Browsing by Subject "mechanical properties"
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Item Effects of Water Content and Alumino-Silicate Sources on the Structure and Properties of Geopolymers(2012-10-19) Lizcano, MaricelaGeopolymers (GPs) are a special class of inorganic polymers with unique properties. Their 3-D amorphous structure and properties are often attributed to SiO2/Al2O3 molar ratios. However; contradictory results reported in literature on the structure and properties, do not conclusively support these reported findings. Furthermore, alternative processing methods are necessary for synthesizing pure geopolymers without impurities often found in precursor material. A rigorous study on chemical composition and processing parameters as well as alternative processing methods are necessary for advancing GPS in various engineering applications. The effects of H2O/(SiO2 + Al2O3) and SiO2/Al2O3 molar ratios , as well as precursor material on the density, open porosity, microstructure and the thermal and mechanical properties in K and Na activated geopolymers is investigated. X-ray diffraction, Nuclear Magnetic Resonance as well as alcohol immersion to determine density and open porosity is utilized for structural characterization. Thermogravimetric analysis and Thermomechanical analysis are used to investigate thermal behavior. Thermal conductivities and mechanical properties were measured using Thermal Constant analysis and compression testing respectively. Conclusive results demonstrate that the amount of water used to process GPs is the governing factor affecting their structure while SiO2/Al2O3 molar ratio plays no significant role. The K- and Na-activated samples have similar amounts of residual water after aging for 21 days at ambient conditions. In addition, the effects of the initial water content, SiO2/Al2O3 ratio, and alkaline activator (Na or K) on the thermal and mechanical properties of GPs, indicate that the dominant factor controlling thermal conductivity is H2O/(SiO2 + Al2O3) ratio used in processing, and to a lesser degree, the type of activation ion (Na or K). The SiO2/Al2O3 ratio did not have an effect on thermal conductivity. However, GPs compressive strengths are strongly affected by H2O/(SiO2 + Al2O3) ratio, especially at higher water ratio. At high and intermediate H2O/(SiO2 + Al2O3) ratios, liquid/solid ratio is the most important factor controlling the strength of GPs. At low H2O/(SiO2 + Al2O3) ratios, SiO2/Al2O3 ratio also plays an important role. Finally, partial geopolymer synthesis was possible using pure SiO2 and Al(OH)3 precursors, providing a possible low temperature alternative to other aluminosilicate precursors.Item Fundamental Scratch Behavior of Styrene-Acrylonitrile Random Copolymers(2011-10-21) Browning, Robert LeeThe present study employs a standardized progressive load scratch test (ASTM D7027/ISO 19252) to investigate the fundamental physical and mechanistic origins of scratch deformation in styrene-acrylonitrile (SAN) random copolymers. Previous findings from numerical simulation using finite element methods are used to establish correlation between mechanical properties and key scratch deformation mechanisms of the SAN model systems. For SAN, the acrylonitrile (AN) content and molecular weight (MW) can be changed to alter mechanical properties such as tensile strength and ductility. The key scratch deformation mechanisms are identified as: scratch groove formation, scratch visibility, periodic micro-cracking and plowing. Groove formation has been correlated to the secant modulus at the compressive yield point while micro-cracking and plowing are related to the tensile strength of the material. The fundamentals and physical origins of scratch visibility are discussed. It is explained how unbiased evaluation is accomplished by means of an automatic digital image analysis software package (ASV?). Frictional behavior and the effects of scratch speed and moisture absorption are also addressed. Increasing the AN content and/or the MW of the SAN random copolymers generally enhances the scratch resistance of the material with regard to the onset of the key deformation mechanisms. Increasing the scratch speed increases the brittleness of the material, resulting in failure at lower applied loads. Moisture absorption increases with AN content and imparts a degree of plasticization as the moisture diffuses into the sub-surface. This plasticization initially results in a degradation of scratch resistance with respect to the key deformation mechanisms, but then, after saturation, the moisture on the surface provides lubrication and improves the scratch resistance. It is important to note that polymers are fundamentally different in nature, but the findings of this study serve as an important stepping stone down the path to a deeper understanding of polymer scratch behavior.Item Mechanical Properties and Radiation Tolerance of Ultrafine Grained and Nanocrystalline Metals(2013-04-26) Sun, ChengAustenitic stainless steels are commonly used in nuclear reactors and have been considered as potential structural materials in fusion reactors due to their excellent corrosion resistance, good creep and fatigue resistance at elevated temperatures, but their relatively low yield strength and poor radiation tolerance hinder their applications in high dose radiation environments. High angle grain boundaries have long been postulated as sinks for radiation-induced defects, such as bubbles, voids, and dislocation loops. Here we provide experimental evidence that high angle grain boundaries can effectively remove radiation-induced defects. The equal channel angular pressing (ECAP) technique was used to produce ultrafine grained Fe-Cr-Ni alloy. Mechanical properties of the alloy were studied at elevated temperature by tensile tests and in situ neutron scattering measurements. Enhanced dynamic recovery process at elevated temperature due to dislocation climb lowers the strain hardening rate and ductility of ultrafine grained Fe-Cr-Ni alloy. Thermal stability of the ultrafine grained Fe-Cr-Ni alloy was examined by ex situ annealing and in situ heating within a transmission electron microscope. Abnormal grain growth at 827 K (600?C) is attributed to deformation-induced martensite, located at the triple junctions of grains. Helium ion irradiation studies on Fe-Cr-Ni alloy show that the density of He bubbles, dislocation loops, as well as irradiation hardening are reduced by grain refinement. In addition, we provide direct evidence, via in situ Kr ion irradiation within a transmission electron microscope, that high angle grain boundaries in nanocrystalline Ni can effectively absorb irradiation-induced dislocation loops and segments. The density and size of dislocation loops in irradiated nanocrystalline Ni were merely half of those in irradiated coarse grained Ni. The results imply that irradiation tolerance in bulk metals can be effectively enhanced by microstructure refinement.Item Mechanical Properties of Bulk Nanocrystalline Austenitic Stainless Steels Produced by Equal Channel Angular Pressing(2012-10-19) Gonzalez, JeremyBulk nanocrystalline 304L and 316L austenitic stainless steels (SS) were produced by equal channel angular pressing(ECAP) at elevated temperature. The average grain size achieved in 316L and 304 L SS is ~ 100 nm, and grain refinement occurs more rapid in 316 L SS than that in 304L. Also the structures are shown to retain a predominant austenite phase. Hardness increases by a factor of about 2.5 in both steels due largely to grain refinement and an introduction of a high density of dislocations. Tensile strength of nanocrystalline steels exceeds 1 GPa with good ductility in both systems. Mechanical properties of ECAPed 316L are also shown to have less dependence on strain rate than ECAPed 304L. ECAPed steels were shown to exhibit thermal stability up to 600oC as indicated by retention of high hardness in annealed specimens. Furthermore, there is an increased tolerance to radiation-induced hardening in the nanocrystalline equiaxed materials subjected to 100 keV He ions at an average dose of 3-4 displacement-per-atom level at room temperature. The large volume fraction of high angle grain boundaries may be vital for enhanced radiation tolerance. These nanocrystalline SSs show promise for further research in radiation resistant structural materials for next-generation nuclear reactor systems.Item Mechanical Properties of Sodium and Potassium Activated Metakaolin-Based Geopolymers(2011-10-21) Kim, HyunsooGeopolymers (GPs) are a new class of inorganic polymers that have been considered as good candidate materials for many applications, including fire resistant and refractory panels, adhesives, and coatings, waste encapsulation material, etc. The aim of this study is to establish relationship between structural and mechanical properties of geopolymers with different chemical compositions. The metakaolin-based geopolymers were prepared by mechanically mixing metakaolin and alkaline silicate aqueous solutions to obtain samples with SiO2/Al2O3 molar ratio that ranges from 2.5 to 5, and Na/Al or K/Al atomic ratios equal to 1. Geopolymer samples were cured in a laboratory oven at 80?C and ambient pressure for different times in the sealed containers. Structural characterization of the samples with different chemical compositions was carried out using X-Ray Diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy, Nuclear Magnetic-Resonance (NMR) spectroscopy and Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS). The mechanical characterization included Micro-indentation, Vickers indentation and fracture toughness measurement, as well as compressive testing. It was found that structure and mechanical properties of GPs depend on their chemical composition. The Na-GPs with ratio 3 have a highest compressive strength and Young?s modulus of 39 MPa and 7.9 GPa, respectively. The results of mechanical testing are discussed in more detail in this thesis and linked to structural properties of processed geopolymers.Item Methods for reduced platen compression (RPC) test specimen cutting locations using micro-CT and planar radiographs(Texas A&M University, 2004-09-30) Lemmon, HeberThis study looks at improving reduced platen compression (RPC) specimen preparation procedures by developing a better method for locating the ideal RPC specimen on each bone. These improvements are aimed at decreasing the amount of time required to complete an RPC analysis and improving the quality of the obtained results. High-resolution micro-CT scans are used to gain a better understanding of rat long bone anatomy by quantifying the location, shape, and orientation of the growth plate, primary spongiosa, and secondary spongiosa. Micro-CT analysis shows that there are easily identifiable external landmarks on the anterior side of both tibias and femurs that identify the end of the growth plate and the point at which the top of an ideal RPC specimen should be located. The landmarks are the most proximal tip of the patellar surface for the femur and the base of the tibial tuberosity for the tibia. This study also analyzes the effect of variations in the actual RPC specimen location from the ideal location and the effect of different platen sizes on test results using BMD as a surrogate for mechanical properties. The analysis shows that the BMD increases as the target RPC specimen location approaches the growth plate and decreases on moving away from the growth plate. The study also indicates that consistency is necessary when obtaining RPC specimens to avoid error due to variation from the specified landmark. Additionally, the BMD decreased as the diameter of the platen is reduced. Choosing platen size then becomes a trade off between testing the greatest amount of cancellous bone possible and potentially higher load sharing by the cortical shell with larger platen sizes as well as the risk of compressing cortical bone during the test.Item Osteogenic effect of electric muscle stimulation as a countermeasure during hindlimb unloading(Texas A&M University, 2007-09-17) Alcorn, Justin DowRats that undergo hindlimb unloading (HU) as a simulation for space flight experience bone changes similar to astronauts in microgravity. The purpose of this research was to assess whether an exercise countermeasure would be effective in preventing or mitigating bone degradation during HU. Controlled electrical muscle stimulation was applied to the lower left hindlimb to simulate resistive exercise. Adult 6-mo. old male rats were assigned to 3 groups of 12 each: hindlimb unloaded (HU), aging cage control (CC), and baseline (BL). The CC group was pair-fed to match the nutritional intake of HU animals during the 28 days of the study. The left leg was exercised 3 days a week for the duration of the study, with the unexercised right leg serving as a contra-lateral control. Mechanical tests were conducted to assess the strength of cancellous bone in the proximal tibia metaphysis. Although isolated specimens of cancellous bone are not feasible, reduced platen compression (RPC) was employed to directly load only the cancellous core region of each specimen. There was no significant difference in ultimate stress or elastic modulus between BL, CC, and HU-Ex (exercised). However, HU-Ex results were dramatically and significantly higher than HU-No Ex (contra-lateral unexercised control) for both ultimate stress (68%) and elastic modulus (81%). It is also notable that ultimate stress was 32% higher (but not statistically significant) for HU-Ex compared to CC. The total bone mineral density in the tibial metaphysis was significantly larger, 11%, in the HUEx compared to the HU-No Ex group's values. The results clearly demonstrate the efficacy of the exercise protocol in preventing the substantial mechanical deterioration induced by HU.Item Osteogenic effect of optimized muscle stimulation exercise as a countermeasure during hindlimb unloading(2009-05-15) Sumner, Lindsay RebeccaItem Polyelectrolyte Multilayers Containing Polyethylene-based Ionomers(2014-08-14) Huang, Hsiu-ChinLayer-by-Layer (LbL) assembly technique is a powerful approach to blend two or more materials to form new materials of thin films on any type of substrate, and the LbL film is called polyelectrolyte multilayers (PEMs). The assemblies are highly influenced by processing conditions and types of incorporated materials, and their morphology and chemical functionality can be controlled and tunable. The advantages allow people to design films with desired properties for a given application. This study uses linear polyethyleneimine (LPEI) and poly(ethylene-co-methacrylic acid) (EMAA) ionomer as main materials. EMAA ionomer is a commercial material known as Surlyn having ethylene as the major component, and dissolved in THF at 65? but becomes a colloidal dispersion when temperature goes down to room temperature. Water soluble LPEI solution has a large and changeable range of ionization degree by adjusting the solution pH, which also changes chain conformation. Oppositely, the ionomer has a low content of charged carboxylic acid groups, only 1.62 mole %, resulting in energetically favorable aggregation of ionic species. This study focuses on complexing the polymers having a drastically large difference in terms of ionization degree using LbL technique at a mixed THF-water solvent system. Via electrostatic interactions between LPEI and EMAA ionomers, the blends are successfully fabricated. Thermal, mechanical, and surface properties of the PEMs are investigated. For thermal properties, a new endothermic peak created in PEMs according to results of DSC overlaps with order-disorder transition peak and melting point of ionomer, resulting in an increase of latent energy. The interactions between the materials influence mechanical behavior; the PEMs exhibit higher stiffness and tensile strength, and are still tough. The most interesting and impressive performance of the blends is surface properties. Micro-sized holes and nano-scale structures (hierarchical morphology) found on the surfaces of LbL assemblies by SEM make the films very hydrophobic and superoleophilic to allow the films to separate water out from an oil-water emulsion. The film surfaces also show rose petal effect to pin water droplet of a high volume even though the surface is turn up side down. The work is the first demonstrator to use EMAA ionomers as a material in the LbL system. Many basic properties of the new complex are investigated and characterized, and these results are believed to benefit the development of novel materials in the future.Item Production, Characterization, and Mechanical Behavior of Cementitious Materials Incorporating Carbon Nanofibers(2012-10-19) Yazdanbakhsh, ArdavanCarbon nanotubes (CNTs) and carbon nanofirbers (CNFs) have excellent properties (mechanical, electrical, magnetic, etc.), which can make them effective nanoreinforcements for improving the properties of materials. The incorporation of CNT/Fs in a wide variety of materials has been researched extensively in the past decade. However, the past study on the reinforcement of cementitious materials with these nanofilaments has been limited. The findings from those studies indicate that CNT/Fs did not significantly improve the mechanical properties of cementitious materials. Two major parameters influence the effectiveness of any discrete inclusion in composite material: The dispersion quality of the inclusions and the interfacial bond between the inclusions and matrix. The main focus of this dissertation is on the dispersion factor, and consists of three main tasks: First a novel thermodynamic-based method for dispersion quantification was developed. Second, a new method, incorporating the utilization of silica fume, was devised to improve and stabilize the dispersion of CNFs in cement paste. And third, the dispersion quantification method and mechanical testing were employed to measure, compare, and correlate the dispersion and mechanical properties of CNF-incorporated cement paste produced with the conventional and new methods. Finally, the main benefits, including the increase in strength and resistance to shrinkage cracking, obtained from the utilization of CNFs in cement paste will be presented. The investigations and the corresponding results show that the novel dispersion quantification method can be implemented easily to perform a wide variety of tasks ranging from measuring dispersion of nanofilaments in composites using their optical/SEM micrographs as input, to measuring the effect of cement particle/clump size on the dispersion of nano inclusions in cement paste. It was found that cement particles do not affect the dispersion of nano inclusions in cement paste significantly while the dispersion of nano inclusions can notably degenerates if the cement particles are agglomerated. The novel dispersion quantification method shows that, the dispersion of CNFs in cement paste significantly improves by utilizing silica fume. However, it was found that the dispersion of silica fume particles is an important parameter and poorly dispersed silica fume cannot enhance the overall dispersion of nano inclusions in cementitious materials. Finally, the mechanical testing and experimentations showed that CNFs, in absence of moist curing, even if poorly dispersed, can provide important benefits in terms of strength and crack resistance.Item Skeletal Response to Simulated Microgravity Exposures and Exercise in the Adult Rat Model(2013-04-29) Shirazi-Fard, YasamanMechanical unloading has deleterious effects on the musculoskeletal system and results in significant reductions in bone density, mass, and strength, which do not fully recover even years after returning to weightbearing. For example, the rate of bone loss in microgravity is 10-fold more rapid than the rate of loss seen in elderly Caucasian females, the population group most predisposed to osteoporosis. This raises concern with individuals who are exposed to multiple bed rest periods or crewmembers who make repeated missions. Exercise offers a way to reduce or reverse these effects. Dual-energy X-ray absorptiometry (DXA) densitometry and bone mineral density (BMD) alone are generally insufficient for capturing the complex changes in bone mass, structure, and integrity and not an accurate predictor of fracture risk. Therefore, it is essential to measure the mechanical properties of bone tissue directly using animal models. The hindlimb unloaded (HU) rat model is a well-established ground-based analog for studying bone response to disuse and effects of spaceflight. The current study is one of the very few that has measured longitudinally densitometric and mechanical properties of bone after repeated simulated microgravity and long-term recovery at multiple anatomic sites in skeletally mature rats. The specific aims were to characterize 1) loss and recovery dynamics of bone following a period of unloading, 2) bone response after a second exposure to 28 days of HU, following an initial 28 days of HU and a recovery period equal to twice the duration of initial exposure, and 3) effects of resistance exercise during recovery period following an initial HU exposure and its effects on a subsequent exposure. In general, our data showed that bone response to unloading and recovery is site-specific. More specifically, we found that: 1) the rat proximal tibia metaphysis modeled the loss and discordant recovery dynamics as seen in the International Space Station (ISS) crewmembers proximal femur better than the rat femoral neck; 2) the initial exposure to HU has minimal effect on the subsequent HU exposure, and detrimental effects of the second HU exposure were milder than the initial due to reduced mechanosensitivity of the bone; 3) exercise significantly enhanced recovery following the initial HU exposure, and losses during the second exposure were not affected by exercise in most cases.Item Synthesis and Properites of Nanotwinned Silver and Aluminum(2013-07-31) Bufford, Daniel CRecent studies of fcc metals with dense twins (~10 nm spacing) have revealed impressive mechanical properties, along with improved ductility and electrical conductivity in comparison to nanocrystalline metals with similar feature sizes. Many important fcc metals could benefit from these ?nanotwinned? microstructures, however, not all fcc metals readily form such twins. The tendency of fcc metals to form twin boundaries is related to the twin boundary energy; those with low twin boundary energy, such as silver (Ag), easily form twins. Increasing twin boundary energy interferes with twin formation, to the point that in metals with high twin boundary energy, like aluminum (Al), twins are quite rare. This thesis focuses on the synthesis of nanotwinned Ag and Al via physical vapor deposition. Nanotwinned Ag is readily fabricated, however, a template approach had to be developed to induce twins in Al. The microstructures and their relationships to observed mechanical properties are also discussed. Grain boundaries interfere with dislocation transmission by posing a slip system discontinuity between grains. Twin boundaries are a special class of grain boundaries in which the grains on either side of the boundary are related by mirror symmetry. Twin boundaries inhibit dislocation transmission, providing strength in the same manner as grain boundaries. However, their symmetrical structure reduces the free volume and grain boundary energy. Accordingly, coherent twin boundaries are often more energetically stable than grain boundaries, and their coherency allows plasticity mechanisms to remain active under conditions where such mechanisms may be inhibited at grain boundaries. Hence, twin boundaries may provide a metal with unique combinations of high strength and good ductility, conductivity, and thermal stability.