Browsing by Author "Mitchell, Larry D."

Now showing 1 - 20 of 71
  • Thumbnail Image
    An analytical study and computer analysis of three-dimensional, steady-state vibration of multishaft geared-rotor systems
    Blanding, James Michael (Virginia Polytechnic Institute and State University, 1985)
    A unique multifrequencied transfer matrix method performs three-dimensional harmonic, steady-state response calculations on geared-rotor systems. The full six degrees-of-freedom method includes physical branching to accommodate multiple shafting and frequency branching to simultaneously accommodate multiple frequencies and their interdependence resulting from time-varying mesh stiffness. Areas of emphasis include development of a modified transfer matrix to handle multiple frequencies and shafting; description of the time-varying stiffness tensor representing the involute spur gear mesh based on bending, shear, compression, and local contact deformation; development of the mesh transfer matrix; development of an automatic system solver to allow the engineer to analyze systems of arbitrary construction; and the development of a matrix solver to efficiently handle large systems. A computer analysis demonstrates the significance of terms included in the stiffness evaluation as compared with less rigorous treatment in the literature. An analytical example problem illustrates the automated model generation through complete rotor system dynamic response analysis produced by the current work with special attention to the significance of parametric excitation due to the gear mesh.
  • Thumbnail Image
    An axisymmetric linear/high-order finite element for filament wound composite structures
    Rogers, Craig A. (Virginia Polytechnic Institute and State University, 1987)
    The development of an axisymmetric linear by high-order finite element to model filament-wound structures is presented. The primary objective of this work was to develop a ’design code' to analyze filament wound spherical pressure vessels. In order to develop a design-oriented analysis capability which can produce accurate results rather quickly with reduced input-data requirements, the total number of system equations must be reduced. To accomplish this task, a linear by high-order element was formulated which uses a single high-order displacement field finite element to model the total thickness of an axisymmetric composite structure. The displacement order for the in-plane direction remains linear, while the transverse order is user selectable. Numerical integration for stiffnesses is evaluated with respect to varying material properties and lamirna thicknesses in each individual element. Results from a computational economy study are presented showing potential time savings of 40 percent when compared to the conventional modeling scheme of using bi-linear elements. Actual test cases indicate that computation time savings may be as great as 55 percent when using linear by fourth-order elements and 45 percent when using linear by sixth-order elements. The accuracy of the element was evaluated by comparing the finite element results to elasticity solutions for isotropic, orthotropic, and filament-wound cylindrical pressure vessels. Most of the finite element results indicated a ±3 percent maximum error of the stresses compared to the elasticity results. The new linear by high order element stress results were nominally within ±2 percent of stresses calculated with conventional bilinear elements. Comparisons of finite element results for an actual filament-wound spherical pressure vessel slowed that linear by third- or fourth-order elements may be adequate for preliminary design purposes while the higher-order elements generally correlated better with the conventional bi-linear elements. Also presented is an outline of the design code and sample results for spherically wound pressure vessels.
  • Thumbnail Image
    A Bayesian statistics approach to updating finite element models with frequency response data
    Lindholm, Brian Eric (Virginia Tech, 1996-08-05)
    This dissertation addresses the task of updating finite element models with frequency response data acquired in a structural dynamics test. Standard statistical techniques are used to generate statistically qualified data, which is then used in a Bayesian statistics regression formulation to update the finite element model. The Bayesian formulation allows the analyst to incorporate engineering judgment (in the form of prior knowledge) into the analysis and helps ensure that reasonable and realistic answers are obtained. The formulation includes true statistical weights derived from experimental data as well as a new formulation of the Bayesian regression problem that reduces the effects of numerical ill-conditioning. Model updates are performed with a simulated free-free beam, a simple steel frame, and a cantilever beam. Improved finite element models of the structures are obtained and several statistical tests are used to ensure that the models are improved.
  • Thumbnail Image
    Characterization and Development of General Material Models for Use in Modeling Structures Bonded with Ductile Adhesives
    Cassino, Christopher (Virginia Tech, 2005-04-22)
    Structural adhesives are materials that are capable of bearing significant loads in shear, and sometimes tension, over a range of strains and strain rates. Adhesively bonded structures can dissipate large amounts of mechanical energy and can be lighter and more efficient than many bolted or vibration welded parts. The largest barrier to using structural adhesives in more applications is the many challenges engineers are presented with when designing and analyzing adhesively bonded structures. This study develops, characterizes and compares several material models for use in finite element analysis of adhesively bonded structures, in general, and a bonded tongue and groove (TNG) joint in particular. The results indicate that it is possible to develop a general material model for ductile adhesives used in structural applications under quasi-static conditions. Furthermore, the results also show that it is also possible to take bulk material data and apply it to an adhesively bonded specimen provided that the mode of failure of the bulk test specimen closely approximates the mode of failure of the bonded joint.
  • Thumbnail Image
    Characterization of a Pressure Sensitive Adhesive (PSA) for Mechanical Design
    Hennage, John B. (Virginia Tech, 2004-10-12)
    This thesis outlines a methodology for formatting and applying stress models, collecting visco-elastic material properties, and presenting the material data for use in adhesive joint designs. There are a number of models/theories that can be applied to the design of Pressure Sensitive Adhesive (PSA) joints. Unfortunately, few design engineers are familiar with these models and the models are not formatted in a manner that can easily be applied to joint designs. By developing a format that is based on the existing knowledge of the designer and presenting them in a familiar manner the theories/models can easily be used in joint designs. This technique was demonstrated with Beam-on-Elastic Foundation, Shear Lag, and Shape Factors. Design examples successfully demonstrated the application of all of these models in the analysis and design of simple adhesive joints. The material properties of PSAs are a function of loading/displacement rate, temperature, relative humidity, and stress state. The Arcanm fixture was used to test VHB™ 4950 over a range loading and stress states including fixed load and displacement rates. Several bond widths were tested to determine the extent of the shape factor effect. A second fixture was used to determine the impact of gradient-tensile stresses on the failure strength. All of the collected data was used to generate design plots. The strength data was presented as allowable strength envelopes with respect to rate. The moduli were calculated from the load-displacement data and plotted with respect to the displacement rate. The failure strength from the fixed load and displacement data were used to transform from one loading case to the other and a plot was generated. These three plots were used in the design and analysis of several adhesive joints. The methods demonstrated in this thesis show a great deal of promises as a design tool, but there is still a large amount of work to be done. The design space for this material is much larger than what was covered by this work. Additional strength testing needs to be conducted to fully characterize the material for all key applications. The principle of time-temperature superposition, beam-on-elastic foundation, shear lag, and shape factors all need to be validated for this material.
  • Thumbnail Image
    Comparing In Situ and Bulk Constitutive Properties of a Structural Adhesive
    Grohs, Joshua Walter (Virginia Tech, 2007-07-20)
    In the continuing quest for more efficient designs, structural adhesives are being used in place of, or with, traditional fastening methods; however designing with adhesives is refined as traditional methods. To obtain the adhesive design properties, tests are often performed on bulk tensile and bonded shear specimens. Questions remain about the relationship between properties obtained from in situ adhesive joints and bulk adhesive specimens. As a result, an experimental plan was developed which characterized both the linear and nonlinear region of bulk and in situ adhesive performance of a two-part acrylic adhesive from Dow Chemical Company. A standard uniaxial tensile test was used for the bulk normal, while an Iosipescu shear test was used to characterize the bulk shear performance. In situ testing was performed on a napkin-ring specimen loaded in both tension and torsion. Stress-strain relationships in both shear and normal were developed and bulk and in situ adhesive performance was compared. Observations from testing were: 1. Bulk shear and in situ shear tests showed similar performance in both the linear and nonlinear regions. 2. Modulus of elasticity in bulk adhesive tests was similar to the effective modulus of elasticity in in situ tests. 3. Prediction of normal yield strengths of the in situ adhesive through simple failure theory models proved to be inaccurate. Stress singularities, loading imperfections, and potentially a hydrostatic sensitivity were considered possible explanations. 4. Adhesive showed sensitivity to voiding and surface flaws when loaded in a tensile configuration, refinement of specimen fabrication minimized these effects.
  • Thumbnail Image
    Computer Aided Analysis of Smoke Stack Designs
    Servaites, Joseph Charles (Virginia Tech, 1996-09-01)
    The purpose of this research is to analyze the statics and dynamics of steel smoke stacks subject to excitation by aerodynamic forces. The wind loads experienced by smoke stacks arise from various phenomenon, the most prominent of which are static drag load, vortex shedding, and atmospheric turbulence. The nature of these loading sources around a cylinder are studied in detail. Both static and dynamic loads are capable of producing large tip deflections, and are of the most prominent design criteria for stack designers. A computer program, STACK1, has been created by modifying an existing analysis code, BEAM8, to be used specifically for stack analysis. This analysis code utilizes the transfer matrix method to perform detailed bending and vibration analyses. This new software has been developed to check stack designs for compliance with appropriate steel stack standards, and provide the designer with information regarding the static and dynamic response of the structure. A detailed analysis is performed to demonstrate the validity of approximating a tapered Timoshenko beam with a series of continuous, constant cross-section beams.
  • Thumbnail Image
    Computer-Aided Design Software for the Undamped Two-Dimensional Static and Dynamic Analysis of Beams and Rotors
    Dolasa, Anaita Rustom (Virginia Tech, 1999-05-03)
    The objective of this research work was to develop a design tool to analyze and design undamped beam and rotor systems in two dimensions. Systems modeled in two dimensions, such as beams with different moments of inertia, could produce varying responses in the each direction of motion. A coupling between the vertical and horizontal motions also exists in rotor systems mounted of fluid film bearings. The computer program called 2DBEAM has been developed to model and provide analyses of such systems in two dimensions. The tool has been based on an existing design package, BEAM9, which in its present state provides the response of beams and rotors in one plane of motion. The 2DBEAM program has the capability of performing the static response, free vibration, forced dynamic response, and frequency response analyses of a system. The Transfer Matrix Method has been used in the development of the software and an explanation of the method is included in this thesis. Mathematical problems and solutions encountered while developing 2DBEAM are also documented in this study. The code has been tested against analytical and published solutions for the types of analysis mentioned above and on coupled and uncoupled system models.
  • Thumbnail Image
    Computer-Aided Design Software for Torsional Analysis
    Griffin, Timothy R. (Virginia Tech, 1998-03-05)
    The goal of this research has been the development of an effective design tool for torsional analysis. In the hopes of achieving this goal the computer program, Torsion 1, has been created. This torsional transfer matrix program provides the user with the ability to easily model multi-rotor systems using a simple user-interface. The program is capable of modeling such components or system characteristics as continuously distributed mass, viscous and structural damping, vibration absorbers, and gear meshes with gear tooth flexibility. The analysis capabilities of the program include forcedresponse and free-vibration analyses. The forced-response analysis module is capable of determining a system’s response to a static or harmonic torsional load. The free-vibration analysis module allows is capable of determining the eigenvalues and eigenvectors for damped and undamped systems. This thesis includes an explanation of the multi-rotor transfer matrix technique employed in Torsion 1. The derivation of transfer matrices for visco-elastic vibration absorbers, pendulum absorbers, flexible gear meshes, and planetary gear trains are included in this work. Finally, the validity of the program results is verified with a set of benchmark examples.
  • Thumbnail Image
    Computer-aided, interactive design routine for the prediction of sound levels in irregularly shaped factory spaces
    Blanding, James M.; Mitchell, Larry D.; Hurst, Charles J. (Acoustical Society of America, 1977)
    An interactive computer algorithm is presented which utilizes modified room acoustics theory to predict dBA sound pressure levels in regularly- and irregularly-shaped factory spaces. Irregularly positioned and nonuniform room absorptions are treated. The user-oriented algorithm, suited to remote interactive terminal operation, employs a conversational format to facilitate input of room dimensions, of absorption data, and of locations and noise levels of machinery. A redesign feature is included which employs three options. These allow the user to change sound source data and acoustic treatments in the process of finding the most economical accommodation of OSHA regulations during any phase of the factory design or redesign. [Work supported by NSF.]
  • Thumbnail Image
    Confidence bands, measurement noise, and multiple input - multiple output measurements using three-channel frequency response function estimator
    Cobb, Richard E. (Virginia Polytechnic Institute and State University, 1988)
    A three-channel Frequency Response Function (FRF) estimator is discussed and statistical relations developed. Methods for estimating the variance of the FRF magnitude and levels of uncorrelated content in the test signals are developed. FRF magnitude variance estimates allow ’confidence bands’ to be placed on FRF magnitude estimates, giving an indication of the variability of the result. Uncorrelated content estimates indicate sources and magnitudes of noise in the measurement system. Both Monte Carlo simulations and experimental work are used to verify the statistical and uncorrelated content estimates. Relations to extend the three-channel FRF estimator to multiple input-multiple output measurements are developed and verified through simulations.
  • Thumbnail Image
    The Creative Entrepreneurs Organization: Developing Innovative Products and Businesses
    Hayes, Thomas J. III (Virginia Tech, 1997-12-04)
    Global socioeconomic trends are changing the nature of the American workplace. To address the challenges brought about by these changes, American engineering education must focus on developing students into future professionals, equipped to thrive in the fast-paced, technologically intense, globally competitive workplace of the future. One of the most effective ways to prepare students to face the future is by teaching them to innovate. This thesis presents the "Creative Entrepreneurs Organization: Developing Innovative Products and Businesses" (CEO) concept as a method by which Virginia Tech could help students learn innovation. The CEO concept is a student-involvement program intended to develop students into successful entrepreneurs as they work together in small teams to develop and market intellectual property. This Program is intended to produce revenue for the University by virtue of the successful commercialization of the intellectual properties it generates. Additionally, the CEO Program will allow faculty and students to share in the financial rewards associated with the intellectual properties they generate. The CEO Program concept is presented in light of current trends in the business and academic worlds. Various issues related to its implementation are addressed. The Program is evaluated for its expected value to students, to the University, to the State, and to the Nation. A survey is presented by which the success of the Program can be measured. For the CEO concept to be successfully realized, several challenges must be overcome. First, the University must embrace this somewhat unorthodox Program in which both educational and financial motives play significant roles. Second, there must be a Program Advocate who will be able to effectively communicate the value and feasibility of the Program. Third, fiscal and physical resources must be available to ensure the successful start-up and operation of the CEO Program. Finally, the Program must find ways to nurture creativity in its participants. I conclude that the effort required to implement the CEO Program is outweighed by its potential benefits to students, to the University, to the State of Virginia, and to the Nation. Therefore, I recommend that the Virginia Tech College of Engineering consider the CEO Program for implementation.
  • Thumbnail Image
    Critical Erosion/Corrosion Piping Wall Thicknesses Under Static and Fatigue Stress Conditions According to ASME Guidelines
    Comeau, Christian R. (Virginia Tech, 2001-07-17)
    The purpose of this project was to show the updated procedures and to make additions to the computer program called Tmin designed by E. I. DuPont De Nemours and Company. This program is used as a screening tool for determining the largest of the minimum pipe-wall thicknesses in a piping system. This project involved several additions that will be released in the next version of the Tmin computer program. The first major additions to be implemented are four alternating Stress-to-Number of cycles curves: Aluminum 1100, Aluminum 3003-0, Aluminum 6061-T6, and Nickel 200. In addition, procedures of the ASME for fatigue curve analysis and implementation of fatigue data were investigated. These four stress-to-number of cycles (S-N) fatigue curves were added to Tmin's internal Microsoft Access® database. Next, a 2-D vertical piping span configuration was incorporated. Finally, DuPont required a Microsoft Word® document output of the pipe-wall thickness data including the piping span model information. Other user-friendly additions were included. Since this computer program was to be American Society of Mechanical Engineers (ASME) compliant, a study of the ASME Pressure Vessel and Piping standards and codes was made to determine how pipe-wall thickness calculations were to be processed. The 2-D vertical piping span calculation procedures were investigated. Once the 2-D vertical piping span analysis was complete, the largest pipe-wall thickness value calculated were passed to a Microsoft Word® document. The last implementation is the inclusion of help files. Help file button additions in all input boxes allowed for the user to know exactly what was needed before a data entry was made.
  • Thumbnail Image
    Design and testing of a nonlinear mechanical advantage demonstration mechanism
    Garguilo, Keith Francis (Virginia Tech, 1995-12-05)
    A four-bar linkage with prescribed nonlinear mechanical advantage has been synthesized and tested. This tool is intended to in1prove cognition of linkage synthesis theory through interactive demonstrations and has both industrial and educational applications. Industrial applications include use as a proof-of-concept demonstrator and sales device. Academic use will focus on interactive classroom instruction. In accordance with current educational theory, this tool requires active participation on the part of the student, whether in academia or industry. Interactive methods have repeatedly demonstrated increased effectiveness in developing cognition and retention of technological subjects. Synthesis of the linkage is based on four-precisian-point Burmester theory. Transformation of the given prescribed mechanical advantage problem into classical synthesis forms, i. e., function generation and body guidance, is necessary during the procedure. The precision points are chosen from the flexion/ extension strength curve of the forearm. Unlike most strength training equipment, this linkage exploits unused muscular capability by allowing variable resistance. Users are capable of dramatic increases in maximum resistance and efficiency over constant mechanical advantage methods. By allowing the full potential of the muscles to be used, the mechanism achieves a work output per repetition that approaches the maximum theoretical work.
  • Thumbnail Image
    Design of User-Weight-based Exercise Machines
    Coombs, Dana Joseph (Virginia Tech, 1997-02-07)
    This thesis describes the process of designing exercise machines that raise the weight of the user as the primary source of resistance. Most strength training machines use weight stacks or springs as the source of resistance. While such machines are highly evolved and provide an excellent workout, they typically have a number of disadvantages including high cost, and large size and weight. A user weight-based exercise design will reduce the cost, size, and weight of the machine. The design process considers some important issues. Parallelogram linkages are implemented to provide non-rotary motion without the disadvantage of linear bearings. The user input is located with respect to the user providing correct relative motion for the exercise. The design also considers proper resistance curves during the design process. Specific examples are given for each step of the design process. These examples include the evolution of ideas and the creation and use of kinematic and automatic tools.
  • Thumbnail Image
    Determination of acoustic ray paths in enclosed spaces
    Mitchell, Bruce Wayne (Virginia Tech, 1975)
    Equations were derived to calculate the reflection points of acoustic rays emanating from a source to a receiver. Reflection points for up to three reflections could be determined if the location of the source, receiver and all reflecting surfaces were known. Assuming a point source, the distances along the calculated ray paths were used along with the power level of the source and the absorptive characteristics of all the reflecting surfaces to determine the sound pressure levels at specified receiver locations. A computer program was developed to perform the necessary calculations for reflection points and sound pressure levels. The output was in the form of ray tracing plots which showed the unique reflection paths for up to three reflections and the sound pressure levels at each receiver location. Comparisons were made between predicted and calculated sound pressure levels in a rectangular parallelopiped shaped room and a long narrow hallway. The results of this investigation showed a promising potential in the area of sound pressure level predictions and the use of ray tracing plots to provide a means of reducing the sound pressure levels. A particularly interesting point of the program was its ability to handle very irregularly shaped rooms which include slanted surfaces.
  • Thumbnail Image
    Determining the moderator temperature coefficient in a PWR by stochastic fluctuation analysis
    Herr, John D. (Virginia Tech, 1990-05-05)
    An on-line, passive method for monitoring the moderator temperature coefficient (MTC) in a pressurized water reactor is described and compared to an analytical model. The method correlates fluctuations in the neutron flux with fluctuations in the coolant temperature. Results show that the frequency response function between the in-core neutron flux and thermocouple signals is proportional to the MTC and can be used to monitor changes in the MTC throughout the fuel cycle. An analytical model is also formulated and used to investigate how the neutron flux and coolant temperature fluctuations are related and change in the axial core direction. The analytical model results agree reasonably well with the experimental results.
  • Thumbnail Image
    Development of a [i.e. an] engineering design procedure for a complex revolute joint
    Hammock, Thomas V. (Virginia Tech, 1988-12-15)
    Design procedures are developed that combine both the finite element method and the transfer matrix method in design. These procedures were drawn up as a result of the analysis of a large continuous mining machine to be produced by Fairchild International. Combining the methods allowed the analysis to be split into smaller jobs. Two examples are given. The first example divides the machine into a number of separate finite element analyses where the internal loads on each component are determined by the transfer matrix method. The second example analyzes the transition region between each of the separate finite element analysis. Utilizing both methods in design reduced computer usage costs.
  • Thumbnail Image
    Development of a laser-based automated mechanical mobility measurement system for one-dimensional experimental modal analysis
    Agee, Barry L. (Virginia Tech, 1992-02-13)
    A laser-based automated mechanical mobility measurement system has been developed that can obtain a very high spatial resolution of frequency response functions (FRFs) for experimental modal analysis where one-dimensional structural motion predominates. With this system FRFs are measured and recorded in a format that is readily imported into a popular modal analysis software. A scanning laser Doppler velocimeter is coupled to a multi-channel FFf analyzer system for efficient data collection and management of spatially distributed FRFs. The objectives in designing this measurement system is to take advantage of the laser's non-contacting nature, its high spatial resolution, and its fast point-to-point movement. By using the scanning capabilities of the laser, experimental mobility FRF data can be collected in those structures that have dominant response in one direction at up to 16,000,000 positions. Moreover, the laser does not mass load the structure thus making the structure time invariant unlike "roving" accelerometers. The design and implementation of this automated mechanical mobility measurement system is presented. Instrumentation interfacing and control issues as well as software development issues are addressed in this thesis. Procedures required to implement the laser-based measurement system are presented. The performance of the laser-based system is presented along with an illustrative example of its use on a free-free rectangular plate where the vibration response is primarily in one direction.
  • Thumbnail Image
    Development of a statistical method for validating ESDM strain-field output
    Doktor, Christopher A. (Virginia Tech, 1995-12-05)
    This thesis addresses the problem of developing a statistical method for validating the ESDM strain-field output. Specifically, it addresses the problems of estimating the ESDM strain-field uncertainty, choosing an appropriate standard measurement, estimating the uncertainty in the standard measurement, and comparing the ESDM strain-fields against a finite set of standard measurements. A solution to each of these problems is presented. Specifically, it is shown that 1) the uncertainty in the strain-field may be estimated from the ESDM dynamic stiffness matrix; 2) strain gages are chosen to validate the ESDM strain-fields; 3) the uncertainty of the strain gage measurements may be estimated from the uncertainty in the system calibration, the output voltage of the system and the tolerance of the strain gage gage factor; and 4) the ESDM strain-fields may be validated against a set of strain gage measurements by a combining a 2-group z-test (hypothesis test on two means with known variances) with a qualitative analysis of the field. This statistical validation method is then evaluated in test-based simulation to determine if it provides reliable assessments of the ESDM strain-field accuracy. The results of the simulation show that the validation method does, indeed, provide reliable assessment of the ESDM strain-field accuracy. The results also show that the ESDM method should be capable of accurately estimating dynamic strain-fields, provided it is not subjected to substantial error in the calculated position and orientation of the laser or nonstationary behavior in the structure.