Maximum Stretch Research Articles

Power generation performance of dielectric elastomer generator with laterally-constrained configuration

A new loading method with laterally-constrained configuration was proposed to study the energy conversion behavior of dielectric elastomer generator (DEG) based on experimental tests and theoretical calculation. The influence of lateral pre-stretch ratio and the loading rate on the energy transformation of DEG (VHB 4905) was investigated under both the quadrangular and triangular harvesting schemes, and the latter shows higher energy density and conversion efficiency than the former. The energy density increases with the lateral pre-stretch ratio at the range of 2 to 4, with the maximum value of 76 mJ g−1 for the quadrangular cycle and 186 mJ g−1 for the triangular cycle at the loading rate of 90 mm s−1 (strain rate of 2 s−1), respectively. The advantage of larger lateral pre-stretch ratio is mainly attributed to the increased capacitance density of the elastomer membrane at the maximum stretch, which enlarges the operational area confined by multiple failure limits on the voltage-charge work-conjugate plane. As the loading rate decreases, the energy density within each cycle decreases monotonically due to the increased charges leakage. Since the mechanical energy loss within a single cycle decreases at the lower loading rate, the conversion efficiency for the triangular cycle shows slight increase at the lower loading rate, with the maximum value of 15.4% for stretching rate of 50 mm s−1 (strain rate of 1.11 s−1).

Extensional rheology of highly-entangled α-olefin molecular bottlebrushes

The transient uniaxial extensional response of a series of ultrahigh molecular weight poly(α-olefins) (PαO) with densely grafted bottlebrush architectures was measured as a function of side chain length, N sc. The PαO bottlebrushes studied here have N sc values ranging from 6 carbons [poly(1-octene)] to 16 carbons [poly(1-octadecene)] (POD). To understand the effect of bottlebrush architecture on the nonlinear response, a linear polyolefin (polypropylene), with N sc = 1, was included in this study. All the PαO bottlebrushes show strong extensional strain hardening (SH) at Weissenberg numbers, W i R, greater than 1. Strain-hardening ratios, S H R = η E + ( t ) / η E , LVE +, show increasing dependence on the rate of extension, described by power laws, with exponents dependent on the N s c values. Moreover, the maximum SHR values, S H R max, are increasing function of the side chain length, reaching values more than one order of magnitude larger for the POD, compared to atactic polypropylene (aPP, e.g., at W i R ∼ 200, S H R max = 3.5 and 75 for PP and POD, respectively). Another remarkable feature of these PαO bottlebrushes is their large extensibility, which is also an increasing function of both the extension rate and the side chain length. For instance, the maximum macroscopic stretch ratio values (before breakup), λ max, reach values of ∼400 (for POD), which is more than an order of magnitude larger than that for aPP ( λ max ∼ 25 ). A mechanism to explain the strong SH and stretchability in the PαO bottlebrushes, based on flow-induced side chain reorientation and interdigitation, is discussed.

Investigation of the Effect of Carbonyl Iron Micro-Particles on the Mechanical and Rheological Properties of Isotropic and Anisotropic MREs: Constitutive Magneto-Mechanical Material Model.

This article focuses on evaluating the influence that the addition of carbonyl iron micro-particles (CIPs) and its alignment have on the mechanical and rheological properties for magnetorheological elastomers (MREs) fabricated using polydimethylsiloxane (PDMS) elastomer, and 24 wt % of silicone oil (SO). A solenoid device was designed and built to fabricate the corresponding composite magnetorheological material and to perform uniaxial cyclic tests under uniform magnetic flux density. Furthermore, a constitutive material model that considers both elastic and magnetic effects was introduced to predict stress-softening and permanent set effects experienced by the MRE samples during cyclic loading tests. Moreover, experimental characterizations via Fourier transform infrared (FTIR), X-ray diffraction (XRD), tensile mechanical testing, and rheological tests were performed on the produced MRE samples in order to assess mechanical and rheological material properties such as mechanical strength, material stiffness, Mullins and permanent set effects, damping ratio, stiffness magnetorheological effect (SMR), and relative magnetorheological storage and loss moduli effects. Experimental results and theoretical predictions confirmed that for a CIPs concentration of 70 wt %, the material samples exhibit the highest shear modulus, stress-softening effects, and engineering stress values when the samples are subject to a maximum stretch value of 1.64 and a uniform magnetic flux density of 52.2 mT.

Anomalous Stretching Dynamics of Tagged Monomer of Branched Polymer in Layered Random Flows

We extend our formalism to theoretically understand the stretch dynamics of single tagged monomer of flexible branched polymer with arbitrary topology in the presence of layered random flows. We derived an expression for the average squared displacement of the m-th bead of an arbitrary polymer structure with respect to its center of mass. The main focus of our study is the dynamics of star and dendrimer, where the effect of the topology, viz. the number and length of branches for stars and the number of generations and spacer lengths for dendrimers, is analyzed. We predict an increase in stretching as a function of time which finally reaches to the steady-state (plateau) value. In addition, we analyze the influence of variation in external flow which is characterized through its flow exponent ($$\alpha $$), root mean square velocity ($$V_0$$) and flow strength ($$W_{\alpha }$$). The composite measure of flow strength $$W_{\alpha }$$ depends upon $$\alpha $$, $$V_0$$ and persistence length of flow. The magnitude of the maximum stretch (plateau region) increases with increase in flow exponent. The anomalous stretch behavior of star polymer with increasing flow strength shows the non-uniform stretch behavior of polymer.

Effect of the deformation dependent permittivity on the actuation of a pre-stretched circular dielectric actuator

Experiments have shown that under large deformation, the permittivity of some dielectric elastomers demonstrates significant deformation dependent behavior. In this paper, we theoretically analyze the effect of this behavior on the actuation of a circular actuator made of a dielectric elastomer, VHB 4910. The dependency of permittivity on deformation is considered through the equations of state. The equilibrium equations of the actuator are solved by MATLAB. The electromechanical instability, loss of tension and dielectric breakdown of the actuator during the actuation process under different pre-stretch levels are discussed. It is found that the dependency of permittivity on deformation can effectively suppress the electromechanical instability of the actuator at low pre-stretch levels. At high pre-stretch levels, however, it lowers the maximum stretch the actuator can achieve. Compared to the case of a constant permittivity, the deformation-dependent effect results in higher voltage needed to achieve the same actuated stretch, and this discrepancy in voltage increases as the pre-stretch increases. Furthermore, it is found that the maximum achievable stretch and the corresponding voltage show a strong dependence on factor c in the permittivity-stretch relation, while the stretch at the onset of loss of tension is independent of c.

Model of Anisotropic Reverse Cardiac Growth in Mechanical Dyssynchrony

Based on recent single-cell experiments showing that longitudinal myocyte stretch produces both parallel and serial addition of sarcomeres, we developed an anisotropic growth constitutive model with elastic myofiber stretch as the growth stimuli to simulate long-term changes in biventricular geometry associated with alterations in cardiac electromechanics. The constitutive model is developed based on the volumetric growth framework. In the model, local growth evolutions of the myocyte’s longitudinal and transverse directions are driven by the deviations of maximum elastic myofiber stretch over a cardiac cycle from its corresponding local homeostatic set point, but with different sensitivities. Local homeostatic set point is determined from a simulation with normal activation pattern. The growth constitutive model is coupled to an electromechanics model and calibrated based on both global and local ventricular geometrical changes associated with chronic left ventricular free wall pacing found in previous animal experiments. We show that the coupled electromechanics-growth model can quantitatively reproduce the following: (1) Thinning and thickening of the ventricular wall respectively at early and late activated regions and (2) Global left ventricular dilation as measured in experiments. These findings reinforce the role of elastic myofiber stretch as a growth stimulant at both cellular level and tissue-level.

Exploring the mechanism of fracture for entangled polymer liquids in extensional flow

The critical strain and stress at fracture are systematically investigated for two groups of nearly monodisperse linear polystyrene liquids in an extensional flow. The samples in group I have similar number of Kuhn segments per entangled strand (Ne) but different number of entanglements per chain (Z), while the samples in group II have similar Z but different Ne. We found that the critical conditions, especially the critical stress, are independent of Z but influenced by Ne. The observations indicate that the fracture in entangled polystyrene liquids occurs at a length scale smaller than an entangled strand. Therefore, the fracture originates more likely from scission of primary bonds in polymer chains, rather than rapid entanglement slipping. The level of the critical stress also suggests that at fracture, the polymer chains approach their theoretical maximum stretch ratio, which is equal to Ne.

Double Torsion Coil Feeding Structure for Patch Antennas

A new feeding structure for patch antennas is proposed in this paper. The feeding structure is formed by a double torsion coil (DTC) with one end electrically connected to the ground. The maximum stretch length of the coil is around half of the wavelength of the center frequency. The winding direction of the coil is reversed at the middle point of the coil. The DTC is able to produce two coaxial equivalent magnetic dipoles polarized in the same direction, which can not only excite the TM01 mode of a patch antenna but also create another resonance, leading to a wideband impedance bandwidth. The working mechanism is explained in this paper by physics intuition. To validate the new feeding structure, a linear-polarized and a dual-polarized patch antenna prototypes working in the 3.5 GHz band are designed, manufactured, and measured. It is found that the linear-polarized patch antenna can achieve a bandwidth of 25% standing-wave ratio (SWR < 2) and 9.5 dBi average gain with stable radiation patterns and low cross polarization over the impedance bandwidth. The new feeding structure has a high potential for wideband massive multiple-input and multiple-output (M-MIMO) arrays of 5G and future wireless communication systems.

Dynamic modeling of a dielectric elastomeric spherical actuator: an energy-based approach

ABSTRACT The present work is concerned with the dynamic modeling of a dielectric elastomeric spherical actuator subjected to an inflation pressure and an electrical DC step voltage across the actuator wall. Herein, we report an alternative method to obtain the static and dynamic instability parameters based on establishing the energy balance principle at the point of maximum stretch in an oscillation cycle. Additionally, we proposed a new energy density function, which accounts the electrostriction phenomenon for a class of an incompressible isotropic dielectric elastomeric material. We first deduce an explicit analytical Lagrange equation of motion based on the Hamiltonian principle of least action for a dielectric elastomeric spherical actuator. Further, a quantitative comparison is presented between the static and dynamic instability parameters. Finally, the numerical results obtained from the developed equation of motion are discussed in line with the parallel results presented in the literature. In summary, the present study brings out the effect of an inflation pressure on the onset of dynamic response in a dielectric elastomeric spherical actuator. In addition, the proposed method is so versatile, and can also be used in other systems of interest.

Influence of long chain branching on fiber diameter distribution for polypropylene nonwovens produced by melt blown process

In this work, linear isotactic polypropylene (L-PP) and long-chain branched polypropylene (LCB-PP) miscible blend, both having comparable weight average molecular weight, zero-shear viscosity, and polydispersity index, were used to produce nonwovens via melt blown technology in order to understand the role of long chain branching in the fiber diameter distribution. Basic morphological characteristics of produced nonwoven samples have been determined using digital image analysis of scanning electron microscope images considering different magnifications to capture nanofibers as well as microfibers. At the same air flow rate, polymer flow rate, and temperature, the average fiber diameters were the same, 1.6 μm, but the coefficient of variation, CV, was greater for the linear PP than for the blend. Material elasticity was assessed by reptation-mode relaxation time, λ, determined by fitting of deformation rate dependent shear viscosity by Cross and Carreau-Yasuda models as well as via fitting of frequency dependent loss and storage moduli master curve by a two-mode Maxwell model. It was found that λ is higher for LCB-PP in comparison with L-PP and the Cross model gives a meaningful relaxation time while the Carreau-Yasuda model does not despite giving a better numerical fit. Extensional rheology was assessed by the strain rate dependent uniaxial extensional viscosity (estimated from the entrance pressure drop using the Gibson method). The infinite shear to zero-shear shear viscosity ratio η∞/η0 (obtained directly from the shear viscosity data measured in a very wide shear rate range) was shown to be proportional to the maximum normalized extensional viscosity at very high extensional strain rates, ηE,∞/(3η0). η∞/η0 was related to temperature and basic molecular characteristics of given polymers via simple equation. It was observed that extensional viscosity for both samples first decreases with increased extensional strain rate to its minimum value at 200 000–400 000 1/s and then increases to plateau value, ηE,∞ (corresponding to the maximum chain stretch) at about 2 ⋅ 106 1/s. At low deformation rates, extensional viscosity is higher for LCB-PP in comparison with L-PP, but the trend is switched at very high deformation rates; ηE,∞ (and also ηE,∞/3η0) becomes lower for LCB-PP in comparison with L-PP. These results suggest that high stability of LCB-PP blend can be explained by its higher stretchability at very high deformation rates (occurring at the die exit where an intensive fiber attenuation takes the place) and its lower stretchability at medium and low deformation rates, at which melt/air inertia driven bending instability called whipping occurs.

Tuning the pull-in instability of soft dielectric elastomers through loading protocols

Pull-in (or electro-mechanical) instability occurs when a drastic decrease in the thickness of a dielectric elastomer results in electrical breakdown, which limits the applications of dielectric devices. Here we derive the criterions for determining the pull-in instability of dielectrics actuated by different loading methods: voltage-control, charge-control, fixed pre-stress and fixed pre-stretch, by analyzing the free energy of the actuated systems. The Hessian criterion identifies a maximum in the loading curve beyond which the elastomer will stretch rapidly and lose stability, and can be seen as a path to failure. We present numerical calculations for neo-Hookean ideal dielectrics, and obtain the maximum allowable actuation stretch of a dielectric before failure by electrical breakdown. We find that applying a fixed pre-stress or a fixed pre-stretch to a charge-driven dielectric may decrease the stretchability of the elastomer, a scenario which is the opposite of what happens in the case of a voltage-driven dielectric. Results show that a reversible large actuation of a dielectric elastomer, free of the pull-in instability, can be achieved by tuning the actuation method.

ВПЛИВ НАНОНАПОВНЮВАЧА НА РЕОЛОГІЧНІ ВЛАСТИВОСТІ РОЗПЛАВІВ ПОЛІМЕРІВ ТА ЇХ СУМІШЕЙ

Studying the influence of additives of nano-sized aluminium oxide on the patterns of Polypropylene (PP) and mixture PP/CPA (copolyamide) melt flow. The mixtures were obtained by the prior injection of the nano-filler to the PP melt with the further mixing of granula with CPA on the worm-disk extruder. Viscosity (η) of melts was examined by the method of capillary viscometry and the elasticity was studied by value of extrudate equilibrium swelling. The melts ability to longitudinal deformation was evaluated by the maximum stretch rating.

Crystallographic preferred orientations of ice deformed in direct-shear experiments at low temperatures

Abstract. Synthetic polycrystalline ice was sheared at temperatures of −5, −20 and −30 ∘C, to different shear strains, up to γ=2.6, equivalent to a maximum stretch of 2.94 (final line length is 2.94 times the original length). Cryo-electron backscatter diffraction (EBSD) analysis shows that basal intracrystalline slip planes become preferentially oriented parallel to the shear plane in all experiments, with a primary cluster of crystal c axes (the c axis is perpendicular to the basal plane) perpendicular to the shear plane. In all except the two highest-strain experiments at −30 ∘C, a secondary cluster of c axes is observed, at an angle to the primary cluster. With increasing strain, the primary c-axis cluster strengthens. With increasing temperature, both clusters strengthen. In the −5 ∘C experiments, the angle between the two clusters reduces with strain. The c-axis clusters are elongated perpendicular to the shear direction. This elongation increases with increasing shear strain and with decreasing temperature. Highly curved grain boundaries are more prevalent in samples sheared at higher temperatures. At each temperature, the proportion of curved boundaries decreases with increasing shear strain. Subgrains are observed in all samples. Microstructural interpretations and comparisons of the data from experimentally sheared samples with numerical models suggest that the observed crystallographic orientation patterns result from a balance of the rates of lattice rotation (during dislocation creep) and growth of grains by strain-induced grain boundary migration (GBM). GBM is faster at higher temperatures and becomes less important as shear strain increases. These observations and interpretations provide a hypothesis to be tested in further experiments and using numerical models, with the ultimate goal of aiding the interpretation of crystallographic preferred orientations in naturally deformed ice.

Performance and perception of haptic feedback in a laparoscopic 3D virtual reality simulator

Background: The benefit of haptic feedback in laparoscopic virtual reality simulators (VRS) is ambiguous. A previous study found 32% faster acquisition of skills with the combination of 3 D and haptic feedback compared to 2 D only. This study aimed to validate perception and effect on performance of haptic feedback by experienced surgeons in the previously tested VRS. Material and methods: A randomized single blinded cross-over study with laparoscopists (>100 laparoscopic procedures) was conducted in a VRS with 3 D imaging. One group started with haptic feedback, and the other group without. After performing the suturing task with haptics either enabled or disabled, the groups crossed over to the opposite setting. Face validity was assessed through questionnaires. Metrics were obtained from the VRS. Results: The haptics for ‘handling the needle’, ‘needle through tissue’ and ‘tying the knot’ was scored as completely realistic by 3/22, 1/22 and 2/22 respectively. Comparing the metrics for maximum stretch damage between the groups revealed a significantly lower score when a group performed with haptics enabled p = .027 (haptic first group) and p < .001(haptic last group). Conclusion: Haptic feedback in VRS has limited fidelity according to the tested laparoscopic surgeons. In spite of this, significantly less stretch damage was caused with haptics enabled.

Determination of a visco-hyperelastic material law based on dynamic tension test data

The objective of this study was to determine the visco-hyperelastic constitutive law of brain tissue under dynamic impacts. A method combined by finite element simulations and optimization algorithm was employed for the determination of material variables. Firstly, finite element simulations of brain tissue dynamic uniaxial tension, with a maximum stretch rate of 1.3 and strain rates of 30 s -1 and 90 s -1, were developed referring to experimental data. Then, fitting errors between the engineering stress-strain curves predicted by simulations and experimental average curves were assigned as objective functions, and the multi-objective genetic algorithm was employed for the optimation solution. The results demonstrate that the brain tissue finite element models assigned with the novel obtained visco-hyperelastic material law could predict the brain tissue's dynamic mechanical characteristic well at different loading rates. Meanwhile, the novel material law could also be applied in the human head finite element models for the improvement of the biofidelity under dynamic impact loadings.

A model for cellular mechanotransduction and contractility at finite strain

AbstractIn this work we introduce a theoretical and computational modeling framework for the contractile response of single cells triggered by external mechanical stimuli. The structural response due to the formation and dissociation of stress fibers is modeled following isotropic anisotropic contractile phases with an orientation that evolves with time and strain. The passive and active structural components are postulated to act in parallel, and the re‐orientation process drives the anisotropic phase of stress fiber orientation to align with the direction of the maximum principal stretch. A reduced form of the Hai‐Murphy model is used to follow kinetics of myosin states considering the combined effect of “latch”‐ and “cross”‐bridge states. The introduction of distinct isotropic and anisotropic activation allows modeling of the contractile intensity of each phase. Tractions on the cell surface initiate bio‐chemical signaling through the RhoA pathway, which in turn controls both myosin contraction and F‐actin polymerization. A signaling model is introduced to effectively connect intracellular events with the tractions on the cell surface. The overall model is defined by a free energy density function that couples the deformation and the activation, and associated equilibrium and kinetic models for evolution. Features of the model are highlighted via implementation in a finite element model and application to benchmark problems. The model captures the dynamic contractile responses of cells and stress fiber re‐alignment under complex load histories. For example, physiologically relevant scenario such as relaxation of cells to their initial state upon removal of applied loads can be simulated.

Design of tunable silicon metasurfaces with cross-polarization transmittance over 80%

We designed highly efficient, mechanically tunable metasurfaces based on an array of crystalline silicon nanoposts on stretchable substrates. The metasurface can continuously tune the wavefront of light by changing the lattice constant of the silicon nanoposts with a high transmission efficiency at all tuning ranges. Systematic numerical simulations were performed to determine the optimized structural parameters of the nanoposts, in which the cross-polarization transmittance was maximized. Transmission efficiency was maintained at over 80% with a maximum stretch ratio of ∼137% (2.9× flat zoom lens) and at over 75% with a maximum stretch ratio of ∼171% (1.9× flat zoom lens) in the tunable metasurface at λ = 680 nm. The simulation also showed that the tunable metasurface could be optimized at other wavelengths between 580 and 730 nm. We believe that the tunable metasurfaces will be useful for a variety of applications including information technology, biomedical sciences, integrated optics, optical communications, imaging, flat displays, or wearable consumer electronics.

Optimisation of hierarchical dielectric elastomer laminated composites

This paper is concerned with the optimisation of the actuation response of electro-elastic, rank-two laminates obtained laminating a core rank-one composite with a soft phase which constitutes the shell. The analysis is performed for two classes of composites that are subjected to traction-free boundary-value problems. The results are compared with those computed in a previous study where the optimisation was carried out at small strains. The non-linear approach allows a better estimation of the geometric layout of the reference configuration to enhance the maximum stretch or shear strain at the operative applied voltage. The optimum layouts are in general characterised by a very low volume fraction of the shell while in the core the two components are almost equally distributed. The amplification of the electric field in each phase of the laminate in the actuated state is also estimated to provide an indication of the effective local electro-mechanical response.

Phase Transition Effects on Mechanical Properties of NIPA Hydrogel.

Due to its excellent temperature sensitivity, the Poly(N-isopropylacrylamide) (NIPA) hydrogel has attracted great interest for a wide variety of applications in tissue engineering and regenerative medicine. NIPA hydrogel undergoes an abrupt volume phase transition at a lower critical solution temperature (LCST) of 30–35 °C. However, the mechanical behaviors of NIPA hydrogel induced by phase transition are still not well understood. In this study, phase transition effects on mechanical properties of NIPA hydrogel are quantitatively studied from experimental studies. The mechanical properties of NIPA hydrogel with the LSCT around 35 °C are systemically studied with varying temperatures (31–39 °C) under a tensile test. We find that the mechanical properties of NIPA hydrogel are greatly influenced by phase transition during the tension process. The maximum nominal stress and maximum stretch above the LCST are larger than those of below the LCST. The Young’s modulus of NIPA hydrogel is around 13 kPa at 31 °C and approximately 28 kPa at 39 °C. A dramatic increase of Young’s modulus values is observed as the temperature increases through the phase transition. The samples at a temperature around the LCST are easy to rupture, because of phase coexistent. Additionally, NIPA hydrogel displays toughening behavior under a cyclic load. Furthermore, the toughening characteristic is different between the swollen state and the collapsed state. This might originate from the internal fracture process and redistribution of polymer chains during the tension process.

Early Shear Failure Prediction in Incremental Sheet Forming Process Using FEM and ANN

The application of incremental sheet forming process as a rapid forming technique is rising in variety of industries such as aerospace, automotive and biomechanical purposes. However, the sheet failure is a big challenge in this process which leads wasting lots of materials. Hence, this study tried to propose a method to predict the early sheet failure in this process using mathematical solution. For the feasibility of the study, design of experiment with the respond surface method is employed to extract a set of experiments data for the simulation. The significant forming parameters were recognized and their integration was used for prediction system. Then, the results were inserted to the artificial neural network as input parameters to predict a vast range of applicable parameters avoiding sheet failure in ISF. The value of accuracy R2 ∼0.93 was obtained and the maximum sheet stretch in the depth of 25mm were recorded. The figures generate from the trend of interaction between effective parameters were provided for future studies.