Issue 38

S. Fu et al, Frattura ed Integrità Strutturale, 38 (2016) 141-147; DOI: 10.3221/IGF-ESIS.38.19 141 Focussed on Multiaxial Fatigue and Fracture Ratcheting of 316L stainless steel thin wire under tension-torsion loading Sichao Fu, Dunji Yu, Gang Chen, Xu Chen School of Chemical Engineering and Technology, Tianjin University fu_sc126@126.com , djyu@tju.edu.cn , agang@tju.edu.cn, xchen@tju.edu.cn A BSTRACT . A series of cyclic tension-torsion tests under symmetric shear strain and asymmetric axial stress control in various loading paths are conducted on 100 μm-diameter 316L steel wires applying a micro tension- torsion fatigue testing apparatus. The ratcheting strain of the thin wire increases with increasing axial mean stress and decreases in a sequence of linear, rhombic and circular paths. The macro-scale based cyclic plastic constitutive models with kinematic hardening rules of the Ohno-Wang (O- W) and the Chen-Jiao-Kim (C-J-K) are evaluated for the thin wire. Comparing with the O-W, the C-J-K predicts more accurately under high axial stress. While the loading path effects on ratcheting for wire specimens are basically simulated, the macro-based models tend to under-estimate the effect of phase difference between axial and torsional loadings and the ratcheting evolution in the initial 50 cycles. K EYWORDS . Thin wire; Tension-torsion; Ratcheting; 316L stainless steel. Citation: Fu, S., Yu, D., Chen, G., Chen, X., Ratcheting of 316L stainless steel thin wire under tension-torsion loading, Frattura ed Integrità Strutturale, 38 (2016) 141-147. Received: 30.05.2016 Accepted: 25.06.2016 Published: 01.10.2016 Copyright: © 2016 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION yclic mechanical behaviors under multiaxial loading conditions at small-scale are key issues in the design of durable micro-devices as well as crucial aspects in characterizing mechanical properties of materials. While the issues at macro-scale have been widely studied for decades, research on multiaxial mechanical properties at micro-scale is quite limited. Most researches on this issue have focused on the biaxial cyclic tensile behaviors of thin films on substrates [1, 2]. Another fundamental multiaxial property, the tension-torsional cyclic behavior, is barely studied experimentally at micro-scale because of the technological problems of precise loading control and measurements. Recently, a novel tension- torsional fatigue apparatus for micro-scale components has been developed [3] and enables the biaxial cyclic tests on thin wires. In this study, a series of cyclic tension-torsion tests under various loading paths are conducted on 100 μm-diameter 316L stainless steel wires. The material is widely used in micro-devices such as cardiovascular stents. In service, small-scale components may undergo multiaxial loading with tensile stress due to low structural stiffness, leaving multiaxial ratcheting C