Ryosuke Matsui

The influence of shape-holding conditions on shape recovery of polyurethane-shape memory polymer foams

The thermomechanical properties of polyurethane-shape memory polymer (SMP) foams and the influence of shape-holding conditions on shape recovery were investigated experimentally. The results obtained can be summarized as follows. (1)?By cooling the foam down to below the glass transition temperature Tg after compressive deformation above Tg, stress decreases and the deformed shape is fixed. By heating the shape-fixed foam up to above Tg under no load, the original shape is recovered. (2)?The shape deformed above Tg is maintained for six months under no load at Tg? 60?K without depending on the maximum strain, and the original shape is recovered by heating thereafter. (3)?If the deformed shape is held at high temperature, the original shape is not recovered. (4)?The ratio of shape irrecovery increases in proportion to the holding strain, holding temperature and holding time.

Transformation-Induced Relaxation and Stress Recovery of TiNi Shape Memory Alloy

The transformation-induced stress relaxation and stress recovery of TiNi shape memory alloy (SMA) in stress-controlled subloop loading were investigated based on the local variation in temperature and transformation band on the surface of the tape in the tension test. The results obtained are summarized as follows. (1) In the loading process, temperature increases due to the exothermic martensitic transformation (MT) until the holding strain and thereafter temperature decreases while holding the strain constant, resulting in stress relaxation due to the MT; (2) In the unloading process, temperature decreases due to the endothermic reverse transformation until the holding strain and thereafter temperature increases while holding the strain constant, resulting in stress recovery due to the reverse transformation; (3) Stress varies markedly in the initial stage followed by gradual change while holding the strain constant; (4) If the stress rate is high until the holding strain in the loading and unloading processes, both stress relaxation and stress recovery are large; (5) It is important to take into account this behavior in the design of SMA elements, since the force of SMA elements varies even if the atmospheric temperature is kept constant.

Fatigue Property and Enhancement of Fatigue Life of TiNi Shape Memory Alloys—An Over View

A shape memory alloy (SMA) is expected to be applied as intelligent or smart material since it shows the functional characteristics of the shape memory effect and superelasticity. Most SMA elements, with these characteristics, perform cyclic motions. In these cases, the fatigue property of SMA is one of the most important subjects in view of evaluating functional characteristics of SMA elements. The fatigue properties are complex since they depend on stress, strain, temperature and their hysteresis. If SMA is implanted by high energy ions, the thermomechanical properties of the material may change, resulting in long fatigue life. If the ultrasonically activated shot peening is applied to SMA, the compressive residual stress may be produced, resulting in an increase in fatigue performance. In the present study, the tensile deformation and bending fatigue properties of a superelastic thin wire, a highelastic thin wire, and a superelastic tape, all made of TiNi alloys, were investigated experimentally. In order to improve the fatigue properties, the nitrogen ion implantation and the ultrasonic shot peening were applied to modify the surface layer of the TiNi SMA tape and the influence of implantation and shot peening treatments on the bending fatigue properties was investigated.

Mechanical Properties of Shape Memory Alloy and Polymer

In order to describe the shape memory effect, superelasticity and recovery stress due to the martensitic transformation and the R-phase transformation of TiNi shape memory alloy, a thermomechanical constitutive equation considering the volume fractions of induced phases associated with both transformations is developed. The conditions for progress of phase transformation and subloop-deformation behavior are discussed. A nonlinear thermomechanical constitutive model of polyurethane-shape memory polymer is developed to describe the thermomechanical properties such as shape fixity, shape recovery and recovery stress, by modifying a linear model. The coefficients in the model are expressed by the single exponential functions of temperature in order to describe the variation in mechanical properties of the material due to the glass transition.

Influence of Strain Ratio on Bending Fatigue Life in TiNi Shape Memory Thin Wire

In this study, we performed the bending fatigue test and investigated the influence of strain ratio on fatigue life in TiNi shape memory thin wire. The pulsating plane bending, alternating plane bending and rotating bending fatigue tests were carried. Additionally, we carried out the observation of the fatigue fracture surface by a scanning electron microscope. The behavior of fatigue crack was investigated. The results obtained are summarized as follows. (1) The martensitic transformation (MT) stress of the superelastic thin wire (SE-NT) is higher than that of the SMA thin wire (SME-NT) and the fatigue life of SE-NT is shorter than that of SME-NT. Maximum bending strain at the fatigue limit is the MT starting strain. (2) The low-cycle fatigue life curve in plane bending for SE-NT is expressed by a power function of maximum strain εmax and the number of cycles to failure Nf. The smaller the strain ratio for the same εmax, the shorter the fatigue life. (3) In both the rotating bending and the plane bending, fatigue cracks nucleate on the surface of the wire and one fatigue crack grows preferentially. The region in which fatigue crack propagated is fan-shaped.

Intelligent Shape Memory Actuators

The shape memory alloy has played an important role to develop the intelligent materials and structures. The shape memory polymer has also been used in practical applications. If the shape memory materials are applied into actuators, the novel intelligent shape memory actuators can be developed. In the present paper, the development of a functionally-graded shape memory alloy actuator, a functionally-graded shape memory polymer actuator and a shape memory composite actuator is discussed. The simple multi-way actuation can be developed by using the functionally-graded shape memory alloy wire and tape. The functionally-graded shape memory polymer board, showing a similar deformation property to a finger, can be applied to the elements coming into contact with body in the medical actuators. The three-way and three-dimensional actuators of simple mechanism can be developed by applying the shape memory composite with various kinds of shape-memory alloy and polymer.

Mechanical Properties of Shape Memory Alloys and Polymers—A Review on the Study by Prof. Tobushi

The mechanical properties of shape memory alloys, shape memory polymers and their composites studied by Prof. H. Tobushi are summarized. The main subjects achieved are as follows. (1) Characteristics of shape memory alloy heat engine, (2) Thermomechanical properties of shape memory alloy, (3) Subloop deformation and influence of strain- and stress-rates on deformation, (4) Bending fatigue properties and enhancement of fatigue life of shape memory alloy, (5) Torsional deformation properties of shape memory alloy thin tape and their application, (6) Thermomechanical properties of shape memory polymer, (7) Thermomechanical properties of shape memory polymer foam and secondary shape forming and (8) Shape memory composite and functionally-graded shape memory polymer. The published papers related to these subjects are explained and listed.

114 Cyclic Deformation Property of Shape Memory Composite Belt by Joule Heating

1.緒言 インテリジェント材料の一つとして形状記憶合金(shape memory alloy, SMA)がある.また,形状記憶ポリマー(shape memory polymer, SMP)も実用において使用されている.SMA では,逆変態温度 Af以下において弾性係数および降伏応力は 低く,Af以上においてそれらの値は高い.SMP では,ガラス 転移温度 Tg 以下において弾性係数および降伏応力は高く, Tg以上においてそれらの値は低い.これらの特性を示すSMA と SMP を組み合わせることで今までにない新しい機能特性 を有する形状記憶複合材料を開発することができる. 本研究では,SMP シートと超弾性(superelasticity,以下 SE)を示す合金(superelastic alloy,SEA)および形状記憶効 果(shape memory effect, 以下 SME)を示す合金(SMA)の 変態温度の異なる 2種類の合金薄帯板を用いて,通電加熱に より動作する SMC ベルトを作製した.往復曲がりを示す SMC ベルトの作製方法の確立および,実用上多く使用され ている曲げ変形に関して,通電加熱と自然冷却の各過程にお ける往復曲がり特性に伴う回復力とたわみの挙動について 検討した.

Improvement of Corrosion Fatigue Strength for TiNi Shape Memory Alloy

In this study, we enhanced the corrosion fatigue life of a TiNi shape-memory alloy wire using a thermal oxidation treatment technique that can generate a passive layer on the wire surface. We followed the following procedure for the thermal oxidation treatment. First, the as-received material with an oxide film was mechanically polished to remove the film using an abrasive paper and a buffing compound. Second, the material was heat-treated in an electrical furnace filled with an N2-20 vol% O2 gas for 1 h at 673 K. Subsequently, the material was allowed to cool in the furnace. The results of this treatment are summarized as follows. (1) A passive layer was uniformly generated on the surface of the TiNi shape-memory alloy wire via thermal oxidation on a macroscopic scale; this significantly improved its corrosion resistance. (2) Thermal oxidation extended the corrosion fatigue life of the treated material more compared with HT in air. In addition, we found that the layer generated via the thermal oxidation treatment can maintain adhesion to the base material even when subjected to a bending strain greater than 1%.

Mechanical testing of shape-memory polymers for biomedical applications

Abstract The mechanical properties of shape-memory polymers depend on stress, strain, temperature, time, and their hysteresis. In order to evaluate the mechanical properties – testing for each property is performed. The tests for the basic mechanical properties, tensile deformation, creep and stress relaxation, shape fixity and shape recovery, shape fixity and shape recovery of foam, recovery stress, and secondary shape forming – are explained. Future trends are also introduced.