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Abstract:
This paper presents experimental and numerical results of a polyurethane shape memory polymer (SMP) subjected to cyclic tensile loading. The goal was to investigate the polymer yielding phenomena based on the effects of thermomechanical coupling. Mechanical characteristics were obtained with a testing machine, whereas the SMP temperature accompanying its deformation process was simultaneously measured in a contactless manner with an infrared camera. The SMP glass transition temperature was approximately 45oC; therefore, when tested at room temperature, the polymer is rigid and behaves as solid material. The stress and related temperature changes at various strain rates showed how the SMP yield limit evolved in subsequent loading-unloading cycles under various strain rates. A two-phase model of the SMP was applied to describe its mechanical response in cyclic tension. The 3D Finite Element model of a tested specimen was used in simulations. Good agreement between the model predictions and experimental results was observed for the first tension cycle.

Keywords:
Shape memory polymer, Tension cyclic loading, Thermomechanical coupling, Yield limit, Thermoelastic effect, Constitutive model

Abstract:
This paper presents experimental and modeling results of the effects of thermomechanical couplings occurring in a polyurethane shape memory polymer (SMP) subjected to tension at various strain rates within large strains. The SMP mechanical curves, recorded using a testing machine, and the related temperature changes, measured in a contactless manner using an IR camera, were used to investigate the polymer deformation process at various loading stages. The effects of thermomechanical couplings allowed the determination of the material yield point in the initial loading stage, the investigation of nucleation and development of the strain localization at larger strains and the estimation of the effects of thermoelastic behavior during the unloading process. The obtained stress–strain and thermal characteristics, the results of the dynamic mechanical analysis and estimated values of the shape fixity and shape recovery parameters confirmed that the shape memory polymer (T g = 45°C) is characterized by good mechanical and shape memory properties, as well as high sensitivity to the strain rate. The mechanical response of the SMP subjected to tension was simulated using the finite element method and applying the large strain, two-phase model. Strain localization observed in the experiment was well reproduced in simulations and the temperature spots were correlated with the accumulated viscoplastic deformation of the SMP glassy phase.

Keywords:
shape memory polymer, thermomechanical coupling, infrared camera, tension test, strain rate, strain localization, constitutive model

Abstract:
In this paper extensive research on the polyurethane shape memory polymer (PU-SMP) is reported, including its structure analysis, our experimental investigation of its thermomechanical properties and its modelling. The influence of the effects of thermomechanical couplings on the SMP behaviour during tension at room temperature is studied using a fast and sensitive infrared camera. It is shown that the thermomechanical behaviour of the SMP significantly depends on the strain rate: at a higher strain rate higher stress and temperature values are obtained. This indicates that an increase of the strain rate leads to activation of different deformation mechanisms at the micro-scale, along with reorientation and alignment of the molecular chains. Furthermore, influence of temperature on the SMP's mechanical behaviour is studied. It is observed during the loading in a thermal chamber that at the temperature 20°C below the glass transition temperature (Tg) the PU-SMP strengthens about six times compared to the material above Tg but does not exhibit the shape recovery. A finite-strain constitutive model is formulated, where the SMP is described as a two-phase material composed of a hyperelastic rubbery phase and elastic-viscoplastic glassy phase. The volume content of phases is governed by the current temperature. Finally, model predictions are compared with the experimental results.

Keywords:
shape memory polyurethane, thermomechanical couplings, infrared camera, temperature change, dynamic mechanical analysis, strain rate, constitutive model

Abstract:
Experimental results of effects of thermomechanical couplings occurring in shape memory polymer subjected to tension are presented. Stress-strain curves were recorded by testing machine, while their related temperature changes were measured with infrared camera. The mechanical and thermal characteristics were used to investigate the polymer properties. Three various stages were distinguished during the deformation process. The first, elastic, is accompanied by a drop in the specimen temperature; the second, plastic, is associated with change of the material structure and the temperature increase; the third stage, related to the specimen rupture and damage mechanisms, is accompanied by the significant increase in temperature.

Keywords:
shape memory polymer, tension, thermomechanical coupling, temperature change, infrared camera

Abstract:
Multifunctional new material—polyurethane shape memory polymer (PU-SMP)—was subjected to tension carried out at room temperature at various strain rates. The influence of effects of thermomechanical couplings on the SMP mechanical properties was studied, based on the sample temperature changes, measured by a fast and sensitive infrared camera. It was found that the polymer deformation process strongly depends on the strain rate applied. The initial reversible strain is accompanied by a small drop in temperature, called thermoelastic effect. Its maximal value is related to the SMP yield point and increases upon increase of the strain rate. At higher strains, the stress and temperature significantly increase, caused by reorientation of the polymer molecular chains, followed by the stress drop and its subsequent increase accompanying the sample rupture. The higher strain rate, the higher stress, and temperature changes were obtained, since the deformation process was more dynamic and has occurred in almost adiabatic conditions. The constitutive model of SMP valid in finite strain regime was developed. In the proposed approach, SMP is described as a two-phase material composed of hyperelastic rubbery phase and elastic-viscoplastic glassy phase, while the volume content of phases is specified by the current temperature.

Keywords:
constitutive model, dynamic mechanical analysis, shape memory polyurethane, strain rate, temperature change, thermomechanical couplings

Abstract:
This paper presents experimental evaluation of a new polyurethane shape memory polymer (PU-SMP) produced by SMP Technologies Inc. It discusses mechanical characteristics and temperature changes of the SMP specimens subjected to tension test performed at room temperature with various strain rates. Basing on the mechanical data and the relevant temperature changes, we have studied the thermomechanical properties of the PU-SMP and influence of the strain rate on the strain localization behavior. Finally, we have identified the material parameters for the one-dimensional rheological model of the SMP.

Keywords:
shape memory polyurethane, tension test, dynamic mechanical analysis, infrared camera, temperature change, thermomechanical properties, rheological model

Abstract:
Thermomechanical and functional properties of shape memory polyurethane are presented. A background of the polymer shape memory effects is described. Elastic modulus at various temperatures and the polyurethane parameters important for the practical applications, called shape fixity and shape recovery, are derived. Taking advantages from the high quality testing machine and infrared camera, mechanical characteristics and temperature changes of the shape memory polyurethane specimens subjected to tension test carried out in various conditions are clarified and analyzed. Stress-strain curves and the relevant temperature changes are recorded both in the elastic and the plastic ranges of deformation. A significant value of a thermoelastic effect is observed. Taking into account the obtained experimental data from the polyurethane tension tests performed at room temperature, followed by the heating above its glass transition temperature, the shape memory polyurethane properties are studied.

Keywords:
shape memory polyurethane, glass transition temperature, tension, mechanical characteristics, infrared camera, temperature change, thermoelastic effect, localization

Abstract:
Experimental results of effects of thermomechanical couplings occurring in polyurethane shape memory polymer (PU-SMP) during tension at different strain rates are presented. Stress-strain curves were recorded by MTS 858 testing machine. The temperature changes were estimated by using a fast and sensitive infrared camera (Phoenix FLIR IR System). The stress and temperature vs. strain characteristics obtained during the tension enable to investigate the SMP deformation process and distinguish 3 different stages: the first, accompanied by a drop in temperature called thermoelastic effect, related to a limit of the material reversible deformation, the second plastic stage, associated with change of the material structure and significant increase in temperature, and the third - related to the mechanisms of damage - a breaking of the polymer chains, leading to the specimen rupture.

Keywords:
shape memory polymer, thermomechanical coupling, tension, infrared camera

Abstract:
Experimental results of effects of thermomechanical couplings occurring in polyurethane shape memory polymer (PU-SMP) subjected to cyclic loading at various strain rates are presented. Stress-strain characteristics were recorded by the testing machine, whereas the specimen temperature changes were measured by a fast and sensitive infrared camera. The influence of strain rate on the polymer thermomechanical behaviour was studied. It was found that the SMP is very sensitive to the strain rate. The higher the strain rate, the higher the values of the stress and temperature changes were obtained. In the initial stage of deformation a drop in temperature called thermoelastic effect, determining a limit of the material reversible deformation, was investigated.

Keywords:
thermomechanical couplings, polyurethane shape memory polymer, cyclic loading, various strain rates, sensitive infrared camera, thermoelastic effect, thermoelastic effect, reversible deformation

Abstract:
In the paper experimental results of the effects of thermomechanical couplings occurring in polyurethane shape memory polymer (PU-SMP), produced by SMP Techno Tokyo, subjected to tension with various strain rates are presented. Stress-strain curves were recorded by MTS 858 testing machine with a high accuracy, while using a fast and sensitive infrared camera (Phoenix FLIR IR System) enables to obtain the temperature distribution on the specimen surface, estimate the average temperature and investigate effects of the strain localization during the deformation process. The higher the strain rate the higher the changes in the stress and in temperature are observed, since the deformation mechanisms occur in more dynamic manner and the loading process is closer to adiabatic conditions.

Keywords:
shape memory polymer, thermomechanical couplings, tension, infrared camera, thermoelastic effect, yield point, temperature distribution, strain localization, strain rate, adiabatic conditions

Abstract:
The paper presents experimental evaluation and modelling of effects of thermomechanical couplings in shape memory alloy (SMA) and shape memory polymer (SMP). TiNi SMA and polyurethane PU-SMP are subjected to tension on MTS Testing machine. Fast infrared camera (IR) Phoenix FLIR System enable obtaining temperature distribution and average temperature changes of the specimens during the deformation process. Mechanical and infrared characteristics recorded during the SMA loading show that after initial, macroscopically homogeneous deformation a localized transformation develops, accompanied by significant temperature changes. Inclined bands of higher temperature accompanying exothermic forward transformation are recorded during the loading, whereas bands of lower temperature related to endothermic reverse transformation are observed during the unloading process. The infrared imaging and average temperature of the SMA sample compared to their mechanical characteristics allow to investigate the current stage of the stress-induced transformation process. A decrease of the specimen temperature reveals the saturation stage of the transformation. Both mechanical and thermal effects significantly depend on the strain rate; the higher the strain rate, the higher the temperature and stress are obtained. Similar experimental methodology is applied to investigate effects of thermomechanical couplings in shape memory polyurethane subjected to tension at various strain rates. Constitutive model valid in finite strain regime is proposed, where the SMP is described as a two-phase material composed of hyperelastic rubbery phase and elastic-viscoplastic glassy phase, while the volume content of phases is specified by the current temperature. Experimental results and modelling show that the SMP deformation process strongly depends on the strain rate, much stronger than for metals and alloys. At higher strain rate higher stress and temperature changes are obtained, since the deformation process is more dynamic and occurs in almost adiabatic conditions. It is shown that during the SMP loading process various deformation mechanisms are active at various strain rates.

Keywords:
shape memory alloy, transformation bands, infrared camera, constitutive model, shape memory polymers, elastic modulus, yield stress, glass transition temperature, shape fixity, shape recovery

Abstract:
Experimental results of effects of thermomechanical couplings occurring in polyurethane shape memory polymer (PU-SMP) subjected to cyclic loading at, are presented. Stress-strain characteristics were recorded by the testing machine, whereas the specimen temperature changes were measured by a fast and sensitive infrared camera. The influence of strain rate on the polymer thermomechanical behaviour is studied. It was found that PU-SMP is very sensitive to the strain rate. The higher the strain rate, the higher the values of stress and temperature changes were obtained. In the initial stage of deformation a drop in temperature called thermoelastic effect was recorded determining a limit of the material reversible deformation.

Keywords:
thermomechanical couplings, polyurethane shape memory polymer, cyclic loading, different strain rates, infrared camera, thermoelastic effect

Abstract:
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.

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

Abstract:
The paper concerns investigation of polyurethane shape memory polymer (SMP) properties. Shape fixity and shape recovery, important parameters for the SMP applications, were quantitatively estimated in thermomechanical cyclic loading; three subsequent thermomechanical loading cycles were performed. It was observed that the shape fixity is proper and does not depend on the cycle number. The obtained mean values of shape fixity parameters are 97-98 %. Although the shape recovery is poor (=83 %) in the first cycle of the thermomechanical loading, it is excellent in the subsequent cycles (=99-100 %). The evaluated parameters confirm good shape memory properties of the SMP.

Keywords:
Shape memory polyurethane, shape fixity, shape recovery, thermomechanical loading, cyclic loading

Abstract:
The paper presents the results of experimental investigation on polymer foam with shape memory properties. The research is focused on characterization of the microstructure of the foam and understanding the mechanisms of deformation under static and dynamic loading. Up till now, selected experimental techniques have been applied. Dynamic Mechanical Analysis (DMA) allows determining the extent of the value of the glass transition temperature under different load conditions, which also reveals the transformation temperature range for the SMP foam. Scanning electron microscopy (SEM) shows the foam microstructure in various scales, while X-ray tomography gave 3D microstructure results presenting in addition mechanism of the cells deformation and changes in their geometry under 30 % and 50% strain. BOSE system enables obtaining the results on dynamic loading.

Keywords:
Shape memory polymer foam, Dynamic mechanical analysis, Glass transition temperature, X-ray tomography

Abstract:
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].

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

Abstract:
Experimental evaluation and modeling of a new polyurethane shape memory polymer (SMP) subjected to cyclic tension and stress-relaxation tests are presented. The influence of effects of thermomechanical couplings on the SMP thermomechanical behaviour for various strain rates was studied, basing on the sample temperature changes measured by a fast and sensitive infrared camera. The constitutive model valid in finite strain regime was developed following [5]. In the proposed approach SMP is described as a two-phase material composed of hyperelastic rubbery phase and elastic-viscoplastic glassy phase while the volume content of phases is specified by the current temperature.

Keywords:
Experimental evaluation, constitutive modeling, polyurethane shape memory polymer, cyclic tension, stress-relaxation tests, effects of thermomechanical couplings, thermomechanical behaviour, various strain rates, temperature changes, sensitive infrared camera, hyperelastic rubbery phase, elastic-viscoplastic glassy phase

Abstract:
In order to contribute to solving the problems of recourses, energy and environment of the earth, the development of multifunctional smart materials is required. In the intelligent materials, investigation of shape memory alloy (SMA) and shape memory polymer (SMP) has attracted high attention due to their functional properties and high potential in practical applications. In SMA, the shape memory property appears based on the martensitic transformation (MT) in which the crystal structure varies depending on the variation in stress or temperature. In SMP, the elastic modulus and the yield stress are high at temperatures below the glass transition temperature Tg and low at temperatures above Tg. The shape memory property appears based on the glass transition in which the characteristics of molecular motion vary depending on the variation in temperature. Among the shape memory polymers, the polyurethane has been most often practically used.
In this paper, investigation of stress-induced martensitic transformation in TiNi SMA and thermomechanical behavior of SMP (Tg = 19 C) in tension at room temperature (22 C) are presented.

Keywords:
shape memory alloy, shape memory polymer, thermomechanical couplings, tensile test, strain localisation

Abstract:
W pracy przedstawiono wyniki badań efektów sprzężeń termomechanicznych poliuretanu z pamięcią kształtu podczas rozciągania z różnymi prędkościami. Zależności zmiany temperatury próbek od czasu wyznaczano za pomocą kamery termograficznej. Zaobserwowano dużą wrażliwość charakterystyk mechanicznych i zmian temperatury poliuretanu na prędkość deformacji. Wykorzystując efekt piezokaloryczny wyznaczano granice plastyczności dla różnych prędkości odkształcenia. Pokazano, że metoda bazująca na minimum temperatury próbki jest przydatna do wyznaczania granicy plastyczności materiałów polimerowych wykazujących nieliniową sprężystość.

Keywords:
poliuretan z pamięcią kształtu, efekty sprzężeń termomechanicznych, kamera termograficzna, efekt piezokaloryczny, granica plastyczności

Abstract:
This paper presents experimental evaluation of a new polyurethane shape memory polymer (PU-SMP) produced by SMP Technologies Inc. It discusses mechanical characteristics and temperature changes of the SMP specimens subjected to tension test performed at room temperature with various strain rates. Basing on the mechanical data and the relevant temperature changes, we have studied the thermomechanical properties of the PU-SMP and influence of the strain rate on the strain localization behavior. Finally, we have identified the material parameters for the one-dimensional rheological model of the SMP.

Keywords:
shape memory polyurethane, tension test, dynamic mechanical analysis, infrared camera, temperature change, thermomechanical properties, rheological model