One-way Shape Memory Effect Research Articles

The device “Klyost” for clipping vessels and soft-elastic tubular structures

A retrospect of earlier elaborated devices applying in clinical practice is presented. A newly invented device for clipping vessels and soft-elastic tubular structures is presented. Traditionally in surgical practice, to stop a blood flow, the surgeons use suturing of vessels (legate) with a silk thread. In laparoscopic surgery this process is accompanied with great difficulties. An alternative method of clipping vessels and structures is also used. The negative feature of this method is that the structure must be first mobilized. Up for to-day there is no device allowing to combine in one method both manipulations: suturing of tissue and laying a clip on a structure. The paper describes the new device, allowing to combine the oversaid manipulations and consequently to simplify the work of the surgeon and shorten the operation time. The device functioning is based on one-way and two-way shape memory effects. The device was applied in treating of 5 patients and manifested fine functional possibilities. The implanted clip can be easily evacuated without any patient trauma.

Shape memory behaviour: modelling within continuum thermomechanics

A phenomenological material model to represent the multiaxial material behaviour of shape memory alloys is proposed. The material model is able to represent the main effects of shape memory alloys: the one-way shape memory effect, the two-way shape memory effect due to external loads, the pseudoelastic and pseudoplastic behaviour as well as the transition range between pseudoelasticity and pseudoplasticity. The material model is based on a free energy function and evolution equations for internal variables. By means of the free energy function, the energy storage during thermomechanical processes is described. Evolution equations for internal variables, e.g. the inelastic strain tensor or the fraction of martensite are formulated to represent the dissipative material behaviour. In order to distinguish between different deformation mechanisms, case distinctions are introduced into the evolution equations. Thermomechanical consistency is ensured in the sense that the constitutive model satisfies the Clausius–Duhem inequality. Finally, some numerical solutions of the constitutive equations for isothermal and non-isothermal strain and stress processes demonstrate that the various phenomena of the material behaviour are well represented. This applies for uniaxial processes and for non-proportional loadings as well.

Repeatable bending actuation inpolyurethanes using opposing embedded one-way shape memoryalloy wires exhibiting large deformation recovery

Shape memory alloys (SMAs) are being used in adaptive structuresfor sensing and actuation because of the large amount ofdeformation and force that these materials can produce uponheating. Of particular interest, is the bending actuation ofpolymers, such as polyurethanes, by embedding opposing SMAwires. Previous investigations on these structures have focused primarily on embedding single or opposing SMA wires that havebeen trained to exhibit the two-way shape memory effect (SME)because of the simplicity with which the actuation behavior canbe modeled and predicted. Alternatively, many potentialadaptive structure applications can employ SMA wires exhibitinga one-way SME to eliminate the need for a priori two-way training of the SMA wires, and to increase the recoverabledeformation in the structure. In this investigation, therepeatable bending actuation of polyurethanes using opposingone-way SMA wires exhibiting large deformation recovery ischaracterized with a full-field deformation measurement technique known as digital image correlation. These results areanalyzed with a one-dimensional actuation model previouslyproposed for predicting the bending actuation of elastomericrods embedded with opposing two-way SMA wires. Usingappropriate recovery strains and constitutive bending responses, this model was modified to characterize an`equivalent two-way SME' recovery strain for the one-way SMAwires. A kinematic model of the bending actuation is alsopresented that attributes the origin of the equivalent two-waySME to the biasing forces acting on the SMA wires. Repeatablebending actuation was achieved in polyurethanes using one-way SMA wires prestrained 5% at an equivalent two-waySME recovery strain level of nearly 5%. These resultsindicate that repeatable bending actuation can be achieved inpolyurethanes by using opposing SMA wires exhibiting theone-way SME instead of the two-way SME at a level 25% greater than reported for the two-way SME. The level of bendingactuation will depend on the constraint of the SMA wiresimposed by the stiffness of the polyurethane matrix. The symmetryof the bending actuation is affected by the symmetry of thelocal volume fraction of SMA wires and the symmetry of theprestrain levels in the wires. Furthermore, symmetric prestrainwill enhance the level of equivalent two-way SME exhibited bythe wires.

A fundamental investigation into large strain recovery of one-way shape memory alloy wires embedded in flexible polyurethanes

Shape memory alloys (SMAs) are being embedded in or externally attached to smart structures because of the large amount of actuation deformation and force that these materials are capable of producing when they are heated. Previous investigations have focused primarily on using single or opposing SMA wires exhibiting the two-way shape memory effect (SME) because of the simplicity with which the repeatable actuation behavior of the structure can be predicted. This repeatable actuation behavior is achieved at the expense of reduced levels of recoverable deformation. Alternatively, many potential smart structure applications will employ multiple SMA wires exhibiting a permanent one-way SME to simplify fabrication and increase the recoverable strains in the structure. To employ the one-way wires, it is necessary to investigate how they affect the recovery of large strains when they are embedded in a structure. In this investigation, the large strain recovery of a one-way SMA wire embedded in a flexible polyurethane is characterized using the novel deformation measurement technique known as digital image correlation. These results are compared with a simple actuation model and a three-dimensional finite element analysis of the structure using the Brinson model for describing the thermomechanical behavior of the SMA. Results indicate that the level of actuation strain in the structure is substantially reduced by the inelastic behavior of the one-way SMA wires, and there are significant differences between the deformations of the matrix material adjacent to the SMA wires and in the region surrounding it. The transformation behavior of the SMA wires was also determined to be volume preserving, which had a significant effect on the transverse strain fields.

Shape memory effect due to magnetic field-induced thermoelastic martensitic transformation in polycrystalline Ni–Mn–Fe–Ga alloy

In Heusler-type alloy Ni 2+ x− y Mn 1− x Fe y Ga, partial substitution of Mn for Ni causes the temperatures of structural (martensitic) T M and magnetic T C (Curie point) phase transitions to converge. Close to the crossover of T M and T C , we have observed the strong strains ( Δl/ l≈2–4%) induced by the external magnetic field. This effect could be classed with colossal magnetostriction. The system exhibits the magnetic field-controlled one-way shape memory effect at fixed temperature as a result of the magnetic field-induced martensite to austenite structural phase transition.

Normally-closed shape memory microvalve

A new type of normally-closed gas microvalve is presented. The valve makes use of two micromachined SMA thin sheet devices, an active SMA device displaying the one-way shape memory effect and a passive superelastic SMA device, which maintains a normally-closed position. The outer diameter and height of the valve are 6 mm. Gas flow experiments in stationary equilibrium reveal a maximum flow of 470 standard ccm (sccm) at a pressure difference of 8000 hPa and a power consumption of 260 mW. The dynamics of heat transfer to and from the active microdevice is investigated by infrared microscopy and coupled FEM simulations. At 300 mW heating power, the heating time is about 1.2 s. Typical cooling times vary between 1.8 and 5 s depending on the gas flow.

Stabilization of the shape memory effect in NiTi: an experimental investigation

Over the last 20 years or so, shape memory alloys have become well known for their ability to undergo large shape changes that traditional metals could never do. One such alloy, NiTi, has been of particular interest due to it’s high efficiency (when compared to other SMA’s) to convert thermal energy into mechanical energy. While NiTi is currently used in industry, its use is hindered by an extreme sensitivity to processing techniques. Certain parameters such as alloy composition, annealing procedures and various loading conditions can improve the shape memory effect or completely make it disappear. Many studies have focused on these variables. Of particular interest to this study is the one-way shape memory effect (OWSME) after the NiTi has been cycled (deformation followed by recovery) many times. It is very desirable to find a processing technique that would stabilize the OWSME and make it repeatable after many cycles. This paper explores these parameters in hopes of finding an optimal process to stabilize the shape memory effect in NiTi. Many studies have been conducted examining the cyclic behavior of NiTi. Most common are studies done on pseudoelastic NiTi (1,2,3) and on the two way shape memory effect (TWSME) (4,5,6,7). However, very few have explored these cyclic effects on the OWSME. One study that was conducted on the OWSME used wire samples and carried tests out to 100 cycles (8). It was reported that the recovery strain was reduced to 93.4% and 81% of the total strain when total strains of 6% and 8% were used, respectively over the 100 cycles. (Note, that these values will differ from ours since they used “hard” cycles with a fixed maximum strain where as we used “soft” cycles with a fixed maximum stress.) This displays the degradation of the shape memory effect that hinders NiTi’s use. After this work, another study was performed that investigated a conditioning technique to try to stabilize the shape memory effect (9). Their research recommended that after SME (strain-heat-recover-cool-repeat) cycling the NiTi 30 times at 6% strain, a stable strain range was obtained for strains less than 6%. This was only shown over a very few cycles and in wire samples. It is our hypothesis that some form of “conditioning” may be performed on NiTi after annealing to improve its cyclic shape memory properties. The result will be a protocol that can be applied to NiTi before its service use. In doing so, it is important to define a few performance criteria that are considered indicative of “good behavior” and clarify what will hopefully be stabilized:

318 Ni_2MnGa スパッタ膜の形状記憶効果

The Ni-Mn-Ga films were deposited on a poly-vinyl alcohol substrate with a radio-frequency magnetron sputtering apparatus using Ni_<50>Mn_<25>Ga_<25> and Ni_<52>Ga_<24> targets. After separating from the substrates, the films were annealed at several temperatures for 3.6ks. The chemical composition of the films depended on the radio-frequency generating power, however, there was no dependence on the heat treatment temperature. Each deposited film has a columnar grain structure. After heat treatment, the width of columnar grains widened and then became indistinct with increasing heat treatment temperature. At more than 1073 K, the precipitation of MnO was observed clearly. The films showed he most crystallinity at 1073 K. The martensitic transformation temperature increased with increasing nickel content of films, while the Curie temperature decreased. The film obtained at 200 W using the Ni_<52>Mn_<24>Ga_<24> target after heat treatment at 1073 K showed one-way shape memory effect through the reverse martensitic structural transformation during heating.

Influence of Cerium, Titanium and Nitrogen on Shape Memory Effect of Fe-Mn-Si-Cr-Ni Alloys

Since the shape memory effect (SME) in Fe-Mn-Si alloy was observed, much research has been done on this kind of shape memory alloy (SMA) because of its particularly good one-way SME and low cost. With the addition of Cr and Ni, it is possible to reach a good corrosion resistance, which confers to this type of alloy a future use in industry. But compared with TiNi and Cu-based shape memory alloys, the incompleteness of shape recovery and less reversible strain are the main disadvantages of this kind of alloy. Yang et al. reported that the fully reversible strain was less than 2% and only 4.5% of the net reversible strain could be obtained under the prestrain of 8%. In the present work, 3% and 5% of the net reversible strain can be obtained in the case of 6.3% prestrain. The paper is mainly concentrated on the influence of these three elements on the SME of Fe-Mn-Si-Cr-Ni alloys.

On the Unified Micromechanics Constitutive Description of One-Way and Two-Way Shape Memory Effects

A micromechanics constitutive model for the two-way shape memory effect (TWSME) of single crystal shape memory alloys is proposed to describe the TWSME behavior during the cooling and heating process after some training of the alloys. The model is established on two physical bases: (1) the micromechanics crystallographic constitutive theory of single crystal recently developed by SUN et al.[3] and (2) the analysis of the stored elastic strain energy due to dislocations that are produced in the training process of a single kind of variant and the analysis of the interactions between the stress fields of dislocations and transformations. Theoretical predictions of the model are in agreement with the available experimental observations. Finally the present model can be used as a theoretical basis to predict the TWSME of polycrystals.

Influence of the presence of pre-existing thermal ε-martensite on the formation of stress-induced ε-martensite and on the shape memory effect of a FeMnCrSiNi shape memory alloy

At present, many investigations are done on Fe-Mn-based shape memory alloys because of their particularly good one-way type shape memory effect and their low cost. With addition of Cr and Ni, it is possible to reach a good corrosion resistance, which confers to this type of alloys a commercial significance. The shape memory effect is associated with the formation of stress-induced [var epsilon]-martensite by deformation of an austenitic ([gamma]) sample. The reversion by heating of the [var epsilon]-martensite provides the shape memory effect. It is generally admitted that the presence of thermal [var epsilon]-martensite before deformation has a negative influence on the formation of the stress-induced martensite and on the shape memory effect. The authors' purpose is to evaluate the real influence of the pre-existing thermal [var epsilon]-martensite on the formation of the stress-induced [var epsilon]-martensite, its recovery and on the shape memory effect.

Transformation behaviour and one-way shape memory effect in Fe-Mn-Si shape memory alloys

Transformation behaviour and one-way shape memory effect in Fe-Mn-Si shape memory alloys

Martensitic transformations and the shape memory effect in Ti 50Ni 10Au 40 and Ti 50Au 50 alloys

Martensitic transformations in Ti 50Ni 10Au 40 and Ti 50Au 50 alloys are of the type B2 a3 B19(2H) as found in a Au-47.5%Cd alloy. The M s temperatures of the Ti 50Ni 10Au 40 and Ti 50Au 50 alloys are 440°C and 610°C, respectively. The martensite in both alloys is orthorhombic (2H) and identified as a modified B19 structure with secondary layer stacking at ( a 2.60 , 0, c 2 ). The morphology of the martensite is spearlike with ( 1 11) M internal twins. In the Ti 50Au 50 martensite, (100) M stacking faults and type II twins are also found. Both Ti 50Ni 10Au 40 and Ti 50Au 50 alloys exhibit a one-way shape memory effect, but not a two-way memory. This may result from the fact that the accumulated “bias” stresses are released when the temperature is raised to above A s during the “training” process.

Formation and reversion of stress induced martensite in Ti-10V-2Fe-3Al

The formation and reversion behavior of stress induced orthorhombic martensite (α″) in Ti-10V-2Fe-3Al has been studied, and several interesting features found. Reversion appears to take place in four stages. First, there is an athermal reshearing of α″ back to the b.c.c. parent phase, which is accompanied by a significant one-way shape memory effect between 165° and 240°C. This athermal reversion competes with an isothermal stabilization of the α″ plates. Further heating results in the isothermal precipitation of α, which seems to reproduce the preferred orientations of the previous α″, and is thus also accompanied by a shape memory effect; an isothermal shape memory effect in the opposite direction as the preceeding athermal shape change. The precipitation of α occurs on two microstructural scales, extremely fine α nucleating in the β matrix on extant isothermal ω particles, and the direct transformation of the unreverted α″ plates to α. Finally, when heating is continued to the β-transus temperature, the α begins to dissolve back to the parent β. This final dissolution is accompanied by a textural loss in the α phase, and consequently gives rise to several final “shape adjustments”, the nature of which have not been fully explained.