Issue 37

N.R. Gates et alii, Frattura ed Integrità Strutturale, 37 (2016) 166-172; DOI: 10.3221/IGF-ESIS.37.23 166 Focussed on Multiaxial Fatigue and Fracture Fatigue crack growth behavior under multiaxial variable amplitude loading 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 Nagaraja Iyyer Technical Data Analysis, Inc. (TDA), 3190 Fairview Park Drive, Suite 650, Falls Church, VA 22042, USA. niyyer@tda-i.com Nam Phan US Naval Air Systems Command, 48110 Shaw Road, Building 2187, Suite 2320A, Patuxent River, MD 20670, USA. nam.phan@navy.mil A BSTRACT . This study compares both uniaxial and multiaxial variable amplitude experimental crack growth data for naturally initiated fatigue cracks in tubular specimens of 2024-T3 aluminum alloy to predictions based on two state-of-the-art analysis codes: UniGrow and FASTRAN. For variable amplitude fatigue tests performed under pure axial nominal loading conditions, both UniGrow and FASTRAN analyses were found to produce mostly conservative growth life predictions, despite good agreement with constant amplitude crack growth data. For variable amplitude torsion and combined axial-torsion crack growth analyses, however, the conservatism in growth life predictions was found to reduce. This was attributed to multiaxial nominal stress state effects, such as T-stress and mixed-mode crack growth, which are not accounted for in either UniGrow or FASTRAN, but were found in constant amplitude fatigue tests to increase experimental crack growth rates. Since cracks in this study were initiated naturally, different initial crack geometry assumptions were also investigated in the analyses. K EYWORDS . Fatigue Crack Growth; Multiaxial; Variable Amplitude; FASTRAN; UniGrow. I NTRODUCTION ost engineering components and structures are subjected to variable amplitude cyclic loadings throughout their service lives. Due to the nature of these loadings, they typically result in multiaxial stress states, and individual stress components can vary in a non-proportional manner. When such components are operating under a damage tolerant design philosophy, being able to predict how fatigue cracks will grow under these complex loading conditions is a topic of particular interest. One of the key requirements for the application of fracture mechanics concepts in a fatigue crack growth analysis is that conditions of similitude should be retained. Similitude implies that for a particular value of driving force parameter (e.g. J- M