Abstract

In this work, a one-dimensional constitutive model is used to study rotary bending fatigue in shape memory alloy beams. The stress and strain distributions in a beam section are driven numerically for both pure bending and rotary bending to show the basic differences between these two loading types. In order to verify the numerical results, experiments are performed on NiTi specimens with an imposed bending angle using a bending apparatus. Since the specimens show significant stress plateau for forward and backward transformation in their stress–strain response, an enhanced stress–temperature phase diagram is proposed in which different slopes are considered for the start and finish of each transformation strip. In order to study low cycle fatigue of shape memory alloys during rotary bending, the stabilized dissipated energy is calculated from numerical solution. A power law for variations of the fatigue life with the stabilized dissipated energy is obtained for the studied specimens to predict their fatigue life. The numerical predictions of the present approach are shown to be in a good agreement with the experimental findings for rotary bending fatigue. Uniaxial tensile fatigue tests are further performed on the studied specimens to investigate effect of loading type on the fatigue lifetime.

Highlights

  • Shape memory alloys (SMAs) are an extraordinary group of materials that have been attracting much attention in recent years

  • The present study focuses on basic understanding of rotary bending fatigue (RBF) in a pseudoelastic SMA beam

  • An enhanced stress–temperature phase diagram is presented for cases where gradual forward and reverse transformations occur, and a one-dimensional constitutive model is utilized to determine moment–curvature response

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Summary

Introduction

Shape memory alloys (SMAs) are an extraordinary group of materials that have been attracting much attention in recent years. Due to a reversible martensitic transformation, they can display the unusual properties of shape memory effect and superelasticity (pseudoelasticity). These unique properties enable them to be used in a wide range of medical (Duerig et al, 1999) and nonmedical (Van Humbeeck, 1999) applications. Many SMA devices are subjected to cyclic loading which may well lead to fatigue. Characterizing the fatigue behavior of these materials in cyclic loading is overriding. SMA components are generally subjected to other kinds of loading such as rotary bending in endodontic instruments (Plotino et al, 2009). When the instrument rotates freely in the tooth root canal, tension/compression cycles are generated at the point of maximum flexure until the fracture occurs

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