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.     

Fatigue resistance of SMA-martensite bars subjected to flexural bending

The fatigue resistance of Ni–Ti shape memory alloy (SMA)-martensite bars in bending subjected to large deformation cycles has been experimentally evaluated. Firstly, fatigue tests under constant displacement amplitude have been carried out, at two different frequencies of loading. Then, the cumulative fatigue damage and fatigue life prediction of specimens loaded under variable load conditions have been investigated. Finally, the effect of low-cycle-fatigue (LCF) and plastic deformations on subsequent high-cycle-fatigue (HCF) has been studied.The experimental results point out that the frequency of loading (i.e. temperature) significantly affects the fatigue life of the specimens. The damage accumulation process seems to follow the “Miner” linear damage theory for low-to-high (L–H) loading sequences, while the same does not hold for high-to-low (H–L) loading sequences. Surprisingly, a small fraction of LCF life consumption seems to enhance the subsequent HCF limits.     

SMA丝缠绕角对扭力驱动器驱动性能影响的试验研究

在智能结构的驱动系统研究中,形状记忆合金(SMA)以其独特的力学性能倍受关注。将含有预拉伸变形的SMA丝与薄壁圆管复合,能够构成具有扭转变形输出或转矩输出功能的驱动器,为飞行器翼面操纵系统等提供新的设计思路。在SMA扭力驱动器中,SMA丝的缠绕角是影响驱动器驱动能力的一个重要参数。针对缠绕角分别为40°、45°、55°、60°和75°的一组驱动器,采用试验手段,获得了自由状态、完全约束状态和弹性约束等三种状态下,上述驱动器的温度－缠绕角－扭转角(或转矩)等多组关系曲线,得到了当缠绕角为60°时,驱动器有最大扭转角和最大转矩的试验结果。研究结果对深入探索SMA扭力驱动器的力学模型和驱动器结构参数的优化具有重要的参考价值。     

Seismic analysis and parametric study of SDOF lead-rubber bearing (LRB) isolation systems with recentering shape memory alloy (SMA) bending bars

Base isolation has been considered one of aseismic design methodologies based on belief that it is feasible to separate the superstructure from the supporting ground, thereby preserving structural members and their internal contents from damage caused by earthquake motions. Although remarkable progress was achieved in this isolation technique towards the end of the century, a number of catastrophic building failures are still found where earthquake with severe ground motion attacked. In particular, high-rise buildings that have been currently constructed in urban area are very vulnerable when exposed to such impulsive events, and thus large relative displacement occurs at the isolator interface. The obstacles in or in the vicinity of the isolator interface destroy seismic performance. In this study, the innovative isolation systems incorporated with superelastic Shape memory alloy (SMA) bars functioning as seismic restrainers are suggested in order to more effectively maintain structural integrity. The superelastic SMA bending bars supply recentering force to the base isolator that accordingly makes a significant contribution to reducing permanent deformation over entire structure. The base Lead-rubber bearing (LRB) isolators with superelastic SMA bars are considered as inelastic behavior reproduced by component spring elements. The response of the proposed base isolators are compared with those of the conventional LRB isolators. Through the observation of the analysis results, it can be concluded that recentering LRB isolators create the improvement of seismic performance in terms of strength, energy dissipation and recentering effect.     

Effects of temperature on the static and dynamic stress-strain characteristics in torsion of 1100-0 aluminum

A series of tests is described in which tubular specimens of a commercially pure polycrystalline aluminum were loaded in torsion at either of two dynamic rates, 300 and 600 sec^?1, up to shear strains of about 2 and 4 per cent, respectively, at temperatures in the range ?180 to 250°C. The tests were performed in a torsional split-Hopkinson bar. In the past when this apparatus was employed at an elevated or reduced temperature multiple reflections were encountered along the bars due to the temperature gradients. These reflections required elaborate computational corrections. In the present tests the reflections were eliminated by tapering the Hopkinson bars appropriately so their mechanical impedance in torsion remained constant in spite of the temperature gradients. The results, in the form of stress-strain curves, are compared to the corresponding “static” curves obtained by testing similar specimens in torsion at about 10^?3 sec^?1. The influence of strain rate and of temperature is generally as expected, i.e. the flow stress increases with an increase in strain rate or a decrease in temperature. The strain rate sensitivity parameter varies non-linearly with temperature, and has a minimum value in the neighborhood of room temperature.     

Performance-based optimal design of self-centering friction damping brace systems between recentering capability and energy dissipation

This paper deals with a novel type of the friction damping brace systems and its development using slip resistance for seismic retrofitting of damaged structures. In the system, slotted bolt holes are placed on the shear faying surface with a view to dissipating a considerable amount of passive energy. Superelastic shape memory alloy (SMA) wire strands that enable self-centering mechanism and enhanced energy dissipation capacity are also installed between brace members. Self-centering friction damping braces (SFDBs) treated in this study have the desirable potential to efficiently reduce residual inter-story drifts in the braced frames during seismic events as compared to conventional passive damping systems. The SFDB system mechanism is described, and then its parametric study accounting for recentering capability and dissipative energy is carried out using numerical models that are calibrated with experimental data. Based upon the parametric investigation, this study suggests an optimal design methodology for establishing the smart self-centering bracing system.     

Prediction of creep failure in aeroengine materials under multi-axial stress states

The creep and creep rupture behaviour of two, significantly different, aeroengine materials, namely a nickel-base superalloy at 700°C and a high temperature titanium alloy at 650°C, were studied. Experimental creep tests were conducted on uniaxial specimens and axisymmetric notched bars under constant tensile loads conditions. From the uniaxial creep test results, a creep continuum damage model was established for each of the materials. The skeletal point stress approach was used to obtain the approximate creep rupture stress criterion in the multi-axial generalization of the creep continuum damage models. This approximation was cross-checked using axisymmetric Finite Element (FE) analyses in a trial and error procedure. Multi-axial creep continuum damage models were then used in further FE creep analyses to predict the creep rupture times in specimens subjected to different tensile loads. The FE predictions of the rupture times in these notched specimens were found to be in good agreement with the experimental results for the nickel-base superalloy (Waspaloy) at 700°C and the titanium alloy (IMI834) at 650°C.     

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。     

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。     

Mode I failure modeling of friction stir welding joints

This paper analyzes mechanical response by finite element method up to the decohesion failure in fracture mode I for joints of friction stir welding (FSW) of an aluminum alloy. It first describes experimental investigations on specimens with FSW embedded, subjected to uniform traction and local punch tests used to characterize local elastic and plastic material parameters. The heterogeneity of the mechanical properties induced by the FSW process is taken into account for the elastic-plastic finite element simulation. The growing damage and the opening failure of the welding zone are described by the adoption of a cohesive interface model with specific mechanical properties.     

合金化对CuZnAl合金低频阻尼性能的影响

CuZnAl合金的形状记忆效应、力学性能及阻尼性能的好坏是决定其工业实用性的关键指标。采用倒扭摆（JN-1型真空扭摆仪）方法,测试了固溶处理后合金的振动应变振幅爿与振动时间t的关系曲线（自由衰减曲线）,并对Zr对CuZnAl合金低频阻尼性能的影响进行了研究,探讨了CuZnAl合金马氏体产生高阻尼特性的微观机制。结果表明,1．2、2．3、1．7三种合金具有最佳阻尼性能的固溶温度分别为800℃、900℃和800℃;相同围溶处理温度8000C下,不含Zr的1．7合全的阻尼性能最佳。含Zr少的1．2合金的阻尼性能好于含Zr多的2—3合金的阻尼性能。     

形状记忆合金材料的应用

作为新型功能材料的形状记忆合金，由于其特有性能--相变的形状记忆效应，在机械、电气、宇航、医疗卫生、日常生活等领域得到广泛应用。列举了近年来国内外形状记忆材料的应用实例，并对其发展前景作了分析。     

Re-centering variable friction device for vibration control of structures subjected to near-field earthquakes

This paper proposes a re-centering variable friction device (RVFD) for control of civil structures subjected to near-field earthquakes. The proposed hybrid device has two sub-components. The first sub-component of this hybrid device consists of shape memory alloy (SMA) wires that exhibit a unique hysteretic behavior and full recovery following post-transformation deformations. The second sub-component of the hybrid device consists of variable friction damper (VFD) that can be intelligently controlled for adaptive semi-active behavior via modulation of its voltage level. In general, installed SMA devices have the ability to re-center structures at the end of the motion and VFDs can increase the energy dissipation capacity of structures. The full realization of these devices into a singular, hybrid form which complements the performance of each device is investigated in this study. A neuro-fuzzy model is used to capture rate- and temperature-dependent nonlinear behavior of the SMA components of the hybrid device. An optimal fuzzy logic controller (FLC) is developed to modulate voltage level of VFDs for favorable performance in a RVFD hybrid application. To obtain optimal controllers for concurrent mitigation of displacement and acceleration responses, tuning of governing fuzzy rules is conducted by a multi-objective heuristic optimization. Then, numerical simulation of a multi-story building is conducted to evaluate the performance of the hybrid device. Results show that a re-centering variable friction device modulated with a fuzzy logic control strategy can effectively reduce structural deformations without increasing acceleration response during near-field earthquakes.     

TiNi形状记忆合金的力传感特性及其机理研究

测试了不同初始组织状态TiTi SMA丝材恒温拉伸过程中的电阻-应力-应变关系，分析了力学及电阻特性的变化机理，得出了马氏体（M）态及奥氏体（A）态TiNi形状记忆合金（SMA）具有力传感性能的结论。为构建TiNi SMA传感元件的智能材料结构提供了理论依据。     

SMA柔性扭转驱动器的结构设计与优化研究

设计了一种基于柔性结构的SMA扭转驱动器,通过加热SMA丝收缩带动柔性结构变形从而实现驱动器的扭转输出。柔性结构在SMA丝拉力作用下的变形情况对驱动器的转角输出有很大的影响,为获得最大的输出转角,需对柔性结构的形状和SMA丝的作用点位置进行优化设计。采用三次B样条曲线描述柔性结构的形状,通过有限元法分析柔性结构的变形,并应用遗传算法进行柔性结构的形状优化和SMA丝作用点位置优化。实际算例表明,利用优化方法可快速有效地获得使SMA柔性扭转驱动器输出转角最大的柔性结构形状与SMA丝作用点位置。     

SMA智能复合材料连接接头钉载自适应分配研究

利用形状记忆合金丝 (SMA)的热 -力耦合特性 ,将 SMA丝埋入复合材料多钉接头钉孔周围 ,通过控制电流 ,加热 SMA丝使之产生回复力 ,从而改变被连接件的钉载分配比例。经过对算例的计算分析 ,证明本文所提方法对改善钉载均匀化程度有显著效果。     

新型SMA驱动器设计与控制

介绍了一种新型SMA驱动器。该驱动器实际上是偏心内嵌有单程SMA丝的弹性硅胶棒。它集成了驱动单元和执行单元的功能，特别适于作为小型和微型机器人系统中的致动元件。以弹性棒变形理论为基础，研究了驱动器的机械和力学特性。最后，实现了基于SMA电阻反馈的变结构滑模控制策略探讨和实验研究。     

形状记忆合金驱动薄壁圆管扭转变形分析 及试验研究

将预拉伸的形状记忆合金(Shap ememory alloy,简称SMA)丝作为驱动器,缠绕并粘贴在薄壁圆管表面。加热SMA,当其发生相变时,会产生很大的恢复力,驱动圆管发生扭转变形。建立了粘有SMA丝应变驱动器的薄壁圆管的力学模型,分析了一个温度循环中,薄壁圆管受SMA驱动发生的压扭复合变形,给出了薄壁圆管扭转角与激励温度之间的关系。同时通过试验对理论结果进行了验证。     

疲劳失效准则在钛合金BT9低周疲劳寿命估算中的应用

在应变控制条件下对钛合金 BT9进行了单轴拉 -压、纯扭及拉 -压与扭转比例加载的低周疲劳试验 ,得到了相应的试验结果。讨论了几种寿命预测准则及其寿命预测精度。在分析 Brown- Miller理论及 Makinde-Neale广义疲劳失效准则的基础上 ,提出了 Makinde-Neale广义疲劳失效准则的变异方程。应用所提出的方程对钛合金 BT9进行了多轴应变条件下的寿命预测 ,并得出了更为符合试验结果的预测结果