Influence of mechanically-induced dilatation on the shape memory behavior of amorphous polymers at large deformation

Mechanics of Time-Dependent Materials - In this study, we explore the influence of mechanically-induced dilatation on the thermomechanical and shape memory behavior of amorphous shape memory...     

Effect of thermomechanical training temperature on the two-way shape memory effect of TiNi and TiNiCu shape memory alloys springs

In this article, the effect of thermomechanical training temperature on the two-way shape memory effect (TWSME) of TiNiCu and TiNi alloys springs were investigated. The results showed that when the springs were thermomechanical-trained at pure martensite, there is an increase of the recovery rate to a saturated value, the maximum recovery rate was about 55% and 45% for TiNiCu and TiNi alloys, respectively. As the springs were thermomechanical-trained at pure austensite and martensite+austensite, there is an increase of the recovery rate to a maximum value and decreased with ongoing training after having passed the maximum value and the maximum TWSME recovery rate is less than that of the shape memory alloys spring-trained at pure martensite. Dislocations generated by martensite reorientation were effective in developing two-way memory effect. Since the amount of the stress-induced martensite variants is less than that of thermal-induced martensite variants, thus, the recovery rate showed a different rule with increasing thermomechanical training cycles at different training temperature.     

Phenomenological description of thermomechanical behavior of shape memory alloy

On the basis of a thermomechanical phenomenological model, we analyze the thermomechanical behavior of polycrystalline NiTi. Pseudoelastic response and strain-temperature response under fixed stress are studied by using finite element simulation. Calculated mechanical and thermal hysteresis behaviors of NiTi sheet are in good agreement with those observed experimentally. Hardening in stress–strain hysteresis loop and sharp change of strain in strain-temperature hysteresis loop are described by numerical simulation. The result from thermomechanically coupled calculation shows the phenomenon that phase transition occurs by nucleation and propagation of transformation fronts.     

A Two-network thermomechanical model of a shape memory polymer

The aim of this work is to demonstrate a Helmholtz potential based approach for the development of the constitutive equations for a shape memory polymer undergoing a thermomechanical cycle. The model is able to simulate the response of the material during heating and cooling cycles and the sensitive dependence of the response on thermal expansion. We notice that the yield-stress of the material controls the gross features of the response of the model, and suggests that the material yields differently depending on not just the current value of the temperature but also on whether the temperature of the material dropped or increased from the previous time-step somewhat similar to the Bauschinger effect in plasticity, except that here the controlling parameter is the rate of temperature change rather than rate of plastic strain. The results of the simulation are in qualitative and quantitative agreement with experiments performed on two different shape memory polymer samples: polyurethane and epoxy resin. We find that modeling the hysteresis of the yield stress of the material during temperature changes is the key to the results.     

Universal relation for the thermal conductivity of amorphous polymers

An invariant relation is derived for the thermal conductivity of amorphous polymers above the vitrification temperature.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 22, No. 4, pp. 729–731, April, 1972.     

成分对多晶Ni-Mn-Ga磁性记忆合金相变行为的影响

研究了Ni50Mn25+xGa25-x和Ni50Mn29Ga21-xTbx两种成分系列磁性记忆合金的相变行为.保持Ni含量不变,增加Mn,降低Ga含量会使马氏体相变温度明显提高,同时相变滞后温区减小,居里温度基本不变.如果添加稀土元素铽、相变温度继续升高,居里温度仍然不变,材料继续保持强的铁磁性及热弹性马氏体相变的特征.     

Modeling the mechanics of light activated shape memory polymers

Shape memory polymers (SMP’s) are a relatively new kind of smart materials and have significant technological applications ranging from biomedical devices to aerospace technology. First generation SMP’s relied on changes in temperature to fix the temporary shape and have been studied quite extensively in the past. In the last few years a new generation of SMP’s have been synthesized in which the temporary shape is fixed by exposure to light at specific wavelengths (typically in the Ultraviolet, UV, range). Exposure to light at certain wavelengths causes photosensitive molecules, which are grafted on the polymer chains comprising the material, to form covalent bonds. These bonds act as crosslinks and are responsible for the temporary shape. On exposure to light at a different wavelength these bonds cleave and the material returns to its original shape. Our research focuses on modeling the mechanics associated with such light activated shape memory polymers (LASMP’s) undergoing complex deformations. The modeling is done using a framework based on the theory of multiple natural configurations taking into consideration the different aspects of modeling this material, which include developing a model for the original virgin network and for the other networks with different stress-free states, formed due to exposure to light. In addition to this, we also model the initiation and the formation of the light activated networks and the reverse transition resulting in the dissolution of these networks. Anisotropy in the mechanical response is also incorporated into the model. The model is then used to simulate results for specific boundary value problems, such as uni-axial extension and inflation of a cylinder.     

Biomedical applications of thermally activated shape memory polymers

Shape memory polymers (SMPs) are smart materials that can remember a primary shape and can return to this primary shape from a deformed secondary shape when given an appropriate stimulus. This property allows them to be delivered in a compact form via minimally invasive surgeries in humans, and deployed to achieve complex final shapes. Here we review the various biomedical applications of SMPs and the challenges they face with respect to actuation and biocompatibility. While shape memory behavior has been demonstrated with heat, light and chemical environment, here we focus our discussion on thermally stimulated SMPs.     

A study on thermomechanical deformation of elastic beam with embedded shape memory alloy wires

The bending deformation of an elastic beam with eccentrically embedded shape memory alloy (SMA) wires under the activation of electrical current has been investigated. A procedure, which accounts for the effect of transient heat transfer and gives rise to the beam bending responding to prescribed input electrical current profile is used for analysis. It is found that influences of ambient temperature, electric current intensity and prestrain of SMA on the bending deformation of beam are significant.     

形状记忆纤维热粘弹性基体复合材料的力学行为

应用热粘弹性理论和Voigt混合律,在变温场中针对马氏体逆相变过程建立了NiTi形状记忆纤维热粘弹性基体复合材料的应力-应变关系,在逆相变过程和基体呈现热粘弹态阶段,由于基体松弛其模量减小,在跃阶拉应力的作用下,复合材料的压缩应变迅速增大,纤维回复应力先增大后减小;在跃阶拉应变的作用下,复合材料的应力增加先变缓然后加快直至稳定,较高的温度和材料参数对NiTi纤维热粘弹性基体复合材料的力学行为和纤维的作动性能有明显的影响.     

A thermodynamic framework for the modeling of crystallizable shape memory polymers

Shape memory polymers are a relatively new class of materials that have the ability to retain a temporary shape, which can be reset to the original shape with the use of a suitable trigger, typically an increase in temperature. The temporary shapes can be very complex and the deformations involved large. These materials are finding use in a large variety of important applications; hence the need to model their behavior. In this paper, we develop constitutive equations to model the thermo-mechanical behavior of crystallizable shape memory polymers. Crystallizable shape memory polymers are called crystallizable because the temporary shape is fixed by a crystalline phase, while return to the original shape is due to the transition of this crystalline phase. The modeling is carried out using a framework that was developed recently for studying crystallization in polymers and is based on the theory of multiple natural configurations. In this paper we formulate constitutive equations for the original amorphous phase and the semi-crystalline phase that is formed after the onset of crystallization. In addition we model the transition of the crystalline phase to capture the return of the polymer to its original shape. These models for shape memory effects in polymers have been developed within a full thermodynamic framework, extending our previous work in which the models were developed within a mechanical setting [G. Barot, I.J. Rao, Constitutive modeling of the mechanics associated with crystallizable shape memory polymers, ZAMP 57 (4) (2006) 652-681]. The model is applied to the problem of inflation and extension of a hollow cylinder. The results are consistent with what has been observed in experiments.     

Experimental investigation on the debonding strength in shape memory alloy wire reinforced polymers

The interfacial shear strength between the shape memory alloy (SMA) wire and epoxy matrix was evaluated experimentally using a single wire pull-out test. Moreover, the effect of pre-strain in SMA wires on the interfacial behavior was studied by pre-straining the SMA wires to 2% and 4% pre-strains. Experiments were conducted in both martensite and austenite phases of SMA. Results showed that pre-straining SMA wire in the martensite phase caused enhancement in interfacial shear strength due to recovery force generation. Further, 9.7% and 33% improvements in the interfacial shear strength were achieved at 2% and 4% of SMA pre-strain, respectively. However, the enhancement of interface behavior did not occur, when the SMA wires were subjected to pre-strain in the austenite phase.     

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。      

A note on constrained shape memory alloys upon thermal cycling

In this paper, we present a systematical study on the constrained shape memory alloys upon thermal cycling. The focuses are the transformation temperature interval and hysteresis. Closed-form expressions are obtained for biased actuators and thin films atop an elastic substrate.     

Analysis and design of smart mandrels using shape memory polymers

Based on the thermomechanical mechanism of shape memory polymers (SMPs), the three-dimensional thermomechanical constitutive equation that can be used in the ABAQUS finite element simulation was derived. Then this paper compiled UMAT subroutine and simulated the thermomechanical behaviors of SMP smart mandrels. In addition, the properties of shape fixity and shape recovery ratio of SMP were considered in detail. Finally, filament winding experiments were proceeded on bottle-shaped and air duct-shaped mandrels and the simple and efficient demoulding of SMP mandrels were verified. The results showed the feasibility of SMP as the smart mandrels from practical application in the future.     

形状记忆聚合物在柔性光/电子器件领域的发展与挑战

随着柔性光/电子技术的不断发展,人们对下一代柔性光/电子器件提出了新的要求。刺激响应型材料尤其是形状记忆聚合物（Shape memory polymer,SMP）近年来得到了广泛的关注与发展。将SMP与柔性光/电子结合不仅能够赋予柔性光/电子器件形状记忆功能,而且还能极大拓展柔性光/电子器件的功能性和应用范围。本文首先综述了使用SMP基板赋予柔性光/电子器件形状记忆功能的SMP种类、驱动方法、制备技术及应用领域;其次综述了具有显著优势的4D打印技术打印的三维SMP结构器件的种类、所使用的SMP原材料、驱动方法及应用领域;最后展望了形状记忆柔性光/电子器件未来发展方向和遇到的挑战。     

Analysis of uniaxial stress-strain-temperature hysteresis in an Fe-based shape memory alloy under thermomechanical loading

The uniaxial thermomechanical behavior in an Fe-based polycrystalline shape memory alloy is analyzed from the phenomenological point of view by means of the rate-type constitutive equation and the newly developed transformation kinetics. The forward and reverse transformations among the martensite phase produced in tension or in compression and the parent phase are shown to be depicted by the transformation zones bounded by the transformation start/finish lines on the stress-temperature plane. The shift of the lines, depending on the extent of prior transformation, is determined from the experimental data by the same authors. The stress-strain-temperature hysteresis loops and the recovery stress during heating are well simulated.