Doctor of Engineering, Cleveland State University, 2014, Washkewicz College of Engineering

The cornea is the outermost layer of the human eye where the tissue meets with the external environment. It provides the majority of the eye's refractive power and is the most important ocular determinant of visual image formation. The refractive power of the cornea derives from its shape, and this shape is a function of the ocular biomechanical properties and loading forces such as the intraocular pressure (IOP). With having the majority of refractive power in the eye, the cornea is the primary tissue of interest for refractive intervention. Globally, the predominant mode of surgical treatment of refractive disorders is photoablation. However, optical power regression over time and under/over correction due to neglected corneal biomechanical properties were still observed following refractive procedures including LASIK, PRK, Astigmatic Keratotomy etc especially at high degree corrections. Also, some evolving procedures such as corneal collagen crosslinking (CXL), a collagen stiffening procedure most commonly performed through UVA photoactivation of riboflavin in the corneal stroma, currently lack surgical guidance for optimizing visual outcomes. Thus, there is a need for methods that explore the patient specific treatment planning strategies for refractive procedures. This work will have a potential impact in translating mechanical principles into corneal surgical planning in order to provide a better guidance and predictive environment to the corneal surgeons. The goals of this thesis are three fold: 1) To develop patient specific models from clinical LASIK cases and to compare the outcomes of these models with clinical outcomes in a patient population. 2) To simulate investigational procedures that utilize CXL. 3) To advance a potential approach to characterize corneal mechanical properties in vivo.

Committee: William J. Dupps Jr., MD, PhD (Committee Chair); Nolan Holland PhD (Committee Co-Chair); Ahmet Erdemir PhD (Committee Member); Antonie Van den Bogert PhD (Committee Member); Andrew Resnick PhD (Committee Member); Abhijit Sinha Roy PhD (Committee Member) Subjects: Engineering

MS, University of Cincinnati, 2012, Engineering and Applied Science: Mechanical Engineering

Space-time finite element method provides a robust and accurate alternative to the traditional FEM based on semi-discrete schemes due to its extended capability in establishing approximations in both space and time. The extended capability, however, requires the simultaneous discretization of spatial and temporal domains. This subsequently results in a system of equations that is considerably larger in size than those obtained in the standard finite element formulation. In general, solving equations generated based on the space-time formulation requires substantially more computing time. In some cases, it becomes a bottle neck for practical implementation due to the large number of degrees of freedom. There is thus a need to explore ways to accelerate the procedure for finding a solution to the system of equations in the space-time method. With the recent developments in the use of Graphics Processing Units (GPUs) for general purpose parallel computations (GPGPU), an effort is made in this thesis to explore the possibility of developing a GPU based solver for the space-time finite element method to accelerate the computation. A two-step approach is taken: In the first step, the GPU version of the direct solver based on LAPACK and a preconditioned conjugate gradient method are tested for systems of linear equations involving both dense and sparse matrices. Both methods are shown to significantly accelerate the process of finding the solution. Subsequently, the developed GPU-based algorithms are implemented on the system of linear equations obtained from space-time FEM and enriched space-time FEM. It is reported that GPU-based implementation yields significant speed up. Based on these implementations, it is concluded that the GPU-based system could serve as an effective platform for the space-time method.

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

Doctor of Philosophy, University of Akron, 2024, Electrical Engineering

An axial flux permanent magnet (AFPM) motor can be considered one of the optimal motor designs due to its higher torque density and compact sizing. To date, most AFPM models have adopted surface mounted and spoke type rotor designs for various applications. However, these designs provide poor protection for the magnet. Furthermore, the low saliency ratio of the surface mounted AFPM motors limits their performance in the flux weakening region. On the other hand, magnets in an AFPM can be placed inside well-designed flux barriers to guard against external forces. The advantages of the proposed design can be described in the following ways. First, the flux barriers provide protection for magnets in harsh operating conditions. Second, this rotor structure generates a satisfactory level of reluctance torque which extends the operating region of the motor. Third, the simpler flux barrier simplifies the manufacturing process of the motor and reduces the manufacturing cost. This paper describes the design, optimization and prototype development of a double stator single rotor (DSSR) axial flux interior permanent magnet (AFIPM) motor with a novel “H” shaped flux barrier. Once the design parameters of the rotor and stator have been pointed out, an initial design of the proposed AFIPM has been developed in finite element analysis (FEA) based on the machining requirements and design specifications. However, the initial AFIPM design exhibited lower average torque and higher ripple. As a result, a novel multistage optimization method has been developed to achieve the desired electromagnetic performance. This optimization method, which includes Taguchi orthogonal array, multivariate regression analysis and a genetic algorithm, calculates the optimal design parameters of the motor. Detailed electromagnetic finite element analysis has been executed to compare the performances of the optimized model with the benchmark design. An AFIPM prototype has been developed to verify th (open full item for complete abstract)

Committee: Dr. Yilmaz Sozer (Advisor); Dr. J. Alexis De Abreu Garcia (Committee Member); Dr. Igor Tsukerman (Committee Member); Dr. Xiaosheng Gao (Committee Member); Dr. J. Patrick Wilber (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, University of Toledo, 2022, Engineering

Anterior cruciate ligament (ACL) injury is quite common among young athletes, with the number of injury cases exceeding 120,000 annually in the United States alone. Over 70% of which account for non-contact injuries. Forces and moments acting on the knee joint play essential roles in these injuries. Motions of the other body segments are effective in increasing or decreasing these loads. In this study, the effect of whole-body (WB) kinematics on the knee joint biomechanics was investigated using in vivo and in silico methods. Motion analysis experiments were done on 14 able-bodied young participants wearing a full-body marker set and performing two variations of single-leg landings, using their left and right limbs (4 tasks). Marker trajectories and force plate data were recorded from the in vivo experiments of these participants. The in silico investigations consisted of two separate parts. First, musculoskeletal simulations were done to obtain whole-body kinematics, kinetics, and muscle forces, using inverse kinematics, inverse dynamics, and static optimization techniques. The next part was non-linear dynamic finite element (FE) analyses. A FE dynamic/explicit knee model was developed from medical images of a healthy young female and validated against in vitro experiments for the knee joints kinematics and ligaments strains. Ligaments' material properties for the knee cruciate and collateral ligaments were obtained through optimization to the experimental tensile test data in the literature. Then, the participants' data from musculoskeletal simulations were used as the input to the FE analyses. The FE outputs included ACL strain, knee joint contact forces, contact pressures, and soft tissue stresses. In order to find the relationship between WB kinematics and knee joint biomechanics, correlation analysis was used. Using Spearman correlation coefficients and P-values, the correlation between WB modifiable parameters and knee biomechanics along with their stat (open full item for complete abstract)

Committee: Vijay Goel Dr. (Advisor) Subjects: Biomechanics; Biomedical Engineering

Doctor of Philosophy (PhD), Ohio University, 2022, Civil Engineering (Engineering and Technology)

Composite adjacent box beams have been utilized to construct bridges as accelerated bridge constructions. These bridges are constantly subjected to the effect of a moving load as vehicles passing over them. Dynamic load allowance (DLA) is a reliable approach to show the amount of that vehicle-bridge interaction. The allowable value of the load allowance factor is 1.33 in AASHTO LRFD (2017), the Bridge Design Specifications. Many researchers have shown that the dynamic load allowance (DLA) value of AASHTO LRFD (2017) does not necessarily yield to a satisfactory account for the dynamic response of highway bridges under moving load. In addition, there is no comprehensive data about adjacent box beams bridges regarding the dynamic load allowance (DLA). Therefore, there is a need to perform a detailed dynamic analysis of vehicle-induced vibration to accurately estimate the dynamic load allowance (DLA) values. This dynamic analysis should include the influence of the most important dynamic parameters, which are related to both the vehicle and bridge circumstances. A field assessment platform was implemented; two adjacent concrete box beams bridges that were dynamically evaluated under the effect of the vehicle-bridge interaction. Then, based on the collected field data, the dynamic load allowance (DLA) values were calculated for all installed instruments under the effect of numerous load cases. The influence of vehicle weights, speeds, brakes, and road roughness conditions on the values of the load allowance factor (DLA) was conducted. In addition, a full scale finite element analysis was performed to complement the field investigation by looking at other dynamic parameters. Finite element method was utilized to model the adjacent box beams bridge of Fairmount Rd according to the asconstructed drawing and ODOT bridge design manual. After modeling, the bridge model was calibrated and validated to analytically investigate the effect of other dynamic p (open full item for complete abstract)

Committee: Issam Khoury (Advisor) Subjects: Engineering

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

In this dissertation, we first investigate the dynamic impact response of tubular composite structures with/without honeycomb sandwich core under transverse low- velocity impact (LVI) and compression after impact (CAI) test. Damage mechanisms, such as matrix cracking, delamination and fiber breakage/rupture in face sheets as well as honeycomb crushing and buckling in the core, are characterized by X-ray micro-computed tomography (µCT) to understand failure processes and their relationship with core material and impactor shape. It is found that sandwich core material helps to absorb impact energy and resist localized damage formation. The benefit of core material includes greater energy absorption capability and higher specific CAI strength. The progressive damage events from damage initiation, failure propagation to final collapse of a tubular composite structure during CAI test are also further discussed. Second, we establish a finite element analysis (FEA) model of LVI of composite sandwich structures. The numerical model is validated by comparing with the experimental results in impact response and major failure modes. Results show that energy absorption and impact damages are less for sandwich structure with thinner top face sheet, while the energy absorption percentage of core material is smaller. Results also show that the increment of core thickness can decrease energy absorption efficiency for asymmetric sandwich structures. iv It is also critical to understand buckling and post-buckling behavior of tubular structures which can essentially be affected by the component geometry such as length, diameter and wall thickness. We use both experimental and numerical method to study aluminum and multilayer carbon fiber reinforced plastic (CFRP) tubes under compressive loading, with the aim to extend the knowledge towards extremely large tubular structures. A pin-ended fixture is designed to examine the influence of different support condition (open full item for complete abstract)

Committee: Kwek-Tze Tan (Advisor); Gregory Morscher (Committee Member); Yalin Dong (Committee Member); Qixin Zhou (Committee Member); J. Patrick Wilber (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, University of Toledo, 2020, Engineering

Using prescriptive design approaches, structures are intended to provide a life-safety level of protection that has been shown by recent natural hazard events to have limited contribution to the post-disaster resilience of a community. The performance-based engineering (PBE) methodology allows the structure to be designed to achieve any pre-defined performance objective. The structures of the future will not only aim at being structurally resilient but also sustainable to natural hazard loads. To contribute to the development of these structures, PBE requires the development of state-of-the-art numerical models for the accurate structural performance assessment and the creation of a framework that can effectively account for this performance when evaluating the environmental impacts of structures. This research has two main goals: i) to create state-of-the-art high-fidelity numerical models for the PBE of structures; and ii) to create a multidisciplinary framework for the resilient-based environmental impact assessment of structures subjected to natural hazard loads. In pursuit of this research's goals, four main objectives were conducted: High-Fidelity Numerical Modeling, PBE, Life Cycle Assessment, and Combined PBE and LCA. This research has been primarily conducted on reinforced concrete (RC) and cross laminated timber (CLT) structures, as the first is a traditional and resilient while the second is a newer and seemingly more sustainable structural alternative. However, the created approach can also be applied to other structural alternatives under natural hazard loads. The high-fidelity numerical models created have demonstrated to satisfactorily capture the structural performance of the considered building structure alternatives and the multidisciplinary framework created provides a powerful means for making science-based decisions when considering newer and seemingly more sustainable building structure alternatives while accounting for their natural hazard (open full item for complete abstract)

Committee: Serhan Guner (Committee Chair); Liangbo Hu (Committee Member); Luis Mata (Committee Member); Mark Pickett (Committee Member); Shiling Pei (Committee Member) Subjects: Civil Engineering; Engineering; Environmental Engineering

Doctor of Philosophy, University of Akron, 2018, Mechanical Engineering

This dissertation aims to develop valid numerical approaches to investigate the micromechanics of ductile fracture process and predict the ductile material failure under various loading conditions. As the first portion of this work, a layered unit cell micromechanics model is proposed. This model consists of three void containing material units stacked in the direction normal to the localization plane. Localization takes place in the middle material unit while the two outer units undergo elastic recovery after failure occurs. Thus, a failure criterion is established as the material is considered failure when the macroscopic effective strain of the outer material units reaches the maximum value. Comparisons of the present model with several previous models suggest that the present model is not only easy to implement in finite element analysis but also more suitable to robustly determine the failure strain. A series of unit cell analyses are conducted for various macroscopic stress triaxialities and Lode parameters to investigate the dependency of failure strain on stress state. The analysis results also reveal the effect of the stress state on the deformed void shape within and near the localization band. Additionally, analyses are conducted to demonstrate the effect of the voids existing outside the localization band. Next, the unit cell model is utilized to investigate the effect of hydrogen on ductile fracture demonstrated by its influence on the process of void growth and coalescence. The evolution of local stress and deformation states results in hydrogen redistribution in the material, which in turn changes the material's flow property due to the hydrogen enhanced localized plasticity effect. The result shows that hydrogen reduces the ductility of the material by accelerating void growth and coalescence, and the effect of hydrogen on ductile fracture is strongly influenced by the stress state experienced by the material, as characterized by the stress tr (open full item for complete abstract)

Committee: Xiaosheng Gao Dr. (Advisor); Chang Ye Dr. (Committee Member); Gregory Morscher Dr. (Committee Member); Ernian Pan Dr. (Committee Member); Chien-Chung Chan Dr. (Committee Member) Subjects: Mechanical Engineering

Master of Science, The Ohio State University, 2017, Civil Engineering

Probability risk assessment (PRA) of nuclear power plants (NPP) has been used since the mid-1970s to evaluate the associated risks or perform a risk-informed design of NPPs. Since its inception, PRA has considered both internal and external events to evaluate the risks to a NPP site. However, external event PRA has historically been recognized as having considerable safety margin until recent events have emphasized the need for a reevaluation. This research is part of a larger project with the goal of incorporating internal and external event PRA in a common platform using state-of-the-art methods. Specifically, the focus of the research in this thesis was to develop and evaluate structural models with different levels of complexity for several structures that are vital for seismic probabilistic risk assessment (SPRA) of NPPs. For SPRA, critical structures, systems, and components (SSCs) are investigated to evaluate their risk during seismic events. To evaluate the risk of critical SSCs, structural models are needed to predict their dynamic behavior. However, due to the large number of analyses required during SPRA, simple yet sufficient models are desired to increase the computational efficiency and reduce the run-time of models. As such, the focus of this research was to develop simple yet sufficiently accurate structural models for SSCs and to evaluate the uncertainty related to those models. Three critical NPP structures are investigated in this research to illustrate the capabilities and limitations of models with varying levels of complexity. The structures included a condensate storage tank (CST), auxiliary building, and containment structure. Realistic geometric and material properties for each structure are introduced, and both detailed three-dimensional (3D) and simplified two-dimensional (2D) models are created. Detailed 3D finite element (FE) models incorporated complex mechanical behavior such as fluid- iii structure interaction and slab flexibility (open full item for complete abstract)

Committee: Halil Sezen (Advisor); Abdollah Shafieezadeh (Committee Member); Jieun Hur (Committee Member) Subjects: Civil Engineering

Doctor of Philosophy, University of Akron, 2016, Mechanical Engineering

In this dissertation, a comprehensive study of ductile damage of metallic materials is presented, covering constitutive modeling, numerical implementation and model calibration and verification. As the first part of this dissertation, a pressure-insensitive plasticity model, expressed as a function of the second and third invariants of the stress deviator (J2 and J3), is presented. Depending on whether the power of the J3 term is odd or even, the proposed model can capture either the tension-compression strength-differential (S-D) effect or the torsion-tension strength-differential effect of the material. The plasticity model with an odd power to the J3 item has been calibrated and validated using measured experimental data of a ß-treated Zircaloy-4 with a wide range of triaxiality and Lode parameter values. Results show that this model captures the strong strength-differential (S-D) effect in the material. The plasticity model with an even power to the J3 item is able to capture the isotropic plastic behavior of a stainless steel Nitronic 40, under various stress states with good accuracy and computational efficiency. Next, the effect of the material's plasticity behavior on the ductile damage process is studied by conducting a series of unit cell analyses of a void-containing representative material volume (RMV), where the plastic response of the matrix material is governed by the J2-J3 dependent plasticity model. To simulate the ductile damage process in anisotropic materials, a new constitutive model, which combines the models proposed by Zhou et al. (2014) and Stewart and Cazacu (2011), is developed and employed to study the plasticity and ductile fracture behavior of a commercially pure titanium (CP Ti). In particular, a Gurson-type porous material model is modified by coupling two damage parameters, accounting for the void damage and the shear damage respectively, into the yield function and the flow potential. The plastic anisotropy and tension-compre (open full item for complete abstract)

Committee: Xiaosheng Gao Dr. (Advisor); Yalin Dong Dr. (Committee Member); Chang Ye Dr. (Committee Member); Ernian Pan Dr. (Committee Member); Kevin Kreider Dr. (Committee Member) Subjects: Mechanical Engineering; Mechanics

MS, University of Cincinnati, 2016, Engineering and Applied Science: Mechanical Engineering

Viscoelastic coatings are currently being used in a variety of industries to provide shock absorption, energy absorption, noise reduction, and vibration isolation. They are commonly used through aerospace, automotive, and electronic industries. These materials have a complex behavior that causes their mechanical properties to vary with both temperature and frequency. Testing historically has been done on a composite material due to the inability of the coatings to support themselves at high temperatures. One common method is the vibrating beam technique, which examines two cantilever beam specimen, an uncoated beam and a coated beam. The change in modal properties is used to determine the properties of the viscoelastic material by itself. However, when the environment is harsh or space is restricted, measurements and excitations cannot be made in a traditional contacting manor. This paper examines methods of non-contact excitation and measurements and explores the effects from these methods. Specifically, magnetic excitation was examined as a source of non-contact excitation on a non-ferrous material. This requires that a magnetic material be attached to the beams in order for the magnetic excitation to be successful. The mass loading effects were examined for the two beam specimen to most accurately define the modal parameters of the beams. This excitation source was also used to test the specimen at a range of temperatures to determine the effect temperature had on the modal parameters of the specimen as well. Finite element models were subsequently created to see if the results from the experimental modal analysis could be confirmed with the FEA results.

Committee: Randall Allemang Ph.D. (Committee Chair); Jay Kim Ph.D. (Committee Member); Allyn Phillips Ph.D. (Committee Member) Subjects: Mechanics

Master of Science (M.S.), University of Dayton, 2014, Mechanical Engineering

Many industrial machines incorporate a multitude of moving and rotating parts necessary for the machinery to perform its intended functions. Rotating machinery, like turbines and compressors, include multiple parts that rotate under heavy loads and high speeds. Shafts are a common medium to transmit these loads and speeds. Quite often, these shafts are required to be stepped to create multiple distinct diameters for carry and located other components. The addition of these steps must be design with care such that a proper radius is selected between two diameters. Parts operating in this field will run for long periods of time and must maintain under multiple start/stop cases and can eventually cause failures. Rotor and torsional dynamic analyses are completed on most if not all rotors in the turbomachinery field. The 45 degree rule is a method of simplification for modeling abrupt changes in diameter. This rule of thumb states a line from the lesser of two steps on a shaft can be drawn at a 45 degree angle to the outside diameter of the greater step. The material outside this line can be modeled with zero modulus and actual density. This region of material does not significantly impact the torsional stiffness of the area. The purpose of this research is to find the effect this modeling approach has on the computed strength, stiffness, and overall rotordynamic properties of a rotating shaft. This will also demonstrate the “Best case” shoulder combination with various fillet radii and/or other angle orientation as well as illustrate additional areas this theory may be applicable. Additional considerations will be made for defective or non-homogeneous material (e.g., inclusions, cracks, and scratches) that may be contained within the region under consideration and their effect on the overall region's computed strength and stiffness. The purpose of this research is split into three claims. The first claim states that after the removal of the material outside the 4 (open full item for complete abstract)

Committee: Thomas Whitney Dr. (Advisor); Dave Myszka Dr. (Committee Member); Steven Donaldson Dr. (Committee Member); Raed Hasan Dr. (Committee Member) Subjects: Mechanical Engineering

MS, University of Cincinnati, 2005, Engineering : Mechanical Engineering

The current research focuses on studying the modal response of a joined wing aircraft based on the Sensorcraft configuration. Sensorcraft, a class of High-Altitude, Long-Endurance (HALE) aircraft, is an Unmanned Air Vehicle (UAV), and is being studied by the AFRL for applications involving telecommunication relay, environmental sensing and military reconnaissance. The Sensorcraft is designed to operate at high altitudes (60,000 ft) with low speed and for long durations of time (60 to 80 hours). At these operating conditions, the density, and hence, the Reynolds number, is low. These conditions require the Sensorcraft to operate with high lift and low drag with high-aspect ratio wings. Moreover, the vehicle must be lightweight and strong, and offer high aerodynamic performance and efficiency. The AFRL has identified a diamond shape joined wing configuration for Sensorcraft due to the primary structural advantage of strength as each wing braces the other against lift loads.The University of Cincinnati (UC), along with its partners, AFRL and Ohio State University are working together to study the complete nonlinear aeroelastic behavior of the joined-wing model. At UC, four different structural modeling approaches were adopted for analysis. The current research focuses on the analysis of an in-house Sensorcraft joined wing model developed by the AFRL. This model is an equivalent representation of the actual 3-D joined wing model. The wing is idealized as a box structure consisting of shells, rods, beams, shear panels and concentrated masses. This box wing structure has the advantage of being computationally inexpensive over the full 3-D model, and has been optimized to minimize the deflections of the antennae equipment in the control surface of the wing. The fluid loads applied on the box-wing structure are obtained from a concurrent aerodynamic analysis for different mach numbers and angles of attack performed at UC.A modal representation is obtained for different oper (open full item for complete abstract)

Committee: Dr. Urmila Ghia (Advisor) Subjects: Engineering, Mechanical

Master of Science, The Ohio State University, 2012, Mechanical Engineering

Face gears are cross-axis gear drives with much simpler geometries and manufacturing process in comparison to other cross-axis gearing such as spiral bevel and hypoid gears. Face gears can provide high power density solutions with high gear ratios. These advantages come with issues associated with rim and web deflections of the large face gear. In aerospace applications where weight of the gearbox is a major concern, web deflections become more challenging. This study explores the feasibility of incorporating a thrust bearing with a face gear drive to remedy rim deflection effects in a compact and light-weight manner. A baseline face gear pair is defined in this study where the face gear is supported by a conical web that is used to preload a rolling element thrust bearing axially to limit gear deflections under load. A state-of-the-art contact mechanics software package is used to analyze this baseline system to identify potential problems. A detailed parametric study covering basic bearing parameters, face gear geometry parameters, axial preload and gear tooth modifications is performed to determine conditions and design approaches that can remedy some of these problems. These results are compiled at the end into simplified guidelines for the face gear drive design.

Committee: Ahmet Kahraman (Advisor); Sandeep Vijayakar (Committee Member) Subjects: Engineering; Mechanical Engineering

Doctor of Philosophy (PhD), Ohio University, 2009, Integrated Engineering (Engineering and Technology)

Carbon foams are porous materials which are attractive for many engineering applications because their thermal and mechanical properties can be customized by varying manufacturing process parameters. However, a highly random geometry at pore level makes it very difficult to analyze the properties and the behavior of this material in an application. Published research work on the analysis of foams has employed various ideal geometries to approximate the pore microstructure. However, these models are unable to predict accurately the foam properties and behavior in engineering applications.The objective of this research work is to determine thermal and mechanical properties of carbon foam on the basis of its true microstructure. A new approach is proposed by creating a three dimensional (3D) solid model based on an accurate representation of the real geometry of carbon foam. Finite element models are then developed to investigate the bulk thermal and mechanical properties of carbon foam using the three dimensional solid model. On the basis of the true 3D model of carbon foam, a study is undertaken to examine the effect of the unique microstructure on the flow field within the foam pores and the resultant convective heat transfer. A finite volume model is developed using the accurate representation of carbon foam microstructure inside a flow channel. The fluid flow and heat transfer is simulated to evaluate pressure drop and heat transfer capabilities. The carbon foam permeability, inertial coefficient and friction coefficient are determined and found to be in good agreement with experimental and semi-empirical models. The results also show a large enhancement in the heat transfer due to the presence of carbon foam in the channel. These results are comparable to the experimental results available in published literature. Another application that has been analyzed in this study is the use of carbon foam as tooling material for manufacturing advanced composite material (open full item for complete abstract)

Committee: M. Khairul Alam (Advisor) Subjects: Engineering; Technology

Doctor of Philosophy (Ph.D.), University of Dayton, 2011, Mechanical Engineering

As the use of composite materials in aerospace structures continues to increase, the need to properly characterize these materials, especially in terms of damage tolerance, takes on additional importance. The world wide failure exercises (WWFE) are an example of the international interest in this issue. But though there has been a great deal of progress in understanding the initiation of damage and modeling damage propagation along known interfaces, methods that can capture the effects of interactions among various failure modes accurately remain elusive. A method of modeling coupled matrix cracks and delamination in laminated composite materials based on the finite element method has been developed and experimentally validated. Damage initiation is determined using the LARC03 failure criterion. Delamination along ply interfaces is modeled using cohesive zones. Matrix cracks are incorporated into the discretization of the problem domain through a robust Mesh-Independent Cracking (MIC) technique. The matrix cracking technique, termed the Regularized Extended Finite Element Method (Rx-FEM), uses regularized forms of the Heaviside and Dirac Delta generalized functions to transform the crack surface into a volumetric crack zone. The Regularized Extended Finite Element method is compared to benchmark cases. The sensitivity of the solution to mesh size and parameters within the cohesive zone model is studied. Finally, the full method with delamination is employed to study a set of experimental tests performed on open-hole quasi-isotropic laminates. The trends of hole-size and ply thickness are well predicted for the laminates. Rx-FEM is also able to simulate the pattern of damage, as demonstrated by comparisons to x-ray images. From the results of this series of analyses it can be concluded that failures occur when delamination originating at the hole links up with delamination originating at the edge along the path of matrix cracks.

Committee: Robert A. Brockman PhD (Committee Chair); Steven L. Donaldson PhD (Committee Member); Endel V. Iarve PhD (Committee Member); James M. Whitney PhD (Committee Member) Subjects: Mechanical Engineering; Mechanics

Master of Science in Mechanical Engineering, Cleveland State University, 2011, Fenn College of Engineering

The temperature distribution and thermal Stresses induced by a temperature difference for steady state heat transfer in silicon carbide (SiC) ceramic tube heat exchanger with circular fins was computationally simulated by a finite element method and probabilistically evaluated in view of the several uncertainties in the performance parameters. Cumulative distribution functions and sensitivity factors were computed for Hoop and Radial stresses due to the structural and thermodynamic random variables. These results are used to identify the most critical design variables in order to optimize the design and make it cost effective. The probabilistic analysis leads to the selection of the appropriate measurements to be used in structural and heat transfer analysis and to the identification of both the most critical measurements and parameters.

Committee: Rama S.R. Gorla PhD (Committee Chair); Asuquo Ebiana PhD (Committee Member); Majid Rashidi PhD (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, University of Akron, 2012, Civil Engineering

In the field of computational mechanics, there has been a very challenging problem, which is the characterization of heterogeneous media with randomly distributed material properties. In reality, no material is homogeneous and deterministic in nature and it has been well-known that randomness in microstructures and properties of materials could significantly influence scatter of structural response at larger scales. Therefore, stochastic characterization of heterogeneous materials has increasingly received attention in various engineering and science fields. In order to deal with this challenging problem, two major challenges need to be addressed: 1) developing an efficient modeling technique to discretize the material uncertainty in the stochastic domain and 2) developing a robust and general inverse identification computational framework that can estimate parameters related to material uncertainties. In this dissertation, two major challenges have been addressed by proposing a robust inverse analysis framework that can estimate parameters of material constitutive models based on a set of limited global boundary measurements and combining the framework with a general stochastic finite element analysis tool. Finally a new stochastic inverse analysis framework has been proposed, which has a novel capability of modeling effects of spatial variability of both linear and nonlinear material properties on macroscopic material and structural response. By inversely identifying statistical parameters (e.g. spatial mean, spatial variance, spatial correlation length, and random variables) related to spatial randomness of material properties, it allows for generating statistically equivalent realizations of random distributions of linear and nonlinear material properties and their applications to the development of probabilistic structural models. First, a robust inverse identification framework, called the Self-Optimizing Inverse Method (Self-OPTIM), has been developed. Unli (open full item for complete abstract)

Committee: Gun Jin Yun Dr. (Advisor); Wieslaw Binienda Dr. (Committee Member); Ernian Pan Dr. (Committee Member); Xiaosheng Gao Dr. (Committee Member); Kevin Kreider Dr. (Committee Member) Subjects: Civil Engineering; Engineering; Experiments; Materials Science; Mathematics

Master of Science, University of Akron, 2006, Applied Mathematics

The research included in this abstract pertains to probabilistic finite-element creep analysis of a composite combustor liner. A composite combustor liner is an aerospace engine component that is subjected to very high temperatures, ranging between 1500 - 2100 degrees Fahrenheit. A creep analysis of this component is essential for rational design as creep (a slow time-dependent information under constant load) is prevalent at high temperatures. In a probabilistic analysis, many, if not all, of the state variables are represented by random variables with appropriate probability distributions incorporating relevant parameters. This formalism is much more realistic, as it more accurately describes the variability in properties and loadings that are inherent in the composition of aerospace materials and loadings encountered by aerospace components.

Committee: Ali Hajjafar (Advisor) Subjects:

MS, University of Cincinnati, 2012, Engineering and Applied Science: Civil Engineering

The objective of this project was to investigate a United States Navy structure and evaluate the potential of the lifting rod, supporting a large mass at the top of the structure, to disengage from the structure due to seismic activity. A concern about the seismic loading of the structure was raised by an inspection team. The concern was that, in the event of an earthquake, the lifting rod would disengage from the structure due to vertical seismic actions. In order to accomplish the objective, seismic analyses of the structure were performed using finite element modeling. The geologic and tectonic settings of the site were investigated and the expected earthquake sources were determined. A probabilistic seismic hazard analysis was performed for the site, including a deaggregation of the earthquake sources affecting the site. The results of the probabilistic seismic hazard analysis were used to select three sets of bedrock acceleration time histories. Artificial acceleration time histories were generated based on the results of the probabilistic seismic hazard analysis and three of the time histories were selected. A finite element model of the structure was created using ABAQUS and seismic analyses were performed by applying the acceleration time histories. The results of the model were analyzed to determine whether or not the lifting rod disengaged from the structure. Plots of contact force vs. time indicated that there were a total of three increments of time where the lifting rod lost contact with the structure. Plots of relative vertical displacement vs. time indicated that there were no increments of time where the lifting rod was lifted up enough to detach. Plots of energy vs. time indicated that there were no increments of time where the kinetic energy of the mass was greater than the potential energy required for detachment. It was determined, based on the information at hand and within the hypotheses discussed in this document, that the lifting rod was not (open full item for complete abstract)

Committee: Gian Rassati PhD (Committee Chair); Ronald J. Ebelhar MS (Committee Member); Thomas Baseheart PhD (Committee Member) Subjects: Civil Engineering