Showing papers by "Brian Evans published in 2009"

The kinetics of microstructural evolution during deformation of calcite

Abstract: [1] There is a strong coupling between the microstructure and the strength of rocks that is thought to play a key role in the evolution of shear zones and in our ability to interpret the mechanics of natural deformation processes. To investigate the microstructural evolution of calcite-rich rocks, we have performed a series of hydrostatic and deformation experiments on synthetic calcite aggregates at 1023 K and 300 MPa. The mechanical data from our experiments were broadly consistent with a composite flow law for concurrent dislocation and diffusion creep. Recrystallization rates responded to the deformation conditions. When the bulk strain rate was dominated by diffusion creep, calcite grains grew at the same rate as occurs by normal grain growth under isostatic conditions. When the dominant deformation mechanism was dislocation creep, the matrix grain size was reduced at a rate that varied directly with product of stress, strain rate, and the square of grain size. Thus, reduction rate was proportional to mechanical work rate. If the stable grain size achieved during deformation depends on the product of stress and strain rate, rather than stress alone, then that grain size is an indication of the work rate and can be used as a paleowattmeter. Following this line of thought suggests that the gradient of recrystallized grain sizes that is often observed in the highly deformed portions of shear zones would not require a gradient in stress but could also be explained by material softening, resulting in locally elevated strain rates under constant stresses.

Hybrid Rocket Investigations at Penn State University's High Pressure Combustion Laboratory: Overview and Recent Results

Abstract: The Pennsylvania State University has actively pursued hybrid rocket fuel regression rate and combustion research for 15 years. Initial work focused on developing and testing a high-pressure slab-burning hybrid motor with X-ray diagnostics for characterizing the local, real-time regression rates of hydroxyl terminated p olybutadiene (HTPB) with gaseous oxygen. Fuel decomposition and pyrolysis investiga tions were also carried out. Follow-on work is continuing to investigate high regression r ate hybrid fuels with various metal additives in center-perforated hybrid motors using both HTPB and paraffin binders. The addition of aluminum powders to paraffin-based solid-fuel formulations was shown to increase the regression rates by a factor of 4 comp ared to neat HTPB. This regression rate increase corresponds to a mass-burning rate increase ~7 times that of HTPB when the increase in fuel density is considered. This artic le reviews some of the more significant results from previous investigations and presents r ecent data from on-going test programs. Nomenclature

Comparison of Nozzle Throat Erosion Behavior in a Solid- Propellant Rocket Motor and a Simulator

Abstract: The performance deterioration of solid-rocket motors caused by nozzle throat erosion becomes more severe with increased operating pressure from higher rates of heat and mass transfer from the core flow to the nozzle surface. Understanding of the rocket nozzle throat erosion processes and developing methods for mitigation of erosion rate can allow motor operation pressures to be substantially higher than those of the existing propulsion systems. Two test rigs have been utilized in the study of nozzle throat erosion phenomena for G-90 grade graphite; an instrumented solid propellant motor (ISPM) and a solid-propellant rocket motor simulator (RMS). The X-ray translucent nozzle assembly used for the RMS and ISPM allows the real-time imaging of the nozzle-throat station. It also has the feature for incorporating a nozzle boundary-layer control system (NBLCS) to mitigate nozzle-throat erosion rates. The RMS is a gaseous reactant combustor, allows for control of product species compositions, their flow rates, and combustor operating pressure. The erosion process of G-90 graphite was also evaluated in the ISPM using both non-metallized and metallized composite solid propellants. Tests conducted at operating pressures around 21 MPa showed greatly reduced nozzle throat erosion rate when the NBLCS was utilized. A dimensionless nozzle-throat erosion rate correlation was developed in terms of the effective oxidizer mass fraction, chamber pressure, Reynolds number, and relative boundary layer thickness. The correlation equation accurately predicts erosion rate data measured in the RMS and the ISPM for both non-metallized and metallized propellants over a wide range of operating conditions. The calculated erosion rates from the correlation showed agreement within ± 0.05 mm/s of the experimentally determined values.

The effect of dissolved magnesium on creep of calcite II: transition from diffusion creep to dislocation creep

Abstract: We extended a previous study on the influence of Mg solute impurity on diffusion creep in calcite to include deformation under a broader range of stress conditions and over a wider range of Mg contents. Synthetic marbles were produced by hot isostatic pressing (HIP) mixtures of calcite and dolomite powders for different intervals (2–30 h) at 850°C and 300 MPa confining pressure. The HIP treatment resulted in high-magnesian calcite aggregates with Mg content ranging from 0.5 to 17 mol%. Both back-scattered electron images and chemical analysis suggested that the dolomite phase was completely dissolved, and that Mg distribution was homogeneous throughout the samples at the scale of about two micrometers. The grain size after HIP varied from 8 to 31 μm, increased with time at temperature, and decreased with increasing Mg content (>3.0 mol%). Grain size and time were consistent with a normal grain growth equation, with exponents from 2.4 to 4.7, for samples containing 0.5–17.0 mol% Mg, respectively. We deformed samples after HIP at the same confining pressure with differential stresses between 20 and 200 MPa using either constant strain rate or stepping intervals of loading at constant stresses in a Paterson gas-medium deformation apparatus. The deformation tests took place at between 700 and 800°C and at strain rates between 10−6 and 10−3 s−1. After deformation to strains of about 25%, a bimodal distribution of large protoblasts and small recrystallized neoblasts coexisted in some samples loaded at higher stresses. The deformation data indicated a transition in mechanism from diffusion creep to dislocation creep. At stresses below 40 MPa, the strength was directly proportional to grain size and decreased with increasing Mg content due to the reductions in grain size. At about 40 MPa, the sensitivity of log strain rate to log stress, (n), became greater than 1 and eventually exceeded 3 for stresses above 80 MPa. At a given strain rate and temperature, the stress at which that transition occurred was larger for samples with higher Mg content and smaller grain size. At given strain rates, constant temperature, and fixed grain size, the strength of calcite in the dislocation creep regime increased with solute content, while the strength in the diffusion creep regime was independent of Mg content. The results suggest that chemical composition will be an important element to consider when solid substitution can occur during natural deformation.

Stress Transfer During Pressure Solution Compression of Rigidly Coupled Axisymmetric Asperities Pressed Against a Flat Semi-Infinite Solid

Abstract: In a previous work, we developed a numerical model of compression by pressure solution (PS) of a single axisymmetric asperity pressed against a flat semi-infinite solid. The dissolution rate at any point along the contact and at any time t was determined by (1) computing the normal stress distribution from the current shape of the asperity, and (2) solving the diffusion equation inside the fluid-saturated solid-solid interface, including local dissolution source terms corresponding to the stress field previously determined. The change in shape of the asperity during an infinitesimal time interval δt can then be calculated and the entire procedure repeated as many times as desired. Our results showed that, as the contact flattens and grows during PS, the initial elastic deformation is partially relaxed and the stress transferred from the contact center to the edge. Our goal in the present paper is to demonstrate that, among a population of asperities, stress can also be transferred from one contact to another and that the overall compaction rate can be significantly affected by this process. For this purpose we extended our previous numerical model to simulate PS of two rigidly coupled spherical asperities simultaneously pressed against a flat semi-infinite solid. We considered two end-member cases: 1) transfer of stress to a newly created, not initially present contact, 2) transfer of stress between asperities with different sizes. In both cases, stress was transferred from the most stressed asperity to the least, and, the overall PS displacement rate was reduced. Thus, formation of new contacts and heterogeneous distribution of asperity sizes, which are both expected to exist in rough fractures with self-affine aperture or in heterogeneous granular materials with variable grain-packing geometry, may significantly slow down PS creep compaction.