Browsing by Subject "Mechanical Properties"
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Item Analytical and Experimental Nanomechanical Approaches to Understanding the Ductile-to-Brittle Transition(2015-10) Hintsala, EricThis dissertation presents progress towards understanding the ductile-to-brittle transition (DBT) using a mixture of nanomechanical experiments and an analytical model. The key concept is dislocation shielding of crack tips, which is occurs due to a dislocation back stress. In order to properly evaluate the role of these interactions, in-situ experiments are ideal by reducing the number of interacting dislocations and allowing direct observation of cracking behavior and the dislocations themselves. First, in-situ transmission electron microscope (TEM) compression experiments of plasma-synthesized silicon nanocubes (NCs) are presented which shows plastic strains greater than 50% in a semi-brittle material. The mechanical properties are discussed and plasticity mechanisms are identified using post-mortem imaging with a combination of dark field and high-resolution imaging. This observations help to develop a back stress model which is used to fit the hardening regime. This represents the first study of its kind where back stresses are used in a discrete manner to match hardening rates. However, the important measurable quantities for evaluating the DBT include fracture toughness values and energetic activation parameters for cracking and plasticity. In order to do this, a new method for doing in-situ fracture experiments is explored. This method is pre-notched three point bending experiments, which were fabricated by focused ion beam (FIB) milling. Two different materials are evaluated: a model ductile material, Nitronic 50, an austenitic steel alloy, and a model brittle material, silicon. These experiments are performed in-situ scanning electron microscope (SEM) and TEM and explore different aspects including electron backscatter diffraction (EBSD) to track deformation in SEM scale experiments, pre-notching using a converged TEM beam to produce sharper notches better replicating natural cracks, etching procedures to reduce residual FIB damage and elevated temperature experiments. Lastly, an analytical method to predict DBTs is presented which can account for effects of strain rate, temperature and impurity presence. The model is tested by pre-existing data on macroscopic compact tension specimens of single crystal Fe-3%Si. Next, application of the model to nano/micro scale fracture toughness experiments is explored and the large number of confounding variables is discussed in detail. A first attempt at fitting is also presented.Item Crystallographic Information Files (CIF) with atomistic models of interacting double-walled carbon nanotubes.(2021-07-19) Dumitrica, Traian; dtraian@umn.edu; Dumitrica, Traian; Computational Nanomechanics LaboratoryThe dataset provides Crystallographic Information Files (CIF) atomistic models of interacting double-walled carbon nanotubes diameter monodisperse and different mean diameter and standard deviations. These models can be used to reproduce Figures 6-10 of the referenced paper. The files can be visualized with molecular visualizers like OVITO and JMOL.Item Effect of Different Heat Treatment Equipment on the Mechanical Properties of Nitinol Wire(2020-12) Kabarowski, KarlNitinol is a nickel-titanium shape memory alloy commonly used in the medical device field for many implanted devices because of its unique properties (shape memory and superelasticity). The purpose of this study is to analyze the mechanical behavior differences between Nitinol wire heat treated in a salt bath and Nitinol wire heat treated in a sand bath. The goal was to determine if one heat treatment method is superior to the other when considering the mechanical properties for the design of a transcatheter aortic valve. Eighteen test groups of Nitinol wire were evaluated by using two different types of heat treatment equipment and varying heat treatment temperature and heat treatment time. Samples from each test group underwent tensile testing with upper plateau strength, lower plateau strength, residual elongation, ultimate tensile strength and elongation recorded. Reducing delivery forces requires a low UPS (UPS less than 89,888 psi). Low chronic outward force was desired for improved fatigue resistance and to reduction of conduction issues caused by high radial force. Therefore, a low LPS (LPS less than 43,220 psi) is desired. A residual elongation less than 0.10% to limit the permanent deformation from loading and unloading. Higher UTS (212,372 psi) and Elongation (18.0%) are also desired for greater design space of stronger and more ductile wire. The tensile data was used to determine what equipment and process parameters yields superior mechanical performance. Results show that there is a limited process space to reach the desired mechanical properties. Heat treatment equipment showed a statistically significant impact in the lower plateau strength, ultimate tensile strength and residual elongation. Sample groups heat treated in the salt bath were more repeatable than the corresponding sand bath groups. There was no combination of tested time and temperature that yielded positive results. The salt bath had a limited design space to obtain an optimal process. A heat treatment for a time of 2 minutes and 20 seconds at a temperature of ~515°C in the salt bath is located approximately in the middle of the acceptable area determined by a contour plot. Conducting a range finding study for salt bath heat treatment is recommended to verify the study and potentially expand the processing parameter options.Item Improving powder tableting performance through materials engineering(2015-08) Osei-Yeboah, FrederickAdequate mechanical strength is a critical requirement to the successful development of a tablet product. Before tablet compression, powders are often engineered by various processes including wet granulation and surface coating, which may improve or adversely affect the powder tableting performance. Such effects, commonly, result from a change in either particle mechanical properties or particulate (size, shape) properties. In this work, tableting performance is interpreted based on the qualitative bonding-area and bonding-strength (BABS) model. The tabletability of the microcrystalline cellulose (MCC) granules deteriorates rapidly with increasing amount of granulating water and eventually leads to over-granulation at high water level. Granule surface smoothing, size enlargement, granule densification and shape rounding are the dominant factors leading to the tabletability reduction of plastic MCC. Incorporation of increasing amounts of brittle excipients, such as lactose or dibasic calcium phosphate reduces the rate of tabletability reduction by promoting more granule fragmentation, introducing more surface area available for bonding. When a sufficient amount of brittle excipients is used, the over-granulation phenomenon can be eliminated. Surface coating of incompressible MCC pellets with highly bonding polymer leads to sufficient surface deformation and adhesion to enable direct compression of the pellets into tablets of adequate mechanical strength. This improvement is enhanced by the presence of moisture, which plasticizes the polymer to allow the development of a larger bonding area between coated pellets. The relationship between mechanical properties and tableting behavior is systematically investigated in polymeric composites using celecoxib-polyvinylpyrrolidone vinyl acetate solid dispersions. Mechanical properties such as indentation hardness of the solid dispersions were measured using nanoindentation. Incorporation of celecoxib up to 60% by weight hardens the polymers, which reduces bonding area but increases bonding strength. On the other hand, moisture softens the solid dispersions and facilitates deformation under pressure to improve tablet mechanical strength. In summary, insights into the deteriorated tabletability of wet granulated powders have been developed and strategies for improving tabletability have been demonstrated. Also, the relationship between particle mechanical properties and tableting performance has been examined using solid dispersions. The BABS model has been further developed to enable its widespread application in interpreting complex tableting behavior.Item Structure and mechanical properties of elastomeric block copolymers.(2010-12) Alfonzo, Carlos GuillermoThis research presents the synthesis (by anionic polymerization and catalytic hydrogenation) and characterization of two types of block copolymers: CMC and XPX. In CMC, C is glassy poly(cyclohexylethylene) and M, the matrix, can be semicrystalline poly(ethylene) E, rubbery poly(ethylene-alt-propylene) P, or rubbery poly(ethylethylene) EE, or a combination to yield: CPC, CEEC, CEC, CPEEC and CEPC, with fC ≈ 0.18 – 0.30. XPX materials have X = CEC, fC ≈ fE, and fP ≈ 0.40 – 0.60. Block copolymer phase behavior and morphology were examined through a combination of DSC, rheology, SAXS, WAXS and TEM. CMC materials are meltordered due to block thermodynamic incompatibility with TODT > Tg (C) ≈ 147 °C and show lamellar or C cylinder morphologies. The design of XPX yields melt disordered materials up to high Mn with microphase segregation induced by E crystallization. Two high Mn XPX polymers are melt ordered above Tm(E) and show two correlation lengths in SAXS assigned to the C – E and X – P length scales. TEM images indicate that all XPX materials, irrespective of melt segregation, are characterized by composite glassy and crystalline hard domains dispersed in rubbery P at room temperature. Tensile and recovery testing at room temperature show that CMC and XPX materials, with the exception of plastic CEC, behave as thermoplastic elastomers with tunable properties. Interestingly, melt disordered XPX materials have competitive mechanical properties comparable to the strongest CMC polymers, but with advantageous processing. For melt ordered CMC, Tprocess > TODT, which is dependent on Mn, while for melt disordered XPX, Tprocess > Tm(E) ≈ 100 °C independent of Mn. The deformation of melt disordered XPX materials, probed by recovery studies and WAXS, suggests that deformation is first taken by P, then E and finally C, which causes ultimate failure, as agreed in the literature for conventional SBS and SIS thermoplastic elastomers. This implies that strain recovery in XPX materials can be comparable to that of CPC if materials contain low hard block content or are stretched to strains below the onset of E deformation. Finally, a collection of data of mechanical properties, namely modulus E, strain at break εb, tensile strength σTS and tension set εs, obtained from CMC, XPX and previously reported materials were examined. Most notably, E and εs were found to be strongly correlated with the volume fractions of C and E, as properties increase with (fC + fE)δ, where δ = 1 – 2.4. Ultimate properties such as σTS and εs are unaffected by changes in composition as failure is dictated by that of the hard domains and values are similar above a minimum amount of hard block. In addition, E, σTS, and εb are inversely correlated to rubber entanglement molecular weight Me, which implies that modulus and ultimate properties are affected by the ability of the rubber network to redistribute stress by entanglement slippage. However, εs is unresponsive to Me variations, which indicates that irrecoverable deformation in these materials results from deformation of the hard domains.Item Utilizing Additive Manufacturing for Surgical Hernia Meshes to Obtain More Desirable Mechanical Properties(2020-05-01) Glenna, Cole SThis research investigated the mechanical properties of 3D modeled hernia meshes and compared them to traditional woven and knitted surgical hernia meshes. The ultimate tensile strength of the hernia meshes varied from 7.7 to 23.1 MPa depending on which direction and which geometrical design. The yield strength of the 3D modeled hernia meshes varied from 4.6 to 16.3 MPa. The stiffness varied from 190 to 770 N/mm. The breaking strain varied from 0.91%-6.0%. The tensile strength was very similar to the six common surgical hernia meshes and mostly showed superiority in strength. The stiffness and breaking strain were quite different in the study compared to the common surgical hernia meshes. The overall project objective was to conclude if this manufacturing technique is more effective and could be a viable option for other long-term implantable devices rather than just surgical meshes. The superiority in the strength of the 3D modeled meshes is promising but does not provide sufficient evidence if 3D printed meshes would be better than traditionally manufactured meshes. More research is needed, including in-person tensile testing and in-vivo proof of concepts.