Browsing by Subject "deformation"
Now showing 1 - 4 of 4
Results Per Page
Sort Options
Item Experimental deformation of natural and synthetic dolomite(Texas A&M University, 2005-11-01) Davis, Nathan ErnestNatural and hot isostatically pressed dolomite aggregates were experimentally deformed at effective pressures of Pe = 50 ?? 400 MPa, temperatures of 400 ?? 850??C, and strain rates of ε& = 1.2x10-4 s-1 to 1.2x10-7 s-1. Coarse- and fine-grained dolomite deformed at low temperature (T ≤ 700??C for coarse-grained natural dolomite, T < 700??C for fine-grained natural and synthetic dolomite) exhibit mechanical behavior that is nearly plastic; differential stresses are insensitive to strain rate, fitted either by a power law no⎟⎟⎠⎞⎜⎜⎝⎛−=??σσεε31&& with n values that range from 12 to 49 or an exponential law ([31exp )] σσαεε−=o&& with exponential law term α values from 0.023 to 0.079 MPa-1. Microstructures of samples deformed at low temperatures include mechanical twins, and undulatory extinction suggesting that twin glide and dislocation slip are the predominant deformation mechanisms. At high temperatures (T ≥ 800??C) flow strengths of coarse- and fine-grained dolomite depend more strongly on strain-rate and exhibit pronounced temperature dependencies. Microstructures of coarse-grained dolomite samples deformed at T ≥ 800??C include undulatory extinction and fine recrystallized grains suggesting that recovery and dynamic recrystallization contribute to dislocation creep at these conditions. By comparison with lower temperature deformation, mechanical twinning is unimportant. Fine-grained synthetic dolomite deformed at high temperature (T ≥ 700??C) exhibits nearly linear (Newtonian) viscous behavior, with n = 1.28 (??0.15) consistent with grain boundary (Coble) diffusion creep. At low temperatures (T ≤ 700??C) coarse-grained dolomite exhibits higher strengths at higher temperatures which cannot be described by an Arrhenius relation, while fine-grained dolomite strengths show little or no temperature dependence. At high temperatures (T ≥ 800??C), dislocation creep of coarse-grained dolomite can be described by a thermally activated power law ⎟⎟⎠⎞⎜⎜⎝⎛−⎟⎟⎠⎞⎜⎜⎝⎛−=RTHno*31exp??σσεε&& with H*/n = 60 kJ/mol, or by an exponential law ()[]⎟⎟⎠⎞⎜⎜⎝⎛−−=RTHo*31expexpσσαεε&& with H*/α = 25447 kJ/mol. At high temperatures, diffusion creep of fine-grained synthetic dolomite can be described by ⎟⎟⎠⎞⎜⎜⎝⎛−⎟⎟⎠⎞⎜⎜⎝⎛−⎟⎠⎞⎜⎝⎛Ω=RTHdno*313exp??σσεε&& with H* = 280 ??45 kJ/mol. Taken together, the flow laws for coarse- and fine-grained dolomites constrain the high temperature conditions over which crystal plasticity, dislocation creep, and diffusion creep dominate.Item Modeling the elastic and plastic response of single crystals and polycrystalline aggregates(Texas A&M University, 2005-02-17) Patwardhan, Parag VilasUnderstanding the elastic-plastic response of polycrystalline materials is an extremely difficult task. A polycrystalline material consists of a large number of crystals having different orientations. On its own, each crystal would deform in a specific manner. However, when it is part of a polycrystalline aggregate, the crystal has to ensure compatibility with the aggregate, which causes the response of the crystal to change. Knowing the response of a crystal enables us to view the change in orientation of the crystal when subjected to external macroscopic forces. This ability is useful in predicting the evolution of texture in a material. In addition, by predicting the response of a crystal that is part of a polycrystalline aggregate, we are able to determine the free energy of each crystal. This is useful in studying phenomena like grain growth and diffusion of atoms across high energy grain boundaries. This dissertation starts out by presenting an overview of the elastic and plastic response of single crystals. An attempt is made to incorporate a hardening law which can describe the hardening of slip systems for all FCC materials. The most commonly used theories for relating the response of single crystals to that of polycrystalline aggregates are the Taylor model and the Sachs model. A new theory is presented which attempts to encompass the Taylor as well as the Sachs Model for polycrystalline materials. All of the above features are incorporated into the software program "Crystals".Item Orienting Deformable Polygonal Parts without Sensors(2012-02-14) Kristek, ShawnParts orienting is an important part of automated manufacturing. Sensorless manipulation has proven to be a useful paradigm in addressing parts orienting, and the manipulation of deformable objects is a growing area of interest. Until now, these areas have remained separate because existing orienting approaches utilize forces that if applied to deformable parts violate the assumptions used by existing algorithms, and could potentially break the part. We introduce a new algorithm and manipulator actions that, when provided with the geometric description and a deformation model of choice for the part, exploits the deformation and generates a Plan that consists of the shortest sequence of manipulator actions guaranteed to orient the part up to symmetry from any unknown initial orientation and pose. Additionally, the algorithm estimates whether a given manipulator is sufficiently precise to perform the actions which guarantee the final orientation. This is dictated by the particular part geometry, deformation model, and the manipulator action path planner which contains simple end-effector constraints and any standard motion planner. We illustrate the success of the algorithm with multiple parts through 192 trials of experiments that were performed with low-precision robot manipulators and six parts made of four types of materials. The experimental trials resulted in 154 successes, which show the feasibility of deformable parts orienting. The analysis of the failures showed that for success the assumptions of zero friction are essential for this work, increased manipulator precision would be beneficial but not necessary, and a simple deformation model can be sufficient. Finally, we note that the algorithm has applications to truly sensorless manipulation of non-deformable parts.Item Quantifying non-axial deformations in rat myocardium(Texas A&M University, 2005-02-17) Aghassibake, Kristina DianeWhile it is clear that myocardium responds to mechanical stimuli, it is unknown whether myocytes transduce stress or strain. It is also unknown whether myofibers maintain lateral connectivity or move freely over one another when myocardium is deformed. Due to the lack of information about the relationship between macroscopic and cellular deformations, we sought to develop an experimental method to examine myocyte deformations and to determine their degree of affinity. A set of protocols was established for specimen preparation, image acquisition, and analysis, and two experiments were performed according to these methods. Results indicate that myocyte deformations are non-affine; therefore, some cellular rearrangement must occur when myocardium is stretched.