Institute of Fundamental Technological Research

Multifunctional polyurethane shape memory polymers (PU-SMPs) have been of increasing interest in various applications. Here we report structure characterization, detailed methodology, and obtained results on the identification of functional properties of a thermoset PU-SMP (MP4510) with glass transition temperature of 45 C. The stable, chemically crosslinked network of this thermoset PU-SMP results in excellent shape memory behavior. Moreover, the proximity of the activation temperature range of this smart polymer to room and body temperature enables the PU-SMP to be used in more critical industrial applications, namely fast-response actuators. The thermomechanical behavior of a shape memory polymer determines the engineering applications of the material. Therefore, investigation of the shape memory behavior of this class of commercial PU-SMP is of particular importance. The conducted structural characterization confirms its shape memory properties. The shape fixity and shape recovery properties were determined by a modified experimental approach, considering the polymer’s sensitivity to external conditions, i.e., the temperature and humidity variations. Three thermomechanical cycles were considered and the methodology used is described in detail. The obtained shape fixity ratio of the PU-SMP was approximately 98% and did not change significantly in the subsequent cycles of the thermomechanical loading due to the stability of chemical crosslinks in the thermoset materials structure. The shape recovery was found to be approximately 90% in the first cycle and reached a value higher than 99% in the third cycle. The results confirm the effect of the thermomechanical training on the improvement of the PU-SMP shape recovery after the first thermomechanical cycle as well as the effect of thermoset material stability on the repeatability of the shape memory parameters quantities.

polyurethane shape memory polymer, thermomechanical loading program, shape fixity, shape recovery

The paper presents experimental and numerical results of the TiNi shape memory alloy (SMA) subjected to a modified program of force-controlled tensile loading. The time-dependent development of transformation strain under the constant-force conditions was investigated to describe transformation-induced creep phenomena. (2) Mechanical characteristics of the TiNi SMA were derived using a testing machine, whereas the SMA temperature changes accompanying its deformation were obtained in a contactless manner with an infrared camera. A 3D coupled thermo-mechanical numerical analysis, realized in a partitioned approach, was applied to describe the SMA mechanical and thermal responses. (3) The stress and related temperature changes demonstrated how the transformation-induced creep process started and evolved at various stages of the SMA loading. The proposed model reproduced the stress, strain and temperature changes obtained during the experiment well; the latent heat production is in correlation with the amount of the martensitic volume fraction. (4) It was demonstrated how the transformation-induced creep process occurring in the SMA under such conditions was involved in thermo-mechanical couplings and the related temperature changes.

TiNi shape memory alloy, phase transformation-induced creep, martensitic transformation, temperature change, thermomechanical couplings, infrared camera, thermo-mechanical coupled numerical analysis

Advanced subjects in mechanical properties of shape memory alloys and polymers are discussed. In the subloop loading under a stress-controlled condition of the shape memory alloy, the transformation-induced stress relaxation appears due to variation in temperature. The enhancement of corrosion and corrosion fatigue life of the shape memory alloy is discussed. The development of a functionally-graded shape memory alloy and polymer is expected to obtain better performance. Three-way motion appears in the shape memory composite with the shape memory alloy and polymer.

shape memory alloy, shape memory polymer, functionally-graded shape memory material, shape memory composite

In order to develop the reciprocating rotary driving actuator with a simple mechanism using shape memory alloy (SMA) tapes, the graphical method to design the actuator was proposed based on the torsional deformation properties of SMA tapes. The torsional deformation properties of the SME tape showing the shape memory effect (SME) and the SE tape showing superelasticity (SE) were obtained. The bias-type reciprocating rotary actuator was composed of the pretwisted SME tape and the flat SE tape in series. The design chart expressed by the relationship between torque and twisting angle of the SME tape and the SE tape was proposed. The rotational angle and torque, which vary depending on temperature, can be estimated based on the design chart. The rotational angle is controlled by adjusting the mounting angle of the SME tape and the heating temperature. The automatically opening and closing blind driven by sunlight was demonstrated. The blind was controlled by using the reciprocating rotary element composed of the SME tape and the SE tape. The behavior of the blind can be achieved based on the proposed design method of the reciprocating rotary driving element

shape memory alloy, tape, torsion, actuator, reciprocating rotation, design chart, bias-type

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

shape-memory alloy, titanium-nickel alloy, superelasticity, nitrogen ion implantation, fatigue, bending

The transformation-induced stress relaxation and stress recovery of TiNi shape memory alloy (SMA) in stress-controlled subloop loading were investigated based on the local variation in temperature and transformation band on the surface of the tape in the tension test. The results obtained are summarized as follows. (1) In the loading process, temperature increases due to the exothermic martensitic transformation (MT) until the holding strain and thereafter temperature decreases while holding the strain constant, resulting in stress relaxation due to the MT; (2) In the unloading process, temperature decreases due to the endothermic reverse transformation until the holding strain and thereafter temperature increases while holding the strain constant, resulting in stress recovery due to the reverse transformation; (3) Stress varies markedly in the initial stage followed by gradual change while holding the strain constant; (4) If the stress rate is high until the holding strain in the loading and unloading processes, both stress relaxation and stress recovery are large; (5) It is important to take into account this behavior in the design of SMA elements, since the force of SMA elements varies even if the atmospheric temperature is kept constant.

shape memory alloy, superelasticity, titanium-nickel alloy, subloop, stress relaxation, stress recovery, martensitic transformation

If a shape memory alloy (SMA) is subjected to the subloop loading under the stress-controlled condition, creep and creep recovery can appear based on the martensitic transformation. In the design of SMA elements, these deformation properties are important since the deflection of SMA elements can change under constant load. The conditions for the progress of the martensitic transformation are discussed based on the kinetics of the martensitic transformation for the SMA. The creep deformation properties are investigated experimentally for TiNi SMA. The creep strain rate increases in proportion to the martensitic transformation strain; the creep recovery strain rate increases in proportion to the reverse transformation strain.

shape memory alloy, superelasticity, subloop, transformation band, creep, creep recovery, strain rate, local deformation

W ramach pracy przeprowadzono bilans energetyczny poliuretanu z pamięcią kształtu PU-SMP o temperaturze zeszklenia Tg = 25 °C w procesie rozciągania. Celem było oszacowanie ilości pracy mechanicznej niezbędnej do odkształcenia, ilości energii dyssypowanej oraz energii zmagazynowanej, tj. ukrytej w elementach zmienionej struktury tego polimeru. Dla wyższej prędkości odkształcania otrzymano wyższe wartości pracy zużytej na odkształcenie nieodwracalne oraz wyższe wartości ciepła dyssypowanego. Przeprowadzona analiza energetyczna procesu odkształcania wykazała, że praca zużyta na odkształcenie nieodwracalne nie magazynuje się w PU-SMP, tylko dysypuje. Może to mieć związek ze specyficzną budową polimeru z pamięcią kształtu, potwierdzoną przez badania strukturalne i odpowiadającą za jego właściwości.

The mechanism of exhibiting shape memory property in polymer is different from that observed in shape memory alloy, since the crystallographic phase transformation does not occur in polymers. The functional characteristics of shape memory polymer (SMP), e.g. rigidity, elastic modulus and coefficient of thermal expansion, change significantly above and below its glass transition temperature (Tg) due to molecular motion of the polymer chains which differs drastically below and above Tg. These properties allow to apply SMP in biomedical, textile, housing, transportation, aviation industries. A goal of this paper is to discuss selected results of the SMP investigation and demonstrate how significantly its mechanical properties change in various conditions; influence of temperature, strain rate and loading history were taken into consideration.

Shape memory polymer, tensile test, strain rate, temperature change

Shape memory alloys (SMAs) are remarkable materials characterized by the thermomechanical properties of shape memory effect and superelasticity. Since the properties like these characteristics are highly conducive to the functions of smart materials, their applications have attracted worldwide attention. The functional properties of an SMA appear based on the martensitic transformation (MT). Research up to now in this area has been mainly concerned with a full loop (or perfect loop) of the MT completion. However, in practical applications, temperature and stress are likely to vary in various ranges. If SMA elements are subjected to loads with a subloop (or partial loop, internal loop) in which temperature or stress varies in an incomplete MT range, the conditions for the start and finish of the MT as in a full loop are not satisfied. The present paper investigates superelastic deformation behaviors of TiNi alloy in various subloop loading conditions, in particular the dependence of the subloop deformation on the loading rate, and the characteristics of transformation-induced creep in the stress plateau region under constant stress.

Shape memory alloy, subloop deformation, temperature distribution, pseudoelastic behaviour

One of the main materials which have activated the research on the smart materials is shape memory alloy (SMA). The main characteristics of SMA are the shape memory effect (SME) and superelasticity (SE). In a recent study using the torsional deformation of a TiNi SMA tube, twist in the blades of rotary aircraft was investigated in order to improve the flight performance. In practical applications making use of SMA tapes, torsional deformation can be obtained simply by gripping both ends without any mechanical process. In the present study, in order to develop the rotary driving actuators of SMA tapes, the torsional deformation properties of TiNi SMA tapes are investigated. The graphical method to design the two-way rotary driving actuator by using torsional deformation of SMA tapes is proposed.

Shape memory alloy, torsion, rotary driving, actuator

If the shape memory alloy (SMA) is subjected to the subloop loading under the stress- or strain-controlled condition, transformationinduced creep or relaxation can appear based on the martensitic transformation (MT). In the design of SMA elements, these deformation properties are important since the deflection or force of SMA elements can change under constant load or constant strain. The conditions for the progress of the MT are discussed based on the kinetics of the MT for the SMA. The creep and relaxation properties are investigated experimentally for TiNi SMA. If the stress is kept constant at the upper stress plateau after loading with a high strain or stress rate up to the stress-holding start strain, the transformation-induced creep occurs due to the spread of the stress-induced martensitic transformation process. If the strain is kept constant at the upper stress plateau after loading with a high stress rate, the transformation-induced relaxation appears due to the exothermic MT until the holding strain and thereafter temperature decreases while holding the strain constant.

shape memory alloy, superelasticity, transformation-induced creep, transformation-induced relaxation

Shape memory polymers (SMP) are new unique and attractive materials which demonstrate shape memory properties. It means that the materials, as a result of an external stimulus such as temperature, can recover their original (permanent) shape from deformed (temporary) shape. The mechanical characteristics of SMP, e.g. the elastic modulus and the yield stress, change significantly below and above their glass transition temperature Tg. It can be explained by differences of molecular motion of the polymer chains below and above Tg [1, 2]. Two phenomena due to this can be observed in the SMP. The first one is a shape fixity which means that it is possible to fix a temporary shape by cooling the deformed SMP below Tg. The second phenomenon, called a shape recovery, denotes the property that the original shape, changed due to deformation, is recovered during subsequent heating above the SMP Tg temperature. Preliminary estimation of these two parameters, crucial to assess SMP potential applications, is the subject of this paper [1].

Shape memory polymers, elastic modulus, yield stress, glass transition temperature, shape fixity, shape recovery