Mechanical Engineering & Applied Mechanics Doctoral Work

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Light-assisted 3D printing of continuous carbon fiber reinforced thermoset composites
(North Dakota State University, 2024) Islam, Md Zahirul
Polymer 3D printing has become an emerging manufacturing technique, due to its design flexibility, however its application to produce structural components is still limited due to the poor mechanical strength and thermal stability of most 3D printed parts. Because of the superior mechanical strength of carbon fiber, 3D printing of continuous carbon fiber reinforced thermoset composites have recently been studied overcome this barrier of mechanical strength and thermal stability. Light- curing based 3D printing of continuous carbon fiber shows a promising potential, however this process also has limitations in making custom object due to fiber loop creation as the nozzle turns at the corner of the object. This study aimed to develop algorithms for light-assisted 3D printing, focusing on custom object fabrication using low-viscosity urethane acrylate and epoxy-acrylate based resins. A novel approach, laser cutting incorporated 3D printing of continuous carbon fiber reinforced thermoset composites, is presented for custom object manufacturing. Furthermore, algorithms were developed to enable the printing of various shapes, including rectangles, triangles, circles, hexagons, and grid structures. A modified algorithm was also introduced and demonstrated to simplify the printing of scalable truss structures. These proposed 3D printing technologies successfully demonstrated the manufacturing of custom objects having comparable mechanical and thermal strength with similar composites manufactured by conventional manufacturing process. Finally, this study presents an experimental approach to determine the minimum light energy required to sustain continuous fiber printing. Proper tuning of the process parameter of this proposed 3D printing technique has great potential to replace conventional manufacturing process of composites by 3D printing.
Influences of Seawater Flows on the Degradation of Organic Coatings Applied on Offshore Wind Turbines
(North Dakota State University, 2022) Vedadi, Amin
The regular protection methods of offshore wind structures consist of the application of two or three layers of epoxy-based coatings with a polyurethane topcoat. The coating systems of offshore wind turbines are mostly tested on-site, where different coated samples are exposed to the sea water at the specific locations planned for the installations of the turbines. Despite several advantages of laboratory testing, the majority of laboratory-based tests have been limited to the exposure of coated or unprotected metals to stationary electrolytic solutions, while the flow-induced corrosion measurements have not received enough attention until recently. The focus of our work is to investigate the influence of applied mechanical stresses due to the water flow on the degradation of organic coatings. In order to resemble the condition of coated monopile structures in shallow water flow, an impingement chamber device and a wave tank were designed and constructed. The Electrochemical Impedance Stereoscopy (EIS) method was utilized for monitoring the electrochemical processes occurred during the degradation of coatings. Computational Fluid Dynamic (CFD) method, as well as Particle Image Velocimetry (PIV) tests were utilized in order to calculate the magnitude of applied stresses on the coating surfaces. Atomic Force Microscopy method (AFM) was employed for characterizations of coating’ surfaces. The theory of thermo-activated processes in combination with the thermoelasticity equations were derived in a way to calculate the influence of applied stresses on different electrochemical parameters of the coatings’ degradation. The afore-mentioned experimental methods and the developed analytical procedure can potentially predict the behavior of organic coatings applied on offshore wind turbines at different exposure zones with respect to the sea water flow.
The Effects of Surface Roughness on the Functionality of Titanium Based Alloy Ti13Zb13Zr Orthopedic Implants
(North Dakota State University, 2021) Jahani, Babak
In this study, the effects of surface roughness on the wettability, cell attachment, and mechanical properties of titanium-based Ti13Nb13Zr orthopedic implants have been investigated. The aim of this multidisciplinary research was to find an optimum range of surface roughness for Ti13Nb13Zr orthopedic implants that could maximize the attachment and the proliferation of cells and improve the wettability of the surface, without adversely affecting the mechanical strength of the implants. There have been some published research works that support the existence of relations between roughness and the functionality of implants, but still, an optimum roughness that can satisfy all of the orthopedic requirements, either is not fully studied or not published. It was seen that the performance of orthopedic implants depends on multiple paradoxical parameters. The results of this study on Ti13Nb13Zr show, even though increasing the value of surface roughness can increase the initial phase of cell attachment onto the surface of Ti13Nb13Zr implants, other major functions such as wettability and mechanical properties can be influenced adversely. Through an experimental methodology, this study proposes an optimum range of roughness, which meets all three major functions of cell attachment, mechanical properties, and wettability. In respect to the recent serious health concerns reported over the implants made of Ti6Al4V which is a common material in the implant industry, scientists and researchers are currently working to introduce a better biomaterial. In this study, Ti13Nb13Zr which is a new and advanced titanium-based biomaterial with improved biocompatibility and more desired mechanical properties was selected and studied. The reason for this selection backs to the fact that Ti13Nb13Zr does not release toxic ions (such as Al and V ions) and its mechanical properties are closer to the bone in comparison to many titanium alloys such as Ti6Al4V.
Simulation-Based Optimization and Artificial Intelligence Techniques for Macromechanical and Micromechanical Characterization of Soft Biological Tissues
(North Dakota State University, 2021) Ramzanpour, Mohammadreza
Traumatic brain injury (TBI) is a serious health and socioeconomic issue which affects thousands of lives annually in the United States. Computational simulations play an important role in better understanding of the TBI and on how it happens. Having accurate material properties of the brain tissue and the elements of the brain will help with more accurate computational simulations. Material characterization is therefore the line on which lots of research have been conducted. In recent years, the emerge of data driven approaches has led to better and more accurate soft tissue characterization. In this dissertation, a metaheuristic search optimization method together with simulation-based optimization framework, and artificial intelligence-based approaches have been employed for macromechanical and micromechanical characterization of brain tissue. First, a constrained particle swarm optimization (C-PSO) technique has been established for soft tissue characterization that overcomes the shortcomings of the classical optimization methods. Through the application of the inherent constraints in the hyperelastic and visco-hyperelastic models, it became possible to reduce the time complexity of this optimization algorithm. Subsequently, the developed constrained optimization method was employed to create simulation-based optimization frameworks for characterizing the micro-level constituents of human brain white matter including axons and extracellular matrix using the hyperelastic and visco-hyperelastic constitutive models. This simulation-based optimization framework helps the researchers to go around the complexities involved with the experimental techniques on micro-level characterization of soft tissues. The final part of this dissertation is devoted to the development of the machine learning and deep learning techniques for classifying the tissue stiffness out of the finite element (FE) simulation results. Through the training of a regularized logistic regression and deep learning convolutional neural networks, it became possible to correctly predict more than 91% of the cases of tissues with high stiffness. The tissues with high stiffness are usually indicative of the pathology and hence are important from medical perspective. The outcome of this part of the work could be useful for qualitative description of the soft biological tissue stiffness and pathology diagnosis which can be used as an alternative to the inversion algorithms.
Dynamic Stall Characteristics of Pitching Finite-Aspect-Ratio Wings
(North Dakota State University, 2021) Ullah, Al Habib
In this study, an experimental investigation was performed to characterize the dynamic stall of pitching wings and provide confirmation of the existence of the arch-shaped vortex for moderate sweep wing. Dynamic stall is a complex flow, which happens because of a sudden change of incident angle during the pitching motion. The pitching motion of a wing can cause instability in the shear layer and generate the separation burst at certain angles. For a pitching wing, the dynamic stall vortex is characterized by the formation of an arch-shaped vortex to the evolution of a ring-shaped vortex. The leg of the arch-shaped vortex causes a negative pressure region on the airfoil surface and can, in fact, generate greater lift. However, in certain conditions, the detachment of the arch-shaped vortex from the airfoil surface can cause high pressure and vibration in the structures. The formation of the arch-shaped vortex and its evolution were systematically investigated using cutting-edge flow diagnostic techniques, and the physics of the dynamic stall is explained in addition to providing the confirmation of the theory developed based on Computational Fluid Dynamics. The study was done using Particle Image Velocimetry (PIV) and Pressure-Sensitive Paint for three sweep angle wings. The wings, with an aspect ratio of AR=4 and a NACA 0012 section assembled with round-tip, are considered for the current experimental study. The sweep angles = 0, 15, and 30 degrees were considered to compare the flow phenomena. The PIV results show the formation of a laminar separation bubble and its evolution to a dynamic stall vortex. The increase of sweep angle causes the formation of such vortices towards the wing tip. In the process of finding the footprint of the vortices and pressure distribution on the surface of the wings, a single-shot lifetime method using fast porous paint was used. The results show the existence of suction pressure and later grows towards the trailing edge of the wing due to the formation of a dynamic stall vortex. In addition, at Re=2x10^5 and reduced frequency k=0.2, a moderate sweep airfoil shows the apparent footprint of the arch-shaped vortex, which confirms the current theory.
Experimental Studies of Pulsatile Flow Characteristics of Aortic Models under Normal and Diseased Conditions
(North Dakota State University, 2021) Zhang, Ruihang
Heart disease is the leading cause of death globally. Aorta is extremely important because of its critical function in blood circulation. Abnormal hemodynamics of aortic valve and arch is related to many severe diseases and has intrigued a growing of fluid dynamic researches over decades. However, due to the complexity of transient flow and fluid-structure interaction, many aspects of aortic hemodynamics have not been fully understood. The goal of this dissertation is to design and construct an in-vitro cardiovascular flow simulator for PIV hemodynamics research and understand the pulsatile flow characteristics of human aortic valve and arch under normal and diseased conditions. First, we investigated the fluid dynamics of a complaint aortic root model under varied cardiac outputs. High turbulence kinetic energy was observed after peak systole. A reduction in cardiac outputs resulted in a lower post-systole turbulence, smaller circumferential deformation, smaller geometric orifice area, and a shortened valve-opening period. Second, we investigated the pulsatile flow through stenotic aortic valve models. Results indicated that a severe prosthetic stenosis causes significant changes in the flow fields downstream. The hemodynamic changes, e.g., increased jet velocity and viscous shear stress, were associated with the stiffened leaflet materials, rather than the stent base structure. Third, we presented a combined experimental and numerical study of the pulsatile flow characteristics within Gothic and Romanesque aortic arch models. The results revealed significantly different primary and secondary flow characteristics between two models. Low and oscillatory wall shear stress and the abnormal secondary flow in the Gothic arch are correlated to vascular endothelial cell remodeling and might provide hints to the increased risks of atherosclerosis, late systemic hypertension, and other cardiovascular complications. Overall, this dissertation provides physical insights into pulsatile flow characteristics through aortic valve and arch models under varied normal and diseased conditions. In-vitro experiments using PIV can capture prominent flow characteristics within prosthetic aortic models, providing better controllability and spatial resolution that complements clinical diagnosis and a source of validation for computational simulations. Future improvements of artificial models’ designs and the advanced flow diagnostic techniques can further enhance the accuracy and credibility of in-vitro flow researches.
Mechanics of Surface Instabilities of Soft Nanofibers and Nonlinear Contacts of Hydrogels
(North Dakota State University, 2020) Ahmadi, Mojtaba
The research of this dissertation is formulated in two fields, i.e., the theoretical and computational studies of circumferential wrinkling on soft nanofibers and the swelling mechanics study of a bi-layered spherical hydrogel containing a hard core. Continuous polymer nanofibers have been massively produced by means of the low-cost, top-down electrospinning technique. As a unique surface instability phenomenon, surface wrinkling in circumferential direction is commonly observed on soft nanofibers in electrospinning. In this study, a theoretical continuum mechanics model is developed to explore the mechanisms of circumferential wrinkling on soft nanofibers under uniaxial stretching. The model is able to examine the effects of elastic properties, surface energy, and fiber radius on the critical axial stretch to trigger circumferential wrinkling and to discover the threshold fiber radius to initiate spontaneous wrinkling. In addition, nonlinear finite element method (FEM) is further adopted to predict the critical mismatch strain to evoke circumferential wrinkling in core-shell polymer nanofibers containing a hard core, as a powerful computational tool to simulate controllable wrinkling on soft nanofibers via co-electrospinning polymer nanofibers incorporated with nanoparticles as the core. The studies provide rational understanding of surface wrinkling in polymer nanofibers and technical approaches to actively tune surface morphologies of polymer nanofibers for particular applications, e.g. high-grade filtration, oil-water separation, polymer nanocomposites, wound dressing, tissue scaffolding, drug delivery, and renewable energy harvesting, conversion, and storage, etc. Furthermore, hydrogels are made of cross-linked polymer chains that can swell significantly when imbibing water and exhibit inhomogeneous deformation, stress, and, water concentration fields when the swelling is constrained. In this study, a continuum mechanics field theory is adopted to study the swelling behavior of a bi-layered spherical hydrogel containing a hard core. The problem is reduced into a two-point boundary value problem of a 2nd-order nonlinear ordinary differential equation (ODE) and solved numerically. Effects of material properties on the deformation, stress, and water concentration fields of the hydrogel are examined. The study offers a rational route to design and regulate hydrogels with tailorable swelling behavior for practical applications in drug delivery, leakage blocking, etc.
Mechanical Characterization and Constitutive Modeling of Rate-dependent Viscoelastic Brain Tissue under High Rate Loadings
(North Dakota State University, 2019) Farid, Mohammad Hosseini
In this dissertation, theoretical, computational, and experimental methodologies are introduced to determine the rate-dependent material properties of the brain tissue. Experiments have shown that the brain tissue is significantly rate-dependent. To examine the range of strain rates at which trauma might happen, a validated finite element (FE) human head model was initially employed to examine the biomechanics and dynamic behavior of the head and brain under impact and blast loads. The strain rates to cause traumatic brain injury (TBI) were found to be in the range of 36 to 241 1/s, under these types of loadings. These findings provided a good estimation prior to exploring the required experiments for characterizing the brain tissue. The brain samples were tested by employing unconfined compression tests at three different deformation rates of 10 (n= 10 brain samples), 100 (n=8), and 1000 mm/sec (n=12). It was found that the tissue exhibited a significant rate-dependent behavior with various compression rates. Two different material characterization approaches were proposed to evaluate the rate-dependent mechanical responses of the brain. In the first approach, based on the parallel rheological framework, a single-phase viscoelastic model which captures the key aspects of the rate-dependency in large strain behavior was introduced. The extracted material parameters showed an excellent constitutive representation of tissue response in comparison with the experimental test results (R^2=0.999). The obtained material parameters were employed in the FE simulations of the brain tissue and successfully verified by the experimental results. In the second approach, the brain tissue is modeled as a biphasic continuum, consisting of a compressible solid matrix fully saturated with an incompressible interstitial fluid. The governing equations based on conservation of mass and momentum are used to describe the solid-fluid interactions. This viscoelastic biphasic model can effectively estimate the rate-dependent tissue deformations, the hydrostatic pressure as well as fluid diffusion through the tissue. Although both single-phasic, as well as bi-phasic models, can successfully capture the key aspects of the rate-dependency in large strain deformation, it was shown the biphasic model can demystify more phenomenological behavior of this tissue that could not be perceived with yet established, single-phasic approaches.
Dynamical Modelling of an Idealized Hemispherical Skull Model with Fluid Pressure Interactions Using Modal Analysis
(North Dakota State University, 2019) Eslaminejad, Ashkan
In this dissertation, a non-invasive intracranial pressure (ICP) monitoring technique is introduced by developing a head dynamic model. The technique is based on modal frequency testing and vibration responses analysis of the skull. To examine and verify this methodology, we conducted vibration tests on a hemispherical shell to stand as a surrogate for human cranium to measure the effect of cerebrospinal fluid (CSF) pressure on human skull dynamic response; we utilized a hammer-impact modal testing methodology on the simulated hemispherical shell to extract its dynamic response characteristics. To be able to examine the CSF-skull dynamics interactions, we measured the skull impulse responses using mechanical tensile tests at different strain rates. The modal analysis by finite elements eigenvalue analysis of the upper cranium skull model was conducted to find the material properties of the skull. Linear elastic, as well as, nonlinear hyperelastic material models were assumed for the skull to find its material parameters. In the simulation of the human head, the cranium was modeled as a closed clamped hemispherical aluminum shell under internal fluid pressure. The interactions of CSF with the simulated cranium were studied and the frequency responses were obtained at different interior pressures. A numerical procedure for dynamic analysis of the systems was developed to measure the modal frequencies of the setup. We examined the changes to the peaks of frequency response under different fluid pressure. The results of modal analyses demonstrate changes in the frequency of bending-wave vibration modes, while longitudinal-wave modes are nominally altered under variable pressure conditions. A single-degree of freedom vibrational model was also developed to fit to the data for the sensitive modes. Linear regression analysis of the results reveals that the dynamic model’s equivalent damping and stiffness parameters are sensitive to fluid pressure variations while the equivalent mass parameter is relatively unaffected. As a result of this study we conclude that variance in CSF pressure has a measurable effect on the dynamic characteristics of the cranium and vice-versa. A calibrating system to connect the dynamic changes of the head can stand as a non-invasive system for ICP changes.
Improving Performance Characteristics of Poly (Lactic Acid) (PLA) Based Nanocomposites by Enhanced Dispersion of Modified Cellulose Nanocrystals (CNCs)
(North Dakota State University, 2018) Shojaeiarani, Jamileh
Poly(lactic acid), (PLA) is a biodegradable and biocompatible polymer which has attracted significant attention as a promising substitute for petroleum-based polymers. To optimize the usage of PLA in a wide range of applications, different methods such as polymer blending and the incorporation of traditional and nanofillers have been extensively explored. Cellulose nanocrystals (CNCs), rod-like nanoparticles with a perfect crystalline structure, are considered as outstanding reinforcing agent owing to the excellent mechanical properties. The optimal characteristics of CNCs as a reinforcing agent in the polymer can be achieved through homogeneous dispersion within the polymeric matrix. However, the strong hydrophilic character of CNCs due to the presence of hydroxyl groups on the surface restricts the uniform dispersion of CNCs in the PLA matrix. In this work, three surface modification treatments along with two different mechanical preparation techniques were employed to improve the dispersion quality of CNCs in the PLA matrix. Polymer adsorption, green esterification, and time-efficient esterification were used as surface modification treatments. Solvent casting and spin-coating method were employed to prepare highly concentrated CNCs masterbatches. Nanocomposites were prepared using melt extrusion, followed by an injection molding process. The morphology of masterbatches indicated better CNCs dispersion through spin-coated thin films, suggesting a high evaporation rate and the effect of centrifugal force and surface tension in the spin-coating process decrease the possibility of CNCs aggregate through the film. Consequently, nanocomposites manufactured using spin-coated masterbatches exhibited higher mechanical strength in comparison with solvent cast ones. In the case of surface modification treatments, the most uniform CNCs dispersion was observed in the nanocomposites reinforced by valeric acid through esterification technique. Higher thermal stability was also achieved through the application of esterification technique. This observation was related to the presence of DMAP on the surface of CNCs which turns into inert materials, prohibiting the thermal degradation. The higher molecular weight and lower molecular number observed in spin-coated samples in comparison with film cast nanocomposites led to the higher damping behavior in spin-coated nanocomposites. This observation indicated the more viscoelastic properties in spin-coated samples owing to the presence of more polymer chain freedom in spin-coated nanocomposites.
Development of an Anti-Corrosion Thermally Sprayed Coating System for Oil and Gas Transmission Pipeline
(North Dakota State University, 2018) Abualigaledari, Sahar
Corrosion, a leading cause of failure in metallic transmission pipelines, significantly impacts the reliability and safety of metallic pipelines. To prevent and mitigate pipeline corrosion, various non-metallic coatings and assessment methods have been implemented with different coating techniques. However, reliable, cost-effective, and environmental friendly corrosion mitigation approaches are yet needed to be achieved. Thermal metallic sprayed coatings have shown to be an effective means for pipeline corrosion prevention in marine environments with low cost, but it is not yet studied for on-shore buried and cased crossing pipelines. In this project, innovative composite self-sensing thermal sprayed coatings are proposed to prevent, monitor, mitigate, and manage pipeline corrosion for on-shore buried metallic transmission pipelines. This project focuses on developing the metallic corrosion resistant coating with thermal spray techniques. The compositions, mechanical properties, corrosion resistance, and effectiveness of composite thermal sprayed coatings have been investigated and studies theoretically, numerically, and experimentally at mechanical engineering department, NDSU, Fargo, ND.
Examination of the Impact of Helmets on the Level of Transferred Loads to the Head Under Ballistic and Blast Loads
(North Dakota State University, 2015) Salimi Jazi, Mehdi
The main causes of human Traumatic Brain Injuries (TBIs) in war zones are ballistic impacts and blast waves. While understanding the mechanism of TBI and the brain injury thresholds are in urgent needs, efficiency of helmets as injury protective is not well-examined. To address these gaps, this study investigates the impact of ballistic helmets and padding systems on the biomechanical responses of the brain under dynamic ballistics and blasts loads. A nonlinear human head-neck finite element modeling procedure has been employed for the analysis. The results are examined against de-facto standard experimental data. The response of the finite element head model (FEHM) in terms of biomechanical parameters of the brain has been examined to measure the influence of padding system materials on the level of the loads transferred to the head. The results show when a bullet hits the front of the helmet vertically, the brain experiences the highest amount of stresses in comparisons with other impact orientations. Also, low stiffness foams cause less amount of load to be transferred to the head, indicating the importance of the mechanical properties of the padding system in helmet design. Parametric studies have also been carried out to examine the efficiency of the helmet under various blast situations and intensities by varying standoff distances and orientation angles of the FEHM. The results indicate that the protected heads experience lower accelerations, and stresses than unprotected heads. In general it was found that the performance of the helmet depends on the extent of the coverage of the head by helmet. To examine the influence of the entire human body in comparison with the only head model, the torso and attached to the head was modeled and the responses of the brain to equivalent loadings were examined. In general for the first few milliseconds of the assault on the head, biomechanical parameters of the brain remain independent of the torso. However, one can iv see the body influence as times goes by. As a conclusion one can rely on the results of the head and neck model to be credible enough for brain injury analysis.
Optimized Evaluation of Bone Tissue Material Properties by Inverse Finite Element Analysis and Femur Fracture Testing
(North Dakota State University, 2015) Javid, Samad
The main objective of this research is to characterize bone inhomogeneous elastic, yield, and post-yield behaviors, using a computational-experimental approach. The current study uses the force-displacement results of one hundred four cadaveric femora that were previously tested to fracture in a fall on the hip loading configuration. Recorded force-displacement data are used to determine stiffness, yield force, and femoral strength values. Finite element (FE) models of the femora are developed from the quantitative computed tomography scans captured before the fracture tests. A power law, or a sigmoid function, is used to determine the elastic modulus from the ash densities for each case modeled. The models are used for FE simulations that mimic the experiments. Inverse finite element analysis is employed to identify the unknown coefficients in the bone density-elasticity relationships. Optimization algorithms are used to minimize the error function between the experimental and FE estimated results in a large subset of female femora. The results of the obtained relationships show a good agreement with the experimental data. This contributes to a coefficient of determination of 70%, which is higher than those of previously proposed density-elasticity relationships on the same set of femora. The parts of the bones with the densities up to 0.5 g/cm3, play an important role in the deformation of the neck and the head of the femur. While power law and sigmoid function show similar correlation in the prediction of stiffness, distribution of stresses and strains are notably different, showing different response in the yield and post-yield behavior. To simulate the material damage, a power density-yield strain relationship is used as the failure criterion in FE models, assuming a ductile and a brittle material behavior for the bone. The unknown coefficients in the density-yield strain relationship are identified for the ductile and brittle material models. The ductile material model shows a more realistic post-yield behavior iv than the brittle model, but it is computationally expensive and may face convergence issues due to nonlinearities. The brittle material model, on the other hand, estimates the bone strength fairly and, due to its simplicity, it seems more applicable for clinical use.
Mechanical and Tribological Properties of Carbon Nanofiber Reinforced High Density Polyethylene
(North Dakota State University, 2014) Xu, Songbo
High density polyethylene (HDPE) is widely used as a bearing material in industrial application because of its low friction and high wear resistance. Reinforcing polymers with the appropriate nanofillers is an effective way to obtain a variety of enhanced material properties. Carbon nanofibers (CNFs) with silane coatings (two thicknesses: 2.8 nm and 46 nm) were added into high density polyethylene (HDPE) to improve the tribological properties of the nanocomposite material. The goal of the present study is to investigate how the mechanical, thermal, and wear behavior of HDPE can be altered by the addition of either pristine or silane treated CNFs at different loading levels (0.5 wt.%, 1 wt.%, and 3 wt.%) and to model the wear of the HDPE/CNF nanocomposites under both dry and lubricated conditions. In this study, the wear and friction tests are performed on a pin-on-disc tribometer under dry, bovine serum, and phosphate buffered saline lubricated conditions. The thermal, mechanical, properties, and biocompatibility of HDPE/CNF nanocomposites are characterized and compared with those of the neat HDPE. The correlations of the wear and factors such as work of fracture, thermal conductivity, and friction force are explored. An energy-based wear model is proposed for the dry sliding condition in which a thermal analysis is derived to trace the friction energy loss in the wear process. A wear model for the lubricated condition is developed with incorporation of elastohydrodynamic lubrication theory and Reye's wear model to predict the long term wear.
Flow Accelerated Organic Coating Degradation
(North Dakota State University, 2014) Zhou, Qixin
Applying organic coatings is a common and the most cost effective way to protect metallic objects and structures from corrosion. Water entry into coating-metal interface is usually the main cause for the deterioration of organic coatings, which leads to coating delamination and underfilm corrosion. Recently, flowing fluids over sample surface have received attention due to their capability to accelerate material degradation. A plethora of works has focused on the flow induced metal corrosion, while few studies have investigated the flow accelerated organic coating degradation. Flowing fluids above coating surface affect corrosion by enhancing the water transport and abrading the surface due to fluid shear. Hence, it is of great importance to understand the influence of flowing fluids on the degradation of corrosion protective organic coatings. In this study, a pigmented marine coating and several clear coatings were exposed to the laminar flow and stationary immersion. The laminar flow was pressure driven and confined in a flow channel. A 3.5 wt% sodium chloride solution and pure water was employed as the working fluid with a variety of flow rates. The corrosion protective properties of organic coatings were monitored inline by Electrochemical Impedance Spectroscopy (EIS) measurement. Equivalent circuit models were employed to interpret the EIS spectra. The time evolution of coating resistance and capacitance obtained from the model was studied to demonstrate the coating degradation. Thickness, gloss, and other topography characterizations were conducted to facilitate the assessment of the corrosion. The working fluids were characterized by Fourier Transform Infrared Spectrometer (FTIR) and conductivity measurement. The influence of flow rate, fluid shear, fluid composition, and other effects in the coating degradation were investigated. We conclude that flowing fluid on the coating surface accelerates the transport of water, oxygen, and ions into the coating, as well as promotes the migration of coating materials from the coating into the working fluid, where coatings experience more severe deterioration in their barrier property under flowing conditions. Pure water has shown to be a much more aggressive working fluid than electrolyte solutions. The flowing fluid over the coating surface could be used as an effective acceleration method.
Nonlinear Fatigue Damage Accumulation in Aircraft Engine Alloys Multiaxial Loading
(North Dakota State University, 2013) Suman, Sandip Kumar
Fatigue is considered to be one of the most frequent phenomena in the failure of many machine parts. Most of the prior studies on fatigue have been limited to uniaxial loading cases with a primary focus on constant amplitude cycles. A detailed exploration of multiaxial fatigue under constant and variable amplitude loading scenarios for a wide variety of aircraft engine alloys has been performed in this study, and a new methodology for the accurate prediction of fatigue damage is developed. A critical-plane based constant amplitude fatigue damage model has been developed in this study which is simple in comparison to prior models developed by other researchers and reduces the computational effort. The constant amplitude fatigue damage model is further used in the development of a multiaxial variable amplitude damage estimation method, with an emphasis on estimating the damage created by both low cycle fatigue (LCF) and high cycle fatigue (HCF) cycles. A significant increase in overall fatigue damage was observed in the tests with the introduction of HCF cycles in the mission histories. The damage due to the HCF cycles has been found to be much greater than predicted by linear damage accumulation theories, although the degree of interaction between the LCF and HCF cycles was found to be very dependent on the multiaxial load paths. In addition, the HCF cycles did not contribute significantly to the accumulation of damage until a certain amount of “pre-damage” had been caused by the LCF cycles. Separate HCF damage computing approaches have been adopted in this study to accurately compute the damage produced by tensile and shear dominant HCF cycles, and a significant improvement in the accuracy of fatigue life prediction has been achieved using the new methodology.
The Physico-Chemical Investigation of Interfacial Properties in Natural Fiber/Vinyl Ester Biocomposites
(North Dakota State University, 2012) Huo, Shanshan
Bast fibers are one of most widely used types of cellulosic natural fibers. Flax fibers, a specific type of bast fiber, have historically been used as reinforcements in composites because they offer competitive advantages, including environmental and economic benefits, over mineral-based reinforcing materials. However, the poor interfacial properties due to the hydrophilicity of flax fibers and the hydrophobicity of most polymer matrices reduce the mechanical performance of flax thermoset composites. On the other hand, the structure of flax fiber is more complex than synthetic fibers, which causes most of traditional mechanical tests from the transverse direction to evaluate the interfacial properties of flax composites are not valid. In this study, the physical and chemical properties of flax fibers, vinyl ester resin and their composites are investigated. A comprehensive understanding of flax fiber, vinyl ester systems and their composites has been established. Surface modifications to the flax fiber and chemical manipulations on vinyl ester systems have been studied to improve the interfacial properties of flax/vinyl ester biocomposites. A new chemical manipulation method for vinyl ester system has been invented. The specific interlaminar shear strength of alkaline treated flax/VE with 1.5% AR shows approximately 149% increase than untreated flax/VE composites. NaOH/Ethanol treated flax/VE with AR shows 33% higher in specific flexural modulus and 73% better in specific flexural strength than untreated flax/VE composites. In addition, AR modified alkaline treated flax composites performs approximately 75% better in specific tensile modulus and 201% higher in specific tensile strength than untreated flax/VE composites. Flax/VE composite with high elastic modulus, which is higher than their theoretically predicted elastic modulus, was achieved. The effects of thermal properties of flax fibers and vinyl ester resin systems on the interfacial properties of their biocomposites were also studied. The theory of modifying the thermal properties of flax and vinyl ester to improve the interfacial adhesion has been proved by the study of the thermal residual stresses in their composites by XRD techniques.
Aerosol-Based Ultrafine Material Deposition for Microelectronics
(North Dakota State University, 2012) Hoey, Justin Michael
Aerosol-based direct-write refers to the additive process of printing CAD/CAM features from an apparatus which creates a liquid or solid aerosol beam. Direct-write technologies are poised to become useful tools in the microelectronics industry for rapid prototyping of components such as interconnects, sensors and thin film transistors (TFTs), with new applications for aerosol direct-write being rapidly conceived. This research aims to review direct-write technologies, with an emphasis on aerosol based systems. The different currently available state-of-the-art systems such as Aerosol Jet™ CAB-DW™, MCS and aerodynamic lenses are described. A review and analysis of the physics behind the fluid-particle interactions including Stokes and Saffman force, experimental observations and how a full understanding of theory and experiments can lead to new technology such as nozzle designs are presented. Finally, the applications of aerosol direct-write for microelectronics are discussed in detail including the printing of RFID antennas, contacts and active material for TFTs, the top metallization layer for solar cells, and interconnects for circuitry.
Fluid Dynamics of Material Micro-Deposition: Capillary-Based Droplet Deposition and Aerosol-Based Direct-Write
(North Dakota State University, 2012) Lutfurakhmanov, Artur
With rapid development of the direct-write technology, in addition to requirement of non-destructive printing, there is a need for non-expensive, robust, and simplified techniques of micro/nano fabrication. This dissertation proposes a new technique of non-invasive lithography called Capillary-Based Droplet Deposition and suggests improvements to existing Aerosol-Jet Direct-Write method that leads to deposition of thinner lines. A hollow capillary filled with liquid is a dispensing tool employed for the Capillary-Based Droplet Deposition method. Due to pressure applied from one side of the capillary, a liquid meniscus is formed at the opposite side of the capillary. After the meniscus touches the substrate, a liquid bridge between the capillary and substrate is formed. The capillary retraction causes the bridge rupturing and liquid droplet deposition. In the first part of this dissertation, the Capillary-Based Deposition method is considered both theoretically and experimentally. From bridge modeling, it is found that the droplet size is dependent on pressure applied, inner radius and wall thickness of the capillary, and liquid-capillary and liquid-substrate equilibrium contact angles. Three deposition scenarios are identified showing that minimum deposited droplet size is about 15% of the capillary inner diameter. Modeling results are verified in experiments with different water-glycerol solutions used as test liquid and with capillaries of wide range of inner diameters. The second part of the dissertation is devoted to theoretical investigation of the Aerosol-Jet Direct-Write method where few micron width lines are created from aerosol droplets that move along with the gas flowing through a converging micro-nozzle. Gas velocity and density profiles inside and outside of the nozzle are obtained from iv ANSYS/CFX simulation. Aerosol droplet trajectories and velocity components are calculated using all forces acting on the particles in the flow. Comparing all forces, it is found that only Stokes and Saffman forces are relevant for simulation of the gas-particle interaction. Original 1D equation for Saffman force is extended to two dimensional gas flows. For some parameter ranges, Saffman force is found to be negligibly small. Based on simulation results, two nozzle designs are proposed in order to collimate aerosol particles with diameters of 1.5-5.0 microns toward the nozzle centerline.
High-Performance Simulations for Atmospheric Pressure Plasma Reactor
(North Dakota State University, 2012) Chugunov, Svyatoslav
Plasma-assisted processing and deposition of materials is an important component of modern industrial applications, with plasma reactors sharing 30% to 40% of manufacturing steps in microelectronics production [1]. Development of new flexible electronics increases demands for efficient high-throughput deposition methods and roll-to-roll processing of materials. The current work represents an attempt of practical design and numerical modeling of a plasma enhanced chemical vapor deposition system. The system utilizes plasma at standard pressure and temperature to activate a chemical precursor for protective coatings. A specially designed linear plasma head, that consists of two parallel plates with electrodes placed in the parallel arrangement, is used to resolve clogging issues of currently available commercial plasma heads, as well as to increase the flow-rate of the processed chemicals and to enhance the uniformity of the deposition. A test system is build and discussed in this work. In order to improve operating conditions of the setup and quality of the deposited material, we perform numerical modeling of the plasma system. The theoretical and numerical models presented in this work comprehensively describe plasma generation, recombination, and advection in a channel of arbitrary geometry. Number density of plasma species, their energy content, electric field, and rate parameters are accurately calculated and analyzed in this work. Some interesting engineering outcomes are discussed with a connection to the proposed setup. The numerical model is implemented with the help of high-performance parallel technique and evaluated at a cluster for parallel calculations. A typical performance increase, calculation speed-up, parallel fraction of the code and overall efficiency of the parallel implementation are discussed in details.

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