigue damage, while different analysis procedures were implemented to study the effect of transient material deformation behavior and crack initiation definition on life estimation accuracy. Part I of this two part paper presents the experimental results and analyses for un-notched specimen tests, while part II applies the same life assessment techniques to situations involving notched specimens. While the best analysis procedures investigated were able to estimate most un-notched specimen fatigue lives within a factor of 3 of experimental results, life estimations had a general tendency to be non-conservative and were not as accurate as those obtained under constant amplitude loading conditions.