Numerical simulation for the behavior of superelastic shape memory alloys

Superelastic shape memory alloys (SMAs) are unique metallic materials that undergo substantial plastic deformations and recover their original conditions when stresses are only removed without any heat treatment. SMAs have currently become prevalent for application in structural engineering because this superelastic property contributes to entire construction system by mitigating the problem of permanent deformation. Notwithstanding many structural advantages, there exist relatively few investigations on the numerical modeling of these smart materials, which had been mostly used for nonlinear analyses. For this reason, this study mainly focuses on a one-dimensional (1D) constitutive model able to simulate the inherent behavior of superelastic SMAs, taking into account phase transformation between austenite and martensite. After discussing a possible approach for the solution scheme, numerical simulation results are compared to experimental data obtained from pull-out tests that are performed on SMA bars in order to validate the adequacy for the 1D constitutive material model presented. Furthermore, the user material model based on the solution algorithm of reproducing this superelastic behavior is applied to the structural analysis with a view to assure adequacy in practical use.     

Mechanical behaviour of shape memory alloys for seismic applications 2. Austenite NiTi wires subjected to tension

The results of cyclic tensile tests on superelastic NiTi shape memory alloy (SMA) wires are presented and discussed. The tests were carried out within a large experimental test programme for the MANSIDE Project, with the scope of verifying the suitability of SMA superelastic wires as kernel components for seismic protection devices.The mechanical behaviour is described by means of four fundamental quantities, namely: secant stiffness, energy loss per cycle, equivalent damping and residual strain. The sensitivity to temperature and strain rate, as well as the influence of strain amplitude and the effects due to repeated cyclic deformation, are analysed in detail.The experimental results show that the characteristics of the superelastic wires are well suited for seismic applications, as both the recentring and the energy dissipating features of the devices can be easily obtained. Moreover, the influence of the investigated parameters, within their usual range of variation in seismic protection devices, is compatible with the use of superelastic wires for practical applications.     

形状记忆合金梁的非线性弯曲变形

形状记忆合金(Shape memory alloy,SMA)梁作为一种新型智能元件,在工程领域的应用日益广泛。基于实测的形状记忆合金材料应力-应变关系曲线及梁的大变形理论,同时考虑SMA材料拉压力学性能的不对称性及简支端移动等因素,建立形状记忆合金梁非线性弯曲变形的控制方程,并采用打靶法、辛普森数值积分等方法对方程进行数值求解。通过梁在不同载荷条件下的挠曲线以及最大挠度—弯矩曲线,分析材料非线性、几何非线性及简支端移动3个因素对SMA超弹性梁弯曲变形的影响规律。结果表明:梁中性层位置随弯矩变化;弯矩较小时,材料性能是线性的,几何非线性及简支端移动对梁的弯曲变形几乎不产生影响;弯矩较大时,材料性能是非线性的,几何非线性及简支端移动对梁的弯曲变形产生明显影响。     

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。     

Characterisation and modelling of behaviour of a shape memory alloys

Shape memory alloys (SMAs) provide an attractive solid-state actuation alternative to engineers in various fields due to their ability to exhibit recoverable deformations while under substantial loads. This feature is of particular importance when utilising the smart composite materials reinforced by SMA. Many constitutive models describing this repeatable phenomenon have been proposed, where some models also capture the effects of rate-independent irrecoverable deformations in SMAs. This paper presents experimental investigations and numerical simulations on shape memory alloys. First, by consisting in determining the transformations of equiatomic Ti–Ni shape memory alloys by differential scanning calorimeter. Then, in order to validate a 3D numerical model of the pseudoelastic behaviour of SMA allowing a finite strain analysis, a set of experimental tests at various initial temperatures is proposed. Finally, the numerical simulations of uniaxial tests performed on shape memory alloys are presented and compared with experimental data, permitting the validation of the proposed modelling. Reasonably good correlation is obtained between the experimental and model predictions.     

A bi-crystal aggregate model of pseudoelastic behaviour of shape-memory alloy polycrystals

A multi-scale model of stress-induced phase transformation and martensite variant reorientation in shape memory alloy (SMA) polycrystals is developed. It is proposed to include neighbouring-grain interaction in a simple manner by introducing an intermediate bi-crystal level into the sequential averaging scheme for SMA. The constitutive relationships are defined by specifying the free energy and dissipation functions. At the level of a single grain, the rate-independent dissipation function is used that incorporates the dissipation due to forward and reverse austenite-to-martensite transformation as well as reorientation of martensite variants. The global response of the model is simulated numerically by minimizing the total incremental energy supply. Specific examples are calculated for a NiTi polycrystal for proportional and non-proportional loading paths.     

Validation of a 3D numerical model of shape memory alloys pseudoelasticity through tensile and bulging tests on CuAlBe sheets

In order to validate a 3D numerical model of the pseudoelastic behaviour of shape memory alloys (SMA) allowing a finite-strain analysis, a set of experimental tests is proposed. First consisting in determining the representative elementary volume (REV) model parameters, tensile tests are performed within a small perturbations context. Therefore, two kinds of structure tests representing different stress states are performed: tensile tests on CuAlBe perforated strips on the one hand and bulging tests on CuAlBe sheets on the other hand. With the update of the material parameters for a finite-strain analysis, it is then possible to compare the experimental and the numerical results obtained from tests on structures submitted to general states of stresses. Besides, pictures correlation and infrared thermography analysis have been used and combined to pinpoint the thermomechanical couplings of SMA behaviour.     

马氏体相变温度对NiTi形状记忆合金抗磨损性能的影响

为了弄清楚N iTi形状记忆合金相与磨损性能之间的关系以及合金的磨损特征,研究了6种N iTi合金在相同条件下马氏体相变温度对磨损量的影响。结果表明:合金在没有发生永久性损坏带来的磨损的情况下,热弹性马氏体的转变,马氏体片的重新取向、合并及阻尼效应都能提高合金适应大变形量的能力,所以在粘着磨损过程中,弹性变形就会累积;对于6种合金来说,抗磨损性能主要取决于Ms转变温度,N i原子的析出和合金的硬度对合金的抗磨损性能也有显著影响。     

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.     

Role of phase transition in the unusual microwear behavior of superelastic NiTi shape memory alloy

The excellent microwear performance of nano-grained superelastic nickel titanium (NiTi) polycrystalline shape memory alloy (SMA) is reported in this paper. The microwear test was conducted at temperatures ranging from 22 to 120 °C by a Hysitron triboindenter. The results showed that the NiTi SMA has superior microwear resistance compared to traditional tribo-materials such as stainless steel AISI 304 and that the material exhibits unusual hardness dependence of wear within certain temperature regimes. With the increase in temperature from 22 to 120 °C, wear resistance was found to decrease anomalously with an increase in hardness. Further investigation and analysis confirmed that the stress-induced phase transition during contact and wear play an essential role in the material's high wear resistance. It is demonstrated through contact mechanics analysis that the increase of hardness with temperature was mainly due to the increase in the phase transition stress. The observed applied threshold load that corresponds to the onset of the plastic deformation in the contact area was strongly influenced by the phase transition process at the tip region. For the investigated superelastic NiTi, the temperature-dependent interplay between reversible phase transition and irreversible plastic yielding plays a key role in the temperature dependence of the wear performance and is responsible for the observed apparent unusual hardness–wear relationships.     

Smart machines with flexible rotors

The concept of smart machinery is of current interest. Several technologies are relevant in this quest including magnetic bearings, shape memory alloys (SMA) and piezo-electric activation. Recently, a smart bearing pedestal was proposed based on SMAs and elastomeric O-rings. However, such a device is clearly relevant only for the control of rigid rotors, for flexible rotors there is a need for some modification on the rotor itself. In this paper, rotor actuation by piezo-electric patches on the rotor is studied. A methodology is presented for the calculation of rotor behaviour and appropriate control strategies are discussed.     

加载速率对超弹性TiNi形状记忆合金棒材力学行为的影响

通过超弹性TiNi形状记忆合金棒材在不同加载速率下的力学性能试验,以相变应力、弹性模量、残余应变、耗能能力等作为该合金棒材的超弹性特征参数,分析了这些特征参数与加载速率的相关性。结果表明:该合金棒材在静态和非静态条件下的室温超弹性能存在一定差异。     

STRUCTURAL HEALTH MONITORING IN COMPOSITE MATERIALS USING EMBEDDED SHAPE MEMORY ALLOY（SMA） WIRE SENSORS

Shape memory alloy (SMA) materials possess completely superelasticity or pseudoelasticity above the austenite finish temperature and many unique mechanical, thermal, thermal-mechanical and electrical properties compared with other conventional materials. Many studies have reported that the superelastic and hysteresis properties of the SMA materials can absorb energies coming from external excitations or sudden impacts. In addition, due to the special electrical properties of NiTi superelastic wires, they can also be used as strain-sensing element to monitor structural health conditions. Composite laminated specimens embedded with SMA wire sensors are fabricated and detailed testing system is designed such as multi-parameters measuring for impact and weak signal processing for SMA sensor. Low velocity impact test shows that SMA wire sensors embedded in fiber-reinforced plastic (FRP) laminate can be well used to monitor impact responses, such as the location of impact damage, impact degree, and strain distribution. Experimental results and theoretical predictions reveal almost the same. Comparing with other method, a simple, economic and reliable technique method monitoring important engineering structures on line is provided.     

SMA-based tension control block for metallic tendons

Metallic tendons are frequently used in arches and vaults to absorb the lateral thrusting forces due to gravity loads or inertial forces caused by an earthquake. Traditional tendons, however, have some limitations (e.g. sensitivity to temperature variations, buckling under compression, etc.). Shape memory alloys (SMAs) can be used to enhance the performances of metallic tendons both in service and seismic conditions. This is achieved with the tensioning control block (TCB) system. The basic components of TCB are pre-strained superelastic SMA wires, in series with the metallic tendon. The number and diameter of the SMA wires is selected based on the force levels to reach. The length is optimised with respect to the thermal behavior of the system. A number of theoretical and experimental studies have been conducted to fully understand the thermal and mechanical behavior of TCB. In the paper, the main outcomes of these studies are described.     

Analytical technique for the two-dimensional stress wave model of memory alloy dampers

In this paper vibration damping capacity of shape memory alloys (SMA) is studied which is based on two-dimensional Oberaigner, Fischer and Tanaka model. The thermodynamic based active and passive control paradigms in SMA are presented. The model solution is presented along with modification based on exact solution for the most general case of a set of conditions based upon general thermal regimes. For the particular cases, four examples of different thermal and mechanical loadings are given. This study incorporates the static thermal changes and dynamic thermal states.     

Micro-end milling of NiTi biomedical alloys,burr formation and phase transformation

This paper focuses on burr formation in micro-end milling of two nickel–titanium shape memory alloys (SMA), an austenitic and a martensitic NiTi. Phase transformation during machining is also examined.     

功能梯度形状记忆合金梁纯弯曲的理论分析

功能梯度形状记忆合金(Functionally graded shape memory alloy, FG-SMA)是一种兼备功能梯度材料(Functionally graded material, FGM)和形状记忆合金(Shape memory alloy, SMA)特性的新型功能材料。根据复合材料力学和已有的SMA本构关系,通过充分考虑组分材料的应力应变关系,建立一个描述FG-SMA力学性能的宏观本构模型。应用该模型,对由弹性材料A和SMA组成的FG-SMA梁在纯弯曲载荷下的力学行为进行研究,详细讨论加载过程的弹性阶段和相变阶段,并给出相应的解析解。通过算例,对截面的应力分布、中性轴的位置和弯矩与曲率的关系进行详细讨论。结果表明,与普通FGM相比,FG-SMA可显著减小载荷作用下的最大应力,避免材料由于应力过大而导致的破坏。研究结果可为FG-SMA材料的设计和进一步研究提供一定的依据。     

Analyzing mechanical properties and nondestructive characteristics of brazed joints of NiTi shape memory alloys to carbon steel rods

Research is being conducted on the use of shape memory alloys, in particular NiTi, in civil engineering, due to the superelastic behavior of NiTi which can be used for damping. In this particular application, NiTi has to be joined to steel, which constitutes a major difficulty due to the considerably different properties of materials involved and the poor weldability of NiTi. Brazing was investigated for this application, since it is an economic and efficient process to joint dissimilar materials. This paper presents a study on the mechanical behavior in the superelastic regime of dissimilar NiTi/steel joints and the feasibility of a nondestructive method based on eddy currents to characterize the weld metal. Brazed joints in lap configuration were produced with a 20 % Ag braze alloy and tested under fatigue conditions in the superelastic regime. Lap joints of steel rods to NiTi ribbons have undergone up to 60 cycles of load/unload without rupture at a maximum load of the superelastic plateau close to the pull-out load of the joint. Measurements of the electrical impedance, with a helicoidal cylindrical coil along the lap joint, allowed the identification of the location of the joint, with a good spatial resolution characterizing the morphology of the brazed joints.     

形状记忆合金线性驱动器的设计及位移特性分析

为克服传统带有偏置装置形状记忆合金驱动器结构复杂、响应速度慢的缺点,利用形状记忆合金(Shape memory alloy,SMA)的单程形状记忆效应,设计并制造一种不带偏置装置且可实现双程运动的线性驱动器,该驱动器由两根形状记忆合金丝、滑轮、部件、导轨、支座及固定螺钉组成。接着基于Brinson一维本构方程及转换方程,推导出两根SMA丝在不同条件下的应变表达式;提出将马氏体相变应力看作体力,分析温度变化与驱动器位移的关系及外载荷变化对驱动器最大位移的影响。对计算结果进行试验验证,结果表明,所设计驱动器可实现往复双程运动;随着SMA丝温度的升高所获得的位移呈非线性增大,当温度超过奥氏体转变结束温度Af,位移达到最大,而初始化过程获得的最大位移为正常运动的一半;增大载荷,驱动器的最大位移逐渐减小,当载荷达到51.0 N时,驱动器停止运动,即位移减小为零。     

新型铜基记忆合金（CuAlMnX）机械性能测试

新型铜基记忆合金（CuAlMnX）在经过布氏硬度测试、拉伸试验、超弹性回复性能测试、超弹状态下的疲劳寿命这4项机械性能的测试后。证实其综合机械性能已达到企业标准,断裂韧性优于NiTi形状记忆合金,弹性回复率、塑性等性能均优于已见报道的其它铜基记忆合金。