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.     

热型连铸CuAlBe超弹性合金丝的力学性能

研究热型连铸柱状晶CuAlBe超弹性合金的性能并与单晶相比较。用热型连铸法制备了柱状晶CuAlBe超弹性合金丝,用循环拉伸试验检测其力学性能,用纯弯曲疲劳试验检测其弯曲疲劳寿命。结果表明,柱状晶CuAlBe合金丝的可恢复应变可达到15％,与单晶接近。超过这一应变量将导致马氏体稳定化,产生残余变形,导致卸载时拉伸曲线上出现锯齿状峰。柱状晶的弯曲疲劳寿命与单晶的为同一数量级,比机械加工表面的单晶高,而比电解抛光表面的单晶低。由此可见,柱状晶CuAlBe的性能与单晶的相近。     

Ultralight and superelastic polyvinyl alcohol/SiC nanofiber/reduced graphene oxide hybrid foams with excellent thermal insulation and microwave absorption properties

Being in the strategic direction of next-generation absorbers, multifunctional microwave absorbing materials possess great application value in military and commercial fields. However, the stringent requirements for performance necessitate the combination of multiple functions in such type of composites, which is still a challenge. This work aims to develop a foam-type absorber composed of multi-dimensional organic and inorganic materials, in which reduced graphene oxide sheets and polyvinyl alcohol membranes serve as the framework and crosslinker to form a three-dimensional skeleton. Meanwhile, SiC nanofibers as a reinforcing component can effectively suppress the over-stacking of reduced graphene oxide and enhance the conductivity and mechanical strength of cell walls. Among the remarkable microwave absorbing properties of the obtained foam, there are the ultra-light (9.85 mg cm^-3), broadband (7.04 GHz), and strong absorption (reflection loss of -61.02 dB), all combined in the ultra-thin (2.5 mm). In addition, the foam possesses superelastic and excellent heat-insulating characteristics that ensure shock resistance, heat preservation, and infrared stealth. The remarkable versatility benefits from the porous structure, as well as from the synergistic effect of multi-dimensional organic and inorganic constituents of the foam. Therefore this study lays the foundation for the design of new-generation microwave absorbers with broad application potential.     

含形状记忆合金(SMA)层柔性梁的振动频响特性

主要研究具有形状记忆合金（shape memory alloy,SMA）层的梁结构的动力响应特性，重点分析了应力诱发马氏体相变的影响。首先采用SMA超弹性分段线性的应力－应变模型表示SMA的超弹性本构特征；其次借助有关粘弹性材料结构动力学分析的复模量方法，推出简谐激励作用下SMA层的面内变形和应力之间的关系，提出具有SMA表层的简支梁横向稳态频率响应求解的数学模型。SMA超弹性非线性的影响使得结构产生复杂的动力学特征，如存在多值稳态解、跳跃性、周期3响应等。上述现象的发生与激振力幅值、温度的变化密切相关。     

Modeling and analysis of the vibration behavior of a shape memory alloy beam

Shape memory alloys have a high potential for passive isolation of vibrations as well as capability of being used as an active vibration isolation system for their shape memory behavior, i.e. having hysteresis along with superelastic behavior. Study of this behavior necessitates recognition of distinct specifications of this alloy and also presentation of a suitable and simple mathematical model. In this paper, a mathematical model based on Auricchio model, considering asymmetry in tension and compression and also temperature effects on hysteresis at superelastic conditions has been presented. Finally, dynamical behavior of a NiTi beam under free vibration as well as application of sinusoidal and impulse loads upon free–clamped and also simply supported conditions have been analyzed.     

Effect of strain rate on properties of superelastic NiTi thin wires

This study deals with the effect of strain rate on tensile and energy absorbing properties of superelastic NiTi thin wires. It also attempts to gain an understanding of the interplay of the ductile behavior, temperature and strain rate effects, energy storage and cycling. The wires are in austenite condition at room temperature and above. The strain rates imposed during testing range from 0.2 to 180%/min (i.e., 0.06–54 mm/min) corresponding to a frequency of 2.77 × 10⁻⁴ to 0.25 Hz for strain amplitudes of 6%. The corresponding frequency for 8% strain amplitude is 2.08 × 10⁻⁴ to 0.18 Hz. It is shown that NiTi SMAs exhibit ductility at both low and high strain rates. This is also true for the cold worked and heat treated conditions both below M_f and above A_f. During tensile testing the stress-induced martensite (SIM) plateau increases in length and translates upwards with increase in strain rate up to a certain value. Similarly, the onset of elastic yield stress also increases with strain rate. At high strain rates the SIM segment and elastically deformed SIM segment overlap. The SIM formation is not able to cope with the externally imposed higher strain rates. This is also the reason for the reduction of hysteresis loop at the high strain rates as observed in the cyclic tests.

The dissipated strain energy density (E_d) increases with increasing strain rate up to a certain value beyond which the E_d decreases. It is clear that the mean point of the superelastic loop shifts to the right and upwards (higher stress and higher strain region) for cyclic testing with increase in strain rates. However, it shifts to the right and downwards (lower stress/higher strain regime) for both the 6 and 8% strain amplitude cycling at constant strain rate. The stabilization of residual strain and E_d is based on the same underlying mechanism relating to SIM formation and occurs at the same numbers of cycles.     

A numerical study on bolted end-plate connection using shape memory alloys

Superelastic shape memory alloys (SMAs) have the ability to recover their original shape after experiencing large strains. A new beam-to-column connection incorporating long shank SMA bolts is presented in this paper. By using the unique characteristics of SMAs, the connection possesses self-centering abilities. The 3D connection model is created using the software ANSYS, and Auricchio’s model is used to simulate the superelastic behavior of the SMA bolts. With cyclic loads applied on the beam ends, the behavior of the connection is studied. The results show the semi-rigid and moderate energy dissipation characteristics of the connection. Since the moment-carrying capacity of bolt cluster controlled below the elastic flexural capacity of connecting beam, a superelastic hinge forms just at the beam-to-column interface. The inelastic interstory drift angle of the connection reaches 0.035 rad, and 94% of the total rotations are recoverable upon unloading.     

形状记忆合金的超弹性行为

超弹性是形状记忆合金最重要的力学性能之一,对于超弹性的研究一直是形状记忆合金材料研究的热点。从基本理论出发,综述了目前国内外描述形状记忆合金超弹性行为的本构模型,并分析总结了影响超弹性的因素,以及各种本构模型在描述形状记忆合金超弹性方面的优缺点。     

形状记忆合金阻尼器在斜拉桥地震控制中的应用

着重介绍和研究一种用在斜拉桥上由形状记忆合金制成的被动抗震控制装置的性能。对形状记忆合金的超弹性和阻尼能力进行探寻,以建立辅助确定中心和能量耗散装置。建立包括土-结构之间的相互作用三维长跨桥模型,并应用在本研究中。形状记忆合金阻尼器被置于桥的桥面板-桥墩和桥面板-塔的连接处。桥受到来自两个历史记录波、三个正交分量的作用,对形状记忆合金阻尼器在控制桥面板位移的效果以及对桥塔上剪力和弯矩限制的作用进行评估。而且,对形状记忆合金阻尼器的滞回特性进行研究,以确定桥响应的敏感性。研究显示:形状记忆合金阻尼器能有效地控制桥的抗震性能,然而,这种新型阻尼器受桥面板-桥墩和桥面板-塔的连接处的相对刚度影响大。研究结果也显示,与逆相变阶段,卸荷应力的变化相比,在相变期间,形状记忆合金阻尼器应变硬化的变化对桥响应的影响较小。     

The FEM simulation of mechanical properties characterization of the stent under the quasi‐static loading/unloading. FEM‐ Simulation zur Charakterisierung der mechanischen Eigenschaften von Stents unter quasistatischer Beanspruchung

In this paper, the detailed stress distributions of NiTi stents have been studied. The primary object of this study is to set up a cell model to reveal the cause of crack initiation for stents. Auricchio’s super‐elastic model is used to analyze the essential stent cell. Two different kinds of models are analyzed. One model is the stent without defect and the other model is a stent with defects caused by laser burrs or TiC inclusions. The structural stress distribution is not only related to a specimen without defect but also to the material of a bulge defect. The maximum stresses of the stent without defect are near the knots, while the one with defects possesses the maximum stress near the surrounding of the bulge defect. It was found that cracks/fractures can easily appear within a distance of 1/3 of the knot to knot spacing. In particular, a stent with TiC inclusion defects is likely to fail near the bulge face defect. Our simulations agree with experimental results.