Localized fluid flow measurements with an He-Ne laser spectrometer

Rayleigh–Taylor and Richtmyer-Meshkov instability induced flow, turbulence, and mixing. II

Ionic polymer metal composites (IPMCs) are active materials that exhibit a bidirectional electromechanical coupling. An IPMC is an electrolytic polymer membrane that is plated by two metallic electrodes. A voltage difference across the electrodes generates structural deformations; and, conversely, a mechanical deformation yields a voltage difference across the electrodes. In this paper, we develop a physics-based model for the sensing and actuation of IPMCs undergoing small deformations. The model describes a variety of phenomena taking place in an IPMC, including counterions, solvent, and polymer motions; electric dipole generation; osmotic effects; boundary layer formation; polymer swelling; and local charge imbalances. We specialize the model to the analysis of linear static deformations of a thin and flat IPMC, for which we derive a plate-like model. The reduced-order linear plate-like model is derived by using the principle of virtual work and a parallel-plate approximation for the electrostatic field inside the IPMC. The proposed plate-like model is equivalent to traditional plate models for moderately thin piezoelectric bimorph plates. The constitutive parameters of the plate-like model are expressed in terms of fundamental IPMC physical quantities, such as polymer hydration level, IPMC dielectric constant, polymer and electrode dimensions and elastic properties, and solute concentration. We validate the reduced-order model by comparing its predictions with available experimental data on mechanical stiffness, electric capacitance, and sensing and actuation capacity of water-hydrated Nafion in Na+ form. The model predictions are in close agreement with experimental findings. The model provides new insights into the design and optimization of IPMCs and into the role of the IPMC electric capacitance on electromechanical performance. More specifically, we show that the IPMC capacitance is largely independent of the IPMC thickness and highly correlated to the thickness of the boundary layers formed by the counterion species in the vicinity of the electrodes. Further, we analytically show that the capacitance strongly influences the sensing and actuation effectiveness of IPMCs.

https://iopscience.iop.org/article/10.1088/0964-1726/18/1/015016/pdf

An electromechanical model for sensing and actuation of ionic polymer metal composites

Ionic polymer metal composites (IPMCs) are electroactive materials composed of a hydrated ionomeric membrane that is sandwiched between noble metal electrodes. Here, we propose a modeling framework to study quasi-static large deformations and electrochemistry of IPMCs. Specifically, IPMC kinematics is described in terms of its mechanical deformation, the concentration of mobile counterions neutralizing the ionomer, and the electric potential. The chemoelectromechanical constitutive behavior is obtained from a Helmholtz free energy density, which accounts for mechanical stretching, ion mixing, and electric polarization. The three-dimensional framework is specialized to plane bending of thin IPMCs. Hence, we propose a structural model, where the moment and the charge stored along the IPMC are computed from the solution of a modified Poisson–Nernst–Planck system, in terms of the through-the-thickness coordinate. For small static deformations, we present a semianalytical solution based on the method of matched asymptotic expansions, which is ultimately used to study IPMC sensing and actuation. We demonstrate that the linearity of IPMC actuation in a broad voltage range could be attributed to the interplay of two competing nonlinear phenomena, associated with Maxwell stress and osmotic pressure. In agreement with experimental observations, our model confirms the possibility of tailoring IPMC actuation by varying the counterion size and the concentration of fixed ions. Finally, the model is successful in predicting the significantly different voltage levels displayed by IPMC sensors and actuators, which are associated with remarkable variations in the ion mixing and polarization energies.

Mechanics and electrochemistry of ionic polymer metal composites

With advances in wireless communications and low power electronics there is an ever increasing need for efficient self-contained power systems. Traditional batteries are often selected for this purpose; however, there are limitations due to finite life-spans and the need to periodically recharge or replace the spent power source. One method to address this issue is the inclusion of an energy harvesting strategy that can scavenge energy from the surrounding environment and convert it into usable electrical energy. Since civil, industrial, and aerospace applications are often plagued with an overabundance of ambient vibrations, electromechanical transducers are often considered a viable choice for energy scavengers. In this study, two classes of transducer are considered: the piezoelectric polymer polyvinylidene fluoride and the ionically conductive ionic polymer transducer. Analytical models are formed for each material assuming axial loading and simulation results are compared with experimental results fo...

An Energy Harvesting Comparison of Piezoelectric and Ionically Conductive Polymers

Recently, Richtmyer–Meshkov Instabilities (RMI) have been proposed for studying the average strength at strain rates up to at least 107/s. RMI experiments involve shocking a metal interface that has initial sinusoidal perturbations. The perturbations invert and grow subsequent to shock and may arrest because of strength effects. In this work we present new RMI experiments and data on a copper target that had five regions with different perturbation amplitudes on the free surface opposite the shock. We estimate the high-rate, low-pressure copper strength by comparing experimental data with Lagrangian numerical simulations. From a detailed computational study we find that mesh convergence must be carefully addressed to accurately compare with experiments, and numerical viscosity has a strong influence on convergence. We also find that modeling the as-built perturbation geometry rather than the nominal makes a significant difference. Because of the confounding effect of tensile damage on total spike growth, which has previously been used as the metric for estimating strength, we instead use a new strength metric: the peak velocity during spike growth. This new metric also allows us to analyze a broader set of experimental results that are sensitive to strength because some larger initial perturbations grow unstably to failure and so do not have a finite total spike growth.

Estimation of Metal Strength at Very High Rates Using Free-Surface Richtmyer–Meshkov Instabilities

On computing the evolution of temperature for materials under dynamic loading

Ionomeric polymers are a promising class of intelligent material which exhibit electromechanical coupling similar to that of piezoelectric bimorphs. ionomeric polymers are much more compliant than piezoelectric ceramics or polymers and have been shown to produce actuation strain on the order of 2% at operating voltages between 1 V and 3 V (Akle et al., 2004, Proceedings IMECE). Their high compliance is advantageous in low force sensing configurations because ionic polymers have a very little impact on the dynamics of the measured system. Here we present a variational approach to the dynamic modeling of structures which incorporate ionic polymer materials. To demonstrate the method a cantilever beam model is developed using this variational approach. The modeling approach requires a priori knowledge of three empirically determined material properties: elastic modulus, dielectric permittivity, and effective strain coefficient. Previous work by Newbury and Leo has demonstrated that these three parameters are strongly frequency dependent in the range between less than 1 Hz to frequencies greater than 1 kHz. Combining the frequency-dependent material parameters with the variational method produces a second-order matrix representation of the structure. The frequency dependence of the material parameters is incorporated using a complex-property approach similar to the techniques for modeling viscoelastic materials. A transducer is manufactured and the method of material characterization is applied to determine the mtaerial properties. Additional experiments are performed on this transducer and both the material and structural model are validated. Finally, the model is shown to predict sensing response very well in comparison to experimental results, which supports the use of an energy-based variational approach for modeling ionomeric polymer transducers.

Characterization and Variational Modeling of Ionic Polymer Transducers

This paper presents a modeling approach for simulating the anisotropic thermal expansion of polycrystalline (1,3,5-triamino-2,4,6-trinitrobenzene) TATB-based explosives which utilizes microstructural information including the porosity, crystal aspect ratio and processing-induced texture. A self-consistent homogenization procedure is used to relate the macroscopic thermoelastic response to the constitutive behavior of single-crystal TATB. The model includes a representation of the grain aspect ratio, porosity and, crystallographic texture attributed to the consolidation process. A quantitative model is proposed for describing the evolution of the preferred orientation of basal planes in TATB during consolidation and an algorithm constructed for developing a discrete representation of the associated orientation distribution function. Analytical and numerical solutions using this model are shown to produce textures consistent with previous measurements and characterization for isostatically and uniaxially ‘die-pressed’ specimens.Predicted thermal strain versus temperature results for textured specimens are shown to be in agreement with corresponding experimental measurements. Results from these simulations are used to identify qualitative trends. Key conclusions from this work include the following. Both porosity and grain aspect ratio have an influence on the thermal expansion of polycrystal TATB, considering realistic material variability. The preferred orientation of the single-crystal TATB [0 0 1] poles within a polycrystal gives rise to pronounced anisotropy of the macroscopic thermal expansion. The extent of this preferred orientation depends on the magnitude of the deformation and, consequently, is expected to vary spatially throughout manufactured components much like the porosity. The modeling approach presented here has utility toward bringing spatially variable microstructural features into macroscale system engineering models.

https://iopscience.iop.org/article/10.1088/0965-0393/22/7/075008/pdf

Miles A. Buechler

Papers

Estimation of Metal Strength at Very High Rates Using Free-Surface Richtmyer–Meshkov Instabilities

On computing the evolution of temperature for materials under dynamic loading

Characterization and Variational Modeling of Ionic Polymer Transducers

Self-consistent modeling of the influence of texture on thermal expansion in polycrystalline TATB

A micromechanical framework and modified self-consistent homogenization scheme for the thermoelasticity of porous bonded-particle assemblies