Modulated interaction in double-layer shape memory-based micro-designed actuators

Abstract

The effect of superposed transitions in actuators with layered shape memory alloy (SMA) films undergoing martensitic phase transformation is analyzed in terms of a model developed for two layers of different composition, deposited at the same temperature on a substrate. A significant difference is observed in the actuation versus temperature relationship, depending on the thermal and elastic properties of the SMA layers and their martensitic transformation temperature. The prediction of the actuation is exemplified using a multilayer model and is verified for a cantilever actuator with NiTi and NiMnGa layers deposited on a Si substrate. The model sets the ground for a smart selection of SMAs in order to achieve a modulated actuation.     

形状记忆合金纤维混杂正交对称铺层板的固有频率

形状记忆合金（SMA）混杂复合材料板是将有预应变的SMA纤维与普通纤维混杂单层内构成的。基于主动应变能调节（ASET）的概念,可实现对板的固有频率的调整。本文采用Galerkin法导出形状记忆合金（SAM）纤维混杂对称正交智能复合材料铺层板自由振动的频率的分析表达式。数值结果表明,SMA纤维的相变激发温度、体积含量、分布方式及其预应变对固有频率均有的影响,尤其是温度、SAM含量及其分布的作用更为显著,是结构振动控制的重要设计参数。     

航天器用记忆合金热开关的设计与理论分析

介绍了一种航天器热控制用记忆合金热开关的设计与分析。该热开关以形状记忆合金弹簧作为感温和驱动元件,当温度超过记忆合金材料的奥氏体相变点时,驱动元件开始变形并推动热开关闭合;温度低于记忆合金材料的马氏体相变点时,偏置弹簧将克服记忆合金元件的弹力,推动热开关使之断开。热开关结构为圆柱体,长度32mm,直径23mm。对热开关的力学参数、传热性能进行了分析计算,表明该热开关的闭合热阻为2．69K／W,断开热阻大于352．64K／W,闭合时间小于30min。     

Ni含量对激冷贫镍TiNi形状记忆合金薄带相变行为的影

用激冷甩带法制备了Til-xNix(x=45％～49.8％)(原子分数)形状记忆合金(SMA)薄带,用示差扫描量热仪研究了Ni含量对铸态及450℃、500℃退火态TiNi SMA薄带相变行为的影响.结果表明,冷却/加热时,铸态和退火态Ti1-xNix(x=45％～49％)SMA薄带发生A→M/M→A一阶马氏体相变;当Ni含量为49.8％时,铸态和退火态TiNi SMA薄带冷却时发生A→ R→M两阶段相变,加热时发生M→A一阶段相变.随Ni含量增加,TiNi SMA薄带马氏体正、逆相变温度范围先增大后减小,Ni含量为48％时相变温度范围最宽.退火态比铸态TiNi SMA薄带相变温度范围窄.随Ni含量增加,TiNi SMA薄带马氏体正、逆相变温度升高,相变热滞增大.当Ni含量为49％时,SMA薄带的马氏体相变温度达最大值,当Ni含量为49.8％时马氏体相变温度迅速下降.     

Carbon nanotube forest growth on NiTi shape memory alloy thin films for thermal actuation

Actuation frequencies in thermally triggered Shape Memory Alloy (SMA) thin films are limited by the slow heat transport into/out of the films. Carbon Nanotubes (CNTs) are known to exhibit an exceptionally high thermal conductivity. Thus, we propose to thermally contact SMA films with CNTs to increase SMA actuation frequencies by enhanced heat transport through the CNTs. The basic requirement for this envisaged nanotube application is to obtain CNT forest growth on a SMA material while retaining a reversible martensitic transformation, as required for Shape Memory Effect exploitation. We show how such growth can be achieved on thin films of the SMA material NiTi. Future work is needed to measure thermal properties and obtainable cycling frequencies of CNT-SMA structures.     

Finite element modeling of cyclic thermal response of shape memory alloy wires with variable material properties

This paper considers the influence of variable thermal conductivity and electrical resistivity on the cyclic thermal response of a shape memory alloy (SMA) wire. The specific boundary value problem of an SMA wire under zero-stress conditions is considered, wherein the wire, in an initially martensitic state, is heated by electrical current and cooled by convection. An isothermal boundary condition is considered at the ends of the wire. The heating is continued until the transformation from martensite to austenite has essentially taken place. A Galerkin finite element method (FEM)-based numerical approach is used to solve the boundary value problem. The two key parameters of interest are the maximum possible transformation achievable (i.e. extent of actuation) and the time period for one cycle. It is seen that for short wires, the assumption of constant thermal conductivity underestimates the actuation and overestimates the cycle time significantly. The assumption of a constant electrical resistivity does not affect the actuation but significantly overestimates the cycle time. For long wires, the thermal conductivity effect is diminished whereas the electrical resistivity effect is very similar to that for the short wires.     

功能梯度形状记忆合金复合梁的力学行为

功能梯度形状记忆合金（Functionally graded shape memory alloy,FGSMA）兼具功能梯度材料和形状记忆合金材料的双重特性,广泛应用于微机电、航空航天等工程领域。为研究FGSMA复合梁的弯曲行为,本文对形状记忆合金（SMA）力学本构方程进行简化处理,并根据复合材料层合板理论建立了FGSMA复合梁的力学模型,据此研究了SMA体积分数沿厚度方向呈线性变化的FGSMA悬臂梁内SMA纤维铺设角度对悬臂梁横截面应变、中面轴向位移、中性面高度和相变层高度的影响以及悬臂梁中面应变、曲率、SMA马氏体相变临界层高度和中性面高度随弯矩载荷的变化规律。研究结果表明:在弯矩载荷作用下,悬臂梁中性面位置与中面位置不重合,且悬臂梁上下层SMA马氏体相变临界层位置不对称;截面轴向应变绝对值随铺设角度增大而增大,截面纵向应变绝对值随铺设角度增大先增大后减小,中面轴向位移随铺设角度增大先增大后减小;随着铺设角度增大,悬臂梁中性面高度逐渐增大,拉伸状态下相变结束临界层高度先减小后增大,压缩状态的趋势相反;随着弯矩载荷绝对值逐渐增大,中性面位置高度表现出先稳定后减小然后逐渐增大的趋势,相变临界层逐渐向中性面位置靠拢;中面正应变和挠曲率随着弯矩载荷绝对值逐渐增大而发生变化,且变化率先增大后减缓。      

Thermomechanical transformation fatigue of TiNiCu SMA actuators under a corrosive environment – Part I: Experimental results

In this two-part paper, the thermomechanical fatigue of TiNiCu shape memory alloy (SMA) wire actuators undergoing thermally induced martensitic phase transformation in a corrosive environment is investigated. The main objective of this work is to evaluate the cyclic response and fatigue behavior of TiNiCu SMA wire under corrosive conditions and to compare it to results obtained for fatigue testing in a corrosion-free environment. Part I focuses on the various experimental aspects of this work, including the presentation of fatigue results as a function of various testing parameters. The variable test parameters are five applied stress levels from about 50 MPa to about 250 MPa, and two different actuation strains, one corresponding to full actuation or complete transformation and the other to partial transformation. The results from fatigue testing in a corrosive environment show a consistent reduction of the fatigue life compared to corrosion-free fatigue results, in both complete and partial transformation conditions. It is also observed that corrosion-assisted fatigue leads to more scattered fatigue data and this spread is mostly attributed to enhanced and accelerated damage mechanisms due to corrosion. From these conclusions, a microstructure evaluation is performed to understand the damage that contributes to lower fatigue limits under corrosion and is presented in Part II of this work. Fracture surfaces, development of fatigue cracks and effect of corrosion are presented and discussed. The conclusion from the microstructure analysis has led to the formulation of a damage accumulation model accounting for a cyclic corrosion mechanism. This modeling approach allows for determining the fatigue life reduction of SMA wire actuators in a corrosive environment. All results of the microstructure analysis and fatigue life modeling are presented in Part II.     

Modelling of residual stresses developed in steel cylinders subjected to surface-layer deposition by welding

A new model for the determination of the residual stress–strain and microstructural state of a cylinder subjected to layer deposition by welding is developed. The growing body of theory and unified the Bodner–Partom viscoplastic theory generalized for the case of coupled thermomechanic processes are involved to describe the mechanical behavior of the material. Continuous cooling transformation (CCT) diagrams are used to account for microstructural transformation. The concept of eigenstrains and temperature state is utilized to satisfy the boundary conditions on the growing surface. A time-stepping procedure and a finite-element technique are used to simulate both the instantaneous and the residual stress–strain and microstructural state of a deposited cylinder. The influence of initial temperature on the residual stress–strain and the structural states of the cylinder deposited by austenitic and martensitic steel layers is investigated. It is discovered that martensite and bainite shares in the heat-affected zone can be effectively controlled by the initial temperature of the cylinder. Numerical and experimental results are compared.     

Size effect on the optimum actuation condition for SMA-composites

Techniques of using shape memory alloy (SMA) wires as actuators inside the hybrid composites have been studied extensively for improving mechanical properties of structures. There were many experimental findings and theoretical predictions agreed the higher the actuation temperature on built-in SMA wires, the greater the improvement in mechanical response of a composite due to recovery action of wires. However, due to the limitation of interfacial shear strength, over-actuation (by means of electrical resistive heating) of SMA wire is able to induce the development of interfacial crack inside the composite structure. When the maximum interfacial shear strength is attained due to vigorous recovery action of SMA wire under relatively high temperature, interfacial debond can be initiated. In this paper, a typical SMA–matrix cylinder model is employed to study the captioned risk of SMA-composite actuation. Applying the criterion of optimum actuation condition (OAC), target actuation level can be determined to prevent structural failure due to over-actuation. Effects of geometric factors including wire embedded length and matrix-to-wire radius ratio on the interfacial shear stress distribution are evaluated prior to the study of size effect on OAC. Results indicate that the size effect become negligible when these two geometric factors are sufficiently large and as a result, the governing equation of OAC can be greatly reduced to a simple relation between externally applied stress and actuation temperature on the SMA wire. This simplified model is able to enhance the application of OAC and provide a simple and explicit solution to determine an ideal range of actuation levels for a large scale SMA-composite structure in the design stage.     

Effect of phase on debond strength in shape memory alloy reinforced composites

Systematic single fiber pullout tests were performed on epoxy composites embedded with nickel titanium shape memory alloy (SMA) wires. The SMA wires were tested in the austenitic or martensitic states to study and decouple the elastic moduli from martensite transformation or reorientation stresses in the analysis of debond loads. The results reveal that the SMA wires that were in the austenite phase consistently produced higher debond loads as compared to that of those wires that started in the martensite phase, likely due to differences in the Poisson’s ratio. Additionally, there appears to be a relationship between the elastic modulus and debond load where reinforcements with a higher elastic modulus displayed lower debond loads. Lastly, for SMA reinforcements that underwent a martensitic phase transformation or reorientation, the debond load was equivalent to the martensite transformation or reorientation load. The results of this work illustrate the sensitivity of SMA reinforced composites on the mechanical behavior and phase transformation characteristics of the constituent materials.     

A statistical mechanics approach describing martensitic phase transformation

A novel theoretical approach modeling martensitic phase transformation is demonstrated in the present study. The generally formulated model is based on the block-spin-approach and on renormalization in statistical mechanics and is applied to a representative volume element which is assumed to be in a local thermodynamic equilibrium. Using fundamental physical properties of a shape memory alloy (SMA) single crystal as input data the model predicts the order parameter “total strain”, the martensitic phase fraction and the stress assisted transformation accompanied by pseudoelasticity without the requirement of evolution equations for internal variables and assumptions on the mathematical structure of the classical free energy. In order to demonstrate the novel approach the first computations are carried out for a simple one-dimensional case. Results for total strain and martensitic phase fraction are in good qualitative agreement with well known experimental data according to their macroscopic strain rearrangement when phase transformation occurs. Further a material softening effect during phase transformation in SMAs is predicted by the statistical physics approach.     

Multiaxial Shape Memory Effect and Superelasticity

Abstract:  The specific behaviour of shape memory alloys (SMA) is due to a martensitic transformation [Shape Memory Materials. Cambridge University press, Cambridge] . This transformation consists mainly in a shear without volume change and is activated either by stress or temperature. The superelastic behaviour and the one-way shape memory effect are both due to the partition between austenite and martensite. The superelastic effect is obtained for fully austenitic SMA: loaded up to 5% strain, a sample recovers its initial shape after unloading with a hysteretic loop. The one-way shape memory effect is obtained when a martensitic SMA, plastically deformed, recovers its initial shape by simple heating. Superelasticity and one-way shape memory effect are useful for several three-dimensional applications. Despite all these phenomena are well known and modelled in 1D, the 3D behaviour, and especially the one-way shape memory effect, remains quite unexplored [Mater. Sci. Res. Int., 1 (1995) 260]. Actually, the development of complex 3D applications requires time-consuming iterations and expensive prototypes. Predictive phenomenological models are consequently crucial objectives for the design and dimensioning of SMA structures. Therefore, a series of 2D proportional and non-proportional, isothermal and non-isothermal tests have been performed. This database will be used to build a phenomenological model within the framework of irreversible processes.     

The effects of solid-state phase transformation upon stress evolution in laser metal powder deposition

To investigate the influences of solid-state phase transformation on stress evolution during multi-pass laser metal powder deposition (LMPD) process, a 3D finite-element (FE) thermo-mechanical model considering phase transformation has been established. The influences of phase transformation such as mechanical properties changes, volume change and transformation induced plasticity (TRIP) are taken into account. Furthermore, the influences of high magnitude stress upon martensitic transformation characteristic temperature and TRIP are considered. The temperature and history (microstructure) dependent material properties used in the present research are obtained by experiments. The stress field during LMPD process is analyzed with and without solid-state phase transformation, respectively. Stress measurement of X-ray diffraction (XRD) method is applied to deposited samples, and the measuring data are compared with the computational predictions. The results show that phase transformation has a dominant effect on the stress evolution, longitudinal residual stresses significantly reduced as a result of solid-state phase transformation. In addition, the effect of stresses on martensitic transformation temperature is important for accurate prediction of residual stresses (stress state after cooling of the clad to ambient temperature). Residual stresses are lower when the phase transformation temperature is reduced.     

SMA纤维混杂层合梁的振动分析

提出一类形状记忆合金(SMA)纤维混杂层合梁的数学模型。采用多胞模型、形状记忆合金一维本构关系分析方法,同时考虑铁木辛柯剪切和马氏体相变的影响。目的是为了更进一步了解层合梁的振动控制。SMA纤维用来作为驱动器,它能够改变弹性模量和回复力,以此改变梁的频率。分析了SMA纤维含量、铺设角度和横向剪切变形的影响。结果表明,通过激活形状记忆合金纤维及改变初始变形,对层合梁的自振频率有很强的控制和调节能力。     

A three-dimensional shape memory alloy/elastomer actuator

This paper presents a mathematical model for analysis of a thermally-driven shape memory alloy (SMA)/elastomer actuator under arbitrary loading and boundary conditions. The actuator is a three-dimensional laminated composite box beam that consists of SMA and elastomer layers with a uniform rectangular cross section. The thermomechanical behavior of SMA layers is modeled utilizing Tanaka and Nagaki's constitutive equation and linear phase transformation kinetics. The behavior of the elastomer layers is assumed to be thermoelastic, in which the elastic modulus is considered to be temperature dependent. The classical laminated beam theory is employed to obtain the force-deformation relationships. The analysis provides explicit solutions to the structural response of the actuator, including strain and curvature of actuator's midplane. A numerical example for a cantilever box beam with uniform square cross-section subjected to a transverse concentrated load is presented. Results demonstrate that significant changes occur in the actuator's responses during phase transformation due to the strain recovery.     

Characteristics of multi-functional composites using elastomer embedded with Shape Memory Alloy wires

Multifunctional composites consist of Shape Memory Alloy and elastomer in the form of a hybrid system, in which every phase performs a different but necessary function. In this study, elastomer embedded with thermally responsive Shape Memory Alloy (SMA) wires forms one kind of multifunctional composites. The hyper elasticity of elastomer and reversing transformation temperature austenitic finish (A_f) of SMA play highly complementary roles in achieving the multifunctional behavior. The composites show good in-plane deformation capability under the external force driving whether the temperature is under martensitic start temperature (M_s), or above austenitic finish temperature (A_f) of SMA. The composites also show out-of-plane deformation ability under the external force driving when the temperature is under martensitic start temperatures (M_s) of SMA. Varying stiffness behaviors of the multifunctional composites can be obtained by changing the environment temperature from M_s. to A_f, by which the grains of SMA wires could be switched from martensitic phase to austenitic phase. In this study, the varying stiffness characteristics of the SMA-elastomer composites are experimentally proved to be effective and theoretically investigated.     

An approach to optimization of shape memory alloy hybrid composite plates subjected to low-velocity impact

The paper presents an approach to the problem of optimum design of composite plates subjected to low velocity impact. The deflections and stresses are reduced by employing prestrained shape memory alloy (SMA) fibers which are in the martensitic phase when embedded within the plate. At an elevated temperature, the SMA fibers transform into the austenitic phase and tend to contract. However, due to a constraint, the contraction is either completely prevented or reduced resulting in significant tensile recovery stresses. This tension reduces deformations and stresses in the plate subjected to low-velocity impact.The solution in the paper addresses an impact of cross-ply plates with SMA fibers embedded within the layers oriented in both directions. An approach to optimization considered in the paper involves variations of the volume fractions of SMA fibers in each direction subject to a constraint on the total volume of the shape memory alloy. It is shown that an application of SMA fibers can significantly reduce deflections and stresses. A further benefit can be achieved by an optimization of a distribution of volume fractions of SMA fibers between the layers.     

Near surface martensitic transformation and recrystallization in a Ti-24Nb-4Zr-7.9Sn alloy substrate after application of a HA coating by plasma spraying

Plasma sprayed hydroxyapatite (HA) coatings on titanium alloy substrate have been used extensively due to their excellent biocompatibility and osteoconductivity. However, the influence of the high temperature process on substrates can not to be ignored, especially for metastable β type titanium alloy substrates. In this paper, martensitic transformation and recrystallization occurred in near surface of the low-modulus Ti-24Nb-4Zr-7.9Sn alloy substrate after application of a HA coating. Various analyses revealed that the martensitic transformation and recrystallization were mainly related to the high temperature and cooling process. The different microstructures can be attributed to inhomogeneous temperature and cooling rate distributions in the substrate. Corresponding to the microstructure, the Young's modulus and micro-hardness of the near surface of the alloy showed a similar graded distribution along the direction perpendicular to the coating/substrate interface. In addition, the martensitic transformation and recrystallization caused remarkable changes of the entire alloy.     

A multi-dimensional constitutive model for shape memory alloys

This paper presents a multi-dimensional thermomechanical constitutive model for shape memory alloys (SMAs). This constitutive relation is based upon a combination of both micromechanics and macromechanics. The martensite fraction is introduced as a variable in this model to reflect the martensitic transformation that determines the unique characteristics of shape memory alloys. This constitutive relation can be used to study the complex behavior associated with 2-D and 3-D SMA structures. A simple example using this constitutive model is also presented, which reveals a new and interesting phenomenon of 3-D SMA structures.