Hisaaki Tobushi

Thermomechanical properties in a thin film of shape memory polymer of polyurethane series

The thermomechanical properties of a thin film of shape memory polymer of polyurethane series were investigated experimentally. Based on the experimental results, the dynamic mechanical properties, cyclic deformation properties at high temperature, thermomechanical cycling properties, creep and stress relaxation are discussed. The shape fixity with loading above the glass transition temperature followed by unloading below does not change under thermomechanical cycling. The residual strain is recovered in the vicinity of during the heating process. Several applications of the polymer are introduced.

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 constitutive modeling in shape memory polymer of polyurethane series

In order to describe the thermomechanical properties in shape memory polymer of polyurethane series, a thermomechanical constitutive model was developed by modifying a standard linear viscoelastic model. The model involved a slip element due to internal friction and took account of thermal expansion. In order to describe the variation in mechanical properties due to the glass transition, coefficients in the model were expressed by a single exponential function of temperature. Several kinds of thermomechanical tests were carried out. The proposed theory expressed well the thermomechanical properties of the material, such as shape fixity, shape recovery and recovery stress. The proposed model is useful for design of shape-memory polymer elements, in which the amount of recovery deformation, the tightening force and the working start and completion temperatures are specified.

Influence of strain rate on superelastic properties of TiNi shape memory alloy

The influence of strain rate on the superelastic properties of TiNi shape memory alloy was investigated. In the case of strain rate higher than 10%/min, the martensitic transformation stress and the dissipated work increased with an increase in strain rate but the reverse transformation stress and the strain energy decreased. In the case of strain rate lower than 2%/min, these characteristic values associated with the martensitic transformation did not depend on strain rate.

Shape fixity and shape recovery in a film of shape memory polymer of polyurethane series

The shape fixity and recovery in a film of shape memory polymer of polyurethane series were investigated by the thermomechanical cycling tests with loading at various temperatures. The results are summarized as follows: (1) Strain is recovered at temperatures in the vicinity of the glass transition temperature Tg for loading above Tg, but it is recovered at temperatures in the vicinity of the midpoint temperature of glass transition for loading below Tg. (2) The rate of strain fixity is 98% for loading above Tg, while it decreases with increasing cycles for loading below Tg. (3) The rate of strain recovery for loading above Tg is 98% except for the early cycles. (4) The thermomechanical properties of materials with different Tg are quite alike in spite of the difference in Tg.

Phenomenological analysis on subloops and cyclic behavior in shape memory alloys under mechanical and/or thermal loads

Abstract A theoretical framework is presented, from the phenomenological point of view, for the cyclic uniaxial deformation in shape memory alloys subjected to the thermal and/or mechanical loads by introducing three internal variables; the local residual stress and strain and the volume fraction of the martensic phase accumulated during cyclic forward and reverse martensitic transformations. The cyclic effect on the stress-strain and strain-temperature hysteresis loops is discussed. The subloops due to incomplete transformations are also analyzed by assuming the transformation starting stress or temperature which depends on the preloading. Numerical results explain qualitatively well the observations on the thermomechanical behaviors of shape memory alloys.

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.

Analysis of the mechanical behavior of shape memory polymer membranes by nanoindentation, bulging and point membrane deflection tests

Abstract The mechanical properties of a thin film of polyurethane shape memory polymer were investigated experimentally. Non-conventional mechanical tests such as nanoindentation, bulging and point membrane deflection are used. The quantitative results obtained by these three experimental investigations are consistent. These tests, performed at room temperature (approx. 35 K below the glass transition temperature T g ), yield a Youngs modulus equal to 2.2±0.4 GPa for the loading sequence and 3.9±0.3 GPa for the unloading sequence. A weak residual stress has been determined. Moreover, the viscoelastic behavior of the material has been established.

Rotating-bending fatigue of a TiNi shape-memory alloy wire

The rotating-bending fatigue of a TiNi shape-memory alloy wire was investigated. The influence of air and water atmospheres, temperature, strain amplitude and rotational speed on the fatigue life was discussed. In the case of the strain amplitude due to the rhombohedral-phase transformation, the fatigue life lengthened above 107 cycles. In the case of the strain amplitude due to the martensitic transformation, the fatigue life in air was shorter than that in water.

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%.