Current Search: Nicholson, David (x)
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(1 - 8 of 8)
- Title
- FINITE ELEMENT ANALYSIS OF A TEST SPECIMEN FOR STRENGTH OF A CO-POLYMER AT A BONE-IMPLANT INTERFACE.
- Creator
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Chhabra, Nitin, Nicholson, David W., University of Central Florida
- Abstract / Description
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The aim of this work is to evaluate the mechanical strength of a co-polymer of 2-hydroxyethylmethacrylate (HEMA) and methylmethacrylate (MMA), so that it can be applied as an interfacial layer between bone cement and steel implants to improve their performance and life. Finite element (FE) analysis techniques are used to assess the behavior of the interface layer under static and dynamic loading conditions. The material property of the co-polymer is a function of its composition and water...
Show moreThe aim of this work is to evaluate the mechanical strength of a co-polymer of 2-hydroxyethylmethacrylate (HEMA) and methylmethacrylate (MMA), so that it can be applied as an interfacial layer between bone cement and steel implants to improve their performance and life. Finite element (FE) analysis techniques are used to assess the behavior of the interface layer under static and dynamic loading conditions. The material property of the co-polymer is a function of its composition and water saturation. The factors affecting the strength of the bone-implant interface are many. Implant interfacial fracture can lead to decreased stability. Fatigue life is a very important process in failure. The results obtained from static and dynamic analyses show that increasing the percentage of HEMA improves the strength of the interface by reducing the stiffness of the implant, absorbing more energy and by reducing the interfacial stress peaks and making the stress distribution more nearly uniform.
Show less - Date Issued
- 2004
- Identifier
- CFE0000076, ucf:46082
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0000076
- Title
- DAMAGE MODELING METHOD FOR TURBINE COMPRESSOR BLADE TUNING.
- Creator
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Afanasiev, Gennadiy, Nicholson, David, University of Central Florida
- Abstract / Description
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The thesis presents a method of evaluation for blade damage in Combustion Turbine Compressor Section. This method involves use of multiple domains within a single Finite Element Model to predict the effect of damage on the blade properties. This approach offers significant time and effort savings when compared to traditional evaluation methods of similar problems. It is demonstrated via examples that the "multi-domain" modeling approach yields acceptable accuracy results. The economical...
Show moreThe thesis presents a method of evaluation for blade damage in Combustion Turbine Compressor Section. This method involves use of multiple domains within a single Finite Element Model to predict the effect of damage on the blade properties. This approach offers significant time and effort savings when compared to traditional evaluation methods of similar problems. It is demonstrated via examples that the "multi-domain" modeling approach yields acceptable accuracy results. The economical implications of described method are readily applicable to both the industrial and the aerospace Combustion Turbine fields. It is economically impractical to replace the blade at each damage occurrence. However, the evaluation time involved in making associated decisions can be extensive if traditional methods of evaluation are used. The specific contributions of this study are twofold: 1. Time savings during evaluation 2. Compressor Blades may be returned to service which are otherwise replaced
Show less - Date Issued
- 2004
- Identifier
- CFE0000009, ucf:46099
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0000009
- Title
- PERFORMANCE OF INTERFACE ELEMENTS IN THE FINITE ELEMENT METHOD.
- Creator
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Rabadi, Kairas, Nicholson, David, University of Central Florida
- Abstract / Description
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The objective of this research is to assess the performance of interface elements in the finite element method. Interface elements are implemented in the finite element codes such as MSC.NASTRAN, which is used in this study. Interface elements in MSC.NASTRAN provide a tool to transition between a shell-meshed region to another shell-meshed region as well as from a shell-meshed region to a solid-meshed region. Often, in practice shell elements are layered on shell elements or on solid elements...
Show moreThe objective of this research is to assess the performance of interface elements in the finite element method. Interface elements are implemented in the finite element codes such as MSC.NASTRAN, which is used in this study. Interface elements in MSC.NASTRAN provide a tool to transition between a shell-meshed region to another shell-meshed region as well as from a shell-meshed region to a solid-meshed region. Often, in practice shell elements are layered on shell elements or on solid elements without the use of interface elements. This is potentially inaccurate arising in mismatched degrees of freedom. In the case of a shell-to-shell interface, we consider the case in which the two regions have mismatched nodes along the boundary. Interface elements are used to connect these mismatched nodes. The interface elements are especially useful in global/local analysis, where a region with a dense mesh interfaces to a region with a less dense mesh. Interface elements are used to help avoid using special transition elements between two meshed regions. This is desirable since the transition elements can be severely distorted and cause poor results. Accurate results are obtained in shell-shell and shell-solid combinations. The most interesting result is that not using interface elements can lead to severe inaccuracies. This difficulty is illustrated by computing the stress concentration of a sharp elliptical hole.
Show less - Date Issued
- 2004
- Identifier
- CFE0000291, ucf:46209
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0000291
- Title
- Thermomechanical Fatigue Life Prediction of Notched 304 Stainless Steel.
- Creator
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Karl, Justin, Gordon, Ali, Bai, Yuanli, Raghavan, Seetha, Nicholson, David, University of Central Florida
- Abstract / Description
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The behavior of materials as they are subjected to combined thermal and mechanical fatigue loads is an area of research that carries great significance in a number of engineering applications. Power generation, petrochemical, and aerospace industries operate machinery with expensive components that undergo repeated applications of force while simultaneously being exposed to variable temperature working fluids. A case of considerable importance is found in steam turbines, which subject blades...
Show moreThe behavior of materials as they are subjected to combined thermal and mechanical fatigue loads is an area of research that carries great significance in a number of engineering applications. Power generation, petrochemical, and aerospace industries operate machinery with expensive components that undergo repeated applications of force while simultaneously being exposed to variable temperature working fluids. A case of considerable importance is found in steam turbines, which subject blades to cyclic loads from rotation as well as the passing of heated gases. The complex strain and temperature histories from this type of operation, combined with the geometric profile of the blades, make accurate prediction of service life for such components challenging. Development of a deterministic life prediction model backed by physical data would allow design and operation of turbines with higher efficiency and greater regard for reliability. The majority of thermomechanical fatigue (TMF) life prediction modeling research attempts to correlate basic material property data with simplistic strain and thermal histories. With the exception of very limited cases, these types of efforts have been insufficient and imprecise in their capabilities. Early researchers did not account for the multiple damage mechanisms that operate and interact within a material during TMF loads, and did not adequately address the extent of the relationship between smooth and notched parts. More recent research that adequately recognizes the multivariate nature of TMF develops models that handle life reduction through summation of constitutive damage terms. It is feasible that a modification to the damage-based approach can sufficiently include cases that involve complex geometry. The focus of this research is to construct an experimentally-backed extension of the damage-based approach that improves handling of geometric discontinuities. Smooth and notched specimens of Type 304 stainless steel were subjected to several types of idealized fatigue conditions to assemble a clear picture of the types of damage occurring in a steam turbine and similarly-loaded mechanical systems. These results were compared with a number of idealized TMF experiments, and supplemented by numerical simulation and microscopic observation. A non-uniform damage-summation constitutive model was developed primarily based on physical observations. An additional simplistic model was developed based on phenomenological effect. Findings from this study will be applicable to life prediction efforts in other similar material and load cases.
Show less - Date Issued
- 2013
- Identifier
- CFE0004870, ucf:49666
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004870
- Title
- Combustion Synthesis and Characterization of Porous NiTi Intermetallic For Structural Application.
- Creator
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Vanterpool, Jessica, Ilegbusi, Olusegun, Gou, Jihua, Nicholson, David, University of Central Florida
- Abstract / Description
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This thesis describes experimental investigation of thermal and combustion phenomena as well as structure for self- propagating combustion synthesis of porous Ni - Ti intermetallic aimed for structural biomedical application. The control parameters for the porosity distribution have been investigated experimentally through varying the preheat temperature, initial porosity, initial elemental particle size, and applied pressure during the fabrication process. Ni and Ti elemental powders are...
Show moreThis thesis describes experimental investigation of thermal and combustion phenomena as well as structure for self- propagating combustion synthesis of porous Ni - Ti intermetallic aimed for structural biomedical application. The control parameters for the porosity distribution have been investigated experimentally through varying the preheat temperature, initial porosity, initial elemental particle size, and applied pressure during the fabrication process. Ni and Ti elemental powders are mixed using a 1:1 ratio. The mixture is compressed using several different compression forces to produce cylindrical samples of 1.1 cm diameter and 2-3cm length, with initial porosity ranging from 30% to 40%. The samples are preheated to various initial temperatures and ignited from the top surface such that the flame propagates axially downwards. The combustion reaction is recorded with a motion camera. An infrared sensor is used to record the temperature profile during the combustion process. The samples are then cut using a diamond saw in both longitudinal and transverse directions. Image analysis software is then used to analyze the porosity distribution in each sample.
Show less - Date Issued
- 2013
- Identifier
- CFE0004768, ucf:49803
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004768
- Title
- A Hybrid Constitutive Model For Creep, Fatigue, And Creep-Fatigue Damage.
- Creator
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Stewart, Calvin, Gordon, Ali, Nicholson, David, Moslehy, Faissal, University of Central Florida
- Abstract / Description
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In the combustion zone of industrial- and aero- gas turbines, thermomechanical fatigue (TMF) is the dominant damage mechanism. Thermomechanical fatigue is a coupling of independent creep, fatigue, and oxidation damage mechanisms that interact and accelerate microstructural degradation. A mixture of intergranular cracking due to creep, transgranular cracking due to fatigue, and surface embrittlement due to oxidation is often observed in gas turbine components removed from service. The current...
Show moreIn the combustion zone of industrial- and aero- gas turbines, thermomechanical fatigue (TMF) is the dominant damage mechanism. Thermomechanical fatigue is a coupling of independent creep, fatigue, and oxidation damage mechanisms that interact and accelerate microstructural degradation. A mixture of intergranular cracking due to creep, transgranular cracking due to fatigue, and surface embrittlement due to oxidation is often observed in gas turbine components removed from service. The current maintenance scheme for gas turbines is to remove components from service when any criteria (elongation, stress-rupture, crack length, etc.) exceed the designed maximum allowable. Experimental, theoretical, and numerical analyses are performed to determine the state of the component as it relates to each criterion (a time consuming process). While calculating these metrics individually has been successful in the past, a better approach would be to develop a unified mechanical modeling that incorporates the constitutive response, microstructural degradation, and rupture of the subject material via a damage variable used to predict the cumulative (")damage state(") within a component. This would allow for a priori predictions of microstructural degradation, crack propagation/arrest, and component-level lifing. In this study, a unified mechanical model for creep-fatigue (deformation, cracking, and rupture) is proposed. It is hypothesized that damage quantification techniques can be used to develop accurate creep, fatigue, and plastic/ductile cumulative- nonlinear- damage laws within the continuum damage mechanics principle. These damage laws when coupled with appropriate constitutive equations and a degrading stiffness tensor can be used to predict the mechanical state of a component. A series of monotonic, creep, fatigue, and tensile-hold creep-fatigue tests are obtained from literature for 304 stainless steel at 600(&)deg;C (1112(&)deg;F) in an air. Cumulative- nonlinear- creep, fatigue, and a coupled creep-fatigue damage laws are developed. The individual damage variables are incorporated as an internal state variable within a novel unified viscoplasticity constitutive model (zero yield surface) and degrading stiffness tensor. These equations are implemented as a custom material model within a custom FORTRAN one-dimensional finite element code. The radial return mapping technique is used with the updated stress vector solved by Newton-Raphson iteration. A consistent tangent stiffness matrix is derived based on the inelastic strain increment. All available experimental data is compared to finite element results to determine the ability of the unified mechanical model to predict deformation, damage evolution, crack growth, and rupture under a creep-fatigue environment.
Show less - Date Issued
- 2013
- Identifier
- CFE0005061, ucf:49985
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005061
- Title
- Creep-Fatigue Crack Initiation and Propagation of a Notched Stainless Steel.
- Creator
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Keller, Scott, Gordon, Ali, Nicholson, David, Raghavan, Seetha, University of Central Florida
- Abstract / Description
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Premature failures of vital gas turbine components, such as blades and vanes, have been the result of increasing demands of power generation facilities. As power needs fluctuate throughout the day, operators are quickly firing up gas turbines as a means of providing instant power. Traditionally, these engines run at constant operating conditions; however, contemporary operating conditions call for these engines to be applied on an (")as necessary(") basis. The result of the cyclic startup and...
Show morePremature failures of vital gas turbine components, such as blades and vanes, have been the result of increasing demands of power generation facilities. As power needs fluctuate throughout the day, operators are quickly firing up gas turbines as a means of providing instant power. Traditionally, these engines run at constant operating conditions; however, contemporary operating conditions call for these engines to be applied on an (")as necessary(") basis. The result of the cyclic startup and shutdown of gas turbines has led to a phenomenon known as creep-fatigue (CF). A coupling of two primary failure mechanisms in gas turbines, CF conditions exacerbate the mechanisms of creep and fatigue, ultimately leading to a premature failure of components. Traditionally, independent creep and fatigue analyses are conducted to determine the limiting life factor of gas turbines. Recently, fracture mechanics approaches have been successfully used in extending the traditional analyses to include fatigue- and creep-crack growth analyses. Founded on existing approaches to creep-fatigue crack growth analyses, including experimental elastic and plastic fracture mechanics approaches, a coupled creep-fatigue crack initiation and propagation model is developed. To bring these models to fruition, the current study utilizes the development of an experimental setup capable of subjecting a modified fracture specimen to creep-fatigue conditions. With two test temperatures key to turbine components, a blunt notch compact tension specimen was subjected to trapezoidal load waveforms with various lengths of holds at maximum load. A developed direct current potential drop (DCPD) system was used to monitor crack initiation and crack lengths throughout the duration of tests. Numerical simulations on a representative specimen were conducted, to correlate and predict key fracture mechanics parameters used in the development of creep-fatigue crack initiation and propagation models. Metallurgical analysis of specimens was conducted, implementing both optical and scanning electron microscopy. From the experimental and numerical studies, a model for both the initiation and propagation of cracks on a single specimen is furnished. Through the use of elastic-plastic fracture mechanics parameters, the proposed models are observed to predict crack initiation and replicate crack propagation rates based on the experimental conditions. Assisting in the implementation of the proposed models, intended uses and applications for the models are provided, simplifying the life prediction analyses for components expected to fail due to creep-fatigue service conditions.
Show less - Date Issued
- 2013
- Identifier
- CFE0004700, ucf:49830
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004700
- Title
- Implementation And Performance Comparisons For The Crisfield And Stiff Arc Length Methods In FEA.
- Creator
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Silvers, Thomas, Gordon, Ali, Nicholson, David, Kassab, Alain, Ilie, Marcel, University of Central Florida
- Abstract / Description
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In Nonlinear Finite Element Analysis (FEA) applied to structures, displacements at which the tangent stiffness matrix KT becomes singular are called critical points, and correspond to instabilities such as buckling or elastoplastic softening (e.g., necking). Prior to the introduction of Arc Length Methods (ALMs), critical points posed severe computational challenges, which was unfortunate since behavior at instabilities is of great interest as a precursor to structural failure. The original...
Show moreIn Nonlinear Finite Element Analysis (FEA) applied to structures, displacements at which the tangent stiffness matrix KT becomes singular are called critical points, and correspond to instabilities such as buckling or elastoplastic softening (e.g., necking). Prior to the introduction of Arc Length Methods (ALMs), critical points posed severe computational challenges, which was unfortunate since behavior at instabilities is of great interest as a precursor to structural failure. The original ALM was shown to be capable in some circumstances of continued computation at critical points, but limited success and unattractive features of the formulation were noted and addressed in extensive subsequent research. The widely used Crisfield Cylindrical and Spherical ALMs may be viewed as representing the 'state-of-the-art'. The more recent Stiff Arc Length method, which is attractive on fundamental grounds, was introduced in 2004, but without implementation, benchmarking or performance assessment. The present thesis addresses (a) implementation and (b) performance comparisons for the Crisfield and Stiff methods, using simple benchmarks formulated to incorporate elastoplastic softening. It is seen that, in contrast to the Crisfield methods, the Stiff ALM consistently continues accurate computation at, near and beyond critical points.
Show less - Date Issued
- 2012
- Identifier
- CFE0004277, ucf:49544
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004277
