Doctor of Philosophy in Engineering, Cleveland State University, 2024, Washkewicz College of Engineering

Nonlinear hereditary inelastic deformation behavior can occur in many materials utilized at elevated service temperatures. This behavior can include creep, rate sensitivity, and plasticity. Accurate assessments of nonlinear stress and deformation behavior are important in predicting the operational life and overall performance of critical engineering systems. Inelastic constitutive models have been developed and deployed to meet these assessment needs. The models must predict nonlinear behavior under complex thermomechanical load paths. This includes capturing phenomena such as Bauschinger's effect, cyclic softening and hardening, stress relaxation, and ratcheting when present in high-temperature applications. This dissertation provides a literature review that details the development of inelastic constitutive modeling as it relates to polycrystalline materials. This review distinguishes between inelastic constitutive models that account for nonlinear behavior at the microstructural level, time independent classic plasticity models, and time-dependent unified models. Emphasis is placed on understanding the underlying theoretical framework for unified viscoplasticity models where creep and classical plasticity behavior are considered the result of applied boundary conditions instead of separable rates representing distinct physical mechanisms. This review also discusses recent topics in constitutive modeling that offer new techniques that bridge the gap between the microstructure and the continuum. Focus has been given to material science models that physically explain nonlinear behavior at the microstructural level. An understanding of material microstructure is always necessary in developing accurate multiaxial continuum-level constitutive models that characterize the responses of engineering components modeled with continuum-level perspective. Many forms of inelastic constitutive models are presented that include differential formulation (open full item for complete abstract)

Committee: Josiah Owusu-Danquah (Committee Chair); Stephen Duffy (Committee Member); Andrew Resnick (Committee Member); Jerzy Sawicki (Committee Member); Nigamanth Sridhar (Committee Member); Anne Campbell (Committee Member) Subjects: Aerospace Engineering; Aerospace Materials; Civil Engineering; Materials Science; Mechanics; Nuclear Engineering

Doctor of Philosophy, University of Akron, 0, Biomedical Engineering

The tricuspid valve, which is located on the right side of the heart, prevents blood backflow from the right ventricle to the right atrium. Regurgitation in this valve occurs when its leaflets do not close normally. Tricuspid valve regurgitation is one of the most common tricuspid valve dysfunctions, often requiring valve repair or replacement. The long-term success rate of the repair surgeries has not been promising; in many cases, reoperations are required within a few years after the first surgery. A limiting factor in understanding the etiology of tricuspid valve repair failure is our lack of knowledge regarding tricuspid valve biomechanics. In particular, tricuspid valve mechanical behavior has not been accurately studied. In addition, there is no precise analytical and/or computerized model to predict the mechanical responses of the valve under normal and pathological conditions. In the current study, we have used biaxial tensile testing, small angle light scattering, ex-vivo passive heart beating simulation, and sonomicrometry techniques to quantify the mechanical characteristics, microstructure, dynamic deformations, and geometric parameters of the tricuspid valve. We aimed to develop a more accurate computerized model of the tricuspid valve for simulation purposes. Our studies are important both for understanding the normal valvular function as well as for development/improvement of surgical procedures and medical devices.

Committee: Rouzbeh Amini Dr. (Advisor); Brian Davis Dr. (Committee Member); Ge Zhang Dr. (Committee Member); Francis Loth Dr. (Committee Member); Rolando Ramirez Dr. (Committee Member) Subjects: Anatomy and Physiology; Biomechanics; Biomedical Engineering; Biomedical Research; Engineering; Surgery; Technology

MS, University of Cincinnati, 2003, Engineering : Aerospace Engineering

A 3D micromechanical model has been developed for Plain Weave Composites (PWC) and implemented in the explicit finite element software DYNA3D. The model accounts for the strain rate dependency, inherent material nonlinearity, and progressive failure of constituents of PWC. The micromechanical equations have been obtained for a Representative Volume Cell (RVC) which is assumed to represent the behavior of a PWC lamina. The model implemented in DYNA3D can be used for the simulation of the mechanical behavior of PWC structures under various loads such as multi-axial and impact. The yarns were assumed to be transversely isotropic till initial failure. A viscoplastic constitutive model was used for the resin constituent as it was the primary reason for the rate dependency of PWC. The nonlinear behavior in shear was modeled by updating the shear moduli of the constituents based on their current stress state. Progressive failure was modeled by defining a set of maximum strain criteria for detecting failure in constituents and degrading the properties depending on the failure mode. The implemented model was validated in different loading conditions by comparing its prediction with experimental results available in the literature. Good correlation was observed between the predicted and the experimental results.

Committee: Dr. Ala Tabiei (Advisor) Subjects:

Master of Science, Miami University, 2011, Computational Science and Engineering

Rate-sensitivity, creep and relaxation behavior of medical grade polymers has been investigated experimentally along with an assessment of different constitutive models. Two types of modified common biomedical material A and B were tested. All materials exhibited rate-sensitivity and rate-reversal behavior during creep and relaxation testing with prior loading and unloading histories for tensile and compressive tests. Numerical modeling was performed through a modified version of the Viscoplasticity Based on Overstress (VBO) model. Simulation and prediction results with good quantitative and qualitative agreement were produced for all materials in tension and compression loading. Parameter fitting using the Hybrid and Three Network Models in PolyUMod software generated material constants for ABAQUS, and the finite element results for a knee joint were verified against experimental data. The parameter fitting was unable to produce acceptable results for MATERIAL C. Implementation of VBO into the PolyUMod library is recommended to enhance modeling capability.

Committee: Fazeel Khan (Advisor); James Moller (Committee Member); Gregory Reese (Committee Member) Subjects: Biomedical Engineering; Materials Science; Mechanical Engineering

Doctor of Philosophy, Case Western Reserve University, 2025, Civil Engineering

This thesis presents an in-depth experimental study on the effect of specimen size on the fracture behavior of geometrically similar concrete notched beams. The research, which is one of the most extensive in this field, tests beams of five different depths. Sixty notched beams were subjected to a three-point bending (TPB) test setup, adhering to the draft recommendations of the ACI/ASCE 446 Technical Committee. The study examines beams with depths of 75 mm, 150 mm, 250 mm, 500 mm, and 1000 mm, and widths of 75 mm and 150 mm. It explores how the width affects the results of concrete notched beam tests and examines variations in TPB test rates to assess their impact on fracture responses. The stress at peak load ($\upsigma_N$) for each depth was plotted on a double logarithmic scale, and the results were compared to Professor Bazant's size-effect law (B-SEL) to analyze the size effect. A new test setup was introduced for larger specimens, and the load responses, peak loads, failure modes, fracture energy, and size-effect analysis are presented. The study utilized digital image correlation (DIC) technology to investigate the fracture behavior of beams with varying depths. DIC was applied to one side of the specimens, providing detailed fracture behavior insights. Fracture energy was measured using the work-of-fracture method, and comparisons between linear variable differential transformer (LVDT) and DIC data were discussed. The fracture process zone (FPZ), which represents the area of tensile softening in concrete, was analyzed using DIC displacement and strain data. This analysis covered the FPZ's length, width, and neutral axis location across different specimen depths. Additionally, a new method to determine the critical crack opening $w_f$ using DIC data was proposed, and variations in $w_f$ and FPZ length along the notch ligament were examined. The evolution and width of the FPZ were analyzed through horizontal strain contour plots. Finally, the cracked hi (open full item for complete abstract)

Committee: Dr. Christian Carloni (Committee Chair); Dr. John Lewandowski (Committee Member); Dr. Xiong (Bill) Yu (Committee Member); Dr. Tommaso D'antino (Committee Member); Dr. Hyoung Suk Suh (Committee Member) Subjects: Civil Engineering; Materials Science; Mechanics

Doctor of Philosophy, The Ohio State University, 2023, Mechanical Engineering

As emission standards of passenger vehicles become more and more strict, automotive manufacturers are seeking lightweight solutions to increase vehicle's fuel economy. Fibrous reinforced polymers (FRPs) are known to have high strength to weight ratios, and thus, made them good candidates for the application in the automotive industry. FRP is a composite material made of a polymer matrix reinforced with fibers. The inhomogeneity, anisotropy, visco-elasticity/plasticity, mechanical degradation due to temperature/damage, brittleness characteristics of the unidirectional FRP composites bring challenges to determine mechanical responses both experimentally and numerically. In this dissertation, mechanical behavior of unidirectional carbon fiber reinforced polymer (CFRP) made of Toray T700S carbon fiber and G83-CM prepreg system is studied. Specimens are fabricated from 8-ply and 16-ply CFRP plates. An experimental series is performed including tension, compression, and shear coupon tests at various strain rates ranging from 0.001 to 1000 s-1. Anisotropy is studied by conducting tension, compression, and shear coupon tests in different fiber orientations. Thermal dependence of the material is investigated by performing coupon tests under temperatures ranging from 25 °C to 120 °C. CFRP has been found that loading in one direction can potentially lead to damage in other directions. Thus, coupled, and uncoupled damage testing is performed to characterize such behavior. Digital image correlation (DIC) is applied for deformation and strain measurement on the surface of the specimens. The coupon test data and damage test data are used to calibrate the deformation and damage sub-models of the constitutive material model, *MAT_COMPOSITE_TABULATED_PLASTICITY_DAMAGE, also called *MAT_213, in LS-DYNA. The deformation sub-model predicts elasto-plastic behavior, and it uses a strain-hardening-based orthotropic yield function with a non-associated flow rule extended from Tsai-Wu fail (open full item for complete abstract)

Committee: Amos Gilat (Advisor); Prasad Mokashi (Committee Member); Kelly Carney (Committee Member); Jeremy Seidt (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, University of Akron, 2021, Civil Engineering

Numerous constitutive models have been developed over the past 5 decades for modeling voided materials under different types of loadings and tests. Since the effective use of these materials rests on the comprehensive understanding of the factors which influence their performance under various practical conditions, different experimental and computational efforts have been made towards such exercise. In this research many aspects of pressure sensitive materials, such as material behavior and its performance in a practical application are studied using a newly developed three-dimensional general model with an eye towards civil engineering voided materials such as different types of soils and concrete. Firstly, the integral part of this study lies in the suitable mathematical formulation of the voided materials model for easy implementation, exact computation and effective utilization in commercial finite element codes. In particular, to embrace all the different levels of complexities regarding the voided materials, i.e., from material point characterization to large-scale analyses of devices, the constitutive model is established for both implicit and explicit-time integration algorithms. This allows smooth adaptation of the model for wide range of initial/boundary value problems, i.e., from simple to complex cases without any severe computational cost and unnecessary loss of accuracy. Secondly, the formulated model is calibrated to characterize a number of the essential responses seen among the different types of voided materials under different loading conditions. These aspects are heavily depended on the availability of experimental data for the various types of voided materials, such as different types of soils and concrete. Finally, the more challenging behavior of these materials is that the shear strength of these materials is dependent on several factors, such as the confining pressure, over consolidation ratio, loading rate, permeability of a particul (open full item for complete abstract)

Committee: Atef Saleeb (Advisor); Ala Abbas (Committee Member); Wieslaw Binienda (Committee Member); Nariman Mahabadi (Committee Member); Jun Ye (Committee Member) Subjects: Civil Engineering

Doctor of Philosophy, University of Akron, 2020, Mechanical Engineering

The objective of this research was to develop a constitutive model for Divinycell PVC H100 foam in order to accurately predict its mechanical behaviors under triaxial crushing. Nowadays, sandwich panels with polymer foam core are widely used because of its low weight-stiffness ratio and high energy absorption capability. Many of these applications require the sandwich panels to have curvature to some extent which causes triaxial stress state in the foam. In this research, a 3D anisotropic elastic-plastic-viscoelastic-damage model was developed to predict the multiaxial crushing behavior of polymer foams used in the core of sandwich structures. This model was based on previous pressure vessel experiments on Divinycell H100, whereby the post-yield response of the foam was characterized by anisotropic hardening during plastic flow, as well as damage and viscoelastic hysteresis. By assuming Tsai-Wu plasticity, post-yield properties from only uniaxial compression/tension and simple shear material responses were used to develop a three-dimensional material constitutive relationship for the foam. This solution methodology was shown to be very effective in predicting the hysteresis response of the foam under triaxial compression, triaxial compression-tension and triaxial compression-shear. Good agreement was found between the theoretical predictions and experimental results. An ABAQUS user-defined subroutine (VUMAT) for the proposed elastic-plastic-viscoelastic-damage model was implemented. By using the VUMAT to simulate the single element and meshed specimen, the proposed material model was verified to be valid and accurate. The VUMAT was then used to simulate the response of a curved sandwich panel with Divinycell H100 foam core under blast load. The results were used to compare with ABAQUS built-in isotropic crushable foam model. Results showed that the commonly-used isotropic crushable foam model overestimated the stiffness, strength and energy dissipation of th (open full item for complete abstract)

Committee: Michelle Hoo Fatt (Advisor); Xiaosheng Gao (Committee Member); Alper Buldum (Committee Member); Wieslaw Binienda (Committee Member); Lingxing Yao (Committee Member) Subjects: Materials Science; Mechanical Engineering; Mechanics

Doctor of Philosophy (Ph.D.), University of Dayton, 2017, Mechanical Engineering

Modern plasticity models contain numerous parameters that no longer correlate directly to measurements, leading to a lack of uniqueness during parameter identification. This problem is exacerbated when using only uniaxial test data to populate a three-dimensional model. Parameter identification typically is performed after all experiments are completed, and experiments using different loading conditions are seldom conducted for validation. Experimental techniques and computational methods for parameter identification are sufficiently advanced to permit real-time integration of these processes. This work develops a methodology for integrating multiaxial experimentation with constitutive parameter calibration and validation. The integrated strategy provides a closed-loop autonomous experimental approach to parameter identification. A continuous identification process guides the experiment to improve correlation across the entire axial-torsional test domain. Upon completion of the interactive test, constitutive parameters are available immediately for use in finite element simulations of more complex geometries. The autonomous methodology is demonstrated through both analytical and physical experiments on Ti-6Al-4V. The proposed approach defines a framework for parameter identification based on complete coverage of the stress and strain spaces of interest, thereby providing greater model fidelity for simulations involving multiaxial stress states and cyclic loading.

Committee: Robert Brockman (Advisor); Steven Donaldson (Committee Member); Thomas Whitney (Committee Member); Andrew Rosenberger (Committee Member); Reji John (Committee Member) Subjects: Engineering; Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2016, Mechanical Engineering

Conducting polymers (CPs) exhibit coupling between electrochemical and mechanical domains, namely, reversible ion exchange with an electrolyte under an applied electrical voltage causes volumetric changes in the polymer matrix. The goal of this dissertation is to develop precise quantification techniques to assess the kinetics of ion transport in CPs. These techniques are based on the mechanics of ion storage in polypyrrole doped with dodecylbenzene sulfonate (PPy(DBS)). In this work, it is postulated that CP response is dictated by the driving force for ion ingress and the accessible ion storage sites in the polymer. Two mechanistic models are founded on this premise: (1) A mathematical constitutive model is derived from the first law of thermodynamics to describe the chemomechanically coupled, structure dependent, input-output relationship in PPy(DBS). The uniqueness of this model is that mechanical expansion of the polymer is predicted without the incorporation of empirical coefficients. (2) A kinetic model is proposed to describe the current and charge response of PPy(DBS) to a step voltage input. The transfer-function based approach used to validate this model offers advantages over traditional lumped parameter models by quantifying the effect of polymer mass and morphology on the magnitude and rate of ion ingress. These metrics are valuable control variables for tuning the performance of CP based sensors, actuators and energy storage devices. This research leads to the development of a calibrated PPy(DBS) sensor for the determination of bulk electrolyte concentration. Additionally, a miniaturized sensor incorporated at the tip of an ultramicroelectrode demonstrates near-field sensing using scanning electrochemical microscopy (SECM) hardware. These electrodes are used in conjunction with shear force imaging to develop a novel imaging technique with potential applications in cell membrane biophysics.

Committee: Vishnu Baba Sundaresan (Advisor); Carlos Castro (Committee Member); Jose Otero (Committee Member); Jonathan Song (Committee Member); Vishwanath Subramaniam (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, University of Akron, 2015, Polymer Engineering

Ability to predict the rheological behavior of materials plays a significant role in polymer manufacturing. Ionomers are a class of polymers that are used in a number of commercial applications. But, as of this moment the only attempts to model the rheology of ionomers were made using a molecular approach. A clear picture of the deformation in transient flows is not available. The problems addressed in this research were to establish an experimental protocol that ensures repeatability and reproducibility of rheology data for ionomers; to understand the changes in the structure of the material during deformation; to study the effect of chemical structure on the rheological behavior of the ionomers, and to propose a constitutive model that describes the rheology of ionomers. Data for the verifying the constitutive model were obtained using a high-molecular weight polystyrene as the starting material for the ionomer. Three different, lightly sulfonated polystyrene ionomers with different levels of sulfonation and three different cations were synthesized and used in the experimental study. Five aspects of the thesis research will be discussed in this talk. • The limits of rheological testing were dictated by the equipment limitations, thermal properties of the materials, and the instabilities that occurred in the rheometrical flow. Due to variations in the microstructure of the ionomer melt and the effect of thermal and deformational history, a complex protocol of testing, using preshearing and material annealing was developed. • Series of strain- and stress-controlled experiments were performed for simple shear and simple elongational flows. Discreet approximations of the linear relaxation spectra were obtained and analyzed to determine the dependency on the ionic group concentration, the temperature and the type of cation. • A constitutive model was developed that consisted of a set of non-linear ordinary differential equations with an additional kinetic equat (open full item for complete abstract)

Committee: Robert Weiss Dr. (Advisor); Arkady Leonov Dr. (Advisor); Kevin Cavicchi Dr. (Committee Member); Avram Isayev Dr. (Committee Member); Trimalai Srivatsan Dr. (Committee Member); Ali Dhinojwala Dr. (Committee Member) Subjects: Materials Science; Polymers

Master of Science, The Ohio State University, 2014, Mechanical Engineering

The ability to accurately computationally predict the end properties of Poly(Ethylene Terephthalate) (PET) based components is of immense use to the packaging industry to reduce product development and lifecycle costs. One activity being undertaken prominently by the industry is the development of PET bottles to maximize the shelf life of beverage-filled bottles. This thesis deals with the development of a material model of PET for use in finite element simulation of blow molding. The mechanical behavior of PET is highly non-linear with temperature dependence, strain-rate dependence, molecular weight (Inherent Viscosity – IV) dependence, strain-state dependence and the tendency when induced by strain to crystallize. Uniaxial compression experiments were conducted on PET samples to characterize the temperature, strain-rate and IV dependence of stress-strain characteristics. The temperature range for the tests was 363K to 383K, the (true) strain rates used were 0.1/s and 1/s and the IVs of samples used were 0.80, 0.86, 0.92 and 0.98. The Dupaix-Boyce (DB) model (Dupaix, 2003) is a complex physically-based material model which can capture the viscoelastic, hyperelastic and plastic aspects of polymer mechanical behavior. This model was fit to the compression test results. Furthermore, uniaxial tension tests were conducted to check the predictive capability of the compression fit DB model in tension. The model under-predicted stress in tension and a different set of constants had to be used to fit the initial portion of the DB model stress strain results to the experimental curves. The temperature range for the tension tests was the same as for the compression tests while the strain rates used were 0.05/s, 0.1/s and 0.425/s engineering strain rate. The IVs used were 0.80, 0.92 and 0.98. A major observation from the uniaxial tension tests was the inability of the DB model to capture drastic strain hardening associated with strain-induced crystallization. Th (open full item for complete abstract)

Committee: Rebecca Dupaix (Advisor); Brain Harper (Committee Member) Subjects: Mechanical Engineering; Polymers

Doctor of Philosophy, The Ohio State University, 2014, Mechanical Engineering

Poly(methyl methacrylate) (PMMA) is a transparent, biocompatible, amorphous thermoplastic with a wide variety of applications. With its relatively low glass transition temperature, Tg, and ability to maintain high aspect ratio features after processing, it is an ideal candidate for the polymer processing technique of hot embossing. Hot embossing takes advantage of the thermomechanical properties of PMMA by applying the die at temperatures greater than Tg. The highly viscous behavior of PMMA at temperatures greater than Tg reduces the required pressure to fully fill the features of the die. After cooling to a temperature less than Tg, PMMA behaves as a solid and maintains the relief shape of the stamp. Despite cooling to temperatures less than Tg, the highly viscoelastic nature of PMMA allows it to recover some of its original shape, or spring-back, after load removal. While these drastic temperature dependent material behaviors are ideal for hot embossing, they make prediction of the final feature shape extremely difficult. Previous simulations have focused on die filling without considering demolding to avoid having to capture these effects. However, without consideration of the spring-back behavior during demolding, it is impossible to know the extent of feature preservation. Therefore, the aim of this work is to develop a large strain constitutive model spanning the glass transition temperature capable of capturing the highly temperature and strain dependent spring-back behavior of PMMA with application in a micro hot embossing simulation. Large strain stress relaxation experiments are used to probe previous constitutive models to find their weaknesses. From this, a new constitutive model is developed to capture the highly temperature and strain dependent relaxation effects. Temperature and strain dependence of spring-back is investigated through modified unconfined recovery tests. Cooling is incorporated into the constitutive model using these results and the (open full item for complete abstract)

Committee: Rebecca Dupaix (Advisor); Soheil Soghrati (Committee Member); Allen Yi (Committee Member); Jose Castro (Committee Member) Subjects: Engineering; Materials Science; Mechanical Engineering

Master of Science (MS), Ohio University, 2013, Mechanical Engineering (Engineering and Technology)

The purpose of this research was to evaluate the mechanical behavior of extruded (UNS-C12200) copper multi-channel tube for HVACR (heating, ventilation, air conditioning and refrigeration) systems. A model was developed to predict the burst pressure of the copper tube. The assumption for the model is based on plane strain plastic deformation to an instant of instability where differential internal pressure is equal to zero. Physical simulations were used to develop a relevant microstructure that is representative of the tube in a manufactured heat-exchanger. To this end, cold rolling was used to simulate post-extrusion straightening and sizing of the tube. A subsequent thermal treatment was performed in a tube furnace to simulate a brazing thermal cycle. Tensile tests were conducted to obtain material data, and to determine material constants for a Voce type constitutive equation. Burst tests were conducted to validate the predictive model. Burst pressures were predicted to within 6% of measured values. The effects from cold working and the simulated brazing cycle were also evaluated in this research.

Committee: Frank Kraft (Advisor) Subjects: Materials Science; Mechanical Engineering; Metallurgy

Master of Science in Engineering (MSEgr), Wright State University, 2011, Mechanical Engineering

The focus of this research is to develop a framework to track damage evolution in a structural model subjected to a fatigue environment. This framework incorporates a micromechanical approach of continuous damage modeling, where damage in a homogenized representative microstructure is introduced at the continuum scale through the material constitutive matrix. In this research, damage in the representative microstructure is simulated utilizing cohesive zone models (CZM) whose properties are a function of the magnitude of applied stresses and the resulting separation. In order to minimize the mesh dependence of the cohesive zone model an adaptive meshing technique is employed. A fatigue simulation is performed to demonstrate the capability of the framework to predict the initiation and evolution of damage.

Committee: Ravi Penmetsa PhD (Advisor); Nathan Klingbeil PhD (Committee Member); Joseph Slater PhD (Committee Member); George Huang PhD (Other) Subjects: Engineering; Mechanical Engineering

MS, University of Cincinnati, 2011, Engineering and Applied Science: Mechanical Engineering

High density polyethylene (HDPE) is a common polymer material that is widely used in industrial applications. While significant amount of efforts have been devoted to understanding the constitutive behavior of HDPE, very little work has been performed to investigate the material response of HDPE at high strain rate and high temperature. The main objective of this research is to develop a constitutive model to bridge this gap by focusing on the non-linear stress-strain behavior in the high strain rate and high temperature range. A series of monotonic uniaxial compressive tests have been conducted at high temperature (100°C) and high strain rate (1/s) to characterize the HDPE behavior. Based on the experimental results, existing hyperelastic material models such as Mooney-Rivlin, Ogden, Arruda-Boyce, are assessed with the use of ABAQUS (a finite element software). Based on extensive comparisons, a new three-dimensional constitutive model for HDPE has been proposed. The constitutive equation integrates the basic mechanisms proposed by Boyce et al. [6] and Shepherd et al. [8]. The total stress is decomposed into an elastic-viscoplastic representation of the intermolecular resistance acting in parallel with a time and temperature dependent network resistance of polymer chains. Material constants involved in the model were calculated by fitting the compressive test results to the proposed constitutive equations. A constitutive solver for the proposed model has been developed. The stress-strain relation resolved from the constitutive model closely matches the corresponding ones from the experiments.

Committee: Dong Qian PhD (Committee Chair); Shepherd Shepherd PhD (Committee Member); Yijun Liu PhD (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2011, Mechanical Engineering

Polymethyl methacrylate (PMMA) is an amorphous thermoplastic used in various industrial applications. PMMA is compatible with human tissues and allows high resolution features to be embossed onto a surface, thus making it highly desirable for use in bio-medical, micro-optics, micro-fluidic devices, electronics, micro-electro-mechanical systems (MEMS), etc. The processes used to fabricate these devices capitalize on the fact that the mechanical behaviour of the polymers changes drastically around the glass transition temperature (Tg). The polymer is deformed at temperatures above the Tg where the material is more fluid-like and then cooled below the Tg where it behaves more like a solid. The changes in physical properties make this temperature regime highly favourable for these warm-temperature deformation processes. The same rationale also makes it more difficult to develop a continuum model which accurately predicts the polymer behaviour with temperature and strain rate dependence across the glass transition temperature. Most of the existing constitutive models do not achieve this task; they either work below or above glass transition, but not in both these regions. Hence, there is a greater need to develop a constitutive model for the polymer that can capture the material behaviour across the glass transition temperature (Tg - 20 to Tg + 60) relevant for hot embossing applications. The aim of this thesis is to develop such a material model for PMMA. First, material characterizations experiments were conducted on PMMA well across its glass transition temperature (Tg). This experimental data along with the existing data in the Dupaix lab was used in developing the material model. In order to develop the new material model for application in hot embossing that will work across the wide range of temperature and strain rates, two existing constitutive models on the polymer PMMA were studied: the Dupaix-Boyce model and the Dooling-Buckley-Rostami-Zahlan model. From the (open full item for complete abstract)

Committee: Rebecca Dupaix PhD (Advisor); Jose Castro PhD (Committee Member); Amos Gilat PhD (Committee Member); Allen Yi PhD (Committee Member) Subjects: Engineering; Mechanical Engineering; Mechanics; Polymers

Doctor of Philosophy, The Ohio State University, 2011, Mechanical Engineering

Springback behavior is an important issue for the automobile industry, particularly with the usage of Advanced High Strength Steels (AHSS). For an accurate prediction of springback, the evolution of Young's modulus and the Bauschinger effect must be considered in the numerical simulations of metal sheet forming processes. The principal objective of this work is to establish a new constitutive model in which the incorporation of nonlinear unloading behavior and the Bauschinger effect enable the model to predict non-proportional hardening and spingback for dual phase steels. Monotonic tension and compression, coaxial tension-compression (T-C), coaxial compression-tension (C-T), and two-stage/non-coaxial tensile tests have been performed for three grades of dual phase steels: DP590, DP780, and DP980. The reverse or second-stage flow curves have three characteristics: reduced yield stress (Bauschinger effect), rapid transient strain hardening over a few percent strain, and long-term or “permanent” softening. The departure of reverse hardening curves from monotonic ones is larger than with other typical sheet forming alloys, presumably because of the large second-phase martensite particles in dual-phase steels. A Modified constitutive model based on the Chaboche approach (M-C) was used to describe above experimental phenomenon. In addition to one or more standard nonlinear components of the back stress, a linear term was added to represent the “permanent” offset of hardening following a stress reversal. The parameters for the model were fit using the monotonic and reverse tensile test results, and the model predictions were then compared with large-strain balanced biaxial bulge test results and with non-coaxial, two-stage tensile tests. All of the effects are captured with reasonable, but not perfect, accuracy. Complex unloading behavior following plastic straining has been reported as a significant challenge to accurate springback prediction. More fundamentally, the na (open full item for complete abstract)

Committee: Robert Wagoner PhD (Advisor); June Key Lee PhD (Committee Member); Stephen Bechtel PhD (Committee Member); Rebecca Dupaix PhD (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2009, Chemical Engineering

In recent years there has been increased interest in the broad areas of micro- and nano-technology due to the potential to create materials with unique properties which were previously unattainable. One area of special interest has been the use of nanoparticles such as nanoclays, nanofibers and carbon nanotubes and microscale carbonyl iron particles. Nanoclays and nanofibers have received attention due to their ability to be incorporated into polymer matrices and impart functionality such as electrical conductivity, increased tensile strength and modulus, and a reduction of gas and moisture permeability at much lower particle loadings when compared to traditional fillers such as carbon black and glass fibers. The addition of these nanoparticles also has a significant effect on the rheological properties of the composite. The rheological behavior of polystyrene/nanoclay composites under steady state shear flow and polystyrene/carbon nanofiber composites under transient shear and uniaxial extension is investigated. A constitutive model is developed that is capable of predicting the shear and extensional rheology of both types of composites and predicts orientation changes to the nanoparticles due to flow. The model is validated through comparison to the experimental rheological measurements of both composite types and experimental measurements of carbon nanofiber orientation in the polystyrene/carbon nanofiber composites under uniaxial extension.The addition of magnetizable carbonyl iron particles to a non-magnetizable carrier fluid has been done to create a smart fluid, known as a magnetorheological fluid, whose rheological properties can be modified through the application of a magnetic field. This added functionality is being utilized in applications such as shock absorbers, dampers, brakes, and clutches. The use of these fluids in engineering applications requires rheological models capable of capturing their complex flow behavior under various flow conditions. Th (open full item for complete abstract)

Committee: Dr. Kurt Koelling (Advisor); Dr. Stephen Bechtel (Committee Member); Dr. L. James Lee (Committee Member) Subjects: Chemical Engineering

Doctor of Philosophy, The Ohio State University, 2007, Biomedical Engineering

Research suggests that in certain types of blunt liver trauma the mechanism of injury is linked to rapid increases in internal pressure within the liver. The objectives of this study were (1) to characterize the relationship between impact-induced pressures and blunt liver injury in an ex vivo organ experimental model; (2) to compare human liver intra-parenchymal pressure and vascular pressure with other biomechanical variables as predictors of liver injury risk; (3) to investigate the feasibility of measuring liver vascular pressure in impacts to pressurized full body post-mortem human subjects (PMHS); and (4) to develop a constitutive model of the mechanical behavior of human liver tissue in blunt impact loading. Test specimens included 19 ex vivo porcine livers, 14 ex vivo human livers, and 2 full body PMHS. Specimens were perfused with normal saline solution at physiological pressures, and a drop tower applied blunt impact at varying energies. Impact-induced pressures were measured by transducers in the hepatic veins and parenchyma (caudate lobe) of ex vivo specimens. Binary logistic regression demonstrated that tissue pressure measured in the parenchyma was the best indicator of serious liver injury risk (p = .002, Pseudo R-square = .78). A peak tissue pressure of 48 kPa was correlated to 50% risk of serious (AIS ≥ 3) liver injury. A burst injury mechanism directly related to hydrostatic pressure is postulated for the ex vivo liver loaded dynamically in a drop test experiment. A constitutive model previously developed for finite strain behavior of amorphous polymers was adapted to model liver stress-strain behavior observed in the ex vivo human liver impacts. The model includes six material properties and captures three features of liver stress-strain behavior in impact loading: (1) a relatively stiff initial modulus; (2) a rate-dependent yield or rollover to viscous “flow” behavior; and (3) strain hardening at large strains. Results of this research could be a (open full item for complete abstract)

Committee: Alan Litsky (Advisor) Subjects: