MS, University of Cincinnati, 2017, Engineering and Applied Science: Mechanical Engineering

This study develops a polygonal finite element solver for 2-D crack propagation simulation along with a meshing algorithm which creates necessary polygonal mesh. The work starts with a brief literature review of historical development of computational fracture mechanics. After reviewing multiple methods employed for modeling fracture problems, Wachspress formulation is selected for constructing the polygonal finite element solver. Polygonal interpolants are developed using Wachspress' framework and validated using published results. A polygonal meshing algorithm is also developed since conventional finite element meshers do not support domain meshing using higher order polygons. The meshing algorithm is then used to create the mesh and input files for the polygonal finite element solver. The polygonal solver is validated using conventional patch tests. The accuracy and convergence of the method is assessed using classical solid mechanics problems with known analytical solutions. Next, ability to include cracks geometrically is added to the meshing algorithm. The polygonal solver is updated with crack tracking and remeshing capability. A fracture problem is solved using the developed subroutines.

Committee: Yijun Liu Ph.D. (Committee Chair); Woo Kyun Kim Ph.D. (Committee Member); Kumar Vemaganti Ph.D. (Committee Member) Subjects: Mechanical Engineering; Mechanics

PhD, University of Cincinnati, 2009, Engineering : Aerospace Engineering

An active field of research that has developed due to the increasing use of computational techniques like finite element simulations for analysis of highly complex structural mechanics problems and the increasing use of composite laminates in varied industries such as aerospace, automotive, bio-medical, etc. is the development of numerical models to capture the behavior of composite materials. One of the big challenges not yet overcome convincingly in this field is the modeling of delamination failure which is one of the primary modes of damage in composite laminates. Hence, the primary aim of this work is to develop two numerical models for finite element simulations of delamination failure in composite laminates and implement them in the explicit finite element software DYNA3D/LS-DYNA.Dynamic fracture mechanics is an example of a complex structural analysis problem for which finite element simulations seem to be the only possible way to extract detailed information on sophisticated physical quantities of the crack-tip at any instant of time along a highly transient history of fracture. However, general purpose, commercial finite element software which have capabilities to do fracture analyses are still limited in their use to stationary cracks and crack propagation along trajectories known a priori. Therefore, an automated dynamic fracture procedure capable of simulating dynamic propagation of through-thickness cracks in arbitrary directions in linear, isotropic materials without user-intervention is first developed and implemented in DYNA3D for its default 8-node solid (brick) element. Dynamic energy release rate and stress intensity factors are computed in the model using integral expressions particularly well-suited for the finite element method. Energy approach is used to check for crack propagation and the maximum circumferential stress criterion is used to determine the direction of crack growth. Since the re-meshing strategy used to model crack growth expli (open full item for complete abstract)

Committee: Dr. Ala Tabiei (Committee Chair); Dr. Jandro Abot (Committee Member); Dr. Shaaban Abdallah (Committee Member); Dr. James Wade (Committee Member) Subjects: Aerospace Materials; Automotive Materials; Mechanical Engineering; Mechanics

PhD, University of Cincinnati, 2015, Engineering and Applied Science: Engineering Mechanics

The smoothed finite element method (S-FEM) was recently proposed to bring softening effects into and improve the accuracy of the standard FEM. In the S-FEM, the system stiffness matrix is obtained using strain smoothing technique over the smoothing domains associated with cells, nodes, edges or faces to establish models of desired properties. In this dissertation, it will introduce several aspects of advanced development and applications of S-FEM in solid mechanics. The idea, main work and contribution are included in four aspects as following: (1) A Generalized Stochastic Cell-based S-FEM (GS_CS-FEM): The cell-based S-FEM is extended for stochastic analysis based on the generalized stochastic perturbation technique. Numerical examples are presented and the obtained results are compared with the solution of Monte Carlo simulations. It is found that the present GS_CS-FEM method can improve the solution accuracy with high-efficiency for stochastic problems with large uncertainties. (2) An effective fracture analysis method based on the VCCT implemented in CS-FEM: The VCCT is formulated in the framework of CS-FEM for evaluating SIF's and for modeling the crack propagation in solids. The one-step-analysis approach of the VCCT is utilized based on the assumption of stress field equivalence under infinitesimal perturbations. The significant feature of the present approach is that it requires no domain integration but attains same level of accuracy compared to the standard FEM using the interaction integral method. Numerical examples are provided to validate the effectiveness of fracture parameter evaluation as well as to predict the crack growth trajectories. (3) Smoothing techniques based crystal plasticity finite element modeling of crystalline materials: A framework and numerical implementation for modeling anisotropic crystalline plasticity using strain smoothing techniques is presented to model anisotropic crystalline plasticity with rate-independence. The edge-ba (open full item for complete abstract)

Committee: Guirong Liu Ph.D. (Committee Chair); Shaaban Abdallah Ph.D. (Committee Member); Yijun Liu Ph.D. (Committee Member); Francesco Simonetti Ph.D. (Committee Member) Subjects: Engineering

Doctor of Philosophy, The Ohio State University, 2010, Mechanical Engineering

The plastic deformation and ductile fracture behavior of 12.7 mm thick 2024-T351 aluminum plate is investigated. Tension, compression and shear experiments are conducted at strain rates ranging from 10-4 s-1 to 5000 s-1 and temperatures ranging from -50 °C to 450 °C. Anisotropy in the plate is studied by conducting tension and compression tests on specimens oriented in multiple directions within the plate. An anisotropic plasticity model is used in numerical simulations of select experiments. Comparison of the simulation results to the actual test data shows that the material behavior can be adequately captured in tension, compression and shear. Anisotropic plastic deformation behavior in an impacted target panel is also investigated. Numerical simulations using both a von Mises and anisotropic yield functions are compared to previously published experimental data. The choice of yield function has a dramatic effect on the predicted projectile residual velocities. Experimental impact data shows evidence of anisotropic behavior, the trends of which can be captured in simulations using the anisotropic yield function. The dependence of equivalent plastic fracture strain on the state of stress is studied through mechanical experiments on specimens with various geometries, subjected to multiple load conditions. Tension tests of plane stress (thin) specimens, axisymmetric specimens and plane strain (thick) specimens are conducted for this purpose. Combined tension – torsion, pure shear and compression – torsion tests as well as dynamic punch experiments are also used. The three dimensional digital image correlation (DIC) technique is used to determine the specimen surface strains in many of the experiments. A coupled experimental – numerical approach is used to generate fracture locus data points for the tension and punch experiments. The equivalent fracture strain dependence on three stress state parameters: stress triaxiality, Lode parameter and product triaxiality is de (open full item for complete abstract)

Committee: Amos Gilat PhD (Advisor); Mark Walter PhD (Committee Member); Brian Harper PhD (Committee Member); Mo-How Herman Shen PhD (Committee Member) Subjects: Aerospace Materials; Engineering; Experiments; Mechanical Engineering; Mechanics

Doctor of Philosophy, University of Akron, 2007, Civil Engineering

In this study, mechanics models for bi-layer beams suitable for bi-material interface characterization are introduced, and their application to mixed mode fracture of wood-FRP bonded interface is studied. First, an engineering approach for evaluating the mixed-mode (Mode-I/II) fracture toughness of wood/FRP composite bonded interfaces is presented. Two four-point bending specimens, i.e., four-point asymmetric end-notched flexure (4-AENF) and four-point asymmetric mixed-mode bending (4-AMMB), are proposed for the study of the mixed-mode fracture. With proper design, the rate of compliance change with respect to crack length can be determined to be independent of the crack length for the specimens. The proposed specimens can be used to determine the mixed-mode fracture toughness without the need of measuring the crack propagation length. Mode decompositions are evaluated for each specimen, and with previously determined Mode-I and Mode-II data, a number of failure criteria and failure envelopes are determined for the wood-FRP interface. Second, an intuitive mechanics-based approach is presented to analyze layered beams using the split beam model, from which the layered beam is modeled as individual sub-beams and the stress acting throughout each sub-beam is used to determine the forces acting on the sub-beams. The split beam model is adapted to evaluate the compliance and ERR of three common groups of fracture specimens and loading conditions: Mode-I dominant (ADCB and DCB), Mode-II dominant (AENF and ENF), and Mixed-Mode (ASLB and SLB). It is demonstrated that the split beam model generates solutions consistent with those of existing specimens found in literature. Also, derived specimens are provided to allow for different ways of viewing the specimen configurations and possible ways of applying loads to achieve certain conditions. The compliance and ERR of two derived specimens are presented. The ERR for these two specimens is shown to be the same as the existing so (open full item for complete abstract)

Committee: Wieslaw Binienda (Advisor); Pizhong Qiao (Advisor) Subjects: Engineering, Civil

Doctor of Philosophy, University of Akron, 2005, Polymer Engineering

In last 50 years, research on elastomer wear has produced qualitative and statistical data regarding wear debris and associated morphologies. However, the exact wear mechanism and the evolution of wear morphologies is not understood to the level where a quantitative prediction or description of wear is possible In this study a numerical analysis (FEA) has been used to understand mechanical interactions related to pattern wear. A blunt surface crack and it's interaction with a single penetrating asperity has been modeled for varying frictional, material and kinematic conditions. An interacting asperity creates a deformation field in an elastomeric body. This stress field is altered in the presence of a crack. The resulting stress relief is quantitatively estimated for varying geometric, material and friction parameters. Consequently, the energy available for a new crack to propagate in the vicinity of the existing crack decreases leading to a characteristic spacing between successive cracks. Energy release rate data from fracture experiments on thin rubber sheets is used to calculate the spacing between the cracks. The approach sheds some light on crack propagation characteristics in pattern wear. Other numerical experiments in this study analyze: (1) Elastomer response to dynamic asperity loading (2) Asperity loading at micro-scale where filled rubber has high degree of non-homogeneity (3) Effect of asperity loading at an angle on a rubber flap. As a result, we now better understand the evolution of wear related morphologies.

Committee: Arkady Leonov (Advisor) Subjects:

Master of Science, The Ohio State University, 1970, Graduate School

Committee: Not Provided (Other) Subjects:

Master of Science, The Ohio State University, 1961, Graduate School

Committee: Not Provided (Other) Subjects:

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, University of Akron, 2024, Polymer Science

The field of fracture mechanics emerged well over 100 years ago and is a subject older than polymer science. Fracture mechanics provides a means to characterize material response to deformation in the presence of flaws or cracks within a specimen, based on either the critical energy release rate (Gc) or the critical stress-intensity factor (Kc) at fracture. While these fracture toughness parameters successfully allow us to rank materials, an understanding as to why they are a material constant and what determines its magnitude remains to be of great interest. Polymers have been studied extensively under this framework for quite a while; in fact, it has been ongoing since prior to the development of theories describing its chain-level physics. While fracture mechanics perfectly describes the criterion surrounding the fracture phenomenon, it cannot explain why a polymer behaves in a brittle or ductile manner in the first place. Such an understanding must come from polymer physics, which also seeks to evaluate its inherent material strength. In this dissertation, an attempt is made to juxtapose the two, showing how polymer's physics affects its fracture behavior in the case of glassy polymers, and vice versa, where we see how the presence of a notch or crack dictates the local stress and failure mechanism using photoelasticity to gain a better insight into the conditions which precede fracture. Microscopic observation of local birefringence allows us to comprehend the origin of the magnitude of fracture toughness. This dissertation also looks at the case of ductile glasses, focusing not only on the conditions surrounding tip-yielding and stability of crack propagation, but also examining the cases where tip-yielding is inhibited solely by the presence of a notch.

Committee: Shi-Qing Wang (Advisor); Christopher Barney (Committee Chair); Wieslaw Binienda (Committee Member); Mesfin Tsige (Committee Member); Ali Dhinojwala (Committee Member) Subjects: Materials Science; Plastics

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

Gigantic off-the-road (OTR) tires used in mining industries are usually damaged and both their damage resistance and damage mechanisms of fiber-reinforced rubbers have not been well understood. In order to optimize OTR tire fillers and mechanical properties, the damage resistance needs to be studied under consideration of size independence and size dependence. In this research, rubbers reinforced with different fiber stiffness and varied fiber amount fabrics were fabricated by vulcanization. The mechanical properties including performance of tensile, scratch, impact, fracture, and debonding were studied under size-independent condition. Natural rubber (NR) having a lower stiffness was better than styrene-butadiene rubber (SBR) in tensile and fracture performance. Increasing fiber stiffness and fiber amount improved tensile, scratch, and impact resistance. Reinforcing Kevlar fabric reduced the matrix crack occurrence in the impact test and the damage level in the scratch test. Carbon fiber-reinforced rubber had the most serious damage level in the scratch and impact test. The fracture toughness decreases with increasing fiber volume ratio and fiber stiffness. The delamination resistance rises as fiber ductility reduces. In the studies of fracture performance, we studied both the thickness effect and the length-width effect. Increasing the SBR thickness to fourfold (3 mm, 6 mm, 9 mm, and 12 mm) exhibited decreased fracture toughness and the varied stress state from plane stress to plane strain in compact tension (CT) tests. For delamination resistance, a higher matrix thickness caused more energy dissipation to increase the adhesion strength. In the study of the length-width effect, specimens tested using single-edge-notched tension (SENT) had increased the same proportion length and width, and unvaried thickness (3mm) to evaluate the size effect under plane-stress condition. The fracture toughness of the unreinforced rubbers was independent of re (open full item for complete abstract)

Committee: Shing-chung Wong (Advisor); Xiaosheng Gao (Advisor); Todd Blackledge (Committee Member); Xiaosheng Gao (Committee Member); Qixin Zhou (Committee Member); Kwek-tze Tan (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, University of Akron, 2023, Polymer Science

Polymeric materials possess many useful properties, especially low density and high toughness. When crystallized, solvent resistance and temperature resistance are imparted. Predrawing can enhance their mechanical strength and toughness. However, whereas semicrystalline polymers (SCPs) like polyethylene (PE) and polypropylene (PP) are ductile, poly(lactic acid) (PLA) and poly (hydroxyalkanoates) (PHA) often show brittle behavior. In the face of a worldwide push for sustainable materials, it has become necessary to uncover the molecular picture that governs the processing-structure-property (P-S-P) relationships regarding the ductility (or lack thereof) of polymers. One example is how various types of predeformation can affect the mechanical properties. It has been shown [1] that pre-melt stretching of PLA in the near-affine limit close to the glass transition temperature Tg was shown to greatly enhance the strength and ductility of otherwise brittle PLA, and to produce a nanoconfined (NC) crystalline state whose length scale is on the order of the mesh size of the deformed chain network. Furthermore, this melt stretched PLA contained significant crystallinity and was transparent. In Chapter I, we show that after pre-melt shearing of amorphous PLA above Tg, the resultant crystalline and transparent PLA remains brittle. Both atomic force microscopy imaging (AFM) and small-angle x-ray scattering (SAXS) verify that melt-shearing has also produced an NC crystalline state. However, the brittle behavior of the melt-sheared PLA demonstrates the importance of having geometric condensation from processing. A similar NC crystalline state was generated in melt-sheared PET, thereby demonstrating the apparent universality of the methodology. In Chapter II, the P-S-P relationships are explored for SCPs that are crystallized under quiescent conditions. Since a disproportionate extent of effort has gone into researching the mechanisms of yielding rather than the origins of brittl (open full item for complete abstract)

Committee: Shi-Qing Wang (Advisor); James Eagan (Committee Chair); Fardin Khabaz (Committee Member); Mesfin Tsige (Committee Member); Toshikazu Miyoshi (Committee Member) Subjects: Physics; Plastics

Doctor of Philosophy, The Ohio State University, 2023, Materials Science and Engineering

Age-hardenable 6xxx-series aluminum-magnesium-silicon (Al-Mg-Si) alloys are commonly used in automotive applications due to their high strength-to-weight ratio and their potential for weight savings. However, there is an increased likelihood of corrosion issues due to the chloride-rich environment produced by using road salts in the winter, as well as coupling with more noble materials, such as steels and carbon fiber reinforced polymer (CFRP), when used in conjunction with Al alloys. The combination of a corrosive environment, the residual stresses induced during forming processes, and the possibility of a strong galvanic couple may increase the stress corrosion cracking (SCC) susceptibility of these normally SCC-resistant Al alloys. This research aims to understand the effect of electrochemical potential on the SCC behavior of AA6111 used in automotive applications. The first part of this work examines the effect of polarization on the threshold stress intensity (KTH) and Stage II crack growth rate (da/dtII) during fracture mechanics-based experiments in 0.6 M NaCl to determine the likelihood of galvanic coupling with CFRP increasing the SCC-susceptibility of AA6111 with an automotive paint bake heat treatment (PB). The measured KTH decreases from near fracture toughness values of ~17 MPa√m at the open circuit potential (OCP) to less than 6 MPa√m when anodically polarized to 100 mV above OCP, and da/dtII increases with increasing anodic polarization of 75 to 100 mV above OCP. This indicates that sufficient anodic polarization induces SCC susceptibility in AA6111-PB. The cracking behavior under cathodic polarization was explored to investigate whether SCC reactivates in AA6111-PB. Results at -1300 mVSCE indicate no significant effect of cathodic polarization on SCC. In the second part of this work, the effect of polarization on the crack tip pH of peak-aged (PA) AA6111 under various polarization levels was investigated to directly link changes in SCC resistan (open full item for complete abstract)

Committee: Jenifer (Warner) Locke (Advisor); Eric Schindelholz (Committee Member); Gerald Frankel (Committee Member) Subjects: Engineering; Materials Science

PhD, University of Cincinnati, 2022, Engineering and Applied Science: Mechanical Engineering

Fracture is a primary cause of catastrophic failure in structures. Since analytical solutions for crack propagation do not exist except for a few special cases, accurate numerical modeling of fracture is essential. Modeling brittle-elastic materials using the variational approach is well-established. One of the well-known challenges resides in the inability of the existing formulations to account for all other micro-scale and mesoscale phenomena that result in the macro-scale behavior that goes under the name of plasticity. This work aims to extend the classic variational model of fracture to present a wide range of behaviors, from brittle to ductile. Many different approaches to address the elastic-plastic fracture mechanics problem can be found in the literature; most of them are based on the cohesive energy originally suggested by Barenblatt. The essential characteristic that most cohesive fracture models have in common is the assumption that the fracture energy Gc is a function of the crack opening instead of the brittle fracture model, where the fracture energy Gc is considered constant. The replacement of Griffith's constant surface energy with surface energy that is a function of the displacement jumps across the crack can theoretically enrich the model but comes with some challenges. A critical challenge embedded in the cohesive model lies in its phenomenological nature. The dependence on postulated surface energy laws creates a disconnection between the model and the underlying physical phenomena. The need to postulate a surface energy law is a significant limitation in the further development of the model, especially in multiscale settings. In this research the goal is to develop a phase-field model of plasticity and fracture capable of capturing mesoscale phenomena and macroscale behavior. The first step is to characterize the error introduced by the regularization and its effects on the predictability of the nucleation phenomenon. Th (open full item for complete abstract)

Committee: Kumar Vemaganti Ph.D. (Committee Member); Ala Tabiei Ph.D. (Committee Member); Gui-Rong Liu Ph.D. (Committee Member); Woo Kyun Kim Ph.D. (Committee Member) Subjects: Mechanical Engineering

Master of Science in Engineering, University of Akron, 2022, Civil Engineering

Stress Intensity Factor (SIF) is a parameter that characterizes the stresses and displacements near a crack tip and is widely used as a measure of how acute the defect caused by the crack(/s) in the material is. Any prediction of crack growth or any choice of a limiting value for the maximum crack dimensions or stress, is contingent upon an accurate determination of the Stress Intensity Factors. This thesis incorporates a detailed description of the analytical derivations of Stress Intensity Factors for an infinite, isotropic, cracked plate, subjected to a tensile far-field state of stress following the method of Singular Integral Equations, based upon the work of W.K. Binienda et al. (1992a). The calculations of local stresses and displacements for the crack problem are done by formulating the problem as a superposition of two auxiliary problems; the one without any crack, being under tension, and the other as a mixed boundary value problem defined for normal compressive tractions within the crack domain and for displacements outside the crack domain. Emphasis is placed on Binienda's (1992a) realization that, at this stage, invoking the stress and displacement boundary conditions into the dual integral equations for the displacement components (u and v), result in a form that is not solvable for the unknown constants C1 through C4. To overcome this difficulty, a form of “auxiliary functions” f1(x) and f2(x) is adopted which was used by Binienda (1992a) following Erdogan (1978) and other researchers. Unknown constants C1 through C4 are expressed and solved in terms of auxiliary functions f1(x) and f2(x) using the continuity conditions that the normal stress in the vertical direction, and the shear stress, for the upper as well as lower halves of the plate, are equal. This is followed by formulation of total stresses for the jth crack defined as the local stresses for the jth crack plus the contribution of the remaining cracks. These are obtained by coordinate and (open full item for complete abstract)

Committee: Wieslaw Binienda (Advisor); Ping Yi (Committee Member); Anil Patnaik (Committee Member) Subjects: Civil Engineering; Mechanical Engineering; Mechanics

Doctor of Philosophy, The Ohio State University, 2022, Mechanical Engineering

Materials experiencing impact loading deform under complex three dimensional states of stress and at high strain rates. Accurately simulating impact events using finite element modeling requires material models capable of depicting the material behavior under these same conditions. In order to create accurate material models, this material behavior must first be determined experimentally. It is of particular interest to determine the equivalent plastic fracture strain at stress states consisting of in-plane biaxial tension and out-of-plane compression, and the plastic stress-strain response at strain rates on the order of 104 s-1. Both of these conditions are found during impact loading, and are outside the scope of current testing techniques. A new test technique is used to investigate Aluminum 2024, Titanium 6Al-4V, and Inconel 718 under in-plane biaxial tension and out-of-plane compression. The test consists of a small spherical or elliptical punch that is advanced into a thin specimen plate to induce in-plane biaxial tension on the back surface of the specimen. A second plate of an appropriate material is placed against the back surface of the specimen plate during loading in order to create out-of-plane compression. The equivalent plastic fracture strain at these stress states is determined from the experimental data and simulations using the commercial finite element software LS-DYNA. The same materials mentioned above are also tested using a modified, direct impact split-Hopkinson bar testing technique to induce strain rates greater than 104 s-1. For these tests, a small cylindrical specimen is placed in contact with the end of a larger cylindrical bar. The specimen is then impacted with a free flying cylindrical projectile to compress the specimen at a high rate of deformation. The stress-strain response of the material at these high strain rates is then investigated from the experimental data and in conjunction with LS-DYNA finite element simulati (open full item for complete abstract)

Committee: Amos Gilat (Advisor); Prasad Mokashi (Committee Member); Jeremy Seidt (Committee Member) Subjects: Aerospace Materials; Engineering; Experiments; Mechanical Engineering; Mechanics

PhD, University of Cincinnati, 2021, Engineering and Applied Science: Engineering Mechanics

Computational fracture mechanics has been an active area of research in the engineering community for decades. The classical objective of fracture mechanics is to determine the damage caused by defects originating from regions of stress intensification. Their response under different loading conditions in different media is of grave importance in structural analysis. In the traditional framework of widely used numerical techniques like the finite element method (FEM), meshfree methods, boundary element method (BEM) several tools have been proposed to solve such problems. However, due to inherent issues associated with the numerical techniques and the computational complexities relating to the study of crack propagation, we are yet to arrive at a standard. Addressing that, this dissertation proposes a robust, e?icient numerical technique to solve fracture mechanics problems in the framework of the smoothed finite element method (S-FEM). The main contributions are: 1.The singular edge-based smoothed finite element method (sES-FEM) using a special layer of five-node singular elements (sT5) connected to the singular point, is proposed to model stress singularity in solids. The aim is develop an analytical means for integration to obtain the smoothed strains. The sT5 element has an additional node at each of the two edges connected to the crack tip, and the displacements are enriched with necessary terms to simulate the singularity. Our analytical integration techniques reduce the dependency on the order of numerical integration during the computation of the smoothed strain matrix. 2.A novel smoothed finite-element and phase-field method (S-FEM+PFM) for simulating fracture paths in brittle materials is proposed. Our S-FEM+PFM is formulated and implemented in the commercial software ABAQUS, using user defined subroutines. The formulation, within the framework of the (CS-FEM), is further extended to formulate a smoothed phase field model (S-PFM). Because gradient smoothi (open full item for complete abstract)

Committee: Gui-Rong Liu Ph.D. (Committee Chair); Yao Fu (Committee Member); Woo Kyun Kim Ph.D. (Committee Member) Subjects: Mechanical Engineering

Master of Science, Miami University, 2020, Chemical, Paper and Biomedical Engineering

Linear Elastic Fracture Mechanics (LEFM) has been modified to account for the role of inherent fracture processing zone in failure of fibrous materials. The presented study further validates the theory using both prepared handsheets with different pressing conditions and literature reported handsheet tensile strength data under different notch size. The analysis shows that modified LEFM can capture the trend of tensile strength as functions of notch size. To further expand the application of the model, we used a simple shifting manipulation coupled with a fitting procedure to indirectly determine characteristic fracture processing zone length. After the treatment, all the tensile strength data were fitted into a unified fracture model. The results show that increasing porosity or decreasing density leads to increase of fracture processing zone length. Lastly, we evaluated the role of tensile strength in affecting the tear strength. We found that tear strength reaches a maximum value as tensile strength increases but drops dramatically once tensile strength or breaking length reaches a critical value for softwood based handsheets. The implication of this results suggests that one should consider the nature of the fibers when preparing high tensile strength and tear strength fibrous materials.

Committee: Douglas Coffin (Advisor); Jessica Sparks (Committee Member); Shashi Lalvani (Committee Member) Subjects: Chemical Engineering; Mechanical Engineering; Mechanics

Master of Science (M.S.), University of Dayton, 2019, Mechanical Engineering

Experimental study accomplished the characterization of fatigue crack growth rates and mechanisms in both hand and die forged Aluminum 7085-T7452. Testing was conducted at various positive and negative loading ratios, primarily focused on L-S and T-S orientations to discover a correlation between crack tip branching or turning mechanisms and stress intensity. Interior delaminations were found to originate in the interior of the specimen and propagate outward to the surface and manifested as splitting cracks parallel to the loading direction. Stress intensity ranges have been correlated with the onset of crack deviation from the primary crack plane, as well as, the transition to branching dominated fatigue crack growth.

Committee: David Myszka (Committee Chair); James Joo (Committee Member); Thomas Spradlin (Committee Member); Mark James (Committee Member) Subjects: Aerospace Materials; Engineering; Materials Science; Mechanical Engineering

Bachelor of Science (BS), Ohio University, 2019, Biological Sciences

Bone is often regarded as a mostly inorganic tissue. Osteoporosis, a skeletal metabolic disorder characterized by increased bone fragility and fracture risk, is currently diagnosed by Dual X-Ray Absorptiometry (DXA) scan. However, DXA scanning is a poor predictor of fracture risk and bone quality, as it only assesses the mineral content of bone. Recent research increasingly suggests that other nonmineral parameters contribute to bone strength, such as size, geometry, and organic collagen. As a result, it is imperative to find a better diagnostic tool that more accurately encapsulates these other factors. One potential solution is Cortical Bone Mechanics Technology (CBMT), a novel technology being developed at Ohio University that uses noninvasive, radiation-free three point mechanical loading test to assess bone flexural rigidity (EI). Because CBMT is a mechanical test, it is believed to better detect changes in nonmineral factors. To assess this, compromising of the organic collagen matrix was induced using potassium hydroxide (KOH), which does not affect bone mineral. Paired cadaveric human forearms (n=16) were treated with either saline (n=8) or KOH (n=8). No statistically significant difference was present between the right and left T-scores of excised ulnas prior to chemical incubation (p= 0.40). No statistically significant difference between the KOH and saline cohorts prior to chemical incubation (p=0.27). Arms were assessed with DXA and CBMT both before and after treatment. Saline immersion did not reduce EIQMT (+0.9±1.2%, p= 0.76) or EICBMT (-0.6±2.3%, p=0.40). By contrast, KOH immersion reduced both EIQMT (-27.2±3.2%, p<0.0001) and EICBMT (-20.6±6.1%, p<0.01), with no difference between the magnitudes of these effects (p=0.21). Ulna BMD at the 1/3 region was not reduced by either saline (-1.4±0.9%, p = 0.09) or KOH (0.2±0.8%, p=0.76). Thus, CBMT detected collagen-mediated effects of KOH on the bending stiffness of whole cadaveric human ulna bones, and DXA (open full item for complete abstract)

Committee: Anne Loucks Ph.D (Advisor); Lyn Bowman Engr (Other) Subjects: Anatomy and Physiology; Biomechanics; Biomedical Research; Technology