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1. Design of a fail-safe magnetorheological-based system for three-dimensional earthquake isolation of structures

   https://doi.org/doi.org/10.1016/j.mechatronics.2019.102296  

   Cesmeci, S; Gordaninejad, F; Ryan, KL; Eltahawy, W (October 2019, Mechatronics)

   This study presents a comprehensive design methodology for a magnetorheological-based damper device for a three-dimensional building isolation. The device acts as a suspension system itself by combining the liquid stiffness and controllable magnetorheological damping features in one unit. The bi-linear liquid stiffness feature enhances resistance to global rocking/overturning of the structural system by increasing the stiffness in the rebound mode compared to the compression mode. In the field, the system is combined with the conventional elastomeric bearings widely employed to mitigate the lateral seismic motions. During a seismic event, the system is subjected to dynamic vertical shaking and large lateral forces. The theoretical and simulation modeling to overcome this major challenge and achieve other system requirements are presented. In addition, a comprehensive optimization program is developed to achieve all design requirements. The modeling procedure is verified with experimental results. Also, the effectiveness of Displacement/Velocity-based control for a single degree-of-freedom system subjected to sinusoidal loading is evaluated. 

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2. Designing and Characterizing Negative Stiffness Devices for Apparent Weakening and Vertical Isolation

   https://doi.org/11244/324272  

   Cain, Thomas M. (May 2020, The University of Oklahoma Libraries)

   null (Ed.)

   Nonlinear systems leveraging the effects of negative stiffness can exhibit beneficial qualities for passive seismic mitigation in structures. Such systems can be achieved by placing nonlinear devices displaying negative stiffness in parallel with linear positive stiffness systems such as a structure or spring. This thesis presents research into two such systems: (i) a device which causes apparent weakening in a structure subjected to horizontal ground motions and (ii) an isolation system to protect building contents from vertical seismic effects. Apparent weakening is the softening of a structure’s apparent stiffness by adding negative stiffness to the overall system via negative stiffness devices. Apparent weakening is an elastic effect that has the benefit of reducing the peak accelerations and base shears induced in a structure due to a seismic event without reducing the main structural strength. The smooth negative stiffness device (SNSD) presented in this thesis consists of cables, pulleys, and extension springs. A nonlinear mathematical model of the load-deflection behavior of the SNSD was developed and used to determine the optimal geometry for such a device. A prototype device was designed and fabricated for installation in a bench-scale experimental structure, which was characterized through static and dynamic tests. A numerical study was also conducted on two other SNSD configurations designed to achieve different load-deflection relations for use in an inelastic model building subject to a suite of historic and synthetic ground motions. In both the experimental prototype and the numerical study, the SNSDs successfully produced apparent weakening, effectively reducing accelerations and base shears of the structures. The buckled-strut vertical isolation system (BSVIS) presented in this thesis combines the non-linear behavior of a laterally-loaded buckled strut with a linear spring. The lateral load-deflection relation for a buckled strut, which is nonlinear and displays negative stiffness, was investigated for various conditions to two- and three-term approximations of the deflected shape of a strut. This relation and the linear positive effect of a spring were superimposed to give the load-deflection relation of a BSVIS. An experimental prototype was fabricated and subjected to static tests. These tests confirmed the validity of the model and the effectiveness of adding a spring in parallel with a buckled strut to achieve isolation-level stiffness. Based on the theoretical and experimental findings, a design guide is proposed for the engineering of a BSVIS to protect a payload from vertical seismic content. 

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3. Mesogen Organizations in Nematic Liquid Crystal Elastomers Under Different Deformation Conditions

   https://doi.org/10.1002/smll.202402305  

   Chung, Christopher; Jiang, Huan; Yu, Kai (November 2024, Small)

   Abstract Liquid crystal elastomers (LCEs) exhibit unique mechanical properties of soft elasticity and reversible shape‐changing behaviors, and so serve as potentially transformative materials for various protective and actuation applications. This study contributes to filling a critical knowledge gap in the field by investigating the microscale mesogen organization of nematic LCEs with diverse macroscopic deformation. A polarized Fourier transform infrared light spectroscopy (FTIR) tester is utilized to examine the mesogen organizations, including both the nematic director and mesogen order parameter. Three types of material deformation are analyzed: uniaxial tension, simple shear, and bi‐axial tension, which are all commonly encountered in practical designs of LCEs. By integrating customized loading fixtures into the FTIR tester, mesogen organizations are examined across varying magnitudes of strain levels for each deformation mode. Their relationships with macroscopic stress responses are revealed and compared with predictions from existing theories. Furthermore, this study reveals unique features of mesogen organizations that have not been previously reported, such as simultaneous evolutions of the mesogen order parameter and nematic director in simple shear and bi‐axial loading conditions. Overall, the findings presented in this study offer significant new insights for future rational designs, modeling, and applications of LCE materials. 

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4. Fundamental Dynamics of 3-Dimensional Seismic Isolation

   Eltahawy, W. (January 2017, 16th World Conference on Earthquake Engineering)

   Seismic isolation systems for buildings are generally selected to achieve higher seismic performance objectives, such as continued operation or immediate occupancy following a design earthquake event. However, recent large scale tests have suggested that these objectives may be compromised if the shaking includes large vertical acceleration components that are damaging to the nonstructural components and contents. Some research has been conducted to develop three dimensional isolation systems that can isolate the structure from both the horizontal and vertical components of ground motion. In several cases, systems have been proposed without much justification of the target design parameters. Rocking has been noted as a potential concern for structures with 3D isolation systems, and complex systems have been proposed to control the rocking. In this study, the fundamental dynamic response of structures with 3D isolation systems is explored. Target horizontal and vertical spectra for a representative strong motion site were developed based on NEHRP recommendations, and horizontal and vertical ground motions were selected that best fit the target spectra when the same amplitude scale factor was applied to all three motion components. Using a simple model of a rigid block resting on linear isolation bearings, the following aspects are evaluated for a wide range of horizontal and vertical isolation periods: response modes and severity of rocking, horizontal and vertical displacement demands in the isolation bearings, and attenuation of both horizontal and vertical accelerations in the structure relative to the ground acceleration. Preliminary results point to a number of useful observations. For example, rocking appears to be an issue only if the horizontal and vertical isolation periods are closely spaced. Helical spring isolation systems that have been applied to a few structures have this characteristic. However, if the horizontal isolation period is large relative to the vertical isolation period, troublesome rocking can be avoided. In addition, other researchers have proposed systems with vertical isolation periods on the order of 2 seconds, which require large displacement and damping capacity. However, preliminary results suggest that vertical isolation periods as low as 0.5 seconds will be effective in attenuating the vertical acceleration. Limiting the vertical isolation period will make design of a 3D isolation system more feasible with respect to vertical displacement capacity and avoiding rocking. 

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5. Hyperelastic continuum models for isotropic athermal fibrous networks

   https://doi.org/10.1098/rsfs.2022.0043  

   Song, Dawei; Oberai, Assad A.; Janmey, Paul A. (December 2022, Interface Focus)

   Many biological materials contain fibrous protein networks as their main structural components. Understanding the mechanical properties of such networks is important for creating biomimicking materials for cell and tissue engineering, and for developing novel tools for detecting and diagnosing disease. In this work, we develop continuum models for isotropic, athermal fibrous networks by combining a single-fibre model that describes the axial response of individual fibres, with network models that assemble individual fibre properties into overall network behaviour. In particular, we consider four different network models, including the affine, three-chain, eight-chain, and micro-sphere models, which employ different assumptions about network structure and kinematics. We systematically investigate the ability of these models to describe the mechanical response of athermal collagen and fibrin networks by comparing model predictions with experimental data. We test how each model captures network behaviour under three different loading conditions: uniaxial tension, simple shear, and combined tension and shear. We find that the affine and three-chain models can accurately describe both the axial and shear behaviour, whereas the eight-chain and micro-sphere models fail to capture the shear response, leading to unphysical zero shear moduli at infinitesimal strains. Our study is the first to systematically investigate the applicability of popular network models for describing the macroscopic behaviour of athermal fibrous networks, offering insights for selecting efficient models that can be used for large-scale, finite-element simulations of athermal networks. 

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