Takaei Yamamoto

Exploitation of shape memory alloy actuator using resistance feedback control and its development

The relationship between phase transformation and electric resistivity of Ti-42.6Ni-7Cu SMA was investigated. In the isothermal tensile tests, stress—strain curve shows large hysteresis and nonlinearity, whereas the resistivity—strain curve can be fitted by linearity. The resistivity of SMAs can be determined from the volume fractions of the martensitic and austenitic phases. This characteristics leads to that the resistance of the SMAs is used as a parameter of strain of SMAs. A SMA actuator using resistance feedback control system was proposed, which can control and retain positioning at any positions without using sensor devices, and the deviation of position of this system was less than 3 μm.

Control Characteristics of Shape Memory Alloy Actuator Using Resistance Feedback Control Method

The actuators using shape memory alloys can work as an actuator to control or retain positioning without using sensor devices. In this work, a position control model with a biasing mechanism is produced. The produced model is controlled by resistance feedback using the method of setting off-time and can be set and retained at an arbitrary position. The effects of input current, control distance and off-time on positioning characteristics such as dynamic behavior and position stability are investigated. The results show that high input current for heating is effective for shortening the rise and settling times. However, the overshoot increases with increasing input current. When the recovery strain is below 2.5%, the rise and settling velocities increase with increasing control distance. Furthermore, the off-time affects position stability. In the case of short off-time, fine position stability is performed regardless of the values of input current and control distance.

Mechanical Behavior of Shape Memory Alloys Under Complex Loading Conditions of Stress, Strain, and Temperature

The transformation behavior of shape memory alloys is simulated for complex loadings of stress, strain, and temperature. Calculations are made by using the “Accommodation Model” which is a constitutive model for shape memory alloys considering the accommodation behavior of the transformation strain. Calculated results are given for the superelastic behavior, the shape memory effect, the transformation behavior under temperature change with stress or strain holding, the structural behavior of a shape memory wire with a bias spring, etc. The effect of the plastic strain on the transformation strain is also investigated.

Numerical Investigations of Thermally Induced Transformation, Reorientation of Martensite Variants and Shape Memory Effect Using Constitutive Model for Accommodation of Transformation Strain

To confirm the constitutive model developed by the present authors, example calculations are conducted for the transformation behavior of shape memory alloys. The constitutive model describes the transformation behavior of the material by calculating the transformation stress and strain of transformation systems of micro structures in the material. In the calculating, the accommodation mechanism acting on microstructures of the material is also taken into consideration. Computational results by the constitutive model for these loading paths are revealed to be reasonable in comparing with experimental observations.

Relationship between Grain Growth and Formation of Fracture Surface of Ultrafine Grained Cu in High-Cycle Fatigue

Fatigue tests were conducted on specimens of ultrafine grained copper produced by equal channel angular pressing. The growth behavior of a fatal crack was monitored successively. The morphological features of the crack growth paths and fracture surfaces were influenced by variation of grain size and damaged areas of the crack tip, depending on the stage of progress of the fatigue damage. The change in fracture surface was discussed by considering the interrelation between the reversible plastic zone size at the crack tip and the microstructure evolved during cyclic stressing.

Fatigue-induced Grain Growth And Formation Of Slip Bands In Cu Processed By Equal Channel Angular Pressing

In order to elucidate the effect of the microstructure on fatigue behavior of ultrafine grained copper, fatigue tests were conducted using copper processed by equal channel angular pressing through 4, 8 and 12 pass numbers. The evolution of microstructure and surface damage during cyclic stressing was monitored. For samples processed by 8 and 12 passes, dynamically recrystallized grains were formed and these grew with subsequent stressing. Slip bands were initiated inside such grains. For samples processed by 4 passes, the formation of slip bands was likely to occur inside dynamically recovered coarse grains. The physical basis on the phenomena leading to the initiation of fatigue cracks was discussed from the viewpoint of microstructure and surface damage evolution.

Growth behavior of fatigue cracks in ultrafine grained Cu smooth specimens with a small hole

In order to study the growth mechanism of fatigue cracks in ultrafine grained copper, stresscontrolled fatigue tests of round-bar specimens with a small blind hole as a crack starter were conducted. The hole was drilled on the surface where an intersection between the shear plane of the final ECAP processing and the specimen surface makes an angle of 45° or 90° with respect to the loading axis. At a low stress ( ? a = 90 MPa), the direction of crack paths was nearly perpendicular to the loading direction regardless of the location of the hole. Profile of crack face was examined, showing the aspect ratio (b/a) of b/a = 0.82. At a high stress ( ? a = 240 MPa), although the growth directions inclined 45° and 90° to the loading-axis were observed depending on the location of the drilling hole, crack faces in these cracks were extended along one set of maximum shear stress planes, corresponding to the final ECAP shear plane. The value of aspect ratios was b/a = 0.38 and 1.10 for the cracks with 45° and 90° inclined path directions, respectively. The role of deformation mode at the crack tip areas on crack growth behavior were discussed in terms of the mixed-mode stress intensity factor. The crack path formation at high stress amplitudes was affected by the in-plane shear-mode deformation at the crack tip.

Transformation Behavior of Shape Memory Alloys in Multiaxial Stress State

The transformation behavior of shape memory alloys is simulated for complex loadings of stress, strain and temperature. Calculations are made by using the “Accommodation Model” which is a constitutive model for shape memory alloys considering the accommodation behavior of the transformation strain. Calculated results are presented for the transformation behavior in the axial and shear stress state. These results are compared with those obtained by the experiment where tube specimens of the Ti-Ni shape memory alloy are subjected to the axial and torsion loading. The proposed constitutive model can predict the transformation behavior including the plastic strain effect of polycrystalline Ti-Ni shape memory alloys under non-proportional multiaxial loading condition.

Effect of Repeated Heat-Treatment under Constrained Strain on Mechanical Properties of Ti-Ni Shape Memory Alloy

It is well-known that the Ti-Ni shape memory alloy (SMA) is applicable to the medical stent. The repeated heat-treatment under the constrained strain is necessary for the manufacturing process of the laser-cut SMA stent. In this research, the effect of heat-treatment under the constrained strain on mechanical properties of the Ti-Ni shape memory alloy wire was investigated. The applied strain at single heat-treatment (εap) was 4, 5 and 8%, and the heat-treatment is repeated so as to became total applied strain 40%. In the case of εap=4 and 5%, partial transformation occurs in the SMA wire, and so a necking appears in the SMA wire. Due to this necking, multi-step martensitic transformation, and decreasing of breaking strength / breaking strain are caused. The necking does not occur because the whole of the SMA wire is transformed for εap=8%. The mechanical properties are improved by increasing of εap. Nevertheless, the mechanical properties of each sample are inappropriate for the medical stent. However, the mechanical properties of the as-manufactured sample are improved greatly by training. In addition, it is desirable that the applied strain during training is slightly larger than the requested strain for application.

Transformation/Deformation Behavior and its Constitutive Equation for Ti-Ni-Cu Shape Memory Alloy

This paper describes the transformation and deformation behavior and its constitutive equation for Ti-41.7Ni-8.5Cu (at%) shape memory alloy. Plastic deformation after pre-deformation is investigated using the volume fraction of slip-deformed martensite. New kinetics and constitutive equations are proposed for the reverse transformation process. The material constants in the proposed equationa are determined from the results of tensile and heating/cooling tests of Ti-41.7Ni-8.5Cu (at%) shape memory alloy. The calculated results describe well the deformation and transformation behavior affected by pre-strain.