Nonequilibrium and Nonlinear Dynamics in Geomaterials I : The Low Strain Regime

Members of a wide class of geomaterials are known to display complex and fascinating nonlinear and nonequilibrium dynamical behaviors over a wide range of bulk strains, down to surprisingly low values, e.g., 10^{-7}. In this paper we investigate two sandstones, Berea and Fontainebleau, and characterize their behavior under the influence of very small external forces via carefully controlled resonant bar experiments. By reducing environmental effects due to temperature and humidity variations, we are able to systematically and reproducibly study dynamical behavior at strains as low as 10^{-9}. Our study establishes the existence of two strain thresholds, the first, epsilon_L, below which the material is essentially linear, and the second, epsilon_M, below which the material is nonlinear but where quasiequilibrium thermodynamics still applies as evidenced by the success of Landau theory and a simple macroscopic description based on the Duffing oscillator. At strains above epsilon_M the behavior becomes truly nonequilibrium -- as demonstrated by the existence of material conditioning -- and Landau theory no longer applies. The main focus of this paper is the study of the region below the second threshold, but we also comment on how our work clarifies and resolves previous experimental conflicts, as well as suggest new directions of research.Comment: 14 pages, 15 figure

The effect of crack orientation on the nonlinear interaction of a P wave with an S wave

Cracks, joints, fluids, and other pore-scale structures have long been hypothesized to be the cause of the large elastic nonlinearity observed in rocks. It is difficult to definitively say which pore-scale features are most important, however, because of the difficulty in isolating the source of the nonlinear interaction. In this work, we focus on the influence of cracks on the recorded nonlinear signal and in particular on how the orientation of microcracks changes the strength of the nonlinear interaction. We do this by studying the effect of orientation on the measurements in a rock with anisotropy correlated with the presence and alignment of microcracks. We measure the nonlinear response via the traveltime delay induced in a low-amplitude P wave probe by a high-amplitude S wave pump. We find evidence that crack orientation has a significant effect on the nonlinear signal

The Relationship between Components of Effective Organizations and Teachers\u27 Instructional Practice

This purpose of this study is to determine whether the components of effective organizations are significantly related to teachers’ instructional practice as perceived by the teacher using data from the Program Capacity Survey (PCS) gathered during Year Two and Three of the STEM Career Awareness Program reform initiative. The PCS utilized Leithwood, Aitken and Jantzi’s (2001) monitoring tool and is based upon both Organizational Learning Theory and the Theory of Sustainability (Coburn, 2003). This study utilized selected questions from the PCS which were categorized into components of effective organizations. A factor analysis was used to establish the validity of the categorization. Using an ordinal logistic regression, data was analyzed to determine whether the components of effective organizations are significantly related to teachers’ instructional practice. In addition, the relationship and strength of association between the level of involvement in the decision making process and teachers’ perception of their instructional practice was analyzed. While the results of the ordinal regression were statistically nonsignificant, there was a statistically significant relationship based upon Cramer’s V for all five of the components of effective organizations at various collection periods throughout the reform implementation. There also was a statistically significant association between the level of involvement in the decision making process to adopt a new reform initiative and teachers’ instructional practice as perceived by the teacher. The findings support attending to and addressing components of effective organizations to create the conditions necessary to ensure a more successful reform implementation and increase the likelihood of sustainability

Evaluation by quantitative 99m-technetium MIBI SPECT and echocardiography of myocardial perfusion and wall motion abnormalities in patients with dobutamine-induced ST-segment elevation

ST-segment elevation during exercise testing has been attributed to myocardial ischemia and wall motion abnormalities (WMA). However, the functional significance of ST-segment elevation during dobutamine stress testing (DST) has not been evaluated in patients referred for diagnostic evaluation of myocardial ischemia. DST (up to 40 μg/kg/min) with simultaneous echocardiography and technetium-99m sestamibi single-photon emission computed tomography (SPECT) was performed in 229 consecutive patients with suspected myocardial ischemia who were unable to perform an adequate exercise test; 127 (55%) had a previous acute myocardial infarction (AMI). ST elevation was defined as ≥ 1 mm new or additional J point elevations with a horizontal or upsloping ST segment lasting 80 ms. Reversible perfusion defects on SPECT and new or worsening WMA during stress on echocardiography were considered diagnostic of ischemia. ST elevation occurred in 40 patients (17%) during the test; 34 of them (85%) had previous AMI. All patients with ST-segment elevation had abnormal scintigrams (fixed or reversible defects, or both) and abnormal wall motion (fixed or transient defects, or both) at peak s

Gender differences in the accuracy of dobutamine stress echocardiography for the diagnosis of coronary artery disease

The accuracy of dobutamine stress echocardiography (DSE) for the diagnosis of coronary artery disease (CAD) has not been yet evaluated in women. We studied the effect of gender on the accuracy of DSE for the diagnosis of CAD in 306 consecutive patients (210 men and 96 women) with limited exercise capacity and suspected myocardial ischemia who underwent coronary angiography within 3 months of DSE. There were no serious complications during DSE. Men had a higher prevalence of nonsustained ventricular tachycardia (7% vs 0.03%, p <0.05) and supraventricular tachycardia (9% vs 0.03%, p <0.05) during the test compared with women. Peak stress rate-pressure product was not different in men and women (18,140 ± 4,187 vs 18,543 ± 4,223). Significant CAD (≤50% luminal diameter stenosis) was present in 171 men (81%) and in 62 women (65%, p <0.005). The sensitivity, specificity, and accuracy of ischemic pattern at DSE for the diagnosis of significant CAD were 76% (confidence interval [CI] 67 to 84), 94% (CI 89 to 99), and 82% (C175 to 90) in women and 73% (CI 67 to 79), 77% (CI 71 to 83), and 74% (Cl 68 to 80) in men, respectively. Overall specificity was higher in women than in men (p <0.05). Regional accuracy of DSE was significantly higher in women than in men in the 3 arterial regions (84% [CI 79 to 88] vs 75% [CI 72 to 79], p <0.005). It is concluded that DSE is a safe and feasible method for the diagnosis of CAD in women.

A High-Precision Electron Emission Model: Computational Methods for Nanoscale Structures

The high-intensity, high-brightness and precision frontiers for charged particle beams are an increasingly important focus for study. Electron microscopy has demonstrated high quality beams from a single nanotip emitter, and cathodes of structured nanoscale arrays show promise as ultracold electron sources. Optimization of the cathode design for precision applications necessitates a detailed treatment of the interplay between the structure geometry, quantum mechanical emission mechanism, and electromagnetic interactions between the emitted electrons and the boundary interface. This dissertation details the numerical tools developed to simulate these processes efficiently with enough fidelity to be accurate even in the ultracold regime. Conventional simulation methods combine a particle-in-cell framework with an ad hoc current density approach to model electron emission, which are inefficient computationally and reliant on assumptions that are not valid for the type of cathodes studied in this work. I designed a novel computational framework called HiPE that is capable of modeling field electron emission from nanoscale structures on a substrate, with the precision to handle the ultracold regime. HiPE incorporates three primary tools: a Poisson integral solver, a collisional N-body numerical integrator, and a first-principles field electron emission routine. I optimized the Poisson Solver to run more accurately and efficiently in parallel on a distributed machine, and developed an adaptive regularization approach to solve an existing numerical instability for near-boundary evaluation. The novel numerical integrator was already extensively optimized to be accurate to machine precision with a relatively modest computational expense. I derived a computationally efficient form of the electron distribution function, and utilize a high-order transformation to map the electrons to the physical structure, resulting in an embarrassingly parallel routine. Consequently, HiPE is the first and only tool of its kind currently available in the whole field, with the potential to aid in increasing the performance of future electron sources by orders of magnitude. This efficacy is showcased by using HiPE to obtain emission characteristics for several cathode designs. Each nanotip emitter in the array generates an independent beamlet initially, and the array of beamlets will then merge to form the final beam. I conclude by presenting analysis of the merger of various beamlet configurations leading to insights regarding the optimal geometric configuration of the cathode and the potential for a method to experimentally infer beam metrics that are not accessible with the current technology