Constitutive modelling and numerical simulation of multivariant phase transformation in superelastic shape‐memory alloys

This work concerns the micromechanical constitutive modelling, algorithmic implementation and numerical simulation of polycrystalline superelastic alloys under multiaxial loading. The model is formulated in finite deformations and incorporates the effect of texture. The numerical implementation is based on the constrained minimization of the Helmholtz free energy with dissipation. Simulations are conducted for thin tubes of Nitinol under tension–torsion, as well as for a simplified model of a biomedical stent. Copyright © 2004 John Wiley & Sons, Ltd.     

Electrochemical micromachining of nitinol by confined-etchant-layer technique

The confined etchant layer technique has been applied to fabricate complex three-dimensional microstructures on nitinol for the first time. HF and HNO₃ were locally and simultaneously electrogenerated at the mold surface to etch a nitinol workpiece. NaOH was used as an efficient scavenger to confine the etchant close to the mold. Cyclic voltammetry was employed to study the electrochemical behavior of a Pt electrode in the etching solution in order to choose an appropriate potential for etchant generation on the mold. The thickness of the confined etchant layer was estimated to be several micrometers by inspecting the deviation of the sizes of the etched spots from the sizes of those on the microelectrode. Thus, the composition of the electrolyte could be optimized for better etching precision. By optimizing the composition of the electrolyte, complex microstructures on a Pt-Ir mold bearing the logo “XMU” of Xiamen University were successfully fabricated on nitinol. The etched patterns were approximately negative copies of the mold, and the precision of duplication could easily reach the micrometer scale.     

Estimating probabilistic fatigue of Nitinol with scarce samples

Current work estimates probabilistic fatigue life efficiently with scarce samples. The underlying idea of the estimation is to approximate the cumulative distribution function of the fatigue life in a transformed space using a third order polynomial subject to monotonicity constraint. The variations associated with the estimated quantiles are quantified using bootstrap. The proposed approach is validated on a data obtained from literature. It is observed that the life quantiles with reasonable accuracy can be estimated even with 10 samples. Finally, the probabilistic fatigue of Nitinol in austenitic condition is obtained with limited experiments.     

Mechanical property analysis of Nitinol defective stent under uniaxial loading/unloading

The vascular stents are important medical devices introduced into a vessel to protect the lumen from unfriendly stenosis. However, Nitinol stent is easy to fail in practice. The present paper focuses on the influence of defects on its mechanical behavior by finite element analysis. The essential stent cell is used with two different type of defects, which includes the face bulge defect resulted from laser burrs or TiC inclusion arises and C‐contained particle voids. Auricchio’s super‐elastic consititutive equations are used in the finite element simulations. It is found that the stress distribution is not only related to the defect type but also to the defect position. With the increasing distance from the TiC defect to the knot’s notch, the influence of defects on stress distribution of stents becomes small. For void defects, those near both the inner fillet and the outer fillet have grand influence on the stent’s global stress distribution. In particular, the higher stress concentrations are undergone near knot’s defects. For all models, the maximum Martensite Volume Fraction is near knots. The finite element analysis shows that cracks/fractures can easily appear near knots. A stent with a TiC inclusion or void defect is likely to fail. All obtained conclusions can be useful to design against stent premature mechanical failure.     

Pulsed Nd:YAG laser cutting of NiTi shape memory alloys—Influence of process parameters

Shape memory alloys (SMAs), in particular Nitinol (NiTi), are of increasing interest in research and industry due to their outstanding properties, e.g. the shape memory effect (SME) and high biocompatibility. Obviously, it is necessary to machine these elements from NiTi sheet materials using suitable processing methods that provide high precision and retain the shape memory effect. Pulsed Nd:YAG laser cutting of 1 mm thick NiTi shape memory alloys for medical applications (SMA-implants) has been investigated. Due to the local energy input only small heat-affected zones (HAZ) occur and the shape memory properties remain. The influence of key parameters like pulse energy, pulse width, and spot overlap on the cut geometry, roughness and HAZ is shown.     

Modeling and simulation of an artificial muscle and its application to biomimetic robot posture control

The shape memory effect exhibited by Nitinol wire can be utilized to construct an artificial muscle. The muscle is activated by an electric current, which produces heat and initiates a phase transformation. The Nitinol artificial muscle stress–strain–power relationship was determined by experiments, and a mathematical model was developed. The artificial muscle model was utilized for the posture control of a biomimetic underwater robot. The optimal activation patterns for height, pitch, and roll postures were determined. Simulation results for the height postures are in agreement with the experiments. The separation between the center of gravity and the centroid of the robot has a stabilizing effect on pitch and roll postures.     

Thermodynamics cycle analysis and numerical modeling of thermoelastic cooling systems

To avoid global warming potential gases emission from vapor compression air-conditioners and water chillers, alternative cooling technologies have recently garnered more and more attentions. Thermoelastic cooling is among one of the alternative candidates, and have demonstrated promising performance improvement potential on the material level. However, a thermoelastic cooling system integrated with heat transfer fluid loops have not been studied yet. This paper intends to bridge such a gap by introducing the single-stage cycle design options at the beginning. An analytical coefficient of performance (COP) equation was then derived for one of the options using reverse Brayton cycle design. The equation provides physical insights on how the system performance behaves under different conditions. The performance of the same thermoelastic cooling cycle using NiTi alloy was then evaluated based on a dynamic model developed in this study. It was found that the system COP was 1.7 for a baseline case considering both driving motor and parasitic pump power consumptions, while COP ranged from 5.2 to 7.7 when estimated with future improvements.     

Evaluation of hydrogen absorption behaviour during acid etching for surface modification of commercial pure Ti, Ti-6Al-4V and Ni-Ti superelastic alloys

The hydrogen absorption behaviour during acid etching for the surface modification of commercial pure Ti, Ti-6Al-4V and Ni-Ti superelastic alloys has been investigated on the basis of the surface morphology, electrochemical behaviour and hydrogen thermal desorption analysis. To simulate the conventional acid etching for the improvement of the biocompatibility of Ti alloys, the specimens are immersed in 1 M HCl, 1 M H₂SO₄ or 0.5 M HCl + 0.5 M H₂SO₄ aqueous solution at 60 °C. Upon immersion, commercial pure Ti absorbs substantial amounts of hydrogen irrespective of the type of solution. In H₂SO₄ or HCl + H₂SO₄ solutions, the hydrogen absorption occurs for a short time (10 min). For Ti-6Al-4V alloy, no hydrogen absorption is observed in HCl solution, whereas hydrogen absorption occurs in other solutions. For Ni-Ti superelastic alloy, the amount of absorbed hydrogen is large, resulting in the pronounced degradation of the mechanical properties of the alloy even for an immersion time of 10 min, irrespective of the type of solution. The hydrogen absorption behaviour is not necessarily consistent with the morphologies of the surface subjected to corrosion and the shift of the corrosion potential. The hydrogen thermal desorption behaviour of commercial pure Ti and Ni-Ti superelastic alloy are sensitively changed by acid etching conditions. The present results suggest that the evaluation of hydrogen absorption is needed for each condition of acid etching, and that the conventional acid etching often leads to hydrogen embrittlement.     

镍钛合金血管支架力学性能测定

支架丝的力学性能参数对于支架的使用至关重要,支撑力是决定血管支架性能好坏的最重要的因素,只有具备良好的支撑性能,才能使支架充分地撑开血管起到使血流畅通的作用。选用近等原子比镍钛合金作为支架材料使用,DSC测定合金丝相变温度,支架丝材拉伸试验测定力学性能,通过支架支撑力的测定及与国外知名支架的对比,发现所选用合金丝制作的血管支架径向力和弹性性能良好,仅需改造端部结构即可达到更优于国外支架的使用效果。     

4D Printing: Multi-Material Shape Change

How might 4D printing overcome the obstacles that are hampering the rolling out and scaling up of 3D printing? Skylar Tibbits , Director of the Self-Assembly Lab at the Massachusetts Institute of Technology (MIT), describes how the Lab has partnered up with Stratasys Ltd, an industry leader in the development of 4D Printing, and is making the development of self-assembly programmable materials and adaptive technologies for industrial application in building design and construction its focus.