Finite element simulation of the influence of TiC inclusions on the fatigue behavior of NiTi shape-memory alloys
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I. INTRODUCTION
SHAPE-MEMORY alloys (SMAs) are well known for their shape-memory effect and superelasticity. These behaviors are due to intrinsic thermoelastic martensitic transformations at different temperatures. Martensitic transformations involve a lattice transformation that, in turn, involves a shear deformation and a coordinated atomic movement; the coordinated atomic movement maintains the one-to-one lattice correspondence between the lattice point in the parent and in the transformed phases. The nearly equiatomic Ni-Ti SMAs have excellent superelastic and shape-memory behavior, long fatigue life, good corrosion resistance, and biocompatibility.[1,2,3] Plentiful work has been done on Ni-Ti SMAs. Most of the work published in the literature focuses on the functional properties. To fully exploit the potential of Ni-Ti SMAs in developing functional structures or smart structures in mechanical and biomechanical engineering, it is important to understand the failure mechanisms of the SMAs. Yi and Gao investigated the fracture toughness of SMAs under different far-field loading conditions by applying Sun and Hwang’s constitutive model in the regime of shape memory.[4,5] In Yan and Wang’s article, the effect of phase-transformation-induced volume contraction on the fracture properties of superelastic SMAs has been studied theoretically.[6] The fatigue life and the fatigue thresholds of Ni-Ti SMAs have also been studied by experimental tests.[7,8] In order to investigate a nonlinear
X.M. WANG and Y.F. WANG, Postdoctoral Students, and Z.F. YUE, Professor, are with the School of Mechanics, Civil Engineering and Architecture, Northwestern Polytechnical University, Xi’an, 710072 People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted September 7, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
drilling device, Takahiro et al. have studied the structural fatigue of superelastic Ni-Ti wires using bending-rotation fatigue (BRF) tests.[9] In some other applications, such as a medical guidewire, the SMAs may also be loaded by bending. So far, BRF has evolved as one standard method for studying the structural fatigue of superelastic NiTi wires. So, it is necessary to study the fatigue behaviors of Ni-Ti SMAs under a bending load. In BRF experiments, it is found that the crack may be initiated at the TiC inclusions at the surface.[9] It is well known that the stress is linearly distributed in the cross section of an elastic beam subjected to pure bending. Since the SMAs have unique functional properties, the bending behavior must be different from that of the elastic material; there are no published works, however, by which the bending behavior of the SMAs can be analyzed quantitatively. In this article, Auricchio’s superelastic model is implemented into the finite element (FE) code ABAQUS[10–13] and calibrated on the basis of a set of uniaxial data obtained for superelastic NiTi wires.[14] The stress distribution on the cross section of the SMA wires with and without TiC inclusions subjected to p
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