纳米单晶NiTi合金单程形状记忆效应的分子动力学模拟

采用基于第二近邻修正型嵌入原子势的分子动力学方法研究了纳米单晶NiTi合金的单程形状记忆效应,详细阐明了温度诱发马氏体相变和应力诱发马氏体重定向过程中纳米单晶的变形行为和微结构演化,进一步分析了加/卸载速率对NiTi合金单程形状记忆效应的影响。结果表明,NiTi纳米单晶在应力加载过程中发生马氏体重定向,卸载后存在残余应变;当加热到奥氏体转变结束温度以上时,马氏体逆相变为奥氏体相,残余应变逐渐减小,但未完全回复;随着应力加载速率的增加,重定向临界应力和模量逐渐增加;再次降温过程中不同加载速率下的原子结构演化各不相同。     

纳米单晶NiTi合金单程形状记忆效应的分子动力学模拟

采用基于第二近邻修正型嵌入原子势的分子动力学方法研究了纳米单晶NiTi合金的单程形状记忆效应,详细阐明了温度诱发马氏体相变和应力诱发马氏体重定向过程中纳米单晶的变形行为和微结构演化,进一步分析了加/卸载速率对NiTi合金单程形状记忆效应的影响。结果表明,NiTi纳米单晶在应力加载过程中发生马氏体重定向,卸载后存在残余应变;当加热到奥氏体转变结束温度以上时,马氏体逆相变为奥氏体相,残余应变逐渐减小,但未完全回复;随着应力加载速率的增加,重定向临界应力和模量逐渐增加;再次降温过程中不同加载速率下的原子结构演化各不相同。     

泡沫材料的应变率效应

对泡沫材料的应变率敏感性进行了系统深入的讨论,认定这种材料是应变率敏感材料,这种敏感性主要是由于泡孔的变形特性产生的。泡沫材料变形的局部化、微观惯性和致密性导致压垮应力明显提高,基体的应变率效应及泡孔的形状大小并不能对泡沫材料应变率敏感性起主导作用。     

基于对数应力率大应变本构关系的参数标定研究

在所有率型弹塑性本构模型中,只有对数应力率对应的本构模型能够满足自适应准则.基于对数应力率,采用实心圆轴扭转实验,对大应变弹塑性本构模型中的参数标定问题进行了讨论.推导出了考虑Swift效应时端部自由实心圆轴扭转变形的变形率、对数旋率、Kirchhoff应力及Kirchhoff应力的对数应力率.对于等向强化大应变弹塑性本构关系,给出了由实心圆轴扭转实验标定的、基于Kirchhhoff应力对数应力率的本构关系中塑性刚度函数的表达式.分析了扭转圆轴的Swift效应对塑性刚度函数的影响.结果表明,实心圆轴扭转的轴向伸长变形和径向变形对基于对数应力率大应变本构关系中的塑性刚度函数都有影响.当不考虑Swift效应时,所得塑性刚度函数表达式与不考虑Swift效应时基于Jaumann应力率的塑性刚度函数表达式相同.     

TiNi合金的动态伪弹性行为和率相关相变本构模型

采用万能材料试验机和SHPB实验技术对TiNi形状记忆合金在10-3s-1和102s-1应变率下的伪弹性相变行为进行了实验研究。实验数据表明:TiNi合金的相变过程具有应变率效应,其原因主要是相变阻力受应变率影响应变率越大,相变阻力越大。在三线性热弹性相变模型的基础上,考虑了应变率对相变阻力的影响,并给出了相变阻力的具体形式,建立了一个一维率相关相变本构模型。模型对TiNi合金相变行为的模拟与实验数据吻合较好。     

应变幅值对循环载荷下NiTi合金力学性质的影响

实验研究了应变幅值对循环载荷下NiTi合金伪弹性退化特征的影响规律,结果表明:当卸载发生在NiTi合金应力诱发马氏体相变阶段时,应变幅值对马氏体相变开始应力的退化规律影响较小,但此时应变幅值的增加会显著增大奥氏体弹性模量的退化程度,而其大变形可回复能力和阻尼特性在应变幅值大于6%时才有大幅度降低。对各参数退化程度进行定量分析,得到了NiTi合金具有较强可回复能力和阻尼性能的应变幅值范围。该研究可为NiTi合金阻尼器的设计提供参考。     

梯度纳米晶NiTi形状记忆合金的超弹性和形状记忆效应相场模拟

通过建立考虑两个马氏体变体的二维相场模型,对梯度纳米晶镍钛(NiTi)合金系统的超弹性、单程和应力辅助双程形状记忆过程进行了模拟和预测.模拟结果显示: 在梯度纳米晶NiTi合金的超弹性过程中,较粗晶粒的区域保留了传统粗晶的马氏体相变和逆相变特征,即局部马氏体带的形核-扩展和缩减-消失, 而随着晶粒尺寸的减小,细晶粒区域表现为均匀相变的特点, 即无局部马氏体带产生; 此外,在超弹性和形状记忆过程中,马氏体相变和重取向都首先在较粗晶粒区域开始并逐步向细晶粒区域传播,而逆相变则相反.马氏体相变和重取向的逐步扩展使梯度纳米晶NiTi合金的应力-应变和应变-温度曲线呈现出“硬化状”,其可归因于纳米多晶NiTi合金中马氏体相变对晶粒尺寸的依赖性,即随着晶粒尺寸的减小, 相变或重取向壁垒逐渐增大,马氏体相变或重取向的形核、扩展越来越困难. 可见,梯度纳米晶结构具有比传统均匀晶粒尺寸NiTi合金更宽的相变应力区间、重取向应力区间和相变温度区间,可显著提高该合金非弹性变形的可控性.      

63Sn-37Pb钎料合金在非比例循环加载下的非弹性流动特性研究

基于63Sn-37Pb钎料舍金在多种非比例应变循环加载下的实验结果,通过考察材料的非弹性应变率与偏应力之间的夹角随累积非弹性应变的变化规律,对63Sn-37Pb钎料合金的非弹性流动特性进行了定量分析。分析结果显示：在相同的非比例加载路径下,当加载等效应变幅值相同时,等效应变率越高,非弹性应变率与偏应力之间夹角平均水平越低,当等效应变率相同时,等效应变幅值越大,相应的夹角平均水平越低;在保持时间范围内,非弹性应变率方向与偏应力方向趋于一致;当非比例路径形状不同时,其非弹性应变率与偏应力之间的夹角随累积非弹性应变的变化趋势明显不同。研究表明,材料的非弹性流动特性强烈依赖于等效应变幅值、等效应变率、保持时间、非比例路径形状。     

低应变速率下形状记忆合金的特性实验研究

形状记忆合金以其特有的形状记忆效应和超弹性,被广泛应用于医学、航空和建筑防震。在耗能性机构中,需要形状记忆合金能够在变速率条件下工作。为了获得非静态条件下的行为特性,比较静态和非静态条件下形状记忆合金的行为特性的差异,本文采用一种新的分析方法,即以形状记忆合金的特性参数为对象,来分析不同应变速率对形状记忆合金特性的影响。在不同的应变速率(0.0005/s,0.001/s,0.005/s,0.01/s,0.05/s,0.1/s)下,对50.8at%-Ni-Ti记忆合金丝的形状记忆效应和超弹性特性进行了实验研究。在低应变速率范围内,由实验结果得到:随应变速率增大,两种特性行为中的各纯相的杨氏模量保持不变;拉伸过程相变起始和终了临界应力会增大,卸载过程相变起始和终了临界应力会减小,滞后环面积增大;相变硬化系数在形状记忆效应行为中会增大,而在超弹性行为中基本不变。     

耐磨TiNi形状记忆合金设计准则的有限元研究

采用有限元方法模拟微突体在TiNi形状记忆合金表面的压入过程，研究了伪弹性应变、伪弹性模量和相变启动应力等参数对TiNi合金抗磨性能的影响，并初步确立了TiNi形状记忆合金耐磨材料设计准则．结果表明：伪弹性应变对TiNi形状记忆合金摩擦学性能的影响最显著；就TiNi形状记忆合金耐磨材料的设计而言，应当强调提高伪弹性应变、降低伪弹性模量、增加相变启动应力；同时满足上述3方面要求的TiNi形状记忆合金的耐磨特性最优．     

TiNi形状记忆合金变形特性实验研究

给出了一种ＴｉＮｉ形状记忆合金（多晶）的单轴拉压、循环拉伸加卸载、形状记忆效果、恢复力、相变塑性变形及拉压。扭转比例、非比例加卸载的实验结果，实验结果显示，其变形特性非常不同于普通的弹塑性材料：如Ｂａｕｓｃｈｉｎｇｅｒ效果提前，甚至于会出现于卸载阶段；低温下的残余变形会在加热后消失；恒载下冷却时产生相当大的塑性变形（相变塑性变形）且在加热时消失等等。多轴加载实验结果表明，比例加载时该材料的加载屈服面及卸载屈服面都基本满足Ｍｉｓｅｓ准则，但非比例加载时则不然。     

超弹性镍钛形状记忆合金单轴相变棘轮行为的宏观唯象本构模型

超弹性镍钛形状记忆合金因其良好的力学性能以及独特的超弹性和形状记忆效应已广泛应用于土木工程、航空航天和生物医疗等多个领域,在实际服役环境中超弹性镍钛合金元件不可避免地会承受不同应力水平的循环载荷作用,亟待建立描述相变棘轮行为(即峰值应变和谷值应变随着正相变和逆相变循环的进行不断累积)的循环本构模型.为此,基于已有的超弹性镍钛形状记忆合金在不同峰值应力下的单轴相变棘轮行为实验研究结果,在广义黏塑性框架下,对Graesser等提出的通过背应力非线性演化方程反映超弹性镍钛形状记忆合金超弹性行为的一维宏观唯像本构模型进行了拓展,考虑了正相变和逆相变过程中特征变量的差异及其随循环的演化,以非弹性应变的累积量为内变量引入了正相变开始应力、逆相变开始应力、相变应变和残余应变的演化方程,同时通过峰值应力与正相变完成应力的比值来确定演化方程中的相关系数,建立了描述超弹性镍钛合金单轴相变棘轮行为的本构模型.将模拟结果与对应的实验结果进行对比发现,建立的宏观唯像本构模型能够合理地描述超弹性镍钛形状记忆合金的单轴相变棘轮行为及其峰值应力依赖性,模型的预测结果和实验结果吻合得很好.     

Effects of matrix inelasticity on the overall hysteretic behavior of TiNi-SMA fiber composites

The effects of the inelastic deformation of the matrix on the overall hysteretic behavior of a unidirectional titanium–nickel shape-memory alloy (TiNi-SMA) fiber composite and on the local pseudoelastic response of the embedded SMA fibers are studied under the isothermal loading and unloading condition. The multiaxial phase transformation of the SMA fibers is predicted using the phenomenological constitutive equations which can describe the two-step deformation due to the rhombohedral and martensitic transformations, and the inelastic behavior of the matrix material using the standard nonlinear viscoplastic model. The average behavior of the SMA composite is evaluated with the micromechanical method of cells. It is observed that the inelastic deformation of the matrix due to prior tension results in a compressive stress in the matrix after unloading of the SMA composite and this residual stress impedes the complete recovery of the pseudoelastic strain of the SMA fibers. This explains that a closed hysteresis behavior of the SMA composite is no longer observed in contrast with the case that an elastic behavior of matrix is assumed. The predicted local stress–strain behavior indicates that the cyclic response of matrix is crucial to the design of the hysteretic performance of the SMA composite under the repeated loading conditions.     

Thermomechanical cyclic behavior modeling of Cu-Al-Be SMA materials and structures

This paper concerns the behavior of Cu-Al-Be polycrystalline shape memory alloys under cyclic thermomechanical loadings. Sometimes, as shown by many experimental observations, a permanent inelastic strain occurs and increases with the number of cycles. A series of cyclic thermomechanical tests has been carried out and the origin of the residual strain has been identified as residual martensite. These observations have been used to develop a 3D macroscopic model for the superelasticity and stress assisted memory effect of SMAs able to describe the evolution of permanent inelastic strain during cycles. The model has been implemented in a finite elements code and used to simulate the behavior of antagonistic actuators based on SMA springs under cyclic thermomechanical loading with a residual displacement appearance.     

Phase-Transformation Fronts Evolution for Stress- and Strain-Controlled Tension Tests in TiNi Shape Memory Alloy

Nucleation and development of phase transformation fronts in TiNi shape memory alloy subjected to the stress- and strain-controlled tension tests were investigated. A thermovision camera was applied to register the distribution of infrared radiation emitted by the specimen and to find its temperature variations. During the loading, narrow bands of considerably higher temperature corresponding to the martensitic phase, starting from the central part of the specimen and developing towards the specimen grips, under both approaches, were registered. The inclined bands of heterogeneous temperature distribution were observed also during the unloading process of the SMA, while the reverse transformation accompanied by temperature decrease took place. Thermomechanical aspects of martensitic and reverse transformations for various strain rates were analyzed under both stress- and strain-controlled tests.     

A finite element model for shape memory alloys considering thermomechanical couplings at large strains

In the present work we propose a new thermomechanically coupled material model for shape memory alloys (SMA) which describes two important phenomena typical for the material behaviour of shape memory alloys: pseudoelasticity as well as the shape memory effect. The constitutive equations are derived in the framework of large strains since the martensitic phase transformation involves inelastic deformations up to 8%, or even up to 20% if the plastic deformation after the phase transformation is taken into account. Therefore, we apply a multiplicative split of the deformation gradient into elastic and inelastic parts, the latter concerning the martensitic phase transformation. An extended phase transformation function has been considered to include the tension–compression asymmetry particularly typical for textured SMA samples. In order to apply the concept in the simulation of complex structures, it is implemented into a finite element code. This implementation is based on an innovative integration scheme for the existing evolution equations and a monolithic solution algorithm for the coupled mechanical and thermal fields. The coupling effect is accurately investigated in several numerical examples including pseudoelasticity as well as the free and the suppressed shape memory effect. Finally, the model is used to simulate the shape memory effect in a medical foot staple which interacts with a bone segment.     

A CONSTITUTIVE MODEL FOR TRANSFORMATION, REORIENTATION AND PLASTIC DEFORMATION OF SHAPE MEMORY ALLOYS

A constitutive model is developed for the transformation, reorientation and plastic deformation of shape memory alloys (SMAs). It is based on the concept that an SMA is a mixture composed of austenite and martensite, the volume fraction of each phase is transformable with the change of applied thermal-mechanical loading, and the constitutive behavior of the SMA is the combination of the individual behavior of its two phases. The deformation of the martensite is separated into elastic, thermal, reorientation and plastic parts, and that of the austenite is separated into elastic, thermal and plastic parts. Making use of the Tanaka’s transformation rule modified by taking into account the effect of plastic deformation, the constitutive model of the SMA is obtained. The ferroelasticity, pseudoelasticity and shape memory effect of SMA Au-47.5 at.%Cd, and the pseudoelasticity and shape memory effect as well as plastic deformation and its effect of an NiTi SMA, are analyzed and compared with experimental results.     

形状记忆合金相变塑性的统一本构理论

本文根据单品相变时其宏观(长程)切应变(等效应变)与外加切应力(等效应力)问的幂函数关系假定,建立了形状记忆合金多晶体相变塑性的统一细观本构理论,它推广了现有细观本构理论的结果,消除了现有理论中存在的当外加偏应力趋于零时自由能函数的不连续性和宏观塑性变形的不连续性。这一本构理论将能统一地描述在不同温度和载荷下记忆合金所呈现的各种行为。