Friction Stir Processing (FSP) is currently being considered for use in manufacture of the Navy's NiAl bronze propellers. Incorporating this technology may improve service performance and enable reduction of manufacturing time and cost. This program of research has employed miniature tensile sample designs to examine the distributions of longitudinal properties through the various regimes in a fusion weld. Also, the distributions of both longitudinal and transverse properties throughout the stir zones for selected FSP conditions were examined. Yield strengths were larger in various FSP conditions by at least a factor of two relative to fusion welds. Ultimate strengths were comparable in the weld pool and stir nugget. WidmanstaÌ tten microstructures and microvoid formation and coalescence in the fracture surface resulted in high ductilities in weld metal and the stir nugget. The thermomechanically affected zone of FSP and the heat affected zone of a fusion weld both exhibit low ductility. This may reflect formation of "a" upon heating to temperatures of 800-850 [degrees]C, followed by rapid cooling and transformation of the "a" to form martensitic transformation products in their respective microstructures. For a single-pass raster pattern, transverse ductility is lower than longitudinal ductility. For a multi-pass raster, transverse ductility is higher than longitudinal ductility. For multi-pass raster and spiral patterns in FSP, the data show that the mechanical properties are more nearly isotropic. / US Navy (USN) author.

Tensile properties, namely the elastic modulus, tensile strength, percent of elongation at yield and at the break were determined for the pure components and blends. The results are connected to the respective phase diagrams and demonstrate that blending makes property manipulation possible. Blends for which the mechanical properties are better than those of pure EPs can be obtained.

Graphene, an atomically-thin layer of hexagonally bonded carbon atoms, is the strongest material ever tested. The unusual electrical and mechanical properties of graphene are particularly useful for next-generation transparent touch screens, flexible electronic displays, and photovoltaics. As such applications arise, it is critically important to characterize the resistance of this material under impact and deformation by nanoscale contact. The objective of this thesis is to study the physics of deformation in graphene sheets on a flat substrate under nanoindentation, as a function of number of graphene layers and applied force. In this work, the nanoindentation behavior of single and few layer graphene sheets was investigated by using atomic force microscopy (AFM). Graphene was created by mechanical exfoliation and deposited on a flat SiO2 substrate. The system of graphene on SiO2 simulates many of graphene’s applications, but its characterization by nanoindentation is not fully understood. Here, it was found that the deformation of the atomically-thin film remains purely elastic during nanoindentation, while the amorphous substrate deforms plastically. Also, both modulus of elasticity and contact stiffness were found to increase by 18% when few layer graphene sheets were added to a SiO2 substrate. However, no pronounced change in nanohardness was observed in the substrate with and without the addition of graphene. Furthermore, three modes of deformation were observed including purely elastic deformation, plastic deformation and an abnormal force-depth step mechanism. Each of these mechanisms was analyzed in detail using force-displacement curves and AFM images, and a deformation mechanism map, as a function of number of graphene layers and contact force, was developed. In addition to nanomechanical experiments, computer simulations by finite element analysis (FEA) were conducted in order to better understand the nanonindentation process and underlying deformation mechanisms in this system.

Specific objective: Guided tissue regeneration (GTR) aims to regenerate the lost attachment apparatus caused by periodontal disease through the use of a barrier membrane. For the GTR procedures to be successful, barrier membranes are required to be present at the surgical site for an extended period of time (weeks to months). Synthetic membranes have the advantage of prolonged presence in a wound site; however, they do not actively contribute to wound healing. Biologic membranes are recognized by the host tissue and participate in wound healing but have the disadvantage of early resorption. Therefore, the goal of this study is to create and characterize a hybrid barrier membrane that contains biologically active fibrin matrix within a synthetic polymeric electrospun scaffold. Method: Fibrin matrices and fibrin-incorporated electrospun scaffold were created from fresh frozen plasma at three different centrifugation conditions 400g for 12 minutes, 1450g for 15 minutes and 3000g for 60 minutes. Following centrifugation, half of the membranes were crosslinked with 1% genipin. Biological stability of these scaffolds was evaluated by resistance to trypsin while their mechanical properties were characterized by MTS Bionix Uniaxial Tensile Testing System. Continuous data was analyzed by ANOVA to detect differences between groups (p=0.05). Results: The addition of an electrospun scaffold to the fibrin matrix led to improvements in the mechanical properties as evidenced by an increase in the modulus (p<0.0001), strain at break (p<0.0001) and energy to break (p<0.0001). The effect of crosslinking was marginal but not statistically significant to the mechanical properties of fibrin matrices or the fibrin incorporated scaffold. However, crosslinking did significantly increase resistance against enzymatic degradation by trypsin (p<0.0001). Lastly, centrifugation speeds at 400g and 1450g showed similar mechanical properties and biologic stability; meanwhile 3000g negatively impacted the properties of the scaffold. Conclusion: Fibrin-incorporated electronspun scaffold exhibits enhanced mechanical and biologic stability compared to fibrin matrices alone. Moreover, crosslinking improves the biologic stability of the novel biomaterial. All these characteristics of the fibrin-incorporated matrix make this membrane a potentially more ideal barrier for GTR procedures to enhance periodontal wound healing.

Friction Stir Processing (FSP) is novel technique for localized modification of the surface layer of materials. FSP produces high local strains, strain rates and local temperatures that are 0.8 - 0.9 Tm, where Tm is the melting point. The processing enhances the microstructural and mechanical properties of materials through intense plastic deformation. This thesis examines the microstructure and tensile properties in FSPed Nickel Aluminum Propeller Bronze (NAB) as a function of position in the stir zone using a unique miniature tensile sample design. Test materials were single and multi-pass FSP runs from both 6 mm and 13 mm tools. Tensile ductility was observed to increase from 11 percent to more than 30 percent elongation to fracture at locations along the center of the stir zone. Yield and ultimate strengths also increased two-fold. These improved properties were associated with the formation of WidmanstaÌ tten [a] and fine, equiaxed [a] at peak temperatures of approximately 1000 [degrees] C in these locations. Some locations in the heat affected zone (HAZ) or thermomechanically affected zone (TMAZ) exhibited ductilities below that of as-cast material. Such regions had microstructures that contained a dark-etching constituent formed by cooling after being heated to approximately 800 [degrees] C. / US Navy (USN) author.

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The characterization of the mechanical behavior of cells has always captured the interest of scientists and, in the last decades, has been facilitated by the development of techniques capable of measuring a cell’s deformability. However, if on one hand, cells are active, living materials that regulate their physiology by generating and transmitting forces throughout their volume, common mechanical characterizations of cells involve material science approaches, which mostly address them as inert materials. As a consequence, although mechanical characterizations of cells have so far provided a wealth of correlations between stiffness and physio-pathological states, they have rarely provided insights into biological function and regulation. In this thesis, a cell nanomechanical platform is presented, whose resolution allows the isolation of the mechanical contribution of load-bearing cellular components. We first demonstrated that tensional forces - rather than the passive viscoelastic properties of the cytoplasm - govern the stiffness of cells at the nanoscale. We then quantitatively characterized the relationship between intracellular forces and the µm-scale patterns of stiffness across the cell surface. This analysis allowed us to calculate multiple physiologically-relevant quantities, such as membrane tension, cortex tension, actin bundle tension, tension-free elastic modulus, and mechanical coupling distances, all from single high-resolution cell stiffness images, providing an unprecedented connection between distinct mechanobiology fields.

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Abstract not available