Modeling of superelastic behavior of porous shape memory alloys

International Journal of Mechanics and Materials in Design - In this study the superelastic behavior of porous shape memory alloys has been simulated in a finite element procedure. The Boyd and...     

Low-energy tensile-impact behavior of superelastic NiTi shape memory alloy wires

Superelastic property of shape memory alloys (SMAs) is becoming increasingly important for impact applications due to their large recoverable strains and high capacity to dissipate energy. In this work, tensile behavior of superelastic NiTi SMA wires at impact strain rates was studied by instrumented tensile-impact technique, which allows to obtain material properties on the order of 1–10² s⁻¹. The results show that even at impact strain rates, martensite can be induced by tension in NiTi. At impact, a plateau stress appears during transformations similar to that at quasi-static strain rates, but 100–150 MPa higher in stress. This is due to the higher temperatures achieved during the deformation due to the close to adiabatic nature of the impact event. The influence of the strain rate over the mechanical behavior of NiTi was spread to the quasi-static strain rates so that the evolution of several parameters was also studied on the range 10⁻⁵–10² s⁻¹. Therefore, forward stress-induced martensitic (SIM) transformation stresses (σ^Ms and σ^Mf) and deformation energy (E_d) increase with strain rate, but they are strain rate independent from 10⁻¹ s⁻¹ at least until 10² s⁻¹. Reverse SIM transformation stresses (σ^As and σ^Af), recoverable strain energy (E_r), and dissipated energy (W_d) depend mainly on maximum strain achieved during the deformation, but for strains corresponding to a load–unload cycle with complete SIM transformation, σ^As, σ^Af and E_r are higher at impact than at quasi-static strain rates, and W_d shows similar values at very low strain rates and at impact.     

超弹性形状记忆合金棒力学性能的实验研究

对超弹性形状记忆合金(SMA)棒进行力学性能实验,研究了荷载循环、应变幅值和荷载频率对其滞回曲线以及耗能量、损耗因子、残余应变的力学参数的影响。结果表明,应力-应变曲线在循环加卸载中逐渐变化并趋于稳定;耗能量和损耗因子均随应变幅值的增加而增加,随荷载频率的增加而减小;稳定的SMA棒的超弹性、耗能能力和阻尼性能均较差,但其输出力较大,故可作为复位或限位装置用于土木工程中。     

Vibrations of plates with superelastic shape memory alloy wires

The article illustrates an approach to the passive vibration control of thin plates utilizing prestressed superelastic shape memory alloy (SMA) wires. The SMA wires can freely slide within protective sleeves that are either embedded within the structure or bonded to its surface. The vibration control mechanism combines an effective continuous elastic foundation representing the support provided by SMA wires to the structure with the energy dissipation as a result of the hysteresis occurring in the wires. The other approach to the vibration control employs superelastic wires attached to the structure at discrete points. The mathematical formulation of the problem presented in the article can be adopted for a rigorous computational analysis. In particular, a closed form expression is obtained for the loss factor in large aspect ratio plates supported at the midspan by a system of parallel SMA wires. As follows from numerical examples presented for such plates, the proposed method offers a significant damping, far exceeding that observed in conventional engineering structures.     

Review Thermo-mechanical fatigue of shape memory alloys

Shape memory alloys (SMA) produce the largest anomalous reversible strains of all crystalline materials: _SM < 10% in comparison to _FM < 1.0% and _FE < 0.1% (FM magnetostrictive, FE electrostrictive). They are considered as active materials for applications in which large scale motions are required or because of their high damping capacity for mechanical vibrations.Their use is however limited if a large number of cycles is applied. A survey is given on fatigue phenomena in SMA:1. classical mechanical fatigue—accumulation of defects, formation and growth of cracks,2. shape memory fatigue—change of transformation temperatures, reduction or loss of memory, loss of pseudo elasticity or damping capacity.Finally, it is shown how a modification of microstructure by thermo-mechanical treatments (TMT) affects fatigue resistance. In this context primary defects (as introduced by TMT) and secondary defects (introduced by mechanical or thermal cycling) have to be distinguished. In case of training for the two-way effect secondary defects are introduced voluntarily.     

超弹性形状记忆合金的神经网络连续本构模型

对超弹性形状记忆合金(SMA)丝在不同应变幅值和荷载速率下进行加卸载单轴拉伸试验,分析其滞回特性随环境因素的变化规律。将径向基函数神经网络(RBFNN)和Graesser模型结合起来,Graesser模型参数取自试验曲线,能由数学式确定的模型参数和应变幅值、荷载速率一起作为网络的输入信息,不能由数学式确定的模型参数作为输出神经元。数值计算表明,RBFNN可以精确地预测Graesser模型参数,且计算的SMA应力-应变曲线与Graesser模型结果吻合很好。     

Continuous mandibular distraction osteogenesis using superelastic shape memory alloy (SMA)

Distraction osteogenesis is a well-established method of endogenous tissue engineering. It is a biological process of bone neo-formation between segments subjected to tension. The concept of this study was to investigate the distraction osteogenesis with a device capable of creating a permanent and constant force during the whole process as if a very large number of small elongations were applied constantly. The mechanical testing of the device used to produce the constant force and the in vivo analysis of the bone growth after it was implanted in rabbits are presented on this work. The device consists of a NiTi coil spring, superelastic at body temperature, in order to have a stress plateau during the austenitic retransformation during the unloading. The in vivo analysis was made on six female rabbits of 12 months old. A segmental mandibulectomy at the horizontal arm of the mandible and a corticotomy at 5mm distant from the gap were made. Next, following a latency period of five days, the SMA springs were implanted to induce the bone neo-formation. The displacement at the unloading plateau shows that it is necessary to have longer springs or to use several (available commercially) in series in order to fulfil the requirements of a human distraction. The temperature variations induced changes in the spring force. However, when the temperature returns to 37 degrees C the distraction force recovers near the initial level and does so completely when the distraction process continues. For the in vivo study, all six rabbits successfully completed the distraction. The radiographies showed the gap as distraction advanced. A continuity in the newly formed bone with similar transversal and horizontal dimensions than the original bone can be observed on the histologies. In conclusion, the application of a constant force on distraction osteogenesis, using SMA springs, may be a successful alternative to the conventional gradual distraction.     

超弹性TiNi记忆合金螺旋弹簧的滞回性能实验研究

通过滞回性能实验研究了大尺寸Ti Ni形状记忆合金(Ti Ni SMA)螺旋弹簧用于结构振动控制的可行性。利用两种Ti Ni合金制作了大尺寸超弹性螺旋弹簧试件。对Ti Ni弹簧试件进行了单轴反复荷载作用下的力学实验,获得了其在不同加载条件下的恢复力-位移曲线。分析了不同循环加卸载次数、加载频率、位移幅值对两种Ti Ni弹簧的滞回环及等效刚度、单位循环耗能、等效阻尼比和残余位移4个力学参数的影响。研究结果表明,超弹性Ti Ni弹簧可提供较大的恢复力、位移以及稳定的复位性能,并具有一定的耗能能力,在工程结构自复位振动控制中具有较好的应用潜力较好。     

Stress block parameters for concrete flexural members reinforced with superelastic shape memory alloys

The unique properties of superelastic shape memory alloys (SMAs) have motivated researchers to explore their use as reinforcing bars. The capacity of a steel reinforced concrete (RC) section is calculated by assuming a maximum concrete strain ε _c-max and utilizing stress block parameters, α ₁ and β ₁, to simplify the non-linear stress–strain curve of concrete. Recommended values for ε _c-max, α ₁, and β ₁ are given in different design standards. However, these values are expected to be different for SMA RC sections. In this paper, the suitability of using sectional analysis to evaluate the monotonic moment–curvature relationship for SMA RC sections is investigated. A parametric study is then conducted to identify the characteristics of this relationship for steel and SMA RC sections. Specific mechanical properties are assumed for both steel and SMA. Results were used to evaluate ε _c-max, α ₁, and β ₁ values given in the Canadian standards and to propose equations to estimate their recommended values for steel and SMA RC sections.     

Low-cycle fatigue life of superelastic NiTi wires

This paper analyzes low-cycle fatigue life under strain control (ε_a–N_f curve) of NiTi wires in bending-rotation tests. These were carried out on stable austenite, superelastic, and stable martensite wires, with strain amplitudes from 0.6% to 12%. For strain amplitudes up to 4%, ε_a–N_f curves of superelastic wires are close to those values reported in the literature, and close to that of the stable austenite wire. For higher strain amplitudes, the fatigue life of superelastic wires increases with strain and approaches the fatigue life of stable martensite wire. This unusual behavior causes a “Z-shaped” curve. Fatigue crack characteristics were studied by scanning electron microscopy. Simple finite element models of the test were also developed. The results suggest that the abnormal shape of the superelastic wire curve is associated to the changes in fatigue properties that occurs when the superelastic material transform to martensite.     

Tensile and fatigue behavior of thin-walled cylindrical specimens under temperature gradient condition

In the present study, a temperature gradient system was designed with the aim of carrying out the tensile and fatigue test of thin-walled cylindrical alloyed steel (30CrMnSi). The tensile test under different temperatures was first carried out to obtain the static mechanical parameters. And then a three-dimensional (3D) finite element model was constructed to further study the deformation behavior under temperature gradient by the finite element analysis (FEA). The FE result was in good agreement with that of the experiment. Following this, the tensile fatigue test was performed under cooled air and no-cooled air to investigate the influence of the temperature gradient on fatigue life-time, respectively. The influence of cooled air under the lower nominal stress on fatigue life was not obvious than that of higher nominal stress. Finally, the scanning electron microscopy (SEM) was employed to investigate the fracture mechanism. The microstructure revealed that the fracture first occurred at the zone where there was a lower temperature.     

Fatigue behavior of Cu-Al-Be shape memory single crystals

The fatigue behavior of Cu-Al-Be shape memory single crystals is studied in cyclic loading mechanical tests. Based on literature and on tensile tests performed at various temperatures, a model is proposed to explain the mechanism of fatigue. This model is based on the idea that during cyclings, the different zones of the samples spend various lengths of time in the martensitic state. During that time, martensite evolves because of the occurrence of some reordering or other diffusional phenomena inside, and consequently, the value of the martensite start temperature for each zone changes. The kinetics of the change in mechanical behavior along with the cycles, as a function of the test temperature, are accurately described by a Johnson–Mehl relation. Relaxation tests suggest that the mechanism described not only depends on time and temperature but is also re-enforced by the movement of the martensite-matrix interface. From the kinetics of this fatigue behavior, an empirical activation energy related to the mechanism is inferred for this Cu-Al-Be alloy.     

Tensile properties of a Fe-32Mn-6Si shape memory alloy

The tensile properties of a Fe-32Mn-6Si shape memory alloy were investigated. It was found that tensile properties depend on temperature, heat treatment and material structure. The relationships of martensitic transformation, tensile properties, and shape memory effect are discussed. Finally, we propose a macroscopic one-dimensional constitutive law describing the thermomechanical behavior in tensile loading. Numerically obtained results are close to the experimental ones. __________ Translated from Problemy Prochnosti, No. 2, pp. 55–65, March–April, 2008.     

Effect of superelastic shape memory alloy wires on the impact behavior of carbon fiber reinforced in situ polymerized poly(butylene terephthalate) composites

Low-velocity impact properties of shape memory alloy (SMA) wires and carbon fiber reinforced poly(butylene terephthalate) obtained by resin transfer molding were characterized. At the subcritical regime the dissipated energy is not affected by the presence of the wires. However SMA has a positive effect on the maximum absorbed energy, since the maximum allowable load is higher. The contribution of the SMA wires to the higher impact performance of the hybrid composite is suggested to be due to their energy absorbing capability, and also to the high reversible force that acts as a healing force.     

Damping,tensile, and impact properties of superelastic shape memory alloy (SMA) fiber-reinforced polymer composites

The potential of superelastic shape memory alloy (SMA) fibers to enhance the damping capacity and toughness of a thermoset polymer matrix was evaluated. A single-fiber winder was designed and built to manufacture a pre-form consisting of 102 μm diameter SMA fibers aligned parallel to each other. This pre-form was loaded to varying amounts of pre-strain and impregnated with vinyl ester to manufacture SMA fiber composites with 20% fiber volume fraction. The composites were tested using a Differential Scanning Calorimeter (DSC) and a Dynamic Mechanical Analysis (DMA), to evaluate the improvement in damping capacity of the polymer matrix due to the SMA fibers. Tensile and instrumented impact testing were carried out to evaluate improvements in mechanical properties and toughness of the composites. Appreciable improvement was observed in damping, tensile, and impact properties of the polymer matrix due to reinforcement with superelastic SMA fibers, highlighting the advantages of their use in polymer composites.     

A finite element model for shape memory behavior

This paper deals with a finite element implementation concerning the shape memory behavior. Shape memory behavior is usually driven by temperature changes. This model allows the simulation of problems integrating complex mechanical loading effects under random temperature variations. According to the relationship between stress and strain, the shape fixation during cooling phases and the memory effect during heating phase are modelised through a hereditary behavior needing incremental formulation developments. The step by step process introduces an additional fixed stress. Simulations request, for complex geometries including boundary conditions, a finite element approach. Thermodynamic developments are presented in order to define energetic balance and dissipations. In this paper, we propose to generalize this dependence of elastic modulus variations. A formulation for random mechanical loading and temperature variations is proposed. An experimental validation is proposed about shape memory alloy polymer DP5.     

Dynamic impact behavior of Ni-rich and Ti-rich shape memory alloys

Stress vs. strain curves of two kinds of Ti–Ni shape memory alloys under dynamic compressive loading condition were obtained by the split Hopkinson pressure bar experiment. One is Ni-rich, 50.75Ni–49.25Ti (at%), alloy and the other is Ti-rich, 49.5Ni–50.5Ti (at%). The former exists as austenite phase to show superelastic behavior at the room temperature, and the latter is martensitic phase at the room temperature. The dynamic stress–strain curves were compared with quasi-static compressive test results. The Ni-rich specimen displayed a much higher stress level at the high strain rate than at the quasi-static test, but the Ti-rich specimen exhibited a stress level alike in the two types of tests. Temperature measurement on the surface of the impact specimens was carried out during the impact test. The temperature rising was about 3–5 °C in the considered test conditions.