Kazumasa Hoshio

Shape recovery and irrecoverable strain control in polyurethane shape-memory polymer

Abstract In shape-memory polymers, large strain can be fixed at a low temperature and thereafter recovered at a high temperature. If the shape-memory polymer is held at a high temperature for a long time, the irrecoverable strain can attain a new intermediate shape between the shape under the maximum stress and the primary shape. Irrecoverable strain control can be applied to the fabrication of a shape-memory polymer element with a complex shape in a simple method. In the present study, the influence of the strain-holding conditions on the shape recovery and the irrecoverable strain control in polyurethane shape-memory polymer is investigated by tension test of a film and three-point bending test of a sheet. The higher the shape-holding temperature and the longer the shape-holding time, the higher the irrecoverable strain rate. The equation that expresses the characteristics of the irrecoverable strain control is formulated.

Characteristics of energy storage and dissipation in TiNi shape memory alloy

Abstract 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 influence of strain rate, cyclic loading and temperature-controlled condition on the characteristics of energy storage and dissipation of the material was investigated. Temperature on the surface of the material was observed and the influence of variation in temperature on the characteristics was clarified. The results obtained can be summarized as follows. (1) In the case of low strain rate, the stress plateaus appear on the stress-strain curves due to the martensitic transformation and the reverse transformation during loading and unloading. In the case of high strain rate, the slopes of the stress–strain curves are steep in the phase-transformation regions during loading and unloading. The recoverable strain energy per unit volume increases in proportion to temperature, but the dissipated work per unit volume depends slightly on temperature. In the case of low strain rate, the recoverable strain energy and dissipated work do not depend on both strain rate and the temperature-controlledcondition. (2) In the case of high strain rate, while the recoverable strain energy density decreases and dissipated work density increases in proportion to strain rate under the temperature-controlled condition, the recoverable strain energy density increases and dissipated work density decreases under the temperature-uncontrolled condition. In the case of the temperature-uncontrolled condition, temperature varies significantly due to the martensitic transformation and therefore thecharacteristics of energy storage and dissipation differ from these under the temperature-controlled condition. (3) In thecase of cyclic loading, both the recoverable strain energy and dissipated work decrease in the early 20 cycles, but change slightly thereafter. (4) The influence of strain rate, cyclic loading and the environment on the characteristics of energy storage and dissipation is important to be considered in the design of shape memory alloy elements.

Influence of strain-holding conditions on shape recovery and secondary-shape forming in polyurethane-shape memory polymer

It was found in previous work on the thermomechanical properties of the polyurethane-shape memory polymer foam that the shape fixity and shape recovery become imperfect and that secondary-shape forming appears, depending on the strain-holding conditions. The main factors of the strain-holding conditions which affect the secondary-shape forming are the holding temperature, holding time and holding strain. In the present study, the influence of the strain-holding conditions on the shape recovery and secondary-shape forming was investigated for the polyurethane-shape memory polymer film. It was found that the secondary-shape forming appears markedly if the holding temperature is higher than the glass transition temperature and does not appear if the holding temperature is lower than the glass transition temperature. The rate of secondary-shape forming increases with an increase in the holding time if the holding temperature is higher than the glass transition temperature. The irrecoverable strain increases in proportion to the holding strain.

Bending Actuation Characteristics of Shape Memory Composite with SMA and SMP

A shape memory composite (SMC) belt with a TiNi-shape memory alloy (SMA) wire fiber and a polyurethane-shape memory polymer (SMP) sheet matrix is fabricated. The bending actuation characteristics of the belt are investigated by the thermomechanical tests. The results obtained are summarized as follows. (1) Residual deflection close to the maximum deflection is obtained by cooling under constant maximum deflection. The residual deflection disappears by heating under no load. Both the rate of shape fixity and the rate of shape recovery are close to 100%. (2) Recovery force appears by heating under constant residual deflection. The recovery force is 93–94% of the maximum force. The development of high functionality of SMC elements is expected by various combinations of SMAs and SMPs.

Shape Memory Composite of SMA and SMP and its Property

The shape-memory composite belt with a TiNi-SMA wire fiber and a polyurethane-SMP sheet matrix was fabricated. The bending actuation characteristics of the belt were investigated by the thermomechanical tests. The results obtained can be summarized as follows. (1) Residual deflection close to the maximum deflection is obtained by cooling under constant maximum deflection. The residual deflection disappears by heating under no load. Both the rate of shape fixity and the rate of shape recovery are close to 100%. (2) Recovery force appears by heating under constant residual deflection. The recovery force is 93-94% of the maximum force. The development of high functionality of shape-memory composite elements is expected by various combinations of SMAs and SMPs.

Influence of Strain-Holding Conditions on Shape Recovery and Secondary-Shape Forming in Polyurethane-Shape Memory Polymer

By applying strain at high temperature to polyurethane-shape memory polymer film, followed by holding the strain under various thermomechanical conditions, the behavior of strain recovery by heating after these processes was investigated. In the experiments, the influence of the strain-holding conditions on shape recovery and secondary-shape forming was discussed. The results obtained are summarized as follows. (1) If strain is held at high temperature for short time, irrecoverable strain starts to appear at the holding time th =0.5 h and strain is not recovered at all at th =8 h in the care of holding temperature Th = Tg + 20 K. In the case of Th = Tg + 10 K, irrecoverable strain appears for short holding time if initial strain is large and the rate of secondary-shape forming S is 42% at th =8 h. (2) If initial strain of 50% is held at low temperature for long time, strain becomes not to be recovered and S is 93% at th=12 h in the care of Th = Tg + 10 K. In the care of Th = Tg, the increasing rate of S increases if th becomes longer than 40 h. If Th is lower than Tg - 10 K for th=72 h, strain is recovered perfectly by heating and secondary-shape forming does not appear.