Institute of Fundamental Technological Research

The shape memory effect and superelasticity appear in shape memory alloy (SMA). The large amount of strain by more than several hundreds percent can be recovered in shape memory polymer (SMP). The shape recovery and shape fixity can be used in SMP elements. These characteristics of shape memory materials (SMMs) can be applied to intelligent elements in various fields. In order to use these characteristics and design the SMM elements properly, it is important to understand the thermomechanical properties of SMAs and SMPs. The deformation behaviors of SMMs differ depending on the thermomechanical loading conditions. The main factors which affect these properties are strain rate, stress rate, temperature, subloop loading, temperature-controlled condition, strain holding condition and cyclic loading. In the present paper, the thermomechanical properties of TiNi shape memory alloy, polyurethane-shape memory polymer and their composite are discussed.

shape memory alloy, shape memory polymer, composite, cyclic loading, strain rate, fatigue

The torsional deformation properties of a TiNi shape-memory alloy thin strip were investigated. The results obtained are summarized as follows. (1) The martensitic transformation starts at the edge of the thin strip. (2) The torsional deformation properties change slightly under thermomechanical cycling. (3) The fatigue life in pul- sating torsion is longer than that in alternating torsion. (4) A simple rotary driving element can be developed by using the SMA thin strip.

shape memory alloy, thin strip, torsion, cyclic deformation, fatigue, rotary driving element

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.

shape-memory alloy, titanium-nickel alloy, subloop, superelasticity, creep, stress relaxation, neutral loading

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.