Bending of Superelastic Wires, Part I: Experimental Aspects

Berg, Brian T.

doi:10.1115/1.2895952

Cited by 26 publications

(21 citation statements)

References 2 publications

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“…Bending is one of the predominant loading modes in many wire-based endovascular devices (Berg, 1995;Wick et al, 2005). As the fatigue life of pseudoelastic Nitinol is straincontrolled, rotary bend testing, which provides a simple method for strain-controlled materials characterization of wires, has been employed extensively over the past decade to investigate the fatigue behavior of medical grade Nitinol.…”

Section: Introductionmentioning

confidence: 98%

High compressive pre-strains reduce the bending fatigue life of nitinol wire

Gupta

Pelton²,

Weaver

et al. 2015

Journal of the Mechanical Behavior of Biomedical Materials

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Prior to implantation, Nitinol-based transcatheter endovascular devices are subject to a complex thermo-mechanical pre-strain associated with constraint onto a delivery catheter, device sterilization, and final deployment. Though such large thermo-mechanical excursions are known to impact the microstructural and mechanical properties of Nitinol, their effect on fatigue properties is still not well understood. The present study investigated the effects of large thermo-mechanical pre-strains on the fatigue of pseudoelastic Nitinol wire using fully reversed rotary bend fatigue (RBF) experiments. Electropolished Nitinol wires were subjected to a 0%, 8% or 10% bending pre-strain and RBF testing at 0.3-1.5% strain amplitudes for up to 10(8) cycles. The imposition of 8% or 10% bending pre-strain resulted in residual set in the wire. Large pre-strains also significantly reduced the fatigue life of Nitinol wires below 0.8% strain amplitude. While 0% and 8% pre-strain wires exhibited distinct low-cycle and high-cycle fatigue regions, reaching run out at 10(8) cycles at 0.6% and 0.4% strain amplitude, respectively, 10% pre-strain wires continued to fracture at less than 10(5) cycles, even at 0.3% strain amplitude. Furthermore, over 70% fatigue cracks were found to initiate on the compressive pre-strain surface in pre-strained wires. In light of the texture-dependent tension-compression asymmetry in Nitinol, this reduction in fatigue life and preferential crack initiation in pre-strained wires is thought to be attributed to compressive pre-strain-induced plasticity and tensile residual stresses as well as the formation of martensite variants. Despite differences in fatigue life, SEM revealed that the size, shape and morphology of the fatigue fracture surfaces were comparable across the pre-strain levels. Further, the mechanisms underlying fatigue were found to be similar; despite large differences in cycles to failure across strain amplitudes and pre-strain levels, cracks initiated from surface inclusions in nearly all wires. Compressive pre-strain-induced damage may accelerate such crack initiation, thereby reducing fatigue life. The results of the present study indicate that large compressive pre-strains are detrimental to the fatigue properties of Nitinol, and, taken together, the findings underscore the importance of accounting for thermo-mechanical history in the design and testing of wire-based percutaneous implants.

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Section: Introductionmentioning

confidence: 98%

High compressive pre-strains reduce the bending fatigue life of nitinol wire

Gupta

Pelton²,

Weaver

et al. 2015

Journal of the Mechanical Behavior of Biomedical Materials

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“…Our goal is to use the moment-curvature relation for a pseudoelastic rod, derived in [1] to formulate the three point bending problem as presented in [3]. Also a specific buckling problem will be analyzed.…”

Section: Introductionmentioning

confidence: 99%

On the Bending and Stability Problems for Shape Memory Materials

Atanacković¹

2000

Journal of the Mechanical Behavior of Materials

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The load deformation curve in uniaxial tension, for pseudoelastic materials (materials with shape memory effects) is characterized by a hysteresis in a process of loading and unloading. On the basis of this results we proposed in [1] a moment curvature relation that describes pure bending (bending by couples) of a memory wire. Recently, in [3] a series of experimental results were reported on pure and three point bending of memory wires. In this work we shall discuss the possibility of using the moment curvature relation obtained in [1] to describe the three point bending problem and the problem of stability of axially loaded wire of memory material.Brought to you by | University of Arizona Authenticated Download Date | 5/29/15 9:26 PM

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“…Studies on small diameter NiTi wire naturally restrict pointbending fixtures as a result of small radii of curvature and large displacements -conditions where undesirable axial loads can develop due to the support constraints. The studies conducted by Berg [13] and Bundara et al [14] have employed momentcontrolled experiments to assess the constitutive relationship between applied pure bending moments and the resulting curvatures. These studies, however, suffer a major limitation due to their lack of control over strain rate, leading to a significant impact on the stress/strain response during phase transformations [12].…”

Section: Fig 1 Concept Sketchmentioning

confidence: 99%

In situ Observation of NiTi Transformation Behaviour: A Micro–Macro Approach

Wickramasinghe

Tomlinson

Rongong

2014

Experimental and Applied Mechanics, Volume 6

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ABSTRACT:A novel experimental investigation is presented of thermally and stress induced transformation behaviour of a Polycrystalline NiTi Shape Memory Alloy (SMA) plate for flexural-type applications: In situ techniques are employed to allow simultaneous macroscopic and microstructural observation of the SMA in a 4-point flexural test. Forming part of a wider research towards realising a NiTi SMA Variable Stator Vane assembly for the gas turbine engine, the study explores variables critical to flexural-type morphing NiTi structures: (1) temperature; (2) strain; and (3) cyclic loading. It builds a relationship between the macro and micro response of the SMA under these key variables and lends critical implications for the future understanding and modelling of shape memory alloy behaviour for all morphing applications. This paper presents the methodological aspects of this study. KEYWORDS:Shape memory; NiTi; In situ; Phase transformations; Micro-macro approach; Cyclic loading INTRODUCTIONGas turbine performance development has been governed traditionally by the Òworst caseÓ deterioration and operating condition [1]. This leads to severe compromises and large safety margins. Active control of the engine operation using smart materials could potentially improve engine efficiency. Shape Memory Alloys (SMAs), a class of smart materials, exhibit several desirable characteristics exploitable for this purpose. NiTi, based on an equiatomic compound of nickel and titanium is the most widely used SMA in commercial applications [2]. Besides the ability of tolerating relatively large amounts of shape memory strain, NiTi shows high stability in cyclic applications, possesses an elevated electrical resistivity, and is corrosion resistant [3].

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Bending of Superelastic Wires, Part I: Experimental Aspects

Cited by 26 publications

References 2 publications

High compressive pre-strains reduce the bending fatigue life of nitinol wire

High compressive pre-strains reduce the bending fatigue life of nitinol wire

On the Bending and Stability Problems for Shape Memory Materials

In situ Observation of NiTi Transformation Behaviour: A Micro–Macro Approach

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