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Engineered Damping: From Mitigating Impacts Through Hierarchical Structured Materials to Enhancing Actuation Forces via Non-Hermitian Metamaterials.

Record Type:	Electronic resources : Monograph/item
Title/Author:	Engineered Damping: From Mitigating Impacts Through Hierarchical Structured Materials to Enhancing Actuation Forces via Non-Hermitian Metamaterials./
Author:	Gupta, Abhishek.
Published:	Ann Arbor : ProQuest Dissertations & Theses, : 2024,
Description:	210 p.
Notes:	Source: Dissertations Abstracts International, Volume: 86-02, Section: B.
Contained By:	Dissertations Abstracts International86-02B.
Subject:	Mechanics. -
Online resource:	https://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=31490021
ISBN:	9798383599518

Engineered Damping: From Mitigating Impacts Through Hierarchical Structured Materials to Enhancing Actuation Forces via Non-Hermitian Metamaterials.
Gupta, Abhishek.

Engineered Damping: From Mitigating Impacts Through Hierarchical Structured Materials to Enhancing Actuation Forces via Non-Hermitian Metamaterials. - Ann Arbor : ProQuest Dissertations & Theses, 2024 - 210 p.

Source: Dissertations Abstracts International, Volume: 86-02, Section: B.

Thesis (Ph.D.)--The University of Wisconsin - Madison, 2024.

This thesis focuses on two interdependent research thrusts: (i) investigating the fundamental structure-property-function relationship in hierarchically architected vertically aligned carbon nanotube (VACNT) foams for mitigating impacts in sports and military that could cause traumatic brain injury, and (ii) investigating the formation of exceptional points (EP) in passive non-Hermitian metamaterials with viscoelastic damping, and experimentally demonstrating the enhancement of an applied actuation force in the vicinity of an EP. Both research thrusts are centered around utilizing engineered damping to create novel materials and structures suitable for extreme engineering applications.VACNT foams are renowned for their exceptional energy absorption capabilities, which rival those of stochastic metallic foams, while they are lightweight with mass densities comparable to polymeric foams. However, transitioning these foams from the laboratory to engineering applications requires addressing fundamental knowledge gaps in the structure-property-function relationship. We performed a comprehensive structural and mechanical characterization of VACNT foams using synchrotron X-ray scattering, quasistatic compression, stress-relaxation, and broadband dynamic mechanical analysis (DMA).When subjected to quasistatic compression, VACNT foams exhibit a preconditioning effect---a softening in the constitutive stress-strain response with increasing compression cycles. Contrary to the prevailing belief that the preconditioning effect arises from mesoscale reorganization of fibers, the synchrotron X-ray scattering experiments we performed as function of the compression cycles revealed that the preconditioning occurs due to a nanoscale permanent strain induced in the walls of multi-walled CNTs, in addition to mesoscale rearrangement of fibers. The range of experiments---from stress-relaxation and quasistatic compression to dynamic mechanical analysis we performed across broad frequency, amplitude, and pre-strain levels---reveal that the VACNT foams exhibit strain-rate-independent behavior. As the viscoelastic models typically used for damping are unsuitable for modeling VACNT foams, we have developed a rate-independent model based on frictional damping, incorporating springs and frictional elements to represent the stiffness of nanotube bundles and van der Waals frictional interactions between nanotubes. This model effectively captures both the quasistatic and dynamic mechanical behavior of VACNT foams, providing a physics-based accurate representation compared to existing models.To enhance the specific energy absorption in VACNT foams, we further introduce mesoscale architectures of thin close packed cylinders, concentric cylinders, and nested fractal cylinders, which exploit the structural characteristics of the additional hierarchy as well as the interactive fiber morphology. We discovered a synthesis-induced size effect that promotes more vertical alignment and denser packing of CNTs in confined regions that lead to significantly enhanced specific energy absorption. Along with this size-effect, tailoring the architecture's geometric parameters in the concentric cylinders allowed us to achieve a desirable linear density-dependent scaling of mechanical properties in VACNT foams. This linear scaling enables achieving much low density foams without significantly reducing their mechanical properties. As designing protective materials require not only the consideration of mechanical properties of the materials, but also the protective pad's geometry, we have developed a scale-free dimensional analysis-guided framework that facilitates the design of thin and lightweight energy-absorbing materials that can effectively absorb kinetic energy while limiting acceleration and compressive strain within specified desirable limits. Our findings and the analytical design framework not only enhance the fundamental understanding of VACNT foams and similar fibrous structured materials but also open up new avenues for the design of architected materials for various protective applications.On contrary to the above utility of energy absorbing materials for impact mitigation, the recent emergence of non-Hermitian physics and the notion of EPs in elastodynamics has promoted damping as a novel design element for creating dynamical symmetries to achieve novel functionalities. While sharp orthogonal bifurcation of eigenmodes at the EP has been utilized to create hyper-sensitive sensors, their realization require gain, typically achieved through active feedback control mechanisms. In our study, we show that viscoelastic materials respecting the Kelvin-Voigt fractional derivative model exhibit a desirable, nearly frequency-independent loss tangent. Such materials used as a non-Hermitian element in an entirely passive dynamical systems can still lead to a desirable sharp bifurcation at an EP. Additionally, we experimentally demonstrate the formation of an EP in a passive elastodynamic system and its ramification in achieving an unusual enhancement of an applied actuation force. Our EP-based actuation enhancement boosts the conventional Purcell amplification by a universal factor of 2 while maintaining the quality (Q-factor) of the signal at a constant value. These findings open up new possibilities for utilizing viscoelastic materials to realize EPs in passive elastic systems and exploit them not only for hypersensitive sensing, but also to enhance actuation in a new class of mechanical indenters and robotic actuators.

ISBN: 9798383599518Subjects--Topical Terms:

525881
Mechanics.
Subjects--Index Terms:

Exceptional points

Engineered Damping: From Mitigating Impacts Through Hierarchical Structured Materials to Enhancing Actuation Forces via Non-Hermitian Metamaterials.
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520 $a This thesis focuses on two interdependent research thrusts: (i) investigating the fundamental structure-property-function relationship in hierarchically architected vertically aligned carbon nanotube (VACNT) foams for mitigating impacts in sports and military that could cause traumatic brain injury, and (ii) investigating the formation of exceptional points (EP) in passive non-Hermitian metamaterials with viscoelastic damping, and experimentally demonstrating the enhancement of an applied actuation force in the vicinity of an EP. Both research thrusts are centered around utilizing engineered damping to create novel materials and structures suitable for extreme engineering applications.VACNT foams are renowned for their exceptional energy absorption capabilities, which rival those of stochastic metallic foams, while they are lightweight with mass densities comparable to polymeric foams. However, transitioning these foams from the laboratory to engineering applications requires addressing fundamental knowledge gaps in the structure-property-function relationship. We performed a comprehensive structural and mechanical characterization of VACNT foams using synchrotron X-ray scattering, quasistatic compression, stress-relaxation, and broadband dynamic mechanical analysis (DMA).When subjected to quasistatic compression, VACNT foams exhibit a preconditioning effect---a softening in the constitutive stress-strain response with increasing compression cycles. Contrary to the prevailing belief that the preconditioning effect arises from mesoscale reorganization of fibers, the synchrotron X-ray scattering experiments we performed as function of the compression cycles revealed that the preconditioning occurs due to a nanoscale permanent strain induced in the walls of multi-walled CNTs, in addition to mesoscale rearrangement of fibers. The range of experiments---from stress-relaxation and quasistatic compression to dynamic mechanical analysis we performed across broad frequency, amplitude, and pre-strain levels---reveal that the VACNT foams exhibit strain-rate-independent behavior. As the viscoelastic models typically used for damping are unsuitable for modeling VACNT foams, we have developed a rate-independent model based on frictional damping, incorporating springs and frictional elements to represent the stiffness of nanotube bundles and van der Waals frictional interactions between nanotubes. This model effectively captures both the quasistatic and dynamic mechanical behavior of VACNT foams, providing a physics-based accurate representation compared to existing models.To enhance the specific energy absorption in VACNT foams, we further introduce mesoscale architectures of thin close packed cylinders, concentric cylinders, and nested fractal cylinders, which exploit the structural characteristics of the additional hierarchy as well as the interactive fiber morphology. We discovered a synthesis-induced size effect that promotes more vertical alignment and denser packing of CNTs in confined regions that lead to significantly enhanced specific energy absorption. Along with this size-effect, tailoring the architecture's geometric parameters in the concentric cylinders allowed us to achieve a desirable linear density-dependent scaling of mechanical properties in VACNT foams. This linear scaling enables achieving much low density foams without significantly reducing their mechanical properties. As designing protective materials require not only the consideration of mechanical properties of the materials, but also the protective pad's geometry, we have developed a scale-free dimensional analysis-guided framework that facilitates the design of thin and lightweight energy-absorbing materials that can effectively absorb kinetic energy while limiting acceleration and compressive strain within specified desirable limits. Our findings and the analytical design framework not only enhance the fundamental understanding of VACNT foams and similar fibrous structured materials but also open up new avenues for the design of architected materials for various protective applications.On contrary to the above utility of energy absorbing materials for impact mitigation, the recent emergence of non-Hermitian physics and the notion of EPs in elastodynamics has promoted damping as a novel design element for creating dynamical symmetries to achieve novel functionalities. While sharp orthogonal bifurcation of eigenmodes at the EP has been utilized to create hyper-sensitive sensors, their realization require gain, typically achieved through active feedback control mechanisms. In our study, we show that viscoelastic materials respecting the Kelvin-Voigt fractional derivative model exhibit a desirable, nearly frequency-independent loss tangent. Such materials used as a non-Hermitian element in an entirely passive dynamical systems can still lead to a desirable sharp bifurcation at an EP. Additionally, we experimentally demonstrate the formation of an EP in a passive elastodynamic system and its ramification in achieving an unusual enhancement of an applied actuation force. Our EP-based actuation enhancement boosts the conventional Purcell amplification by a universal factor of 2 while maintaining the quality (Q-factor) of the signal at a constant value. These findings open up new possibilities for utilizing viscoelastic materials to realize EPs in passive elastic systems and exploit them not only for hypersensitive sensing, but also to enhance actuation in a new class of mechanical indenters and robotic actuators.
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