Johnson, Timothy Mason (M.S. Mechanical Engineering)
High Strain Rate Testing of Cement Paste with a Split Hopkinson Pressure Bar
An important technique used for the characterization of dynamic properties of materials subjected to high rates of strain is the Split Hopkinson Pressure Bar, which uses one dimensional wave propagation in cylindrical bars to generate uniaxial stress in the material under investigation
Portland Types III and IV cement pastes, which represent the extremes of cement mineralogies commonly available, were tested at strain rates ranging from 10-4 to 103 s-1 to determine their failure mechanisms' dependence on loading rate. Four different size specimens with three water-to-cement ratios for each were used to investigate the pore size and macroporosity. While the specimen size had no apparent effect the water content greatly influenced the failure strength by increasing the pore size, resulting in a 50% decrease in the strength.
Over the seven decade strain rate range the cement pastes' ultimate strengths were found to triple for several of the water-to-cement ratios. The greatest increase was in the range of 10 to 200 s-1.
Osterman, Michael (M.S. Mechanical Engineering)
Component Placement for Reliability and Routability
The placement of electronic components on a printed wiring board (PWB) is a complex problem which requires tradeoffs between several goals. Traditionally, placement techniques have focused on improving routability based on minimizing the total wirelength between interconnected components. However, PWB reliability, which is measured by the total failure rate of the individual components on the PWB, is also affected by component placement.
This thesis examines the problem of component placement for reliability. Placement procedures are developed so as to minimize the total failure rate of the components on a PWB cooled by either convection or conduction cooling technologies. A one-dimensional placement problem is initially examined. The single row problem is then extended to placement for an entire board. Finally, a placement procedure based on force directed placement procedures is developed to combine placement for reliability and routability. the force directed placement procedure employs a system of equations which describe the interaction topology amongst interconnected components and acts to minimize the potential energy of the system. From the reliability placement procedure, pseudo-force equations are developed based on the reliability placement configuration. These force equations are developed in such a way as to assign preferred movement to components into the best placement configuration based on reducing the total failure rate of the system. The advantage of this approach is that it can be implemented into an existing force directed placement techniques for minimizing total wirelength. Then, controlling the influence of both placement procedures, components are placed in such a way as to provide reliability management and good routability.