Browsing by Subject "Composite materials"
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Item A quantitative approach to modeling polymer blend compatibility(Texas Tech University, 1997-05) Mikel, Natausha J.Some of the difficulties in designing polymer blends have been hinted at in the previous section. Polymer blending, including blending of virgin and recycled polymers, is a relatively new area. Even so, a great deal of scientific research has been undertaken pertaining to this area mainly due to the recognition of the importance of this field. As a result, there is a theoretical understanding of many aspects of polymer blending. The goal of this work was to organize some of these theoretical aspects into a modeling tool to aid in the development and/or improvement of polymer blends. The emphasis of this model was on polymer/polymer blends and does not include polymer solutions which differ slightly from polymer/polymer blends in behavior. The model which was developed is based on first principals and is meant to be used as a directional tool. It does not address some aspects of blending, such as rheology, crystallization, or the kinetics of phase separation. The main focus of the modal is on determination of blend compatibility with additional exploration of methods for the prediction of physical properties ofa blend. Because these two areas of interest can be approached separately depending on the needs of the user, the model developed is essentially presented as an algorithm using different modaling approaches to address the two specific areas mentioned. This allows a user to bypass compatibility modeling in a case where property predictions are dasu-ad for a wall-defined compatible blend.Item Development of composite materials for non-leaded glove for use in radiological hand protection(2002-05) Doodoo-Amoo, David Nii; Landsberger, SheldonItem Development of neutron beam analytical techniques for characterization of carbon fiber composite materials(2003) Dorsey, Daniel John; Charlton, William S.Item Discrete wavelet analysis of acoustic emissions during fatigue loading of carbon fiber reinforced composites(Texas Tech University, 1996-05) Kamala, Girish P.Acoustic Emissions (AE) are generated during operational loading of Fiber Reinforced Composite (FRC) materials due to various sources of fracture. These sources which include matrix fracture, fiber fracture, splitting and delamination could be generated individually or simultaneously. The multiplicity of defects and failure modes creates problems in identifying and distinguishing various sources of emissions. This analysis is further complicated due to the friction related emissions (generated during grating of newly generated damage surfaces and fretting of broken fibers with matrix) that mask the actual signal and in most cases, exceeds the emissions from actual damage. The purpose of this thesis is to investigate the appUcability of discrete wavelet transforms in the analysis and time-frequency breakdown of AE signals detected during fatigue loading of FRC materials. The first objective is to decompose the AE signals into different levels based on the central frequency around which the emissions are generated. The second objective is to identify the frequency at which friction based emissions are generated. The final objective is to determine if a possibility exists to associate various failure modes with specific frequencies.Item Effects of moisture on the dimensional and viscoelastic properties of glassy polymers(Texas Tech University, 2003-12) Zheng, YongOne cause of long-term dimensional changes in glassy polymers is the gradual evolution of the glassy structure and viscoelastic properties through aging processes in the glassy state. Many applications of polymers involve changes in relative humidity (RH), under which the materials exhibit aging processes that may differ from those in constant RH conditions. In this work, results from a study of a glassy epoxy subjected to isothermal RH-jumps are reported. Similar to the temperature jump experiments of Kovacs, the volume recovery responses in different histories, intrinsic isopiestic (constant RH), memory effect and the asymmetry of approach, are obtained. In addition, the effects of structural recovery on sorption and desorption of water and the physical aging responses of this epoxy resin are also measured. The experimental results qualitatively support the hypothesis that water has an effect similar to temperature on structure (volume, enthalpy) of the glass-forming material. However, quantitatively, at the same RH-temperature states, the glasses formed by RH jumps show anomalous differences from those formed by temperature jumps. The conventional TNM-KAHR model is modified to fit the experimental results of isothermal RH jump experiments. Furthermore, application of the potential energy landscape theory to describe the glasses formed by relative humidity jumps, and especially to the observed differences between temperature glass and concentration glass, is discussed.Item Electromechanics of dielectric particles in dielectric liquids acted on by a microelectrode array(Texas A&M University, 2006-04-12) Seo, Cheong SooArrays of microelectrodes were used to apply forces to dielectric (soda lime glass) spheres in a thin (200 micrometer thick) layer of a dielectric liquid polymer (EOPN 8021). The microelectrodes were fabricated using standard photolithographic methods of evaporating and electroplating gold onto a glass substrate. The objective is to use the electric body forces in the sphere and the electric surface tractions on the sphere to position the spheres in a microscale pattern, in this case a square array in-plane. Three sizes of spheres were used: 30, 90, and 170 microns in diameter. The 30 micron spheres formed clusters associated with the regions of highest electric energy density, whereas single 90 micron spheres were located at the regions of highest electric energy density. The 170 micron spheres generally did not form patterns. The experiments indicated that free charges, either in the volume of the sphere and/or on the sphere surface, significantly influence the motion of the sphere. A finite element analysis was performed to study the electro-fluid mechanics. The liquid velocity and streamlines were plotted, and the force resultants due to the liquid acting on the sphere were calculated. Also, the electric body force and surface tractions resultants were calculated. In general, the forces on the sphere and the liquid velocity are in agreement with the experimental results.Item Extreme energy absorption : the design, modeling, and testing of negative stiffness metamaterial inclusions(2013-08) Klatt, Timothy Daniel; Seepersad, Carolyn C.; Haberman, Michael R.A persistent challenge in the design of composite materials is the ability to fabricate materials that simultaneously display high stiffness and high loss factors for the creation of structural elements capable of passively suppressing vibro-acoustic energy. Relevant recent research has shown that it is possible to produce composite materials whose macroscopic mechanical stiffness and loss properties surpass those of conventional composites through the addition of trace amounts of materials displaying negative stiffness (NS) induced by phase transformation [R. S. Lakes, et al., Nature, 410, pp. 565-567, (2001)]. The present work investigates the ability to elicit NS behavior without employing physical phenomena such as inherent nonlinear material behavior (e.g., phase change or plastic deformation) or dynamic effects, but rather the controlled buckling of small-scale structural elements, metamaterials, embedded in a continuous viscoelastic matrix. To illustrate the effect of these buckled elements, a nonlinear hierarchical multiscale material model is derived which estimates the macroscopic stiffness and loss of a composite material containing pre-strained microscale structured inclusions. The nonlinear multiscale model is then utilized in a set-based hierarchical design approach to explore the design space over a wide range of inclusion geometries. Finally, prototype NS inclusions are fabricated using an additive manufacturing technique and tested to determine quasi-static inclusion stiffness which is compared with analytical predictions.Item Mechanical properties of glass fiber reinforced, rubber toughened nylon 6(2002-05) Laura, David Matthews; Paul, Donald R.Item Novel composite materials synthesized by the high-temperature interaction of pyrrole with layered oxide matrices(2005) Pavel, Alexandru Cezar; McDevitt, John T.The initial goal of the research presented herein was to develop the very first synthetic metal – high-temperature superconductor ceramic composite material, in the specific form of a polypyrrole - Bi2Sr2CaCu2O8+δ nanocomposite. In the course of scientific investigation, this scope was broadened to encompass structurally and compositionally similar layered bismuthates and simpler layered oxides. The latter substrates were prepared through novel experimental procedures that enhanced the chance of yielding nanostructured morphologies. The designed novel synthesis approaches yielded a harvest of interesting results that may be further developed upon their dissemination in the scientific community. High-temperature interaction of pyrrole with molybdenum trioxide substrates with different crystalline phases and morphologies led to the formation of the first members of a new class of heterogeneous microcomposites characterized by incomplete occupancy by the metal oxide core of the volume encapsulated by the rigid, amorphous permeable polymeric membrane that reproduces the volume of the initial grain of precursor substrate. The method may be applied for various heterogeneous catalyst substrates for the precise determination of the catalytically active crystallographic planes. In a different project, room-temperature, templateless impregnation of molybdenum trioxide substrates with different crystalline phases and morphologies by a large excess of silver (I) cations led to the formation of 1-D nanostructured novel Ag-MoO ternary phase in what may be the simplest experimental procedure available to date that has yielded a 1-D nanostructure, regardless the nature of the constituent material. Interaction of this novel ternary phase with pyrrole vapors at high reaction temperatures led to heterogeneous nanostructured composites that exhibited a silver nanorod core. Nanoscrolls of vanadium pentoxide xerogel were synthesized through a novel, facile reflux-based method that employed very acidic pH levels and long reaction times. The nanoscrolls proved to be an excellent precursor for the synthesis of reduced vanadium oxide nanosheets by the redox intercalation of long chain monoamine molecules. In a related development, the very first synthetic metal – mixed-valence polyoxovanadate salt hybrid material was synthesized in the form of a polypyrrole – tetrammonium hexavanadate microcomposite by a redox simultaneous co-precipitation in an aqueous solution. The novel material displayed good mechanical properties towards solid lubricant applications and tunable electronic conductivity. Nanocomposites of polypyrrole – layered bismuthates were produced by the topotactic intercalation of pyrrole and its subsequent in situ polymerization. Insulating and superconducting layered bismuthates were used in a similar experimental procedure that used pre-intercalated iodine species as sacrificial topotactic oxidizing agents. A novel method of iodine intercalation by a solution-based transport procedure was used in the process. Interaction of pyrrole with layered bismuthates at high reaction temperatures led to the formation of polymer-covered metal nanorods as a result of intrinsic lattice templating effect. The successful synthesis of the 1-D heterogeneous nanostructures represents the first example in which nanocomposites were used as precursors. Appropriate doping of the initial layered ceramic substrates led to polymer-covered metal alloy nanorods.Item Thermal analysis for the recovery and quenching of disturbed composite superconductors(Texas Tech University, 1993-05) Seol, Seoung YunThermal stability is one of the major issues in the design and operation of superconducting devices. Due to a thermal disturbance, the superconductor may experience a transition from the superconducting state to the normal resistive state a phenomenon known as quenching. The high electrical resistivity of the normal state superconductor and large energy stored in magnetic field contribute to an excessive amount of heat generation which may cause an irrecoverable damage. In a composite superconductor, a stabilizer is provided to alleviate the problem through a current sharing process. For a low disturbance energy, the conductor can reinstate its superconducting state. However for a large disturbance energy, irrecoverable quenching stiU occurs. The critical energy is referred to as a maximum energy required to initiate quenching. The one-dimensional heat balance equation based on the assumption of uniform cross-sectional temperature distribution has been used to calculate the critical energy in previous studies. Cryogenic stability criterion and the Minimum Propagation Zone (MPZ) theory have been typical tools to analyze the stabihty of composite superconductors. The present study investigates the effect of non-uniform temperature distribution in a cross-section of a composite superconductor. Mathematical models of current sharing and Joule heat generation in the superconductor and the stabilizer are formulated. The transient solution by finite-difference method reveals the scenario of the behavior of the conductor, starting from the deposition of initial disturbance energy, current sharing, quenching, and possible recoveryo f superconductivity. The analytical solutions of the critical energies in the superconductor and the stabilizer are also obtained for special geometries, such as a tape/film superconductor sandwiched between two stabilizers, and a wire superconductor imbedded in a stabilizer. Based on the analytical calculation of the critical energies, a new stabihty criterion for the composite superconductor is proposed. In addition, a new approach to the one-dimensional stabihty analysis has been conducted by analyzing the effect of disturbance temperature profile. The concept of critical Joule heat generation is introduced to remedy the drawback of existing critical energy theory. To achieve a low hehum boiloff in the high-temperature superconducting current lead without danger of burnout the analytical and numerical analyses are performed.Item Viscoelastic{Viscoplastic Damage Model for Asphalt Concrete(2010-10-12) Graham, Michael A.This thesis presents a continuum model for asphalt concrete incorporating non- linear viscoelasticity, viscoplasticity, mechanically-induced damage and moisture- induced damage. The Schapery single-integral viscoelastic model describes the nonlinear viscoelastic response. The viscoplastic model of Perzyna models the time- dependent permanent deformations, using a Drucker-Prager yield surface which is modified to depend on the third deviatoric stress invariant to include more complex dependence on state of stress. Mechanically-induced damage is modeled using continuum damage mechanics, using the same modified Drucker-Prager law to determine damage onset and growth. A novel moisture damage model is proposed, modeling moisture-induced damage using continuum damage mechanics; adhesive moisture- induced damage to the asphalt mastic-aggregate bond and moisture-induced cohesive damage to the asphalt mastic itself are treated separately. The analytical model is implemented numerically for three-dimensional and plane strain finite element analyses, and a series of simulations is presented to show the performance of the model and its implementation. Sensitivity studies are conducted for all model parameters and results due to various simulations corresponding to laboratory tests are presented. In addition to the continuum model, results are presented for a micromechanical model using the nonlinear-viscoelastic-viscoplastic-damage model for asphalt mastic and a linear elastic model for aggregates. Initial results are encouraging, showing the strength and stiffness of the mix as well as the failure mode varying with moisture loading. These initial results are provided as a an example of the model's robustness and suitability for modeling asphalt concrete at the mix scale.Item Wavelet-based acoustic emission analysis of composite materials(Texas Tech University, 1996-08) Qi, GangIn this dissertation, a methodology for time-frequency analysis of acoustic emission (AE) signals generated due to static loading of composite specimen is presented. The tool is based on a recently developed mathematical transform called the wavelet transform. Two aspects of AE-based nondestructive evaluation (NDE) are failure mode identification and residual strength prediction. In this work, the wavelet-based AE method is applied to these two aspects of AE-based NDE. Presently, the public literature review indicates that AE techniques are dominated by time domain analysis methods. It can be seen that these methods have matured into tools which provide satisfactory results. There are limited results available that use frequency domain techniques, however, there is valuable information available in the frequency domain. Thus, it is evident that there is a need for an AE analysis technique that simultaneously utilizes both the time and frequency domains. In this dissertation, a hybrid technique is developed. With the application of wavelet transforms to the failure mode identification, the AE signals are decomposed into different wavelet levels. A general trend is observed by investigating the energy-frequency distribution of the decomposed AE signals. This trend indicates that the energy in the AE signals is essentially concentrated in three levels (seven, eight, and nine), representing frequency rages of 50-150 kHz, 150-250 kHz, and 250-310 kHz. Furthermore, the energy percentages in levels seven, eight, and nine are determined to be 8%, 15%, and 75%, respectively. The analysis indicates that the three dominant wavelet levels may be related to different failure modes associated with the fracture of CFR composites. In the prediction of residual strength, the ability of the wavelet transform to enhance the signal to noise ratio is employed. The exponential constant in value used to determine the relationship between stress and stress intensity factor are compared relative to classical fracture mechanics and AE techniques. In the comparison study, the conventional and wavelet-based AE techniques are presented side-by-side to show the advantage of wavelet-based methods. The results verify that the wavelet-based method improves on the results relative to classical fracture mechanics methods.