Kayo Okumura

Thermomechanical constitutive model of shape memory polymer

A nonlinear thermomechanical constitutive model of shape memory polymer (SMP) is developed 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. The proposed theory expresses well the thermomechanical properties of polyurethane-shape memory polymer, such as shape fixity, shape recovery and recovery stress. The proposed model is useful for design of shape-memory polymer elements.

Thermomechanical Properties of Polyurethane-Shape Memory Polymer Foam

The thermomechanical properties of polyurethane-shape memory polymer foam were investigated by the compressive tests. The results are summarized as follows. (1) The material contracts uniformly in the axial direction with the ratio of lateral strain to axial strain 0.4 in the early stage, but about 0.15 thereafter. (2) The deformation resistance is large at low temperature and at high strain rate. (3) In the case of thermomechanical loading, strain is recovered at temperatures in the vicinity of Tg. The rate of strain recovery is 99%. The rate of strain fixity is 100%.

Superelastic deformation of a TiNi shape memory alloy subjected to various cyclic loadings

Abstract Superelasticity is one of the important mechanical properties of shape memory alloys (SMAs). In the present paper, the superelastic deformation behaviour of a TiNi SMA under various loading conditions with different experimental temperatures and strain rates was investigated. The characteristics of transformation stress during cyclic deformation were analysed. The relationship between the residual stress and strain accumulated during cyclic loading is expressed by a straight line. After mechanical training with cyclic deformation, two-way deformation appears on account of R-phase transformation. The starting and finishing conditions of the transformation become unclear after cyclic deformation at high temperature.

Thermomechanical Properties of Polyurethane-Shape Memory Polymer

The development of applications of shape memory polymers (SMPs) is currently of great interest [1–4]. Because SMPs are light, high in shape recovery, easy to manipulate and economical compared with shape memory alloy, their development is being actively promoted. In many SMPs, the phase transition temperature is around room temperature. Characteristics such as shape recovery and shape fixity exist due to the difference in mechanical properties of materials above and below the phase transition temperature. SMPs possessing these characteristics are being used as temperature sensors and actuators.

Deformation behavior of TiNi shape memory alloy under strain- or stress-controlled conditions

The deformation properties of TiNi shape memory alloy subjected to strain control and stress control were investigated experimentally. The result obtained are summarized as follows. In the case of full loop, the stress- strain curves under stress-controlled condition are similar to those under strain-controlled condition with high strain rate. The overshoot and undershoot do not appear at the start points of the stress-induced martensitic transformation in these curves. In the case of subloop under stress-controlled condition, temperature decreases and therefore strain increases owing to the martensitic transformation at the early stage in the unloading process. At the early stage in the reloading process, temperature increase and therefore strain decease owing to the reverse transformation. In the case of subloop under stress- controlled condition, the starting stresses of the martensitic transformation and the reverse transformation in the loading and unloading processes coincide with the transformation stresses under strain-controlled condition with low strain rate, respectively. The deformation behaviors for subloop under stress-controlled condition are prescribed by the condition for progress of the martensitic transformation based on the transformation kinetics. The deformation behaviors subjected to cyclic loading under stress-controlled condition at constant temperature are also prescribed by the condition for progress of the martensitic transformation.

Fatigue Properties of TiNi Shape-Memory Alloy Wire

A shape memory alloy (SMA) is expected to be applied as intelligent material since it shows the unique characteristics of the shape memory effect and superelasticity. Most SMA elements, with these characteristics, perform cyclic motions. In these cases, fatigue of SMA is one of the important properties in view of evaluating functional characteristics. The fatigue properties are complex since they depend on stress, strain, temperature and time. If SMA is implanted by high energy ions, the thermomechanical properties may change, resulting in long fatigue life. In the present study, the nitrogen ion implantation was applied to modify TiNi SMA wire surface and the influence of implantation treatment on the tensile deformation and bending fatigue properties was investigated.