VTechWorks Repository :: Browsing by Author "Lesko, John J."

Browsing by Author "Lesko, John J."

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Ac-dc Bus-interface Bi-directional Converters in Renewable Energy Systems
Dong, Dong (Virginia Tech, 2012-07-25)
This dissertation covers several issues related to the ac-dc bus-interface bi-directional converters in renewable energy systems. The dissertation explores a dc-electronic distribution system for residential and commercial applications with a focus on the design of an ac-dc bi-directional converter for such application. This converter is named as the "Energy Control Center" due to its unique role in the system. First, the impact of the unbalanced power from the ac grid, especially the single-phase grid, on the dc system operation is analyzed. Then, a simple ac-dc two-stage topology and an advanced digital control system is proposed with a detailed design procedure. The proposed converter system significantly reduces the dc-link capacitor volume and achieves a dynamics-decoupling operation between the interfaced systems. The total volume of the two-stage topology can be reduced by upto three times compared with the typical design of a full-bridge converter. In addition, film capacitors can be used instead of electrolytic capacitors in the system, and thus the whole system reliability is improved. A set of ac passive plus active filter solutions is proposed for the ac-dc bus-interface converter which significantly reduces the total power filter volume but still eliminate the total leakage current and the common-mode conducted EMI noises by more than 90%. The dc-side low-frequency CM voltage ripple generated by the unbalanced ac voltages can be eliminated as well. The proposed solution features a high reliability and fits three types of the prevalent low-voltage ac distribution systems. Grid synchronization, a critical interface control in ac-dc bus-interface converters, is discussed in detail. First, a novel single-phase grid synchronization solution is proposed to achieve the rejection of multiple noises as well as the capability to track the ac voltage amplitude. Then, a comprehensive modeling methodology of the grid synchronization for three-phase system is proposed to explain the output frequency behaviors of grid-interface power converters at the weak grid, at the islanded condition, and at the multi-converter condition. The proposed models provide a strong tool to predict the grid synchronization instabilities raised from industries under many operating conditions, which is critical in future more-distributed-generation power systems. Islanding detection issues in ac-dc bus-interface converters are discussed in detail. More than five frequency-based islanding detection algorithms are proposed. These solutions achieve different performances and are suitable for different applications, which are advantageous over existing solutions. More importantly, the detailed modeling, trade-off analysis, and design procedures are given to help completely understand the principles. In the end, the effectiveness of the proposed solutions in a multiple-converter system are analyzed. The results drawn from the discussion can help engineers to evaluate other existing solutions as well.
Advanced Polymeric Membranes and Multi-Layered Films for Gas Separation and Capacitors
Shaver, Andrew Thomas (Virginia Tech, 2016-06-30)
The following studies describe the synthesis and properties of a family of poly(arylene ether ketone)s which are well known to have good thermal stability, mechanical durability, and other film properties. These poly(arylene ether ketone)s were functionalized with fluorine, oxidized, blended, and crosslinked to increase performance with focus on materials for polymeric capacitors and gas separation membranes. There is a need for polymeric capacitors with improved energy storage density and thermal stability. In this work, the affect of polymer molecular structure and symmetry on Tg, breakdown strength, and relative permittivity was investigated. A systematic series of four amorphous poly(arylene ether ketone)s was compared. Two of the polymers had symmetric bisphenols while the remaining two had asymmetric bisphenols. Two contained trifluoromethyl groups while the other two had methyl groups. The symmetric polymers had Tg's of approximately 160 °C while the asymmetric polymers showed higher Tg's near 180 °C. The symmetric polymers had breakdown strengths near 380 kV/mm at 150 °C. The asymmetric counterparts had breakdown strengths near 520 kV/mm even at 175 °C, with the fluorinated polymers performing slightly better in both cases. The non-fluorinated polymers had higher relative permittivities than the fluorinated materials, with the asymmetric polymers being better in both cases. Two amorphous, high glass transition, crosslinkable poly(arylene ether)s for gas purification membranes have been studied. The polymers were polymerized via step growth and contained tetramethyl bisphenol F and either 4,4'-difluorobenzophenone or 4,4'-dichlorodiphenylsulfone. The benzylic methylene group in tetramethyl bisphenol F can undergo oxidation reactions and crosslinking with UV light. The polymers were oxidized under two different conditions, one by chemical treatment using oxone and KBr and one by elevated thermal treatment in air. Thermogravimetric analysis, 1H-NMR and attenuated total reflectance Fourier transform infrared spectroscopy revealed the progress of the thermal oxidation reactions. Both polymers produced tough, ductile films and gas transport properties of the non-crosslinked linear polymers and crosslinked polymer was compared. Crosslinking was performed by irradiating polymer films for one hour on each side in air under a 100W high intensity, long-wave UV lamp equipped with a 365-nm light filter. The O2 permeability of tetramethyl bisphenol F containing non-crosslinked poly(arylene ether ketone) was 2.8 Barrer, with an O2/N2 selectivity of 5.4. Following UV crosslinking, the O2 permeability decreased to 1.8 Barrer, and the O2/N2 selectivity increased to 6.2. Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) is a commercial polymer that is utilized for gas separation membranes. It has a relatively high free volume with high gas permeabilities but suffers from low gas selectivities. In this study, PPO polymers with number average molecular weights of 2000, 6000, 17,000, 19,000 and 22,000 were synthesized and blended with a poly(arylene ether ketone) synthesized from bisphenol A and difluorobenzophenone (BPA-PAEK) to make UV-crosslinkable films. The ketone and benzylic methylene groups on the BPA-PAEK and the PPO polymers respectively formed crosslinks upon exposure to broad wavelength UV light. The crosslinked blends had increased selectivities over their linear counterparts. DSC thermograms showed that the blends with all but the lowest molecular weight PPO had two Tg's, thus suggesting that two phases were present, one high in PBA-PAEK and the other high in PPO composition. The PBA-PAEK blend with the 2000 Mn PPO showed only one Tg between the two control polymers. Despite the immiscibility of these films, the gel fractions after UV exposure were high. Gel fractions as a function of the amount of the 22,000 Mn PPO were explored and did not show any significant change. UV spectroscopy of the individual components and the blends showed that more broad wavelength light was transmitted through the PPO component, so it was reasoned that films that was high in PPO composition crosslinked to deeper depths. The O2/N2 permeabilities and selectivities were measured for the linear and crosslinked films. Between the 33/67, 67/33, and 90/10 22k PPO/BPA PAEK crosslinked blended films, the 90/10 PPO/BPA PAEK gained the most selectivity and maintained a larger amount of its permeability. In comparison to commercial gas separation polymers, the non-crosslinked 33/67 22,000 Mn PPO/BPA PAEK blend outperformed polysulfone and cellulose acetate with a 2.45 degree of acetylation. Overall, we were able to blend a small amount of BPA PAEK with the commercially used PPO to create a mechanically robust crosslinked polymer film.
An Algorithm and System for Measuring Impedance in D-Q Coordinates
Francis, Gerald (Virginia Tech, 2010-01-25)
This dissertation presents work conducted at the Center for Power Electronics Systems (CPES) at Virginia Polytechnic Institute and State University. Chapter 1 introduces the concept of impedance measurement, and discusses previous work on this topic. This chapter also addresses issues associated with impedance measurement. Chapter 2 introduces the analyzer architecture and the proposed algorithm. The algorithm involves locking on to the voltage vector at the point of common coupling between the analyzer and the system via a PLL to establish a D-Q frame. A series of sweeps are performed, injecting at least two independent angles in the D-Q plane, acquiring D- and Q-axis voltages and currents for each axis of injection at the point of interest. Chapter 3 discusses the analyzer hardware and the criteria for selection. The hardware built ranges from large-scale power level hardware to communication hardware implementing a universal serial bus. An eight-layer PCB was constructed implementing analog signal conditioning and conversion to and from digital signals with high resolution. The PCB interfaces with the existing Universal Controller hardware. Chapter 4 discusses the analyzer software. Software was written in C++, VHDL, and Matlab to implement the measurement process. This chapter also provides a description of the software architecture and individual components. Chapter 5 discusses the application of the analyzer to various examples. A dynamic model of the analyzer is constructed, considering all components of the measurement system. Congruence with predicted results is demonstrated for three-phase balanced linear impedance networks, which can be directly derived based on stationary impedance measurements. Other impedances measured include a voltage source inverter, Vienna rectifier, six-pulse rectifier and an autotransformer-rectifier unit.
Assessment of Infrared Thermography for NDE of FRP Bridge Decks
Miceli, Marybeth (Virginia Tech, 2000-12-11)
Statistics released in the fall 1989 showed that 238,357 (41%) of the nation's 577,710 bridges are either structurally deficient or functionally obsolete. New materials, such as fiber reinforced polymeric composites (FRP), are being suggested for use in bridge systems to solve some of the current problems. These materials are thought to be less affected by corrosive environmental conditions than conventional civil engineering materials. Therefore they may require less maintenance and provide longer life spans. More specifically, glass fiber reinforced vinyl ester matrix composites are considered possible replacements for deteriorating conventional bridge decks due to their durability, decreased weight, and relative affordability. In order to facilitate rapid acceptance of FRP structural components into the world of civil structural engineering, effective and efficient NDE techniques must be explored and documented in these situations. This thesis will discuss the use of Infrared Thermography (IRT) as a means of detecting debonds and voids caused by conditions encountered both in fabrication and in the field. As forced convective hot air is applied within the bridge deck, debonds between bridge deck components near the riding surface appear cold while imperfections near the bottom of the deck give rise to concentrations of heat. These variations in thermal propagation patterns are observed by the infrared camera and indicate possible structural deficiencies. Results of experimentation and thermal analyses from laboratory studies of a model bridge deck and some from in situ full-scale investigations are presented.
C-Grid as Shear Reinforcement in Concrete Bridge Girders
Ward, John Charlton III (Virginia Tech, 2016-03-28)
Corrosion of reinforcing steel causes shorter life spans in bridges throughout the United States. The use of carbon fiber reinforced polymer (CFRP) materials as the flexural reinforcement in bridge girders has been extensively studied. However, CFRP transverse reinforcement has not been as rigorously investigated, and many studies have focused on CFCC stirrups. The use of C-Grid as an option for transverse reinforcing has not been previously investigated. This thesis concludes that C-Grid is a viable shear design option and presents the initial recommendations for design methods. These methods provide a basis for the design of C-Grid shear reinforcing that could be used as a starting point for future testing of full scale specimens. This testing program first determined the mechanical properties of C-Grid and its development length. Four 18 ft long 19 in. deep beams, modeled after prestressed Bulb-T beams, were created to test the C-Grid, as well as steel and CFCC stirrups. The beams were loaded with a single point load closer to one end to create a larger shear load for a given flexural moment. Overall beam displacement was measured to determine when flexural reinforcement yielding was reached, and beams were fitted with rosettes and instrumentation to capture initiation of shear cracking. Shear capacity calculations following four methods were compared to test results. The design method should follow the AASHTO modified compression field theory with equations for β and θ. The manufacturer's guaranteed strength should be used for design as long as that strength is the average reduced by three standard deviations. Shear crack widths are controlled to a similar size as steel stirrups when using at least two layers of grid.
Carbon Nanotube Mechanics: Continuum Model Development from Molecular Mechanics Virtual Experiments
Sears, Aaron Thomas (Virginia Tech, 2006-11-07)
Carbon Nanotubes (CNTs) hold great promise as an important engineering material for future applications. To fully exploit CNTs to their full potential, it is important to characterize their material response and ascertain their material properties. We have used molecular mechanics (MM) simulations to conduct virtual experiments on single-wall and multi-wall carbon nanotubes (SWNTs and MWNTs respectively) similar to those performed in the mechanics of materials laboratory on a continuum structure. The output (energy and deformation rather than the load and deflection) is used to understand the material response and formulate macroscopic constitutive relations. From results of MM simulations of axial and torsional deformations on SWNTs, Young's modulus, the shear modulus and the wall thickness of an equivalent continuum tube made of a linear elastic isotropic material were found. These values were used to compare the response of the continuum tube, modeled as an Euler-Bernoulli beam, in bending and buckling with those obtained from the MM simulations. MM simulations have been carried out to find energetically favorable double-walled carbon nanotube (DWNT) configurations, and analyze their responses to extensional, torsional, radial expansion/contraction, bending, and buckling deformations. Loads were applied either to one wall or simultaneously to both walls of an open-ended DWNT. These results were compared against SWNT results. It was found that for simple tension and torsional deformations, results for a DWNT can be derived from those for its constituent SWNTs within 3% error. Radial deformations of a SWNT were achieved by considering a DWNT with the SWNT as one of its walls and moving radially through the same distance all atoms of the other wall of the DWNT thereby causing a pseudo-pressure through changes in the cumulative van der Waals forces which deform the desired wall. Results of radial expansion/contraction of a SWNT were used to deduce an expression for the van der Waals forces, and find through-the-thickness elastic moduli (Young's modulus in the radial direction, Er, and Poisson's ratio ?r?) of the SWNT. We have found four out of the five elastic constants of a SWNT taken to be transversely isotropic about a radial line. MWNTs were studied using the same testing procedures as those used SWNTs. Based on the results from those simulations a continuum model is proposed for a MWNT whose response to mechanical deformations is the same as that of the MWNT. The continuum structure is comprised of concentric cylindrical tubes interconnected by truss elements. Young's modulus, Poisson's ratio, the thickness of each concentric tube, and the stiffness of the truss elements are given. The proposed continuum model is validated by studying its bending and buckling deformations and comparing these results to those from MM simulations. The major contributions to the field on nanotubes and the scientific literature is a simple and robust continuum model for nanotubes. This model can be used to study both SWNTs and MWNTs in either global or local responses by applying different analytic techniques. This model was developed using a consistent engineering methodology that mimicked traditional engineering testing, assumptions and constraints.
Characterization and Modeling of a Fiber-Reinforced Polymeric Composite Structural Beam and Bridge Structure for Use in the Tom's Creek Bridge Rehabilitation Project
Hayes, Michael David (Virginia Tech, 1998-12-12)
Fiber reinforced polymeric (FRP) composite materials are beginning to find use in construction and infrastructure applications. Composite members may potentially provide more durable replacements for steel and concrete in primary and secondary bridge structures, but the experience with composites in these applications is minimal. Recently, however, a number of groups in the United States have constructed short-span traffic bridges utilizing FRP members. These demonstration cases will facilitate the development of design guidelines and durability data for FRP materials. The Tom's Creek Bridge rehabilitation is one such project that utilizes a hybrid FRP composite beam in an actual field application. This thesis details much of the experimental work conducted in conjunction with the Tom's Creek Bridge rehabilitation. All of the composite beams used in the rehabilitation were first proof tested in four-point bending. A mock-up of the bridge was then constructed in the laboratory using the actual FRP beams and timber decking. The mock-up was tested in several static loading schemes to evaluate the bridge response under HS20 loading. The lab testing indicated a deflection criterion of nearly L/200; the actual field structure was stiffer at L/450. This was attributed to the difference in boundary conditions for the girders and timber panels. Finally, the bridge response was verified with an analytical model that treats the bridge structure as a wood beam resting upon discrete elastic springs. The model permits both bending and torsional stiffness in the composite beams, as well as shear deformation. A parametric study was conducted utilizing this model and a mechanics of laminated beam theory to provide recommendations for alternate bridge designs and modified composite beam designs.
Characterization of damage mechanisms and behavior in two-dimensionally braided composites subjected to static and fatigue loading
Burr, Scott T. (Virginia Tech, 1996-05-20)
In the present research project, four braided composite architectures consisting of graphite fibers in an epoxy matrix were tested under static and fatigue loading conditions to determine damage mechanism types and progressions. The braided architectures consisted of straight axial fiber bundles, which were surrounded by braider fiber bundles oriented at ±a 0 with respect to the axial fiber bundles. Static tension and compression testing was completed first to determine material strengths and basic damage modes for each of the architectures. Under static tension loading, cracking in the braider fiber bundles occurred first, and was followed by splitting in curved regions of the axial fiber bundles. Matrix cracking and kink band formation were found to occur under static compression loading.
Characterization of Mixed-Mode Fracture Testing of Adhesively Bonded Wood Specimens
Nicoli, Edoardo (Virginia Tech, 2010-07-19)
The primary focus of this thesis was to investigate the critical strain energy release rates (G) for mixed-mode (I/II) fracture of wood adhesive joints. The aims of the study were: (1) quantifying the fracture properties of two material systems, (2) analyzing the aspects that influence the fracture properties of bonded wood, (3) refining test procedures that particularly address layered orthotropic systems in which the layers are not parallel to the laminate faces, of which wood is often a particular case, and (4) developing testing methods that enhance the usefulness of performing mixed-mode tests with a dual-actuator load frame. The material systems evaluated experimentally involved yellow-poplar (Liriodendron tulipifera), a hardwood of the Magnoliaceae family, as adherends and two different adhesives: a moisture-cure polyurethane (PU) and a phenol/resorcinol/-formaldehyde (PRF) resin. The geometry tested in the study was the double cantilever beam that, in a dual-actuator load frame, can be used for testing different levels of mode-mixity. The mixed-mode loading condition is obtained by applying different displacement rates with the two independently controlled actuators of the testing machine. Characteristic aspects such as the large variability of the adhesive layer thickness and the intrinsic nature of many wood species, where latewood layers are alternated with earlywood layers, often combine to confound the measures of the critical values of strain energy release rate, Gc. Adhesive layer thickness variations were observed to be substantial also in specimens prepared with power-planed wood boards and affect the value of Gc of the specimens. The grain orientation of latewood and earlywood, materials that often have different densities and elastic moduli, limits the accuracy of traditional standard methods for the evaluation of Gc. The traditional methods, described in the standards ASTM D3433-99 and BS 7991:2001, were originally developed for uniform and isotropic materials but are widely used by researchers also for bonded wood, where they tend to confound stiffness variations with Gc variations. Experimental analysis and analytical computations were developed for quantifying the spread of Gc data that is expected to be caused by variability of the adhesive layer thickness and by the variability of the bending stiffness along wooden beams.
Characterization of the Interfacial Fracture of Solvated Semi-Interpenetrating Polymer Network (S-IPN) Silicone Hydrogels with a Cyclo-Olefin Polymer (COP)
Murray, Katie Virginia (Virginia Tech, 2011-02-02)
As hydrogel products are manufactured and used for applications ranging from biomedical to agricultural, it is useful to characterize their behavior and interaction with other materials. This thesis investigates the adhesion between two different solvated semi-interpenetrating polymer network (S-IPN) silicone hydrogels and a cyclo-olefin (COP) polymer through experimental, analytical, and numerical methods. Interfacial fracture data was collected through the application of the wedge test, a relatively simple test allowing for the measurement of fracture properties over time in environments of interest. In this case, the test was performed at discrete temperatures within range of 4Ë C to 80Ë C. Two COP adherends were bonded together by a layer of one of the S-IPN silicone hydrogels. Upon the insertion of a wedge between the two adherends, debonding at one of the two interfaces would initiate and propagate at a decreasing rate. Measurements were taken of the debond length over time and applied to develop crack propagation rate versus strain energy release rate (SERR) curves. The SERR values were determined through the application of an analytical model derived for the wedge test geometry and to take into account the effects of the hydrogel interlayer. The time-temperature superposition principle (TTSP) was applied to the crack propagation rate versus SERR curves by shifting the crack propagation rates with the Williams-Landel-Ferry (WLF) equation-based shift factors developed for the bulk behavior of each hydrogel. The application of TTSP broadened the SERR and crack propagation rate ranges and presented a large dependency of the adhesion of the system on the viscoelastic nature of the hydrogels. Power-law fits were applied to the master curves in order to determine parameters that could describe the adhesion of the system and be applied in the development of a finite element model representing the interfacial fracture that occurs for each system. The finite element models were used to validate the analytical model and represent the adhesion of the system such that it could be applied to future geometries of interest in which the S-IPN silicone hydrogels are adhered to the COP substrate. [Files modified per J. Austin, July 9, 2013 Gmc]
Characterization of UHMWPE Laminates for High Strain Rate Applications
Cook, Frederick Philip (Virginia Tech, 2010-01-04)
The research presented in this thesis represents an effort to characterize the properties of ultra-high molecular weight polyethylene (UHMWPE). As a composite of polymers, the properties of UHMWPE are time-dependent. It is desired by research sponsors to know the properties of the material at high strain rates, in order to simulate the use of these materials in computer models. Properties believed to be significant which are investigated in this research are the tensile properties of lamina and laminates, and the interlaminar shear properties of laminates. The efficacy of using time-temperature superposition to shift tensile properties of the composite is investigated, and a novel apparent shear strength test is proposed and demonstrated. The effects of processing the material at various temperatures and pressures are also investigated.
Characterizing the Mechanical Properties of Composite Materials Using Tubular Samples
Carter, Robert Hansbrough (Virginia Tech, 2001-07-16)
Application of composite materials to structures has presented the need for engineering analysis and modeling to understand the failure mechanisms. Unfortunately, composite materials, especially in a tubular geometry, present a situation where it is difficult to generate simple stress states that allow for the characterization of the ply-level properties. The present work focuses on calculating the mechanical characteristics, both on a global and local level, for composite laminate tubes. Global responses to axisymmetric test conditions (axial tension, torsion, and internal pressure) are measured on sections of the material. New analysis techniques are developed to use the global responses to calculate the ply level properties for tubular composite structures. Error analyses are performed to illustrate the sensitivity of the nonlinear regression methods to error in the experimental data. Ideal test matrices are proposed to provide the best data sets for improved accuracy of the property estimates. With these values, the stress and strain states can be calculated through the thickness of the material, enabling the application of failure criteria, and the calculation of failure envelopes.
Composites of thermosetting resins and carbon fibers having polyhydroxyether sizings
(United States Patent and Trademark Office, 2000-02-01)
Composite material having carbon fibers embedded in a polymeric matrix comprising a thermoset resin, where the carbon fibers are precoated with a sizing agent comprising a saturated polyurethane or polyhydroxyether before being embedded in the resin. The invention also contemplates processes for making this composite material and intermediate products thereof.
Compression Creep Rupture of an E-glass/Vinyl Ester Composite Subjected to Combined Mechanical and Fire Loading Conditions
Boyd, Steven Earl (Virginia Tech, 2006-11-20)
Polymer matrix composites are seeing increasing use in structural systems (e.g. ships, bridges) and require a quantitative basis for describing their performance under combined mechanical load and fire. Although much work has been performed to characterize the flammability, fire resistance and toxicity of these composite systems, an understanding of the structural response of sandwich type structures and laminate panels under combined mechanical and thermal loads (simulating fire conditions) is still largely unavailable. Therefore a research effort to develop a model to describe the structural response of these glass/vinyl esters systems under fire loading conditions is relevant to the continuing and future application of polymer matrix composites aboard naval ships. The main goal of the effort presented here is to develop analytical models and finite element analysis methods and tools to predict limit states such as local compression failures due to micro-buckling, residual strength and times to failure for composite laminates at temperatures in the vicinity of the glass transition where failure is controlled by viscoelastic effects. Given the importance of compression loading to a structure subject to fire exposure, the goals of this work are succinctly stated as the:(a)Characterization of the non-linear viscoelastic and viscoplastic response of the E-glass/vinyl ester composite above Tg. (b)Description of the laminate compression mechanics as a function of stress and temperature including viscoelasticity.(c)Viscoelastic stress analysis of a laminated panel ([0/+45/90/-45/0]S) using classical lamination theory (CLT). Three manuscripts constitute this dissertation which is representative of the three steps listed above. First, a detailed characterization of the nonlinear thermoviscoelastic response of Vetrotex 324/Derakane 510A - 40 through Tg was conducted using the Time - Temperature - Stress - Superposition Principle (TTSSP) and Zapas - Crissman model. Second, the modeling approach and viscoelastic relaxation mechanism is validated by substituting the shear relaxation modulus into a compression strength model to predict lifetimes for isothermal and one sided heating of unidirectional laminates. Finally, viscoelastic stress analysis using CLT is performed for a general laminated panel to predict lifetimes under one sided heating. Results indicate that when temperatures remain in the vicinity of Tg, the laminate behavior is controlled by thermoviscoelasticity.
Conducted EMI Noise Prediction and Filter Design Optimization
Wang, Zijian (Virginia Tech, 2016-10-04)
Power factor correction (PFC) converter is a species of switching mode power supply (SMPS) which is widely used in offline frond-end converter for the distributed power systems to reduce the grid harmonic distortion. With the fast development of information technology and multi-media systems, high frequency PFC power supplies for servers, desktops, laptops and flat-panel TVs, etc. are required for more efficient power delivery within limited spaces. Therefore the critical conduction mode (CRM) PFC converter has been becoming more and more popular for these information technology applications due to its advantages in inherent zero-voltage soft switching (ZVS) and negligible diode reverse recovery. With the emerging of the high voltage GaN devices, the goal of achieving soft switching for high frequency PFC converters is the top priority and the trend of adopting the CRM PFC converter is becoming clearer. However, there is the stringent electromagnetic interference (EMI) regulation worldwide. For the CRM PFC converter, there are several challenges on meeting the EMI standards. First, for the CRM PFC converter, the switching frequency is variable during the half line cycle and has very wide range dependent on the AC line RMS voltage and the load, which makes it unlike the traditional constant-frequency PFC converter and therefore the knowledge and experience of the EMI characteristics for the traditional constant-frequency PFC converter cannot be directly applied to the CRM PFC converter. Second, for the CRM PFC converter, the switching frequency is also dependent on the inductance of the boost inductor. It means the EMI spectrum of the CRM PFC converter is tightly related the boost inductor selection during the design of the PFC power stage. Therefore, unlike the traditional constant-frequency PFC converter, the selection of the boost inductor is also part of the EMI filter design process and EMI filter optimization should begin at the same time when the power stage design starts. Third, since the EMI filter optimization needs to begin before the proto-type of the CRM PFC converter is completed, the traditional EMI-measurement based EMI filter design will become much more complex and time-consuming if it is applied to the CRM PFC converter. Therefore, a new methodology must be developed to evaluate the EMI performance of the CRM PFC converter, help to simplify the process of the EMI filter design and achieve the EMI filter optimization. To overcome these challenges, a novel mathematical analysis method for variable frequency PFC converter is thus proposed in this dissertation. Based on the mathematical analysis, the quasi-peak EMI noise, which is specifically required in most EMI regulation standards, is investigated and accurately predicted for the first time. A complete approximate model is derived to predict the quasi-peak DM EMI noise for the CRM PFC converter. Experiments are carried out to verify the validity of the prediction. Based on the DM EMI noise prediction, worst case analysis is carried out and the worst DM EMI noise case for all the input line and load conditions can be found to avoid the overdesign of the EMI filter. Based on the discovered worst case, criteria to ease the DM EMI filter design procedure of the CRM boost PFC are given for different boost inductor selection. Optimized design procedure of the EMI filter for the front-end converter is then discussed. Experiments are carried out to verify the validity of the whole methodology.
Construction of a Virginia Short-Span Bridge with the Strongwell 36-Inch Double-Web I-Beam
Cousins, Thomas E.; Lesko, John J. (Virginia Center for Transportation Innovation and Research, 2005-10-01)
The Route 601 Bridge in Sugar Grove, VA, spans 39 ft over Dickey Creek. The bridge is the first to use the Strongwell 36-in-deep fiber-reinforced polymer (FRP) double-web beam (DWB) in a vehicular bridge superstructure. Construction of the new bridge was completed in October 2001, and field testing was undertaken shortly thereafter as well as in June of 2002 to assess any potential changes in structural performance. This paper details the field evaluation of the Route 601 Bridge. Using midspan deflection and strain data from the October 2001 and June 2002 field tests, AASHTO bridge design parameters were determined, namely wheel load distribution factor g, dynamic load allowance IM, and maximum deflection. The wheel load distribution factor was determined to be S/4, a dynamic load allowance was determined to be 0.50, and the maximum deflection of the bridge was L/1110. Deflection results were lower than the AASHTO L/800 limit. This discrepancy is attributed to partial composite action of the deck-to-girder connections, bearing restraint at the supports, and contribution of guardrail stiffness. It was found that diaphragm removal had a small effect on the wheel load distribution factor. An examination of the 36-in DWB capacity and failure mode indicates that the strength of the girder is controlled by compression failure in the flange and not shear failure, as originally thought. An attempt to predict the girder fatigue performance shows that small losses in bending stiffness would be expected at fatigue loads 26% of the ultimate capacity, which was confirmed through experiments. Moreover, there is no concern that fatigue alone will cause a failure during the reasonable life of the structure as presently operated.
Continuation of Field and Laboratory Tests of a Proposed Bridge Deck Panel Fabricated from Pultruded Fiber-Reinforced Polymer Components
Coleman, Jason Thomas (Virginia Tech, 2002-02-20)
This thesis presents research completed on the experimental performance of two 6 3/4 in thick bridge deck panels fabricated by the Stongwell Corporation of Bristol, Virginia. The panels are made of off-the-shelf, pultruded glass fiber-reinforced polymer elements, bonded and mechanically fastened together. The testing involved laboratory stiffness tests performed on one deck panel which afterwards, was placed in a field test site at the I-81 Troutville Weigh Station facility. The daily truck traffic over the deck panel at this site is approximately 5400 vehicles. The second deck panel was constructed as a prototype to test benefits of steel thru-rod mechanical connectors. Further, a rubber tire loading patch was developed for the laboratory stiffness and strength tests performed on this second specimen to investigate modes of failure. Both decks made use of a hook bolt type connection to steel support beams in order to reduce damage seen in previous methods of connection.
Crack path selection and shear toughening effects due to mixed mode loading and varied surface properties in beam-like adhesively bonded joints
Guan, Youliang (Virginia Tech, 2014-01-17)
Structural adhesives are widely used with great success, and yet occasional failures can occur, often resulting from improper bonding procedures or joint design, overload or other detrimental service situations, or in response to a variety of environmental challenges. In these situations, cracks can start within the adhesive layer or debonds can initiate near an interface. The paths taken by propagating cracks can affect the resistance to failure and the subsequent service lives of the bonded structures. The behavior of propagating cracks in adhesive joints remains of interest, including when some critical environments, complicated loading modes, or uncertainties in material/interfacial properties are involved. From a mechanics perspective, areas of current interest include understanding the growth of damage and cracks, loading rate dependency of crack propagation, and the effect of mixed mode fracture loading scenarios on crack path selection. This dissertation involves analytical, numerical, and experimental evaluations of crack propagation in several adhesive joint configurations. The main objective is an investigation of crack path selection in adhesively bonded joints, focusing on in-plane fracture behavior (mode I, mode II, and their combination) of bonded joints with uniform bonding, and those with locally weakened interfaces. When removing cured components from molds, interfacial debonds can sometimes initiate and propagate along both mold surfaces, resulting in the molded product partially bridging between the two molds and potentially being damaged or torn. Debonds from both adherends can sometimes occur in weak adhesive bonds as well, potentially altering the apparent fracture behavior. To avoid or control these multiple interfacial debonding, more understanding of these processes is required. An analytical model of 2D parallel bridging was developed and the interactions of interfacial debonds were investigated using Euler-Bernoulli beam theory. The numerical solutions to the analytical results described the propagation processes with multiple debonds, and demonstrated some common phenomena in several different joints corresponding to double cantilever beam configurations. The analytical approach and results obtained could prove useful in extensions to understanding and controlling debonding in such situations and optimization of loading scenarios. Numerical capabilities for predicting crack propagation, confirmed by experimental results, were initially evaluated for crack behavior in monolithic materials, which is also of interest in engineering design. Several test cases were devised for modified forms of monolithic compact tension specimens (CT) were developed. An asymmetric variant of the CT configuration, in which the initial crack was shifted to two thirds of the total height, was tested experimentally and numerically simulated in ABAQUS®, with good agreement. Similar studies of elongated CT specimens with different specimen lengths also revealed good agreement, using the same material properties and cohesive zone model (CZM) parameters. The critical specimen length when the crack propagation pattern abruptly switches was experimentally measured and accurately predicted, building confidence in the subsequent studies where the numerical method was applied to bonded joints. In adhesively bonded joints, crack propagation and joint failure can potentially result from or involve interactions of a growing crack with a partially weakened interface, so numerical simulations were initiated to investigate such scenarios using ABAQUS®. Two different cohesive zone models (CZMs) are applied in these simulations: cohesive elements for strong and weak interfaces, and the extended finite element method (XFEM) for cracks propagating within the adhesive layer. When the main crack approaches a locally weakened interface, interfacial damage can occur, allowing for additional interfacial compliance and inducing shear stresses within the adhesive layer that direct the growing crack toward the weak interface. The maximum traction of the interfacial CZM appears to be the controlling parameter. Fracture energy of the weakened interface is shown to be of secondary importance, though can affect the results when particularly small (e.g. 1% that of the bulk adhesive). The length of the weakened interface also has some influence on the crack path. Under globally mixed mode loadings, the competition between the loading and the weakened interface affects the shear stress distribution and thus changes the crack path. Mixed mode loading in the opposite direction of the weakened interface is able to drive the crack away from the weakened interface, suggesting potential means to avoid failure within these regions or to design joints that fail in a particular manner. In addition to the analytical and numerical studies of crack path selection in adhesively bonded joints, experimental investigations are also performed. A dual actuator load frame (DALF) is used to test beam-like bonded joints in various mode mixity angles. Constant mode mixity angle tracking, as well as other versatile loading functions, are developed in LabVIEW® for use with a new controller system. The DALF is calibrated to minimize errors when calculating the compliance of beam-like bonded joints. After the corrections, the resulting fracture energies ( ) values are considered to be more accurate in representing the energy released in the crack propagation processes. Double cantilever beam (DCB) bonded joints consisting of 6061-T6 aluminum adherends bonded with commercial epoxy adhesives (J-B Weld, or LORD 320/322) are tested on the DALF. Profiles of the values for different constant mode mixity angles, as well as for continuously increasing mode mixity angle, are plotted to illustrate the behavior of the crack in these bonded joints. Finally, crack path selection in DCB specimens with one of the bonding surfaces weakened was studied experimentally, and rate-dependency of the crack path selection was found. Several contamination schemes are attempted, involving of graphite flakes, silicone tapes, or silane treatments on the aluminum oxide interfaces. In all these cases, tests involving more rapid crack propagation resulted in interfacial failures at the weakened areas, while slower tests showed cohesive failure throughout. One possible explanation of this phenomenon is presented using the rate-dependency of the yield stress (commonly considered to be corresponding to the maximum traction) of the epoxy adhesives. These experimental observations may have some potential applications tailoring adhesive joint configurations and interface variability to achieve or avoid particular failure modes.
Crack Path Selection in Adhesively Bonded Joints
Chen, Buo (Virginia Tech, 1999-11-15)
This dissertation is to obtain an overall understanding of the crack path selection in adhesively bonded joints. Using Dow Chemical epoxy resin DER 331Â® with various levels of rubber concentration as an adhesive, and aluminum 6061-T6 alloy with different surface pretreatments as the adherends, both symmetric and asymmetric double cantilever beam (DCB) specimens are prepared and tested under mixed mode fracture conditions in this study. Post-failure analyses conducted on the failure surfaces indicate that the failure tends to be more interfacial as the mode II component in the fracture increases whereas more advanced surface preparation techniques can prevent failure at the interface. Through mechanically stretching the DCB specimens uniaxially until the adherends are plastically deformed, various levels of T-stress are achieved in the specimens. Test results of the specimens with various T-stresses demonstrate that the directional stability of cracks in adhesive bonds depends on the T-stress level. Cracks tend to be directionally stable when the T-stress is compressive whereas directionally unstable when the T-stress is tensile. However, the direction of crack propagation is mostly stabilized when more than 3% mode II fracture component is present in the loading regardless of the T-stress levels in the specimens. Since the fracture sequences in adhesive bonds are closely related to the energy balance in the system, an energy balance model is developed to predict the directional stability of cracks and the results are consistent with the experimental observations. Using the finite element method, the T-stress is shown to be closely related to the specimen geometry, indicating a specimen geometry dependence of the directional stability of cracks. This prediction is verified through testing DCB specimens with various adherend and adhesives thicknesses. By testing the specimens under both quasi-static and low-speed impact conditions, and using a high-speed camera to monitor the fracture sequence, the influences of the debond rate on the locus of failure and the directional stability of cracks are investigated. Post-failure analyses suggest that the failure tends to be more interfacial when the debond rate is low and tends to be more cohesive when the debond rate is high. However, this rate dependence of the locus of failure is greatly reduced when more advanced surface preparation techniques are used in preparing the specimens. The post-failure analyses also reveal that cracks tend to be more directionally unstable as the debond rate increases. Finally, employing interface mechanics and extending the criteria for the direction of crack propagation to adhesively bonded joints, the crack trajectories for directionally unstable cracks are predicted and the results are consistent with the overall features of the crack paths observed experimentally.
Creep and Elevated Temperature Mechanical Properties of 5083 and 6061 Aluminum
Allen, Benjamin William (Virginia Tech, 2012-12-20)
With the increasing use of aluminum in naval vessels and the ever-present danger of fires, it is important to have a good understanding of the behavior of aluminum at elevated temperatures. The aluminum samples 5083-H116 and 6061-T651 were examined under a variety of loading conditions and temperatures. Tensile testing was completed on both materials to measure strength properties of elastic modulus, yield strength, and ultimate strength as well as reduction of area from room temperature to 500 deg C taking measurements every 50 deg C. These tests showed how much the material weakened as temperature increases. Low temperatures had a minimal effect on strength while exposure to temperatures between 200 and 300 deg C had the most significant impact. Creep testing was also completed for these materials. These tests were completed at temperatures between 200 and 400 deg C in 50 deg C increments. Stresses for these tests were in the range of 13 to 160MPa for 5083 aluminum and between 13 to 220MPa for 6061 aluminum. These tests showed a significant relationship between stress and temperature and how changes to one can cause a very different resulting behavior. In addition to the creep testing, three creep models were examined as a means of predicting creep behavior. These models included a power law, exponential, and hyperbolic-sine versions and were able to predict creep results with decent accuracy depending on the stress used in the model.

Browsing by Author "Lesko, John J."

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