331 Matching Results

Search Results

Advanced search parameters have been applied.

Developmental plasticity of mitochondrial function in American alligators, Alligator mississippiensis

Description: This article investigates the effects of chronic developmental hypoxia on cardiac mitochondrial function in embryonic and juvenile American alligators (Alligator mississippiensis).
Date: October 5, 2016
Creator: Galli, Gina L. J.; Crossley, Janna; Elsey, Ruth M.; Dzialowski, Edward M. (Edward Michael); Shiels, Holly A. & Crossley, Dane A., II
Partner: UNT College of Arts and Sciences

Initial Stages of Yield in Nanoindentation

Description: We have used the Interracial Force Microscope" to perform nanoindentations on Au single- crystal surfaces. We have observed two distinct regimes of plastic deformation which are distinguished by the magnitude of discontinuities in load relaxation. At lower stresses, relaxation occurs in small deviations from elastic behavior, while at the higher stresses they take the form of large load drops often resulting in complete relaxation of the applied load. These major events create a relatively wide plastic zone that subsequently deepens more rapidly than it widens. We discuss these findings in terms of contrasting models of dislocation processes in the two regimes.
Date: November 24, 1998
Creator: Houston, J.E.; Jarausch, K.F.; Kiely, J.D. & Russell, P.E.
Partner: UNT Libraries Government Documents Department

Simulations of a stretching bar using a plasticity model from the shear transformation zone theory

Description: An Eulerian simulation is developed to study an elastoplastic model of amorphous materials that is based upon the shear transformation zone theory developed by Langer and coworkers. In this theory, plastic deformation is controlled by an effective temperature that measures the amount of configurational disorder in the material. The simulation is used to model ductile fracture in a stretching bar that initially contains a small notch, and the effects of many of the model parameters are examined. The simulation tracks the shape of the bar using the level set method. Within the bar, a finite difference discretization is employed that makes use of the essentially non-oscillatory (ENO) scheme. The system of equations is moderately stiff due to the presence of large elastic constants, and one of the key numerical challenges is to accurately track the level set and construct extrapolated field values for use in boundary conditions. A new approach to field extrapolation is discussed that is second order accurate and requires a constant amount of work per gridpoint.
Date: June 5, 2010
Creator: Rycroft, Chris H. & Gibou, Frederic
Partner: UNT Libraries Government Documents Department

LLNL's program on multiscale modeling of polycrystal plasticity

Description: At LLNL a multiscale modeling program based on information-passing has been established for modeling the strength properties of a body-centered-cubic metal (tantalum) ,. under conditions of extreme plastic deformation. The plastic deformation experienced by an explosively-formed shaped-charge jet is an example of �extreme deformation�. The shaped charge liner material undergoes high strain rate deformation at high hydrostatic pressure. The constitutive model for flow stress, which describes the deformation, is highly dependent on pressure, temperature, and strain-rate. Current material models can not be extrapolated to these extreme conditions because the underlying mechanisms of plastic deformation are poorly reflected in the models and laboratory experiments are limited to pressures orders of magnitude less than actual pressures. This disparity between actual deformation conditions and those that can be attained in laboratory experiments is the principle motivation behind the multiscale modeling program. The fundamental elements of LLNL� s multiscale modeling program are distinct models at the atomistic, microscale and mesoscale/continuum length scales. The information that needs to be passed from the lower to higher length scales has been carefully defined to bound the levels of effort required to ''bridge'' length scales. Information that needs to be generated by the different simulations has been specified by a multidisciplinary steering group comprised of physicists, materials scientists and engineers. The ultimate goal of the program is to provide critical information on strength properties to be used in continuum computer code simulations. The technical work-plan involves three principle areas which are highly coupled: 1) simulation development, 2) deformation experiments and 3) characterizations of deformed crystals. The three work areas are presented which provide examples of the progress of LLNL's program.
Date: April 27, 1998
Creator: Diaz De La Rubia, T.; Holmes, N. H.; King, W. E.; Lassila, D. H.; Moriarty, J. A. & Nikkel, D. J.
Partner: UNT Libraries Government Documents Department

Void growth and softening of a single crystal with strain gradient effects

Description: The strain gradient crystal plasticity theory is applied to study the deformation of planar single crystal with a void under a nominally uniaxial tension. The crystal theory assumes elevated strain hardening due to slip gradients and has a constitutive length scale. The effects of the void size with respect to the constitutive length scale on the single crystal deformation are investigated.
Date: February 6, 1997
Creator: Shu, J.Y.
Partner: UNT Libraries Government Documents Department

An Experimental Investigation of a Prescription for Identifying Plastic Strain

Description: A series of experiments is described in which a novel prescription for the identification of plastic strain is tested to determine its validity in the context of the strain-space formulation of rate-independent plasticity. Biaxial experiments were performed on several thin-walled aluminum 1100-O cylindrical specimens.
Date: February 29, 2000
Creator: Brown, A.A.; Casey, J. & Nikkel, D.J.
Partner: UNT Libraries Government Documents Department

On the Elasto-Plastic Response of a Large-Tow Triaxial Braided Composite

Description: The elastic-plastic response of a large-tow 0{sup o}/{+-}{theta}{sup o} tri-axially braided composite is numerically simulated to determine the elastic coefficients and post-yield behavior. The ratios of extensional to flexural effective Young's moduli vary from 0.30 to 0.52 in the longitudinal direction and 0.90 to 0.95 in the transverse direction. Measurements on a 2-ply 0{sup o}/{+-} 30{sup o} braid support these numerical trends. The onset of macro yield in uniaxial extension coincides with the experimental values in the longitudinal direction while it is nearly twice the experimental values in the transverse direction. In simple shear, matrix plasticity around the undulations facilitates local rotation of the braiders at the onset of macro yield. Under uniaxial flexure, modest stiffening occurs prior to strain softening in both the principal directions.
Date: June 14, 2000
Creator: Zywicz, E.; O'Brien, M.J. & Nguyen, T.
Partner: UNT Libraries Government Documents Department

Connecting the micro to the mesoscale: review and specific examples

Description: Historically, dislocation are thought of and treated as dual objects. The large lattice distortions inside the core region warrant an atomistic treatment, whereas the slightly distorted crystal outside of the core is well represented within a linear elastic framework. Continuum dislocation theory is powerful and elegant. Yet, it is unable to fully account for the structural differentiation of dislocation behavior, say, within the same crystallography class. The source of these structural variations is mostly in the dislocation core (see [1] for an excellent review). In the past several years, the gap between the two approaches (atomistic and continuum-mesoscopic) for modeling dislocation behavior has started to close, owing to the overlap of the time and length scales accessible to them [2]. The current trend in dislocation modeling is to try to abstract the local rules of dislocation behavior, including their mobility and interactions, from the atomistic simulations and then incorporate these rules in a properly defined continuum approach, e.g. Dislocation Dynamics. The hope is that, by combining the two descriptions, a truly predictive computational framework can be obtained. For this emerging partnership to develop, some interesting issues need to be resolved concerning both physics and computations. It is from this angle that I will try to discuss several recent developments in atomistic simulations that may have serious implications for connecting atomistic and mesoscopic descriptions of dislocations. These are intended to support my speculations on what can and should be expected from atomistic calculations in the near future, for further development of dislocation theory of crystal plasticity.
Date: August 26, 1999
Creator: Bulatov, V
Partner: UNT Libraries Government Documents Department

Experiments for calibration and validation of plasticity and failure material modeling: 304L stainless steel.

Description: Experimental data for material plasticity and failure model calibration and validation were obtained from 304L stainless steel. Model calibration data were taken from smooth tension, notched tension, and compression tests. Model validation data were provided from experiments using thin-walled tube specimens subjected to path dependent combinations of internal pressure, extension, and torsion.
Date: January 1, 2006
Creator: Lee, Kenneth L.; Korellis, John S. & McFadden, Sam X.
Partner: UNT Libraries Government Documents Department

GENERIC THERMO-MECHANICAL MODEL FOR JOINTED ROCK MASSES

Description: A new nonlinear thermo-mechanical model for heavily jointed rock masses is presented. The model uses correlation functions between the porosity and the basic rock properties such as elastic moduli, tensile and compressive strength. The model assumes that the media is isotropic and is characterized by two variable parameters: insipient porosity and in-situ-to-intact modulus ratio.
Date: March 8, 2007
Creator: Vorobiev, O
Partner: UNT Libraries Government Documents Department

Indentation induced deformation in metallic materials.

Description: Nanoindentation has brought in many features of research over the past decade. This novel technique is capable of producing insights into the small ranges of deformation. This special point has brought a lot of focus in understanding the deformation behavior under the indenter. Nickel, iron, tungsten and copper-niobium alloy system were considered for a surface deformation study. All the samples exhibited a spectrum of residual deformation. The change in behavior with indentation and the materials responses to deformation at low and high loads is addressed in this study. A study on indenter geometry, which has a huge influence on the contact area and subsequently the hardness and modulus value, has been attempted. Deformation mechanisms that govern the plastic flow in materials at low loads of indentation and their sensitivity to the rate of strain imparted has been studied. A transition to elastic, plastic kind of a tendency to an elasto-plastic tendency was seen with an increase in the strain rate. All samples exhibited the same kind of behavior and a special focus is drawn in comparing the FCC nickel with BCC tungsten and iron where the persistence of the elastic, plastic response was addressed. However there is no absolute reason for the inconsistencies in the mechanical properties observed in preliminary testing, more insights can be provided with advanced microscopy techniques where the study can be focused more to understand the deformation behavior under the indenter. These experiments demonstrate that there is a wealth of information in the initial stages of indentation and has led to much more insights into the incipient stages of plasticity.
Date: December 2005
Creator: Vadlakonda, Suman
Partner: UNT Libraries

Experimental assessment of unvalidated assumptions in classical plasticity theory.

Description: This report investigates the validity of several key assumptions in classical plasticity theory regarding material response to changes in the loading direction. Three metals, two rock types, and one ceramic were subjected to non-standard loading directions, and the resulting strain response increments were displayed in Gudehus diagrams to illustrate the approximation error of classical plasticity theories. A rigorous mathematical framework for fitting classical theories to the data, thus quantifying the error, is provided. Further data analysis techniques are presented that allow testing for the effect of changes in loading direction without having to use a new sample and for inferring the yield normal and flow directions without having to measure the yield surface. Though the data are inconclusive, there is indication that classical, incrementally linear, plasticity theory may be inadequate over a certain range of loading directions. This range of loading directions also coincides with loading directions that are known to produce a physically inadmissible instability for any nonassociative plasticity model.
Date: January 1, 2009
Creator: Brannon, Rebecca Moss (University of Utah, Salt Lake City, UT); Burghardt, Jeffrey A. (University of Utah, Salt Lake City, UT); Bauer, Stephen J. & Bronowski, David R.
Partner: UNT Libraries Government Documents Department

Genomic Plasticity in Ralstonia eutropha and Ralstonia pickettii: Evidence for Rapid Genomic Change and Adaptation

Description: The proposed foci of our investigations were on Ralstonia eutropha and Rasltonia pickettii. We have 18 derived lineages of the former as well as their progenitor and eleven isolates of the latter. Our goal was to measure the level of plasticity in these strains and attempt to derive a mechanistic understanding of how genomic plasticity formed. Extensive attempts to reproducibly induce conformational changes in the genome of R. eutropha were unsuccessful. We thought that we had a reasonable lead on this inasmuch as we had shown that the ancestral strain along with many of the derivative lineages exhibited “temperature induced mutation and mortality akin to R. metallodurans. However we were unable to get subtractive hybridization working to the degree that it revealed differences between the lineages. During this time the R. pickettii analysis was proving quite fruitful and so we concentrated our efforts on our analyses of R. pickettii. These strains were isolated from a copper-contaminated lake sediment and were resistant to copper at 800 µg/ml (CuSO4). Our results in the investigation of R. pickettii permitted a view into the adaptation of a beta-proteobacteria to an extreme environment. Our worked revealed that within the same ecosystem two genomovars with structurally different genomes and genome sizes were present and apparently filling similar if not identical niches. The genomovars were detected with REP & BOX-PCR, pulse field gel electrophoresis, and DNA:DNA hybridizations. Moreover there were different metal resistance patterns associated with the different genomovars, one showing resistance to Zn and Cd while the other had resistance to Ni. Five of the isolates had a high-copy number extrachromosomal element that was identified as the replicative form of a filamentous phage. Mature virions were isolated from culture broth using PEG precipitation and CsCl density centrifugation. The DNA associated with the filamentous particles was single ...
Date: June 27, 2007
Creator: MArsh, Terence L.
Partner: UNT Libraries Government Documents Department

KAYENTA : theory and user's guide.

Description: The physical foundations and domain of applicability of the Kayenta constitutive model are presented along with descriptions of the source code and user instructions. Kayenta, which is an outgrowth of the Sandia GeoModel, includes features and fitting functions appropriate to a broad class of materials including rocks, rock-like engineered materials (such as concretes and ceramics), and metals. Fundamentally, Kayenta is a computational framework for generalized plasticity models. As such, it includes a yield surface, but the term 'yield' is generalized to include any form of inelastic material response including microcrack growth and pore collapse. Kayenta supports optional anisotropic elasticity associated with ubiquitous joint sets. Kayenta supports optional deformation-induced anisotropy through kinematic hardening (in which the initially isotropic yield surface is permitted to translate in deviatoric stress space to model Bauschinger effects). The governing equations are otherwise isotropic. Because Kayenta is a unification and generalization of simpler models, it can be run using as few as 2 parameters (for linear elasticity) to as many as 40 material and control parameters in the exceptionally rare case when all features are used. For high-strain-rate applications, Kayenta supports rate dependence through an overstress model. Isotropic damage is modeled through loss of stiffness and strength.
Date: March 1, 2009
Creator: Brannon, Rebecca Moss (University of Utah, Salt Lake City, UT); Fossum, Arlo Frederick (BP America, Inc., Houston, TX) & Strack, Otto Eric
Partner: UNT Libraries Government Documents Department

History of ultrahigh carbon steels

Description: The history and development of ultrahigh carbon steels (i.e., steels containing between 1 and 2.l percent C and now known as UHCS) are described. The early use of steel compositions containing carbon contents above the eutectoid level is found in ancient weapons from around the world. For example, both Damascus and Japanese sword steels are hypereutectoid steels. Their manufacture and processing is of interest in understanding the role of carbon content in the development of modern steels. Although sporadic examples of UHCS compositions are found in steels examined in the early part of this century, it was not until the mid-1970s that the modern study began. This study had its origin in the development of superplastic behavior in steels and the recognition that increasing the carbon content was of importance in developing that property. The compositions that were optimal for superplasticity involved the development of steels that contained higher carbon contents than conventional modern steels. It was discovered, however, that the room temperature properties of these compositions were of interest in their own right. Following this discovery, a period of intense work began on understanding their manufacture, processing, and properties for both superplastic forming and room temperature applications. The development of superplastic cast irons and iron carbides, as well as those of laminated composites containing UHCS, was an important part of this history.
Date: June 20, 1997
Creator: Wadsworth, J. & Sherby, O.D.
Partner: UNT Libraries Government Documents Department

Superplastic deformation in two microduplex stainless steels

Description: The deformation behavior and mechanisms of superplastic flow in two microduplex stainless steels (SuperDux64 and Nitronic 19D) were studied at {similar_to}0.7T{sub m}. The two steels differed in initial grain size by a factor of 3. Both steels exhibited solute-drag-controlled grain boundary sliding in a high temperature {gamma}+{delta} phase field. In a lower temperature {gamma}+{sigma} phase field, the fine-grained steel ({bar L}=5{mu}m) exhibited climb-controlled grain boundary sliding and the coarser- grained steel ({bar L}=15{mu}m) exhibited solute-drag-controlled slip creep.
Date: September 1, 1996
Creator: Lesuer, D.R.; Nieh, T.G.; Syn, C.K. & Taleff, E.M.
Partner: UNT Libraries Government Documents Department

MECHANICAL BEHAVIOR OF MOLYBDENUM DISILICIDE-BASED ALLOYS

Description: We have investigated the mechanical behavior of the following single-phase polycrystalline alloys with the MoSi{sub 2} body-center tetragonal structure: MoSi{sub 2} alloyed with {approximately}2.5 at.% Re, MoSi{sub 2} alloyed with 2 at.% Al, MoSi{sub 2} alloyed with 1 at.% Nb, and MoSi{sub 2} alloyed with 1 at.% Re and 2 at.% Al. Several anomalies in the mechanical behavior of alloyed materials were observed. For example, (1) addition of only {approximately}2.5 at. % Re results in an order of magnitude increase in compressive strength at 1600 C, (2) additions of Nb and Al cause solution softening at near-ambient temperatures, and (3) quaternary MoSi{sub 2}-Re-Al alloys show strengthening at elevated temperatures and reduction in flow stress with enhanced plasticity at near-ambient temperatures in compression. The mechanisms of anomalous solution hardening and softening are discussed.
Date: December 1, 2000
Creator: MISRA, A.; SHARIF, A. & AL, ET
Partner: UNT Libraries Government Documents Department

Microstructure for Enhanced Plasticity and Toughness

Description: Magnesium is the lightest metal with a very high specific strength. However, its practical applicability is limited by its toughness and reliability. Mg, being HCP has low ductility. This makes the improvement of toughness a grand challenge in Mg alloys. Friction stir processing (FSP) is a thermomechanical technique used to effect microstructural modification. Here, FSP was utilized to affect the toughness of WE43 sheets through microstructural modification. Room temperature Kahn-type tests were conducted to measure the toughness of WE43 sheets. Microscopic techniques (SEM, TEM) was utilized to study the effect of various microstructural factors like grain size, texture, constituent particles, precipitates on crack initiation and propagation. Tensile properties were evaluated by mini-tensile tests. Crack growth in WE43 sheets was also affected by mechanics and digital image correlation (DIC) was utilized to study the plastic zone size. The underlying mechanisms affecting toughness of these sheets were understood which will help in formulating ways in improving it. WE43 nanocomposites were fabricated via FSP. Uniform distribution of reinforcements was obtained in the composites. Improved mechanical properties like that of enhanced strength, increased hardness and stiffness were obtained. But contrary to other metal matrix composites which show reduction in ductility with incorporation of ceramic reinforcements, the nanocomposites showed good strength-ductility combination. The composites were precisely characterized and mechanisms governing this property were studied. The nano-length of the reinforcements was observed to be the main criteria and the dislocation-particle interaction, the main reason behind the strength-ductility property.
Access: This item is restricted to UNT Community Members. Login required if off-campus.
Date: August 2016
Creator: Das, Shamiparna
Partner: UNT Libraries

Ductile Fracture of Cracked Steel Plates

Description: A simple relationship between loading for crack initiation, or onset of ductile tear, and crack length is presented for center-cracked plates of mild steel. Formulation of the nonlinear boundary-value problem is based on incremental theory of plasticity for Prandtl-Reuss materials. Quasi-static solutions corresponding to a series of incremental loading conditions are obtained by the method of finite elements. Tests conducted on plates of two types of mild steel agree with numerical results.
Date: October 23, 2001
Creator: Yau, W.F.
Partner: UNT Libraries Government Documents Department