Free Recovery Behaviors Research Articles

Modeling the thermomechanical behaviors of particle reinforced shape memory polymer composites

Shape memory polymer composites (SMPCs) possess superior thermomechanical properties while keep good shape memory capabilities, so SMPCs attract great research interest from academia and industry. In the paper, a modified thermoviscoelastic finite deformation constitutive model is developed to investigate the thermomechanical properties and shape recovery behaviors of particle reinforced SMPCs. The model is incorporated with the modified Adam–Gibbs model and the Eyring model to describe the structural relaxation and yielding behaviors. Also, a non-Gaussian chain molecular network model is used to capture the hyperelastic properties in the large deformation situation. Then, the constitutive model is employed to estimate the free recovery behaviors of SMPCs with different particle fractions, and the comparison between the simulation results and the test data shows good agreement. Furthermore, the model is applied to predict the constrained recovery properties and uniaxial tensile stress–strain response of the composites. The thermomechanical model provides an efficient method for designing and optimizing particle reinforced SMPCs.

A multi-branch thermoviscoelastic model based on fractional derivatives for free recovery behaviors of shape memory polymers

A multi-branch thermoviscoelastic model based on fractional derivatives is proposed for the prediction of temperature-dependent free recovery behaviors of amorphous shape memory polymers (SMPs). The model is similar to the generalized fractional Maxwell model, which gives an approximation of the temperature-dependent storage modulus, loss modulus and loss factor by introducing the shift factor, usually derived from time-temperature superposition of isothermal data obtained at different temperatures. The simulation results show a satisfactory agreement with the experimental free recovery curves from shape memory tests. Moreover, this method is also proved valid for the triple-shape memory effect and multi-shape memory effect of shape memory polymers. In addition, it is an effective approach for the prediction of free recovery behaviors of the SMP-based syntactic foam consisting of hollow glass microspheres dispersed in a shape memory polymer matrix. The application of the fractional derivatives to the thermoviscoelastic constitutive model can effectively reduce the number of the required parameters while give an accurate prediction of viscoelastic behaviors and free recovery behaviors of SMPs.

Influence of mechanically-induced dilatation on the shape memory behavior of amorphous polymers at large deformation

In this study, we explore the influence of mechanically-induced dilatation on the thermomechanical and shape memory behavior of amorphous shape memory polymers (SMPs) at large deformation. The uniaxial tension, glass transition, stress relaxation and free recovery behaviors are examined with different strain levels (up to 340% engineering strain). A multi-branched constitutive model that incorporates dilatational effects on the polymer relaxation time is established and applied to assist in discussions and understand the nonlinear viscoelastic behaviors of SMPs. It is shown that the volumetric dilatation results in an SMP network with lower viscosity, faster relaxation, and lower $T_{g}$ . The influence of the dilatational effect on the thermomechanical behaviors is significant when the polymers are subject to large deformation or in a high viscosity state. The dilation also increases the free recovery rate of SMP at a given recovery temperature. Even though the tested SMPs are far beyond their linear viscoelastic region when a large programming strain is applied, the free recovery behavior still follows the time-temperature superposition (TTSP) if the dilatational effect is considered during the transformation of time scales; however, if the programming strain is different, TTSP fails in predicting the recovery behavior of SMPs because the network has different entropy state and driving force during shape recovery. Since most soft active polymers are subject to large deformation in practice, this study provides a theoretical basis to better understand their nonlinear viscoelastic behaviors, and optimize their performance in engineering applications.

A Fractional Calculus Approach to the Prediction of Free Recovery Behaviors of Amorphous Shape Memory Polymers

AbstractA fractional model generalized from the Zener model is proposed for the prediction of temperature-dependent free recovery behaviors of amorphous shape memory polymers (SMPs). This model differs from the Zener model in that it involves nonlinear differential equations of fractional, not integer, order. The theoretical solution based on this fractional model is utilized to simulate the isothermal and nonisothermal free recovery of an amorphous SMP compared with the one based on the Zener model. The results show a reasonable improvement in the prediction of the strain recovery response of SMP by the fractional calculus method.

Effects of stretch induced softening to the free recovery behavior of shape memory polymer composites

During a cyclic tension test, many elastomeric materials exhibit an appreciable softening in their mechanical properties resulting from the previous stretch, known as the Mullins effect. This paper explores the influence of the stretch induced softening effect to the free recovery behavior of an acrylate shape memory polymer (SMP) composite by incorporating carbon black (CB) as filler materials. The observed softening effect in this SMP composite is considered to be a consequence of stretch induced alternation of filler–polymer interactions inside the composite. Further experiments find that a larger prior stretch gives a larger increase in material softening, which in turn decreases the shape recovery speed. To capture the experimental observations, a multi-branch one dimensional (1D) model is applied, where the modulus in the equilibrium branch is modeled to decrease with stretching deformation following a damage-like softening function. It is found that the loss in modulus due to softening consequently reduces the driving force for recovery and thus results in a slow recovery. Parametric studies further demonstrate that the discounted shape recovery speed will finally reach a saturated level when gradually increasing the programmed strain level in a shape memory cycle.

Influence of molecular architecture on the isothermal time-dependent response of amorphous shape memory polyurethanes

The thermomechanical response of a series of thermally activated shape memory polyurethanes (SMPUs), determined by dynamic mechanical analysis (DMA), has been adjusted by systematic modification of the molecular architecture. It is argued that the free recovery behavior of these SMPUs at temperatures in the vicinity of the calorimetric glass transition temperature is dependent not only on the recovery temperature, but also on the form of the corresponding peak in tan δ in DMA temperature scans at constant frequency. On the basis of simple correlations between recovery rates and the width and shape of the tan δ peak, it is suggested that DMA may provide a relatively simple and rapid means of assessing the potential of the SMPUs with respect both to recovery and shape fixity at a given storage temperature. This in turn allows establishment of a direct link between the shape memory performance and molecular architecture.

Stepwise behavior of free recovery processes after diffused vacuum arc extinction

The objective of this paper is to gain insight into free recovery processes in vacuum interrupters after diffused vacuum arcs extinction. A sub-microsecond voltage impulse, which peaked at 90 kV with a rising time of 150 ns) (rate of rise 480 kV/μs), were electronically controlled to apply upon a switching gap of a vacuum interrupter at a predetermined interval after a power frequency current zero. Based on the breakdown voltages at different intervals after arc extinction, we can obtain the free recovery behavior of the vacuum interrupter. The contact materials under study included Cu, CuCr25 and CuCr50. The contacts were butttype, and the contact diameters were 12 and 25 mm, respectively. The arc current peaked at 2.1 kA with a frequency of 50 Hz and the arcing time was ~9 ms. The results show the free recovery behavior stepped from ~28 kV to ~70 kV at ~4 μs after arc extinction for the three kinds of contact materials with the two contact diameters given. By calculation of the dissipation of metal vapor density and ion density, we conclude that the decay of ions, rather than metal vapor, may be the main reason for the stepwise behavior of the free recovery process after diffused vacuum arcs extinction.

Prediction of temperature-dependent free recovery behaviors of amorphous shape memory polymers

Shape memory polymers (SMPs) are active materials that can fix a temporary shape and recover the permanent shape in response to environmental stimuli such as temperature, light, moisture or magnetic field. In order to provide insight into the mechanism for shape memory behavior and to predict the behaviors of targeted design, several constitutive models were developed in the past. Most of these models are complicated and require time-consuming experiments to obtain model parameters. However, for many engineers, an estimation of key features of shape memory behaviors, such as time for free recovery, is sufficient. Such estimation should be based on a simplified model involving only a few key parameters that can be quickly identified experimentally. In this paper, a simple theoretical solution was developed to predict the temperature dependent free recovery behaviors of amorphous SMPs. This solution is based on a modified standard linear solid (SLS) model with a Kohlrausch–Williams–Watts (KWW) stretched exponential function and requires only eight parameters that can be determined by stress relaxation tests. The theoretical predictions of free recovery behaviors show a good agreement with experimental results. Parametric studies using this solution reveal that the free recovery time can be reduced by increasing the equilibrium modulus (E0) or KWW stretching parameter (β), or by decreasing the nonequilibrium modulus (E1) or the relaxation time (τ0) and is most sensitive to the KWW stretching parameter β.

Strain‐Based Temperature Memory Effect for Nafion and Its Molecular Origins

AbstractA shape memory polymer traditionally refers to a polymer that can memorize one temporary shape and recover to its permanent shape upon exposure to an external stimulus. Although this basic concept has been known for at least half a century, recent advances have led to the discoveriy of previously uncovered memory properties that challenge the traditional concept of shape memory polymers. In particular, a temperature memory effect refers to the capability of a polymer to memorize temperatures instead of shapes. Thus far, the reported temperature memory effect has been established under iso‐strain stress recovery conditions, in which the maximum recovery stress appears at a temperature roughly identical to the deformation temperature. This effect can be called recovery stress based temperature memory effect. In this work, experiments were designed in an attempt to establish a temperature memory effect based on the stress free strain recovery behaviors of Nafion. The results show that, under carefully selected conditions, the temperature at which a maximum strain recovery rate is observed can indeed be quantitatively related to the deformation temperature. In addition, indications that the polymer is capable of memorizing more than one deformation temperature (i.e., multi‐temperature memory effect) are shown. The molecular origin of Nafion’s temperature memory effect is elucidated through small angle neutron scattering study.

Time and Temperature Dependent Recovery of Epoxy-Based Shape Memory Polymers

Shape memory polymers (SMPs) are a group of adaptive polymers that can recover the permanent shape from a temporary shape by external stimuli on demand. Among a variety of external stimuli for polymer actuation, temperature is the most extensively used. In SMP applications, one of the major design considerations is the time necessary to recover the shape without external deformation constraints, or free recovery, and the amount of the recoverable strain. This paper investigates the amount of the recoverable strain and the recovery rate of an epoxy-based SMP (Veriflex® E, VFE1-62 (CRG, Dayton, OH)) under different thermal conditions. In particular, the free recovery behaviors of the SMPs under two experimental protocols, isothermal and shape memory (SM) cycle, are studied. It is found that free recovery in isothermal experiments is much faster than that in a SM cycle at the same recovering temperature and the material is fully recoverable at the temperature above differential scanning calorimetry Tg. Furthermore, for the recovery in SM cycle experiments, reshaping the sample at a low temperature and recovering from the deformation at a high temperature yield the fastest recovery rate, while reshaping at a high temperature and recovering at a low temperature cannot recover the original shape within this work’s experimental time frame. The possible mechanism for these observations is discussed.

Analysis of the finite deformation response of shape memory polymers: I. Thermomechanicalcharacterization

This study presents the analysis of the finite deformation response of a shape memorypolymer (SMP). This two-part paper addresses the thermomechanical characterization ofSMPs, the derivation of material parameters for a finite deformation phenomenologicalmodel, the numerical implementation of such a model, and the predictions from the modelwith comparisons to experimental data.Part I of this work presents the thermomechanical characterization of the materialbehavior of a shape memory polymer. In this experimental investigation, the visionimage correlation system, a visual–photographic apparatus, was used to measuredisplacements in the gauge area. A series of tensile tests, which included nominalvalues of the extension of 10%, 25%, 50%, and 100%, were performed on SMPspecimens. The effects on the free recovery behavior of increasing the value of theapplied deformation and temperature rate were considered. The stress–extensionrelationship was observed to be nonlinear for increasing values of the extension, and theshape recovery was observed to occur at higher temperatures upon increasing thetemperature rate. The experimental results, aided by the advanced experimentalapparatus, present components of the material behavior which are critical forthe development and calibration of models to describe the response of SMPs.

A New Thermoelastic Model for Analysis of Shape Memory Alloy Hybrid Composites

A constitutive model and a finite element formulation are developed for predicting the thermomechanical response of SMA hybrid composite structures subjected to combined thermal and mechanical loads. The constitutive model is valid for constrained, restrained, or free recovery behavior with appropriate measurements of basic SMA material properties. The model captures the material nonlinearity of the SMA with temperature and more accurately captures the mechanics of composites with embedded SMA actuators as compared to other recently developed approaches. The constitutive and finite element models are amenable to commercial code implementation. The fundamental thermoelastic behavior of such structures is described in physical terms and related to the governing equations. It is shown that alloy selection is imperative for achieving the desired performance with respect to the application environment. It is also shown that fundamental efforts to strategically place actuators can produce dramatic performance improvements. Numerical results are shown for glass-epoxy beam specimens with embedded Nitinol actuators. Control of critical buckling temperature, thermal post-buckling deflection, and random response are demonstrated.