Teacher absenteeism is a major concern in many school districts around the country. There have been numerous studies on the topic, but we as educators are still dealing with the problems of teacher absenteeism. To get a good understanding of teacher and administrator perceptions of this problem, a quantitative research approach was used. Quantitative data was gathered using a questionnaire with 28 Likert scale questions. One hundred teachers and three administrators participated in this part of the study. In addition, structured questions were used to interview three teachers and three administrators. Multiple regression analyses were conducted to determine which predictor variables presented in the demographic section of the questionnaire contributed to variation in the various dependent variables, e.g. attendance. All multiple regression analyses were performed at the .05 level of statistical significance. The t-test for attendance indicated that the predictor variable, age, was statistically significant and could be concluded that the older the teacher, the less absenteeism. The t-test for attendance indicated that the predictor variable, degree, was statistically significant and could be concluded that the higher the degree, the less absenteeism. The t-test for operating procedures indicated that the predictor variable, years of experience, was statistically significant and could be concluded that the more experience, the more familiar were teachers with the operating procedures of the school. The t-test for training indicated that the predictor variable, years experience, was statistically significant and could be concluded that the more experience, the more inclined regular teachers were for substitute teacher training. The survey responses revealed many similarities between the teachers and administrators. Teachers and administrators believe there is reduced student achievement taking place when there are substitute teachers in the classroom. Because of this, all teachers and administrators feel the need for extensive professional development for substitute teachers. Teachers and administrators agree that student attendance is affected by substitute teachers in the classrooms. From all points of view, it is strongly believed that operational procedures are greatly affected when several regular teachers are absent at the same time and substitutes are in their classrooms.

No description available.

No description available.

No description available.

No description available.

<p>In this thesis the dynamic response of wide-flange steel beams and columns to blast loading was experimentally evaluated. A total of twenty six steel members were field tested using live explosives, where the charge size ranged from 50 to 250 kg of ANFO and the ground stand-off distance from 7.0 to 10.3 m. Blast wave characteristics, including incident and reflected pressures were recorded. In addition, time-dependant displacements, accelerations, and strains at different locations along the steel members were measured, and the post-blast damage and mode of failure of the test specimens were observed. This study also presented detailed analysis of the experimental data. The blast load characteristics were compared with those obtained using the Technical Manual UFC 3-340-02 model (UFCM). The spatial and temporal variations of strain rate were computed from the recorded strain time histories and analyzed. In addition, time-dependant deformations were analyzed to study the contributing modes of vibration in the dynamic response using Power Spectral Density (PSD) function. Moreover, the effect of the axial load on the maximum deformations, vibration periods, strain rates, and contributing modes in the dynamic response were study by comparing the beam results with the column results tested in the same blast shots.</p> <p>The experimental results were compared with those obtained from an equivalent Single-Degree-of-Freedom (SDOF) model, which included material nonlinearity, strain rate effect, and <em>P-δ</em> effect. To account for strain rate effect on member stiffness and strength, its full moment-curvature response is determined by dividing its cross-section into a number of layers and a strain rate-dependent stress-strain relationship, based on the Cowper-Symonds strain rate model, was used to capture the nonlinear stress distribution over the section. The <em>P-δ</em> effect was modelled using the equivalent lateral load (ELL) method to simulate the secondary moment due to axial load. To determine the effects of higher modes of vibration and the variation of steel member mechanical properties along its length on its dynamic response, the test steel members were also analyzed using Multi-Degree-of-Freedom (MDOF) models, based on Finite Element Modelling (FEM). These dynamic models were also used to investigate the effect of axial-bending interaction and dynamic stability of columns. In addition, the results of the dynamic models were used to evaluate the results of the Moment Magnification Factor (MMF) commonly used in the interaction formulas to design steel beam columns under blast. Moreover, the effect of strain rate caused by the blast loading on the local stability of steel columns was also evaluated insofar as it might lead to a shift in the governing mode of failure.</p> <p>Results showed the UFCM pressure predictions compared reasonably well with the measured pressure in the positive phase in terms of both the peak pressure and overall time variations. Results also showed that when proper accounting for secondary-moment due to axial load and strain rate effect on the member resistance function, the SDOF model adequately captured both the overall response, such as the time-dependant deformations and internal forces, and instability behaviour of steel columns under blast loading. It is also shown that using MMF method overestimates the column capacity for ductility ratios <em>µ</em> greater than one, irrespective of the axial load to Euler elastic buckling load ratio (<em>P</em>/<em>P</em>_e). Also for <em>P</em>/<em>Pe</em> > 0.5, even if <em>µ</em> >1.0, the UFC method still overestimates the actual column capacity. The results of the dynamic models were used to generate stability diagrams for the assessment of the critical load and Pressure-Impulse (PI) diagrams for checking the column performance against the allowable deflection limits, which can be implemented in design standard of steel structures under blast loading.</p> / Doctor of Philosophy (PhD)

<p> The use of AA 5754 Al-Mg alloy for automotive applications is limited by its rapid shear failure process, due to shear banding. This failure mechanism is further complicated by the presence of inhomogeneous plastic deformation, so-called Portevin-Le Chatelier (PLC) effect, during deformation. Therefore, the purpose of this study was primarily to investigate the impact of Portevin-Le Chatelier (PLC) banding towards shear banding in this commercial alloy. The second objective was to study the PLC banding as a function of prior deformation under positive strain rate sensitivity condition.</p> <p> The experimental work involved pre-straining experiments coupled with a non-contact strain measurement technique. Pre-straining experiments were carried out by deforming the sample at 223 K, at which the PLC effect is significantly suppressed, up to a prescribed amount of true strain prior to room temperature testing. A non-contact strain measurement technique, based on digital image correlation (DIC), was utilized in order to observe PLC band behavior during tensile tests at room temperature and subsequently to measure the amount of plastic strain carried within the band.</p> <p> The results showed the appearance of random nucleation deformation bands, associated with type B PLC banding, with short distance propagation during constant strain rate tensile test at room temperature. A change in the nature of PLC banding, marked by distinct band propagation, was observed once a critical amount of pre-strain is given. However, there is no evidence of a relationship between two existing phenomena, PLC banding and shear banding, in this alloy.</p> / Thesis / Master of Applied Science (MASc)

The response of wide flange steel columns subjected to constant axial loads and lateral blast loads was examined. The finite element program ABAQUS was used to model W8x40 sections with different slendernesses and boundary conditions. For the response calculations, a constant axial force was first applied to the column and the equilibrium state was determined. Next, a short duration, lateral blast load was applied and the response time history was calculated. Changes in displacement time histories and plastic hinge formations resulting from varying the axial load were examined. The cases studied include single-span and two-span columns. In addition to ideal boundary conditions, columns with linear elastic, rotational supports were also studied. Non-uniform blast loads were considered. Major axis, minor axis, and biaxial bending were investigated. The effects of strain rate and residual stresses were examined. The results for each column configuration are presented as a set of curves showing the critical blast impulse versus axial load. The critical blast impulse is defined as the impulse that either causes the column to collapse or to exceed the limiting deflection criterion. A major goal of this effort was to develop simplified design and analysis methods. To accomplish this, two single-degree-of-freedom approaches that include the effects of the axial load were derived. The first uses a bilinear resistance function that is similar to the one used for beam analysis. This approach provides a rough estimate of the critical impulse and is suitable only for preliminary design or quick vulnerability calculations. The second approach uses a nonlinear resistance function that accounts for the gradual yielding that occurs during the dynamic response. This approach can be easily implemented in a simple computer program or spreadsheet and provides close agreement with the results from the finite element method. / Ph. D.

The research reported in this dissertation focuses on the response of grain boundaries in polycrystalline metallic nanostructures to applied strain using molecular dynamics simulations and empirical interatomic force laws. The specific goals of the work include establishing how local grain boundary structure affects deformation behavior through the quantitative estimation of various plasticity mechanisms, such as dislocation emission and grain boundary sliding. The effects of strain rate and temperature on the plastic deformation process were also investigated. To achieve this, molecular dynamics simulations were performed on both thin-film and quasi-2D virtual samples constructed using a Voronoi tessellation technique. The samples were subjected to virtual mechanical testing using uniaxial strain at strain rates ranging from 105s-1 to 109s-1. Seven different interatomic embedded atom method potentials were used in this work. The model potentials describe different metals with fcc or bcc crystal structures. The model was validated against experimental results from studying the tensile deformation of irradiated austenitic stainless steels performed by collaborators at the University of Michigan. The results from the model validation include a novel technique for detecting strain localization through adherence of gold nanoparticles to the surface of an experimental sample prior to deformation. Similar trends with respect to intergranular crack initiation were observed between the model and the experiments. Simulations of deformation in the virtual samples revealed for the first time that equilibrium grain boundary structures can be non-planar for model potentials representing fcc materials with low stacking fault energy. Non-planar grain boundary features promote dislocation as deformation mechanisms, and hinder grain boundary sliding. This dissertation also reports the effects of temperature and strain rate on deformation behavior and correlates specific deformation mechanisms that originate from grain boundaries with controlling material properties, deformation temperature and strain rate. / Ph. D.

Die Festigkeitssteigerung von Betonen unter hohen Belastungsgeschwindigkeiten ist seit über 100 Jahren im Fokus der Forschung. Bisher konnten bereits eine Vielzahl von möglichen Er-klärungen dieser Festigkeitssteigerung benannt werden, wobei die Heterogenität des Betons im Allgemeinen als eine wesentliche Ursache angenommen werden kann. Die Heterogenität des Betons resultiert aus den im Zementstein eingebetteten Zuschlägen, aber auch durch da-rin eingeschlossene Luft- und Wasserporen, welche Hauptgegenstand dieser Arbeit sind. Es wurde untersucht, inwieweit vorhandenes Porenwasser die Festigkeitssteigerung von Be-ton unter hohen Belastungsgeschwindigkeiten beeinflusst. Auf Basis von experimentellen Ver-suchen im Split-HOPKINSON-Bar an verschiedenen Normalbetonen konnte gezeigt werden, dass vorhandenes Porenwasser die statische und dynamische Betondruckfestigkeit in gleicher Weise signifikant reduziert. Diese Abnahme der Betondruckfestigkeit kann als unabhängig von der Belastungsgeschwindigkeit angesehen werden. Des Weiteren wurden numerische Untersuchungen zum Einfluss von Poren auf die Ausbrei-tung von Belastungswellen durchgeführt. Dabei rückten sowohl das globale Wellenausbrei-tungsverhalten in einer porendurchsetzten Struktur als auch die lokale Wellenbrechung an ei-ner Einzelpore, sowie das Porenmedium (Luft oder Wasser) in den Fokus. Es konnte gezeigt werden, dass vorhandene Poren die Wellenausbreitungsgeschwindigkeit reduzieren und dass das Verhältnis aus Porengröße zur Belastungswellenlänge einen wesentlichen Einflussfaktor für die transiente Spannungsverteilung im Porenbereich darstellt. Insbesondere konnte mit ab-nehmender Belastungswellenlänge eine Reduzierung der örtlichen Kerbspannungen am Po-renrand beobachtet werden, woraus festigkeitssteigernde Effekte resultieren können.