No abstract

The main characteristics which appear in shape memory alloys (SMAs) are the shape memory effect and superelasticity. In applications of SMAs, the thermomechanical properties of SMAs are most important. The return-point memory does not appear under the stress-controlled conditions. Creep and stress relaxation can be induced due to the phase transformation in the subloop loading under the stress-controlled conditions. In order to design the SMA elements properly, it is important to understand the influence of the thermomechanical loading conditions on the nucleation and progress of the phase transformation and the corresponding deformation behaviors. In the present paper, the conditions for the nucleation and progress of the phase transformation are investigated for SMAs subjected to the subloop loadings under the stress-controlled conditions. The uniaxial tension tests for the TiNi SMAs were carried out in the superelastic region under the various thermomechanical loading conditions. The thermomechanical conditions for the progress of the phase transformation are discussed in the subloop loading under the stresscontrolled conditions. Strain increases during unloading and decreases during reloading under the stress-controlled subloop loading. These pseudoviscoelastic behaviors are important for the precise control of SMA elements.

Experiments were conducted to investigate the deformation behaviors and fatigue properties of a superelastic thin tube (SE-tube) and a high-elastic thin wire (HE-wire) of TiNi alloy under conditions of pulsating-plane, alternating-plane and rotating bending. The main results obtained are summarized as follows. (1) The stress-strain curve of the SE-tube in tension describes a superelastic hysteresis loop with an elastic modulus of 35 GPa. It is thus suited for use as a medical catheter tube with flexibility and shape recovery. The stress-strain curve of the HE-wire stays close to a straight line up to a strain of 4% and a stress of 1500 MPa with an elastic modulus of 50 GPa, and is suited for use as a medical guide wire with flexibility, high pushability and a good torque transmission performance. (2) With respect to fatigue, the SE-tube and the HEwire in air both have a longer life in pulsating-plane than in alternating-plane and rotating bending, whereas the difference in fatigue life between alternating-plane and rotating bending is small. The relationship between the maximum bending strain and the number of cycles to failure in the region of low-cycle fatigue can be expressed by a power function for each kind of bending fatigue. The fatigue life in the body is longer than that obtained in air. (3) The maximum bending strain at the fatigue limit of the SE-tube is 0.8%-1.0% which is close to the starting strain of the stressinduced martensitic transformation. The maximum bending strain at the fatigue limit of the HE-wire is 0.7%-0.8%.

The characteristics of energy storage and dissipation in TiNi shape memory alloys were investigated experimentally based on the superelastic properties under various thermomechanical loading conditions. The results obtained can be summarized as follows. (1) The recoverable strain energy increases in proportion to the rise in temperature, but the dissipated work per unit volume depends slightly on temperature. In the case of low strain rates, the recoverable strain energy and dissipated work do not depend on both the strain rate and the temperature-controlled condition. (2) In the case of high strain rates, while the recoverable strain energy decreases and the dissipated work increases in proportion to the rise in strain rate under the temperature-controlled condition, the recoverable strain energy increases and the dissipated work decreases under the temperatureuncontrolled condition.

If a shape-memory alloy (SMA) thin strip is applied to elements subjected to torsion, a rotary driving element with a simple mechanism can be developed. With this purpose in view, torsion, recovery torque and torsion fatigue tests were carried out for a TiNi SMA thin strip, and the basic torsion characteristics of the strip were obtained. It was found that torque and recovery torque both increase in proportion to the angle of twist and temperature, and that the recoverable strain energy increases in proportion to temperature, while dissipated work decreases slightly with an increase in temperature. Fatigue life decreases in proportion to the angle of twist, and the fatigue life of a heat-treated material is longer than that of an as-received material. The angle of twist at the fatigue limit for the heat-treated material is almost the same as for the as-received material. As a practical application for a rotary driving element of this type, a means of opening and closing a door with an element driven by an SMA thin strip is demonstrated.