Miles A. Buechler

Characterization and Variational Modeling of Ionic Polymer Transducers

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.

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

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.

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

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.

Residual Stress Characterization in a Dissimilar Metal Weld Nuclear Reactor Piping System Mock Up

Time-of-flight neutron diffraction, contour method, and surface hole drilling residual stress measurements were conducted at Los Alamos National Lab (LANL) on a lab sized plate specimen (P4) from phase 1 of the joint U.S. Nuclear Regulatory Commission and Electric Power Research Institute Weld Residual Stress (NRC/EPRI WRS) program. The specimen was fabricated from a 304L stainless steel plate containing a seven pass alloy 82 groove weld, restrained during welding and removed from the restraint for residual stress characterization. This paper presents neutron diffraction and contour method results, and compares these experimental stress measurements to a WRS finite element (FE) model. Finally, details are provided on the procedure used to calculate the residual stress distribution in the restrained or as welded condition in order to allow comparison to other residual stress data collected as part of phase 1 of the WRS program.

Electromechanical Model of an Active Polymer Thin Circular Disk

Ionic polymers exhibit an electromechanical response similar to a piezoelectric bender. It has been shown that material properties similar to piezoelectric properties can be used to effectively model ionic polymer devices. One proposed ionic polymer device is a circular disk fabricated from the ionic polymer material for shape and vibration control can be accomplished through electrical boundary conditions applied to the ionic polymer rather than by adding external actuators. This paper extends the formulae for natural frequencies and mode shapes of a thin disk to include quasi-piezoelectric properties and electrical boundary conditions. An electromechanical model for ionic polymers using equivalent circuit representation has been previously developed. Three materials properties, which are compatible with accepted piezoelectric actuator and transducer relationships were derived and experimentally verified. The equivalent Young’s modulus, dielectric permittivity, and the strain coefficient were found to be frequency dependant over the range 0.1 Hz to 500 Hz. In this paper the variational energy method is applied to develop a two-dimensional model of a thin electro-active polymer disk. The variational model relies on an extension of the electromechanical material properties derived for ionomeric materials. An example of a disk with simply supported geometric boundary conditions is presented and operational deflection shapes are simulated for electrical excitation between 0.1 and 500 Hz. The model predicts that the frequency dependence of the material properties will produce modal responses with both real and imaginary components, indicating the existence of travelling waves in the disk. Voltage to deflection transfer functions are also developed for several geometric boundary conditions using this model and then compared to experimental results. The model correctly predicts damped resonant frequencies as a result of the viscoelastic properties. It also accurately predicts low frequency phase lag and resonant frequencies of the electromechanical response.Copyright

A variational model of ionomeric polymer actuators and sensors

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. Their high compliance is also advantageous in low force sensing configurations because ionic polymers have a very little impact on the dynamics of the measured system. This paper presents a variational approach to the dynamic modeling of ionic polymer actuators and sensors. The 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. A model of a cantilever beam incorporating this frequency dependence has been developed. 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 was manufactured and the method of material characterization is outlined. Additional experiments are performed on this transducer and both the material and structural model are validated. The modeling method is then used to simulate the performance of actuators and sensors in a cantilever configuration.

Variational modeling of ionic polymer plate structures

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 5% at operating voltages between 1 V and 5 V. This performance indicates the potential for self-actuating devices manufactured from ionomeric polymers, such as deformable mirrors or low pressure pump diaphragms. This paper presents a variational approach to the dynamic modeling of ionic polymer plates in rectangular coordinates. A linear matrix equation, which relates displacement and charge to applied forces and voltage, is developed to determine the response of the structure to applied forces and applied potentials. The modeling method is based on the incorporation of empirically determined material properties, which have been shown to be highly frequency dependent. The matrices are calculated at discrete frequencies and solved frequency-by-frequency to determine the response of the ionomeric plate structures. A model of a thin rectangular plate is developed and validated experimentally. Simulated frequency response functions are compared to experimental results for several locations on the plate. The response of the plate at certain frequencies is computed and compared to experimentally-determined response shapes. The results demonstrate the validity of the modeling approach in predicting the dynamic response of the ionomeric plate structure. These spatial solutions are also compared to experimentally determined response shapes.

Characterization of a Plate Specimen From Phase I of the NRC/EPRI Weld Residual Stress Program

Time-of-flight neutron diffraction and contour method residual stress measurements were conducted at Los Alamos National Lab (LANL) on a lab sized plate specimen (P4) from Phase I of the joint U.S. Nuclear Regulatory Commission and Electric Power Research Institute Weld Residual Stress (NRC/EPRI WRS) program. The specimen was fabricated from a 304L stainless steel plate containing a seven pass Alloy 82 groove weld, restrained during welding and removed from the restraint for residual stress characterization. This paper presents neutron diffraction and contour method results, and compares these experimental stress measurements to a WRS Finite Element (FE) model. Finally details are provided on the procedure used to calculate the residual stress distribution in the restrained or as welded condition in order to allow comparison to other residual stress data collected as part of the EPRI lead Phase I WRS program.Copyright