Browsing by Author "Domann, John P."
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- The Development of a Novel Figure of Merit to Analyze Strain-Mediated Magnetoelectric AntennasGoforth, Michael Emory (Virginia Tech, 2021-11-09)Strain-mediated magnetoelastic composite materials are being considered for communication in lossy environments. Their consideration is attributable to predictions stating order of magnitude improvements over current antenna technology. The magnetic antenna design considered herein consists of three layers: 1) a piezoelectric layer, 2) a linear elastic layer, and 3) a magnetoelastic layer. The antenna operates by mediating strain through the device in a resonant bending mode. The magnetoelastic layer is stressed which results in a changing magnetization ultimately leading to a changing magnetostatic field in free space which acts as a signal for information transfer. In order to prove the efficacy of this approach finite element models have been developed to aid in the design and optimization process. Where these models fall short is in their overall run-time to fully resolve the coupled dynamics. It is for this reason that the work presented in this thesis focuses on the development of a figure of merit capable of predicting optimal bias conditions and geometries needing only the data from a static bias study from FEA. The material level magnetomechanical coupling factor is chosen as the foundation for the figure of merit. The figure of merit is then augmented to include structure level information regarding the demagnetizing field and the non-uniform stress distribution. The main results presented are the effects of including demagnetization and stress distributions, and most importantly the ability of the metric to predict the change in magnetization of the device. It is shown that for aspect ratios greater than roughly 2.5 the metric trends the same as the change in magnetization predicted by finite element simulations. The region of disagreement between the metric and the fully resolved finite element simulation is explained by tying back to underlying assumptions made during the formulation of the magnetometric demagnetization factor used in the analysis. The case is made for the figure of merit to be included in the analysis of strain-mediated antennas for its ability to find optimum designs while reducing the overall simulation run-time by an order of magnitude.
- Dispersion Curve Estimation for Longitudinal Rail Stress MeasurementCorbin, Nicholas Allen (Virginia Tech, 2021-08-13)There currently exists no reliable, non-destructive method for measuring stress in railroads and other similar structures without the need for a calibration measurement. Major limitations which have hindered previous techniques include sensitivity to boundary conditions, insensitivity to stress, and intolerance for material and geometry uncertainty. In this work, a technique is developed which seeks to solve these challenges by extracting the spectrum relation, or dispersion curve, of a waveguide from dispersive wave propagation meaasurements. The technique is based on spectral analysis of waves in structures modeled as beams, and as such is based on relatively low frequency vibrations, as opposed to other techniques which use nonlinear elastic modeling of structures at ultrasonic frequencies. The major contribution of this work is the development of a frequency-domain based signal processing technique which is capable of compensating for the dispersive, long wavelength reflections which have limited the ability of previous techniques to go low enough in frequency to achieve high stress sensitivity. By compensating for reflections, the present work is able to automate the process of analyzing wave propagation signals such that the entire dispersion curve can be extracted, enabling the identification of various parameters including stress, stiffness, density, and other material and geometry properties. This in turn enables measuring stress, performing model-updating for material and geometry uncertainty, and being indifferent to boundary conditions. The theory and algorithmic implementation is presented, along with simulations and experimental validation on a rectangular beam.
- Multi-component Elastomer Composites for Next Generation Electronics and MachinesBarron III, Edward John (Virginia Tech, 2023-12-14)Multi-component soft materials offer innovative solutions for traditional and emerging technologies by possessing unique combinations of tunable functionality and adaptive mechanical response. These materials often incorporate functional inclusions such as metals or ceramics in elastomers to create deformable composite structures with high thermal or electrical conductivities, magnetic material response, or stimuli-responsive shape and rigidity tuning. In recent years, these materials have become enabling for wearable electronics and soft machines which has led to the development of new material architectures that provide advanced functionalities while maintaining a low mechanical modulus and high extensibility. In this work, we develop methods for the fabrication and utilization of advanced material architectures which integrate room temperature liquid metals (LM), low melting point alloys (LMPA), and magnetic powders and fluids with soft elastomers to introduce multifunctionality to electronic and machine systems. LM-elastomer composites which have high thermal and electrical conductivities are enabling for heat transfer applications and soft, extensible wiring for wearable electronics and soft robots. These materials have been utilized to create emerging devices such as electronics that are capable of improving human health and efficiency, as well as robots capable of adapting their functions based on environmental need. One possible area where LM composites could be applied is in marine environments, where wearable electronics can improve safety for divers, and soft machines could be utilized for underwater exploration. In Chapter 2, we provide the first study to quantify the effects of underwater aging in freshwater and saltwater environments on the important mechanical and functional properties of LM composites for long-term underwater use. It is found that LM composites are largely resistant to changes in their mechanical properties, as well as both thermal and electrical functionality due to long-term underwater aging. In Chapter 3, we introduce a new chemical approach for the tough bonding of LM composites to diverse substrates, which increases adhesion by up to 100x, improving the integration of these materials with rigid electronics. It is shown that the fracture energy and thermal conductivity of these materials can be tuned by controlling the size and volume loading of the LM inclusions. The utility of this method is then shown through the permanent bonding of LM composites to rigid electronics for use as thermal interface materials. \\ Chapter 4 introduces a multi-component shape morphing material that leverages an LMPA endoskeleton and soft LM resistive heaters to produce rapid (< 0.1 s) and reversible shape change. The morphing material utilizes a unique 'reversible plasticity' mechanism enabled by patterned kirigami cuts that allows for instantaneous shape fixing into load bearing shapes without the need for sustained power. The material properties are enabling for the creation of shape morphing robots, which we show through by integration of on board power and control to create a multi-modal morphing drone capable of land and air transport as well as through an underwater machine that can be reversibly deployed to obtain cargo. For magnetic elastomers, the magneto-mechanical properties of state-of-the-art magnetorheological elastomers (MREs) with diverse structures are studied. These materials have long been studied for their ability to rapidly tune stiffness in the presence of a magnetic field. Chapter 5 introduces a new form of hybrid MRE material architecture which utilizes a combination of magnetic powders and fluids to achieve high magnetic permeability and low stiffness for wearable electronic applications. The zero-field magneto-mechanical properties of MREs with rigid particles, magnetic fluids, and a combination of the two are studied. The inclusions are modeled through an Eshelby analysis which demonstrates magnetic fluids can be utilized to increase magnetic response while decreasing the stiffness of the composite material. The stiffness tuning capabilities of these material architectures are then explored in Chapter 6, where we introduce a predictive model that captures the stiffness tuning response of MREs across diverse microstructures and compositions. This model guides the creation of materials with rapid (~ 20 ms) and extreme stiffness tuning (70x) which we utilize to create a soft adaptive gripper capable of handling objects of diverse geometries.
- Nonlinear one-dimensional constitutive model for magnetostrictive materialsImhof, Alecsander N.; Domann, John P. (2022-02-08)This paper presents an analytic model of one dimensional magnetostriction. We show how specific assumptions regarding the symmetry of key micromagnetic energies (magnetocrystalline, magnetoelastic, and Zeeman) reduce a general three-dimensional statistical mechanics model to a one-dimensional form with an exact solution. We additionally provide a useful form of the analytic equations to help ensure numerical accuracy. Numerical results show that the model maintains accuracy over a large range of applied magnetic fields and stress conditions extending well outside those produced in standard laboratory conditions. A comparison to experimental data is performed for several magnetostrictive materials. The model is shown to accurately predict the behavior of Terfenol-D, while two compositions of Galfenol are modeled with varying accuracy. To conclude we discuss what conditions facilitate the description of materials with cubic crystalline anisotropy as transversely isotropic, to achieve peak model performance.
- Optimal Blast-Resistant Sandwich Structures with Transversely Isotropic, Elasto-plastic Polymeric Foams as CoresKim, Dong Ho (Virginia Tech, 2023-01-26)Polymeric foam cores are widely used as core materials in sandwich panels subject to blast loads, where high strain rates of the order of 4000 /s are observed. Unlike metallic foams polymeric foams exhibit transversely isotropic response when tested in a laboratory setting. More specifically, they exhibit different hardening along the foam thickness than that in a direction transverse to the thickness. Furthermore, polymeric foams harden differently in tension and compression. In this thesis we adopt ideas from the constitutive model developed by Hoo Fatt et al. cite{hoofatt2}, which captures strain hardening, transverse isotropy and distinguishes the response in tension and in compression, to include isotropic strain rate hardening in our constitutive model. A one dimensional prototype of the model is used to aid in the physical explanation of various variables, and the model is generalized to three dimensions. The material model is implemented as a VUMAT (user defined) subroutine in the commercial finite element software ABAQUS Explicit. We show that the model works robustly in uniaxial deformations as well as in sandwich problems using the test data available in the literature. We provide values of the 39 material parameters for H45, H60, H80, H100, H130 and H200 foams. The constitutive relation is utilized in an optimization problem in which the surrogate optimizer is utilized to minimize the backface deflection of a blast loaded clamped sandwich plate of a fixed mass. The core in the optimized sandwich structure has a stratified configuration (not functionally graded) and has 24% less maximum back face deflection as compared to that in which the six core layers vary from highest density to lowest density or vice a versa. For a sandwich panel subject to a blast load, when the strain rate hardening effect are neglected, we observed a 12% reduction in the predicted peak deflection from that when strain rate effects are considered. It is counter intuitive and needs further investigation.
- Transport geometry of the restricted three-body problemFitzgerald, Joshua T. (Virginia Tech, 2023-07-05)This dissertation expands across three topics the geometric theory of phase space transit in the circular restricted three-body problem (CR3BP) and its generalizations. The first topic generalizes the low energy transport theory that relies on linearizing the Lagrange points in the CR3BP to time-periodic perturbations of the CR3BP, such as the bicircular problem (BCP) and the elliptic restricted three-body problem (ER3BP). The Lagrange points are no longer invariant under perturbation and are replaced by periodic orbits, which we call Lagrange periodic orbits. Calculating the monodromy matrix of the Lagrange periodic orbit and transforming into eigenbasis coordinates reveals that the transport geometry is a discrete analogue of the continuous transport geometry in the unperturbed problem. The second topic extends the theory of low energy phase space transit in periodically perturbed models using a nonlinear analysis of the geometry. This nonlinear analysis relies on calculating the monodromy tensors, which generalize monodromy matrices in order to encode higher order behavior, about the Lagrange periodic orbit. A nonlinear approximate map can be obtained which can be used to iterate initial conditions within the linear eigenbasis, providing a computationally efficient means of distinguishing transit and nontransit orbits that improves upon the predictions of the linear framework. The third topic demonstrates that the recently-discovered "arches of chaos" that stretch through the solar system, causing substantial phase space divergence for high energy particles, may be identified with the stable and unstable manifolds to the singularities of the CR3BP. We also study the arches in terms of particle orbital elements and demonstrate that the arches correspond to gravity assists in the two-body limit.