Tensile tests have been carried out on plain and notched ultra high molecular weight polyethylene and polyoxymethylene specimens over a range of quasistatic strain rates and stress triaxiality conditions. Numerical simulations of the experiments have been carried out using the finite element code NIKE2D in order to give accurate predictions of the triaxial state of stress at the fracture initiation site as a function of initial geometry and axial strain. The predicted axial load-time curves obtained from the numerical simulations were in a good agreement with the experimental curves demonstrating that the NIKE2D code has the ability to model the deformation behaviour of these polymers accurately. The experimental results for plain cylindrical specimens show that the materials under investigation are sensitive to changes in strain rate, with plastic flow stress increasing with increasing strain rate. The results from the tests on notched specimens show that the local failure strain decreases with reducing specimen notch profile radii (i.e. increasing stress triaxiality) but this dependence is less clear for ultra high molecular weight polyethylene as a result of its much higher ductility leading to large axial strains and consequent molecular orientation.
This paper considers the steady-state bubble-thermohydrodynamic behavior of rigid circular pad thrust bearing and presents an iterative numerical scheme to solve the governing equations. The Reynolds equation, the energy equation of the oil film, and the heat conduction equation of the pad are converted by means of finite difference method and solved numerically. The air/gas bubbles included in the lubricant are assumed to be evenly dispersed. The variation in the oil density and viscosity due to bubble presence as well according to pressure differentials and temperature rise is considered. The surface tension of the bubbles is taken into account in the analysis. Typical graphs showing the influence of the bubble content on the most important design criteria of the bearing: load W, friction loss F, pressure center location Xp and temperature rise within the oil film are presented. The load carrying capacity, outlet temperature or friction loss does not change much with increasing bubble content. The pressure distributions predicted in earlier work by To̸nder [1–4] etc. is confirmed. The pressure center location, however, shifts more downstream, making the bearing more unstable for smaller loads.
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