Browsing by Author "Dr. Kara Peters, Committee Member"

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The Aerodynamic Analysis and Aeroelastic Tailoring of a Forward-Swept Wing
(2006-05-08) Roberts, David William; Dr. Kara Peters, Committee Member; Dr. Charles E. Hall, Committee Chair; Dr. James Selgrade, Committee Member
The use of forward-swept wings has aerodynamic benefits at high angles of attack and in supersonic regimes. These consist of reduction in wave drag, profile drag, and increased high angle of attack handling qualities. These increased benefits are often offset due to an increase in structural components, to overcome flutter and wing tip divergence due to high loading of the wing tips at high angles of attack. The use of composite materials and aeroelastic tailoring of the structures eliminates these instabilities without a significant increase in weight. This work presents the design of an aeroelastic wing structure for a highly forward-swept wing, and the verification of the aerodynamic and structural finite element analysis through experimental testing.
Atomistic Simulations of Fracture of 2d Graphene Systems and the Elastic Properties of Carbon Nanotubes
(2004-11-15) Jin, Yun; Dr. Kara Peters, Committee Member; Dr. Eric C. Klang, Committee Member; Dr. Jeffrey W. Eischen, Committee Member; Dr. Fuh-Gwo Yuan, Committee Chair
The essential feature of all the macroscopic mechanical properties of a material are governed by constitutive atoms and the basic laws of physics. The traditional descriptions of fracture phenomena in solids have been developed almost exclusively from continuum concepts. These continuum approaches have successfully described many fracture mechanism in solid materials and continue to be of great use. Nevertheless, they still have some shortcomings, such as the spurious singularity of the stress field at crack tip. Hence, atomistic predictions may open new avenues in the studies of microscopic origins of material failure behavior. Also the highly active researches on the lightweight nano-structured materials in the past decade revealed the emergence of interests in predicting the properties of materials in the atomic level before they are synthesized. In this dissertation the fracture mechanism of a nanostructure material has been investigated by atomistic simulations. Macroscopic fracture parameters have been examined from both atomistic simulation and continuum models. There is a very good agreement between atomistic simulation and theoretical results from LEFM for the energy release rate in a semi-infinite graphene sheet containing cracks. The atomic description of the stress field in this case is also obtained and matches very well with the linear elastic solutions. Then another case in which a center-cracked graphene sheet with finite width is proposed. The energy release rates are obtained in both global energy approach and local force approach. These simulations show that, in macroscopic fracture mechanics under small deformation, linear elastic fracture mechanics is sufficient for the description of cracking behavior for this covalently bonded material. The results merge the discrete (atomistic) and continuum (macroscopic) description of facture. Meanwhile, the method to calculate J integral in the atomic system is successfully developed. The numerical results of J integral agree very well with the energy release rate in the same systems. Then after a necessary modification on the Tersoff-Brenner potential, the critical value of J integral, denoted by , is eventually reached as the measure of the fracture toughness of graphene sheet. The mechanical properties of single-walled carbon nanotube (SWNT) have also been evaluated in this dissertation. Several elastic moduli of SWNTs using the MD simulations were obtained at the atomic scale. The values of the in-plane Young¬°¬Øs modulus, rotational shear modulus, and in-plane Poisson's ratio are in the range of existing theoretical and experimental results. It has been shown from simulations that overall the elastic constants of SWNTs are insensitive to the morphology pattern such as nanotube radius and thus the effect of curvature on the elastic constants can be neglected. Assumption of the transversely isotropic properties on the cylinder surface of the single-walled nanotube was confirmed by numerical calculations. Besides the conventional energy approach, a new method, which denoted as force approach in the dissertation, has been developed to analysis the elastic properties of carbon nanotubes. The results from two approaches matched very well. The advantage of the force approach is that it can provide more accurate prediction than the energy approach. Furthermore, the force approach can predict the nonlinear behavior without assumption of assumed total potential energy in quadratic form described for small-strain deformation in the energy approach.
Behavior of FRP Repair/Strengthening Systems for Prestressed Concrete
(2006-08-23) Rosenboom, Owen Arthur; Dr. Mervyn Kowalsky, Committee Member; Dr. Kara Peters, Committee Member; Dr. Paul Zia, Committee Member; Dr. Sami Rizkalla, Committee Chair
This research study examines the behavior of prestressed concrete beams retrofitted with Fiber Reinforced Polymer (FRP) materials. Due to deficiencies in the built environment, engineers may be asked to retrofit or upgrade the capacity of an existing concrete structural member. This could be a result of new demands on the structure, or a repair of damage from an unforeseen event. Retrofits are possible using the traditional building materials of concrete and steel. The cross-section of the structural element can be increased, or steel plates can be bolted or adhesively affixed to the structure to increase capacity. Many of these techniques are costly, and some perform poorly under service conditions. The main benefit for using FRP materials for the strengthening of existing structures is the lightweight nature of the composite material, which makes the use of extensive scaffolding (required in the installation of steel plates) obsolete. The objectives of this research are twofold. First, the overall structural behavior of an FRP strengthened or repaired concrete beam is studied. Two different loading conditions are examined: extreme loading simulated by a monotonic load to failure, and fatigue loading designed to simulate service loads. The structural behavior of the system is evaluated under these different conditions, and an analytical model is presented which predicts the flexural behavior of the system assuming certain failure modes. The second objective of this research is to evaluate the bond behavior of an FRP strengthened reinforced or prestressed concrete flexural member. A database of experimental failures was constructed, and an analytical model is proposed which predicts the bond failure of the FRP strengthening system.
Bond Characteristics and Environmental Durability of CFRP Materials for Strengthening Steel Bridges and Structures
(2008-08-19) Dawood, Mina Magdy Riad; Dr. Sami Rizkalla, Committee Chair; Dr. Emmett Sumner, Committee Member; Dr. Murthy Gudatti, Committee Member; Dr. Kara Peters, Committee Member
This dissertation presents the findings of a research program that was conducted in two parts to investigate the bond behavior and environmental durability of carbon fiber reinforced polymer (CFRP) materials for strengthening steel bridges and structures. The first part of the research consisted of an experimental and analytical research program to investigate the bond characteristics of CFRP lap-splice joints. The experimental program included a total of eight double-lap shear tests and ten steel beam tests. The main parameters considered include the geometric configuration of the plate ends, the length of the splice plates and the use of mechanical anchorage near the plate ends. A finite element analysis was conducted to determine the distribution of the stresses within the adhesive layer for different splice configurations. The findings indicate that the presence of the reverse tapered plate end reduced the magnitude of the peak stresses in the adhesive layer thereby increasing the tension strength of the splice joints. Increasing the splice length and installing additional mechanical anchorage did not enhance the strength of the joints. Based on the findings, a method is proposed to design lap-splice joints for implementation of the proposed CFRP system on longer-span flexural members. The second part of the research consisted of a total of 44 steel-CFRP double-lap shear tests to study the environmental durability of the proposed CFRP strengthening system. The specimens were exposed to accelerated corrosion conditions and subsequently loaded monotonically to failure. The additional use of a silane adhesion promoter or a glass fiber insulating layer, to enhance the bond durability, was also studied. The findings indicate that the presence of the glass fiber layer enhanced the initial bond strength of the system, while the use of a silane adhesion promoter was essential to ensure the durability of the system. The findings of this research program demonstrate that, with proper detailing, the proposed CFRP system can be effectively used for strengthening of steel bridges and structures.
Database for Real-Time Loading Path Prediction for Tube Hydroforming
(2009-11-02) Deshmukh, Karan; Dr. Gracious Ngaile, Committee Chair; Dr. Jeffrey Eischen, Committee Member; Dr. Kara Peters, Committee Member
Development of the Metal Foam Electrical Resistance Heater
(2003-04-14) Cookson, Edward James; Dr. Albert Shih, Committee Chair; Dr. Kara Peters, Committee Member; Dr. John Strenkowski, Committee Member
This thesis presents a novel concept using a radial heating element made from porous Fe-Cr-Al metal foam in an air heater. Electrical resistance heating has been used extensively to convert the electrical energy into thermal energy. An analytic heat transfer model is first developed to estimate dimensions of the heating element. Four prototype Fe-Cr-Al metal foam electrical heaters with different levels of porosity and density are built. A more detailed computational fluid dynamics modeling of prototype heaters to include the temperature loss to the surroundings is developed. Experiments were conducted to evaluate effects of airflow rates and electrical current and measure the change of air inlet and outlet temperatures. The temperature rise in the airflow is directly proportional to electric current, and inversely proportional to the weight density of the foam. The temperature appears directly proportional to airflow rate in low density foams, while it is inversely proportional in foams of higher relative density. Experimental temperature measurements show reasonable agreement with modeling predictions. Finally, possible improvements to the initial concept are discussed.
Elastic Wave Propagation in Composites and Least-squares Damage Localization Technique
(2004-07-29) Wang, Lei; Dr. Fuh-Gwo Yuan, Committee Chair; Dr. Kara Peters, Committee Member; Dr. Jeffrey W. Eischen, Committee Member
The main objective of Structural Health Monitoring (SHM) is to be able to continuously monitor and assess the status of the integrity of a structure or its components with a high level of confidence and reliability. In general, the common techniques employed in SHM for monitoring structures and detecting damages can be divided into two categories: (1) vibration-based approach and (2) wave-based approach. Since wave-based approach can provide better local health status information and has higher sensitivity to damages than vibration-based approach, this thesis focuses on damage localization of plate structure using wave-based approach by first characterizing elastic waves in composite laminates; then using a time-frequency signal processing technique to analyze dispersive stress waves; and lastly a least-squares technique is proposed for damage localization. Exact solutions of dispersive relations in a composite lamina and composite laminate are first deduced from three-dimensional (3-D) elasticity theory. The dispersion relations containing infinite number of symmetric and antisymmetric wave modes are numerically solved. Then, to make dispersive wave solutions tractable in composites, a higher-order plate theory is proposed. The dispersion relations of three antisymmetric wave modes and five symmetric wave modes can be analytically determined. The dispersion curves of phase velocity and group velocity are obtained from the two theories. From the results of the 3-D elasticity theory and higher-order plate theory, it can be seen from dispersion curves that the higher-order plate theory gives a good agreement in comparison with those obtained from 3-D elasticity theory in the relatively high frequency range; and especially for the lowest symmetric and antisymmetric modes, dispersion relation curves obtained from the two theories match very well. In the Chapter of time-frequency analysis of dispersive waves, a Wavelet Transform (WT) is directly performed on a transient dispersive wave to extract the time-frequency information of transient waves. Consequently, the dispersion relations of group velocity and phase velocity can be mathematically obtained. Experiments are set up to verify the proposed WT method, in which a lead break is used as a simulated acoustic emission source on the surface of an aluminum plate. The dispersion curves of both phase and group velocities of the lowest flexural wave mode obtained from the experiments by using WT show good agreement with theoretical prediction values. Having group velocities verified from the experiments, a least-squares method is proposed to SHM field for iteratively searching damage location based on elastic wave energy measurements. The method is suitable for achieving automated SHM system since the proposed method is based on active damage detection technique and deals with the entire sensor data in the least-squares algorithm without the need of ambiguously measuring the time-of-flights. The simulated data are obtained from finite difference method in conjunction with Mindlin plate theory. Simulated examples for damage detection are demonstrated by using the least-squares method. Moreover, an active SHM system is set up to validate the feasibility of the least-squares damage localization technique. From the simulated and experimental results, it is shown that the estimated damage position by least-squares method gives good agreement with the targeted damage location.
Influence of Weld Sequence on the Seismic Failure of Welded Steel Moment Connections in Building Structures
(2009-05-04) Syed, Sammiuddin Quadri; Dr. Mervyn J. Kowalsky , Committee Member; Dr. Emmett. A. Sumner, Committee Co-Chair; Dr. Tasnim Hassan, Committee Chair; Dr. Kara Peters, Committee Member
Over a decade of research activities after the Northridge earthquake have developed modified designs of welded steel moment connections (WSMCs) with improved ductility performance. Various modifications have been made, since then, to weld access hole and weld details towards improvement of structural performance with emphasis on global parameters such as drift, strength and stability. However the recent research by Castiglioni demonstrates that new designs of WSMCs may still fail in a brittle manner. While test results on the new connections demonstrate significant improvement with regards to ductility, little work has been done to fully understand the localized failure mechanism of WSMCs. Previously a study at NC State by Lu(2003) demonstrated that the localized failure mechanism and fatigue life of welded piping joints is directly influenced by the welding procedure and welding sequence. Hence this current research makes an effort to investigate the influence of weld sequence on the seismic failure of WSMCs. Further an attempt is made to demonstrate a correlation between the localized failure mechanism and global structural performance. The primary goal of this research is to observe and understand the local behavior of the WSMCs through analysis and a set of structural experiments on specially designed tee-joint specimen and full-scale welded joints. A tee-joint specimen consists of a short section of bottom beam flange welded to the column flange on both ends using a complete joint penetration weld. Further a tee-joint specimen yields two set of data for analysis when compared to that of full-scale joint. Two different types of weld sequences are employed in the fabrication of the test specimens to gain an insight of the effect of weld sequence on the fatigue life of the WSMCs. In addition, the cyclic loading tests performed on full-joint welded steel moment connections with weld sequences similar to that of the tee-joints for understanding the relationship between the local and global responses. Finite element simulations of the full-scale WMSCs are conducted using ANSYS with Chaboche and multi-linear material models. These pretest analyses are utilized to develop the loading protocol for the experimental program. Both the full-joint and tee-joint specimens showed brittle failures when subjected to constant amplitude cyclic loading. It is also observed that the fatigue cracks in all the experiments occurred at the weld toe of the complete joint penetration weld. Recorded strain data from the strain gages located near the complete joint penetration welds demonstrated the presence of ratcheting. This observation is further supported by the symmetric strain response (no ratcheting) in the strain gages located away from the welded joint. This strain ratcheting response may also influence the formation of cracks near the welds leading to the brittle failure of the WSMCs. The two tee-joint and full-scale specimens have shown varied fatigue life indicating the affect of weld sequence used in their fabrication. In conclusion, this research investigates local and global failure responses of welded steel moment connections with different weld sequences.
An Integrated Microoptical Microfluidic Device for Agglutination Detection and Blood Typing
(2007-07-31) Alexander, Stewart Parks IV; Dr. M. K. Ramasubramanian, Committee Chair; Dr. William Roberts, Committee Member; Dr. Kara Peters, Committee Member
Blood type identification is an important requirement for many medical procedures, especially blood transfusions. Currently, medical professionals have several ways for determining a person's blood type; however the potential for human error is a factor in all these ways. No automated process exists that takes this human error out of the equation without great expense. The accuracy of testing methods used on a large scale relies heavily upon the experience of the technician performing the test. Pervious work performed at NC State University in this area made use of the light-scattering properties of particles with a macoscopic sample. The device described in this paper uses a much smaller sample and overall can be miniaturized significantly. The focus is a microfluidic device that is able to detect blood type compatibility. It specifically does this by identifying agglutinated blood cells vs. free non‐agglutinated blood cells. The fluid portion of the apparatus is a polymer based two dimensional microfluidic device. It provides channels for the fluid flow but also holds and very accurately aligns two fiber optic cables that are used for agglutination detection. In short the device has a fluid channel perpendicular to two fiber optic cables. The fluid, a blood/saline mixture, flows in between the two cables. When a red blood cell passes across the beam of light some amount of the light is absorbed by the cell and some it is scattered, the rest continues on to the receiver fiber. When an agglutinated cell passes through the gap between the fibers more of the light is absorbed and scattered than as with the individual cell. This larger reduction in amplitude of light transmitted to the receiver fiber is indicative of red blood cell agglutination and is ultimately how the device determines blood type compatibility. Another way to setup the device makes use of Mie light scattering to detect agglutination. This device is solely a research piece of equipment in its current configuration but has very appealing qualities that would allow it to easily be scaled down into a microelectromechanical system (MEMS) device. From the results obtained one can clearly see that the device is able to detect an agglutinated sample vs. a non agglutinated sample.
Investigation of Tube Hydroforming Process Envelope for Macro/Meso Scalability
(2008-06-11) Gibson, M. Christopher; Dr. Kara Peters, Committee Member; Dr. Jeffrey Eischen, Committee Member; Dr. Gracious Ngaile, Committee Chair
A Method for the Encapsulation of MicroSpherical Particles
(2007-08-15) Fisher, Sarah M; Dr. M. K. Ramasubramanian, Committee Chair; Dr. William Roberst, Committee Member; Dr. Kara Peters, Committee Member
Microstructural Modeling and Design Optimization of Adaptive Thin-Film Nanocomposite Coatings For Durability and Wear
(2009-02-17) Pearson, James Deon; Dr. Mohammed Zikry, Committee Chair; Dr. Kara Peters, Committee Member; Dr. Yong Zhu, Committee Member; Dr. Jeffrey Eischen, Committee Member
Adaptive thin-film nanocomposite coatings comprised of crystalline ductile phases of gold and molybdenum disulfide, and brittle phases of diamond like carbon (DLC) and ytrria stabilized zirconia (YSZ) have been investigated by specialized microstructurally-based finite-element techniques. A new microstructural computational technique for efficiently creating models of nanocomposite coatings with control over composition, grain size, spacing and morphologies has been developed to account for length scales that range from nanometers to millimeters for efficient computations. The continuum mechanics model at the nanometer scale was verified with molecular dynamic models for nanocrystalline diamond. Using this new method, the interrelated effects of microstructural characteristics such as grain shapes and sizes, matrix thicknesses, local material behavior due to interfacial stresses and strains, varying amorphous and crystalline compositions, and transfer film adhesion and thickness on coating behavior have been investigated. A mechanistic model to account for experimentally observed transfer film adhesion modes and changes in thickness was also developed. One of the major objectives of this work is to determine optimal crystalline and amorphous compositions and behavior related to wear and durability over a wide range of thermo-mechanical conditions. The computational predictions, consistent with experimental observations, indicate specific interfacial regions between DLC and ductile metal inclusions are critical regions of stress and strain accumulation that can be precursors to material failure and wear. The predicted results underscore a competition between the effects of superior tribological properties associated with MoS2 and maintaining manageable stress levels that would not exceed the coating strength. Varying the composition results in tradeoffs between lubrication, toughness, and strength, and the effects of critical stresses and strains can be controlled for desired behavior. The analysis also indicates that coating strength increases at a higher rate than the internal coating stresses with decreasing grain size. For transfer films, the present study underscores the beneficial material effects of increasing transfer film thickness and reducing transfer film extrusion due to increased thickness.
Performance of Post-Tensioned Clay Brick Masonry with Openings
(2008-08-12) Ewing, Bryan Darnell; Dr. Mervyn Kowalsky, Committee Chair; Dr. James Nau, Committee Member; Dr. Paul Zia, Committee Member; Dr. Kara Peters, Committee Member
This dissertation aims to advance the understanding of unbonded post-tensioned masonry wall systems. Previous research has shown that unbonded post-tensioned masonry walls can adequately resist in-plane loading but their possible use in regions of high seismic activity has not been widely accepted. The research described in this dissertation focuses primarily on clay brick masonry. The first study is on the in-plane cyclic behavior of unbonded post-tensioned masonry walls with openings. Openings can interrupt the standard path of the compression strut. The compression strut is how unbonded post-tensioned masonry walls distribute the lateral load to the foundation, and without it the wall can become unstable. The results show that the size and location of the opening has a major effect of the overall response of the wall. As the opening size increases the compression strut becomes more unstable.. Experimental studies involved the construction and testing of three walls. A parametric study was conducted to determine the effect of opening size and aspect ratio on the behavior of unbonded post-tensioned masonry walls with openings. Several tables are proposed for the initial design of these walls depending on the opening size and aspect ratio of the wall. The latter part of the dissertation focuses on the Direct Displacement-Based Design (DDBD) of unbonded post-tensioned clay brick masonry walls. A unique problem of the use of clay brick masonry walls arose and was studied. Because clay brick masonry and the concrete foundation's Young moduli are different, the interaction between the two surfaces was analyzed. It is shown that the foundation locally confines the clay brick masonry, thereby increasing its compressive strength. Without including this confinement, effect the lateral resisting strength is greatly underestimated. Previous methods are modified to predict the compressive strength of clay brick masonry at the wall/foundation interface. The method is verified against previous unbonded post-tensioned clay brick masonry walls and established methods of calculating the compressive strength of masonry prisms. Then using the proposed method of calculating the compressive strength of clay brick masonry at the interface, a design methodology is proposed for unbonded post-tensioned clay brick masonry walls.
Strengthening of Steel Structures with High Modulus Carbon Fiber Reinforced Polymer (CFRP) Materials
(2005-06-15) Schnerch, David; Dr. Sami Rizkalla, Committee Chair; Dr. Emmett Sumner, Committee Member; Dr. Mervyn Kowalsky, Committee Member; Dr. Kara Peters, Committee Member
Transportation departments and the telecommunications industry are currently demanding cost-effective rehabilitation and/or strengthening techniques for steel structures, including bridges and monopole towers. Rehabilitation is often required due to cross-section losses resulting from corrosion damage and strengthening may be required due to changes in the use of a structure. Current strengthening techniques, have several disadvantages including their cost, need to match the surface configuration of the existing structure, poor fatigue performance and the need for ongoing maintenance due to continued corrosion attack. The current research program makes use of new high modulus types of carbon fiber for strengthening steel structures. The experimental program was developed in four phases. These phases included the selection of suitable resins and adhesives for bonding the CFRP sheets and strips to the steel, characterization the bond to the steel through testing of the development length, performing large-scale tests on strengthened steel monopole towers and also determining the behavior of strengthened steel-concrete composite beams that are typical of bridge structures. The result of the experimental program was the demonstration of sizeable strength and stiffness increases for the steel structures, strengthened with the developed system. Analytical work has also been completed to predict these strength and stiffness increases as well as to determine the bond stresses to ensure the avoidance of a debonding failure, which is detrimental to the effective use of the high modulus CFRP material.
Stresses in the Cranial Cruciate Ligament Deficient Canine Stifle Following Three Tibial Osteotomy Procedures: A Finite Element Analysis.
(2008-10-03) Crimi, Christina Marie; Dr. Ola Harrysson, Committee Co-Chair; Dr. Andre P. Mazzoleni, Committee Chair; Dr. Denis J Marcellin-Little, Committee Member; Dr. Kara Peters, Committee Member
Wrinkling analysis of thin membranes
(2008-01-09) Adilapuram, Srinivas; Dr. Kara Peters, Committee Member; Dr. Mohammed .A. Zikry, Committee Member; Dr. Jeffrey W. Eischen, Committee Chair