Two-way Shape Memory Behavior Research Articles

Two-way actuation behavior of shape memory polymer/elastomer core/shell composites

Semi-crystalline shape memory polymers (SMPs) show net two-way shape memory (2W-SM) behavior under constant stresses by the recoverable creep strain upon heating and stress-induced crystallization under the application of creep stress upon cooling. The applied constant stress is the key factor in this 2W-SM behavior. A core/shell structure is manufactured for the purpose of imparting a constant stress upon SMPs. An SMP in film or fiber form is dipped into a solution of an elastomer, photoinitiator, and curing agent and then dried out. After this dip coating process is repeatedly carried out, the SMP/elastomer core/shell composite is deformed into a temporary shape after being heated up above the transition temperature of the SMP. Under constant strain conditions, the composite is cooled down, after which the shell elastomer is cured using ultraviolet light. Then, the SMP/elastomer core/shell composite extends and contracts upon cooling and heating, respectively, without any external load. This cyclic deformation behavior is characterized, demonstrating that the current method offers a simple macroscopic processing technique to manufacture 2W-SM polymer composites.

Semi-crystalline two-way shape memory elastomer

Most shape memory polymers are rigid and exhibit only one way (irreversible) shape memory effect. This work explored the two-way (reversible) shape memory effect for semi-crystalline elastomeric networks. These semi-crystalline networks with various gel fractions and crosslinking densities were synthesized via radical initiated crosslinking of poly(ethylene-co-vinyl acetate). The thermal, mechanical, and the two-way shape memory properties of the resulting materials were investigated. The critical impact of the gel fraction on the two-way shape memory behaviors was revealed. The results from this study establish the basis for potential applications involving soft reversible actuation.

In situ temperature tunable pores of shape memory polyurethane membranes

Conventional shape memory polymers, such as shape memory polyurethanes (SMPU), canexhibit net two-way shape memory behavior (2WSM), i.e., upon heating and subsequentcooling, their macroscopic shapes change reversibly under an applied bias load. This paperis aimed at reporting similar 2WSM behavior, especially by focusing on the size ofnanopores/micropores in SMPU membranes, i.e., the size of the pores can be reversiblychanged by up to about 300 nm upon repeated heating and cooling. The SMPU membraneswere prepared by electrospinning and elongated at temperatures higher than the transitiontemperature of the SMPU. Under the constant stress, the size change of the pores inthe membranes was measured by applying cyclic temperature change. It wasobserved that the pore size changed from 150 to 440 nm according to the temperaturechange, demonstrating that the SMPU membrane can be utilized as a smartmembrane to selectively separate substances according to their sizes by just controllingtemperature.

The effect of internal and external stress on two-way shape-memory behaviour in Co49Ni21.6Ga29.4 single crystals

The effect of the internal stress on the two-way shape memory in Co49Ni21.6Ga29.4 single crystals has been investigated. We found that the internal stress generated natively by the solidifying process works as a tensile force along the growth direction. Applying different compressive pre-stresses along the [0 0 1] direction, the shape-memory strain can be continuously changed from +1.0% to −2.3%. In the [1 1 0] direction, the strain monotonically increases from −2.0% to −4.0% due to a strong detwinning produced by the consistent effect of the external and internal stresses.

Two-way shape memory behavior of shape memory polyurethanes with a biasload

Thermo-responsive shape memory polyurethane (SMPU) is a smart material that canrespond to external heat by changing its macroscopic shape from a temporary configurationto a memorized permanent one. The temporary shape can be processed usingmechanical forces above a certain temperature (the transition temperature) and can bemaintained until the material acquires a certain thermal energy. Thereafter, thematerial will recover its memorized permanent shape. However, it is unclear whatwill occur if the thermal energy is then dissipated, i.e., the material temperaturedecreases. There are two possibilities: the material will respond to the dissipatedenergy, resulting in another macroscopic shape change; or nothing will happenbeyond the thermal contraction. The former is called two-way shape memory(TWSM) behavior and the latter is called one-way shape memory behavior. Thispaper reports novel findings showing that TWSM behavior can be imparted toSMPUs using a thermo-mechanical treatment, i.e., imposing a constant stress onthem after their temporary shaping. A series of experiments were carried outto characterize the TWSM behavior of SMPUs and to explain its mechanism.

Properties and mechanism of two-way shape memory polyurethane composites

Recently, smart materials have drawn wide attention from many researchers. In this paper, laminated shape memory polymer composite (SMPC) exhibiting two-way shape memory effect (SME) has been prepared by combining the pre-elongated shape memory polyurethane (SMPU) with the un-elongated elastic polyurethane (PU). Then the shape memory behaviors, laminated structure and two-way SME mechanism are investigated systematically. The results show that two-way shape memory behaviors, i.e., bending upon heating and reverse bending upon cooling, are achieved in the resulted SMPU-based-SMPCs. In addition, the SMPCs show the compositive thermal properties and dynamic mechanical properties due to their physical combination of layer-by-layer. Furthermore, the shape memory mechanism can be ascribed to the reversible change of two forces, i.e., recovery force in the SMPU layer and bending force in the elastic PU layer.

Coupling between martensitic phase transformations and plasticity: A microstructure-based finite element model

A microstructural finite element (MFE) model is developed to capture the interaction between martensitic transformations and plasticity in NiTi shape memory alloys (SMAs). The interaction is modeled through the grain-to-grain redistribution of stress caused by both plasticity and phase transformation, so that each mechanism affects the driving force of the other. A unique feature is that both processes are modeled at a crystallographic level and are allowed to operate simultaneously. The model is calibrated to pseudoelastic data for select single crystals of Ti–50.9at.%Ni. For polycrystals, plasticity is predicted to enhance the overall martensite volume fraction at a given applied stress. Upon unloading, residual stress can induce remnant (retained) martensite. For thermal cycling under load bias, plasticity is observed to limit the net transformation strain/cycle and increase the hysteretic width. Deformation processing, via plastic pre-straining at elevated temperature, is shown to dramatically alter subsequent pseudoelastic response, as well as induce two-way shape memory behavior during no-load thermal cycling. Overall, the model is suitable at smaller imposed strains, where martensite detwinning is not expected to dominate.

Two-way shape memory in a nanoscale sample of Ti49.5Ni25.5Cu25.0 alloy with a partially ordered structure

The dependence of the thermomechanical properties and structure of rapidly quenched Ti49.5Ni25.5Cu25.0 alloy on the annealing conditions has been studied. A sample with a partially ordered structure is found to have two-way shape memory effect during single deformation. An alloy sample 17 × 10 × 0.07 μm in size has been prepared and trained. It is shown to demonstrate two-way shape memory behavior.

Micromechanical prediction of the two-way shape memory effect in shape memory alloy composites

The two-way shape memory effect in monolithic shape memory alloys has been widely investigated both theoretically and experimentally. In the present study, this effect is analyzed for shape memory alloy composites by employing a micromechanical model. To this end, the responses of polymeric matrix and metal matrix unidirectional composites with embedded shape memory alloy fibers are determined. For the polymeric matrix composite, the effect of axial, transverse and shear loadings as well as the fiber volume fraction on the resulting two-way shape memory behavior are studied. The local distributions of stresses among the shape memory alloy fiber and epoxy matrix in the low- and high-temperature shapes of the composite are also investigated. Two training procedures that generate the two-way shape memory effect in the metal matrix composite are offered. The present analysis shows that the two-way shape memory effect in the chosen type of metal matrix composite is not as useful as in the polymeric matrix one. Finally, for a polymeric matrix composite that is subjected to a transverse normal loading, the effect of imperfect bonding between the shape memory alloy fibers and the neighboring matrix is investigated.

Two-way shape memory effect in polymer laminates

Novel polymer laminate exhibiting two-way shape memory effect has been prepared by layer technique with the shape memory polymer and elastic polymer. In this paper, we demonstrate the two-way shape memory behavior, i.e., bending on heating and reverse bending on cooling; describe the preparation procedure; and investigate its two-way shape memory mechanism. Finally, it suggests that the mechanism can be ascribed to the release of elastic strain of shape memory polymer layer upon heating, and the elastic strain recovery induced by the bending force of substrate layer upon cooling.

On the two-way shape memory behavior in NiTi alloy—An experimental analysis

The present research aims to understand the mechanism of the two-way memory effect (TWME) in NiTi alloy through experimentally investigating shape recovery in the course of various thermomechanical processes. Particular attention was paid to the effect of martensitic strain, either due to pre-deformation or constrained forward transformation, on the TWME. Attempts were made to clarify the correlation of the stress-assisted two-way memory effect (SATWME) and the TWME to martensitic strain resulting from pre-straining and/or constrained cooling. When the martensitic strain resulting from constrained forward transformation exceeds the initial pre-strain, it directly influences the SATWME and TWME. Partial reverse transformation through thermal arrest during heating leads to accommodation of stress-assisted and detwinned martensite variants that result in formation of internal forward and back stresses. TWME is promoted through dominant internal forward stress formation, while the dominance of internal back stress reduces the TWME by reducing the martensitic strain.

Two-way Shape Memory Behavior of Sputter-deposited Ni-rich Ni2MnGa Films

The properties of the Ni-rich Ni2MnGa films and their shape memory behavior are investigated with respect to various sputtering conditions. The films are deposited on a poly-vinyl alcohol substrate with a radio-frequency magnetron sputtering apparatus using Ni50Mn25Ga25 and Ni52Mn24Ga24 targets. The sputtering power is in the range of 50–400 W. After separating from the substrate, the films are vacuum-annealed at 1073 K for 3.6 ks. The chemical composition of the films depends on the sputtering power. The martensitic transformation temperature increases with the valence electron density of the films, while the Curie temperature slightly decreases. The film obtained with 50 W using the Ni50Mn25Ga25 target after heat treatment at 1073 K shows a two-way shape memory effect through the martensitic transformation and its reversion.

Thermally and stress-induced martensitic transformation in Co–Ni–Al ferromagnetic shape memory alloy single crystals

In the present study, the shape memory characteristics of Co 38Ni 33Al 29 (numbers indicate at.%) alloy single crystals are investigated. Transformation behavior under constant stress levels, two-way shape memory behavior, the effect of cyclic deformation and pseudoelasticity as a function of strain and temperature were studied. The single crystals demonstrate large two-way shape memory strain, a near perfect pseudoelastic behavior with up to 4% pseudoelastic strain, low pseudoelastic stress hysteresis, a large pseudoelastic temperature window and high resistance for dislocation slip.

207 TiNi合金の二方向形状記憶特性に関する実験的研究(OS9(1) 形状記憶合金の特性と応用)

The two-way shape memory effect is expected as unique mechanical property for new engineering applications of shape memory alloys such as micro actuators. The two-way shape memory effect occurs by giving internal stress field to shape memory alloys. The mechanical loading training is the method to give internal stress field. The mechanism of the two-way shape memory behavior have not been understood yet. In this study, the two-way shape memory behavior obtained by the mechanical loading training was investigated experimentally. The obtained results are presented and discussed in this paper.

Martensitic transformation and shape memory effect in a ferromagnetic shape memory alloy: Mn2NiGa

Heusler alloy Mn2NiGa has been developed by synthesizing a series of ferromagnetic shape memory alloys Mn25+xNi50−xGa25 (x=0–25). Mn2NiGa exhibits a martensitic transformation around room temperature with a large thermal hysteresis up to 50 K and a lattice distortion as large as 21.3% and has a quite high Curie temperature of 588 K. The martensite shows a high-saturated field up to 2 T. The excellent two-way shape memory behavior with a strain of 1.7% was observed in the single crystal Mn2NiGa. The magnetic-field-controlled effect created a total strain up to 4.0% and changed the sign of the shape deformation effectively.

Characterization of preferential orientation of martensitic variants in a single crystal of NiMnGa

We report the detailed observation of martensitic variants in NiMnGa single crystals. The variants that are twinned with each other in different ways can be clearly identified in our single crystals by optical observation. We also investigated the preferential orientation of the martensitic variants in NiMnGa single crystals. We observed the motion of the variant boundary in response to application of a magnetic field. This observation can be used to explain phenomenologically the magnetic-field-induced strain. In the single crystal with composition Ni 52Mn 24Ga 24, martensite with seven modulated layers (7M) shows preferentially oriented variants. A completely recoverable two-way shape-memory behavior was also observed by measuring the free sample in three different directions during a complete temperature cycle. It was found that the largest strains in the [001] and [010] directions occur in different temperature ranges.

TiNi形状記憶合金の二方向挙動に関する実験的研究(J09-4 形状記憶,J09 知的材料・構造システム)

The two-way shape memory effect is expected as prospective property for new engineering applications of shape memory alloys, hi this study, the two-way shape memory behavior obtained by the mechanical loading training was investigated experimentally. The obtained results are presented and discussed in this paper.

Two-way shape memory effect of TiNi alloys induced by hydrogenation

An investigation on the two-way shape memory effect (TWSME) induced by an electrochemical hydrogenation has been conducted on two TiNi shape memory alloys (SMAs) (Ti49.2Ni50.8 and Ti51.1Ni48.9 denoted as materials A and B, respectively). While a bending constraint has been applied during hydrogenation, the effects of current density, solution temperature, and charging period are studied. A two-way shape memory behavior was observed on the bent, hydrogenated specimens. The measured ratio of TWSME of specimen A is greater than that of specimen B. The variation of the TWSME of the specimens is relatively small during the first 50 thermal cycles. The TWSME increases first and then decreases with increasing either current density or charging period of the electrochemical hydrogenation. The X-ray diffraction peak corresponding to hydrides in the hydrogenated specimen still exists but becomes weaker after three months of “resting,” and the TWSME obviously decreases with decreasing amount of hydrides.

Two-way shape memory effect of NiTi alloy induced by constraint aging treatment at room temperature

A new optimal training procedure to induce a two-way shape-memory effect (TWSME) in bending mode has been investigated in a Ni-rich NiTi alloy that had underwent a preaging heat treatment followed by constraint aging at room temperature. The aim of the present investigation is to develop a new training procedure to induce TWSME as well as to examine the relationship between the obtained properties and microstructures of specimens prepared by various preaging temperatures. It is suggested that the R-phase plays an important role in the establishment of two-way shape-memory behavior on cooling.

Two-Way Shape Memory Behavior Obtained by Combined Loading Training

The two-way shape memory effect obtained by combined loading training is investigated experimentally by applying the combined loads of axial force and torque to the thin-walled tubular specimen of Cu-based shape memory alloy. In this paper, some unique two-way behaviors are shown and discussed.