Effects of mean stress and stress concentration on fatigue behavior of short fiber reinforced polymer composites S. MORTAZAVIAN and A. FATEMI Mechanical, Industrial and Manufacturing Engineering Department, The University of Toledo, 2801 West Bancroft Street, Toledo, OH, 43606, USA Received Date: 10 December 2014; Accepted Date: 26 June 2015; Published Online: 2015 ABSTRACT An experimental study was conducted to evaluate the effect of mean stress on fatigue behavior of two short glass fiber reinforced thermoplastic composites and the effect of stress concentration on fatigue behavior of an unreinforced and a short glass fiber reinforced thermoplastic. Load-controlled fatigue tests were conducted on unnotched (smooth) specimens at R ratios of À1, 0.1, and 0.3 in different mold flow directions or fiber orientations and at a range of temperatures between À40 and 125 °C. Effect of mean stress on fatigue life was found to be significant at all temperatures. Several mean stress parameters including modified Goodman, Walker, and Smith–Watson–Topper were evaluated for their ability to correlate mean stress data. A general fatigue life prediction model was also used to account for the effect of mean stress, temperature, and fiber orientation. Notched fatigue tests of an unreinforced polymer and a short glass fiber thermoplastic composite were also conducted using plate type specimens with a central circular hole and with or without the presence of mean stress. Effect of stress concentration was found to be considerable, with or without mean stress and in both the longitudinal and transverse directions. The commonly used Neuber’s rule for metallic materials, nonlinear finite element analysis, as well as critical distance approaches were utilized for notch deformation and fatigue life analyses. Keywords fatigue of short fiber polymer composite; mean stress effect; notch deformation; R ratio; stress concentration effect. NOMENCLATURE a Peterson’s material characteristic length A fatigue strength intercept b fatigue strength exponent B S–N line slope c fatigue ductility exponent D damage E elastic modulus E′ cyclic elastic modulus K′ cyclic strength coefficient K f fatigue notch factor K t elastic stress concentration factor ΔK th threshold stress intensity range L critical distance length n′ strain hardening exponent N applied cycles N f cycles to failure q fatigue notch sensitivity r notch radius R stress ratio S 11 stress component in specimen axis direction S 22 stress component in specimen width direction S 33 stress component in specimen thickness direction Correspondence: A. Fatemi. E-mail: afatemi@eng.utoledo.edu © 2015 Wiley Publishing Ltd. Fatigue Fract Engng Mater Struct 00 1–18 1 ORIGINAL CONTRIBUTION doi: 10.1111/ffe.12341