Date of Award
Yun B. Yi, Ph.D.
Convective cooling on frictionally excited thermoelastic instability, Finite element analysis of thermoelastic instability in intermittent sliding contact, Thermoelastic instability (TEI)
This thesis involves three related topics in the area of thermal stress resulting from sliding contact of frictional materials. These three contributions collectively and the discussion of them herein form a collective basis furthering the research and understanding within this field. Firstly, the effect of convective cooling on thermoelastic instability is evaluated using finite element analysis involving insertion of a thermal convection term in the formula for frictional heat generation. It has been found that convection or radiation heat dissipation can stabilize the thermal-mechanical feedback process, leading to a raised critical sliding velocity. Two representative models for brake and clutch systems are studied. The computational results reveal that the effect of thermal convection on critical sliding speed is significant for liquid cooling, but negligible for air convection. With a practical range of convection coefficients estimated from fundamental heat transfer theories, critical speed in the presence of convection can be doubled or tripled. However, the wave number for the lowest critical speed remains nearly unchanged regardless of convective dissipation. Comparisons between linear and quadratic finite element interpolations are also made via a set of convergence studies. The results show that implementing quadratic elements in the friction layer has an obvious advantage over implementing linear elements due to rapidly-oscillating temperature-variations across the thermal skin layer. This is particularly important for future studies when higher-dimension problems are of interest. Secondly, a finite element model is developed for the fractionally excited thermoelastic instability problem in intermittent sliding contact with finite geometries and realistic friction materials. Existing analytical solutions are used to validate the method in several limiting cases. It is concluded that some caution must be taken for the commonly-used strategy of assuming time-averaged, frictional heat generation for intermittent contact. Predictions made by half-plane analytical solutions that assume thermally-nonconductive, rigid frictional surfaces considerably overestimate dimensionless critical speeds of realistic brake and clutch systems. Long wavelength perturbations become unstable at a dimensionless sliding speed approaching zero, which opposes the convergence of two unity in half-plane solutions. Averaging the heat input over the entire circumference is appropriate only when the period of frictional contact is longer than that of separation. These results merit the use of finite element analysis in more general applications involving intermittent contact. Thirdly, in the automotive world, the usage of sliding-disk mechanical systems that produce friction has ever led engineers to address problems regarding friction, heat, and distortion of materials, particularly friction discs themselves, with many examples found when disassembling working systems. Engineers have witnessed the phenomenon of thermal buckling, the conditions of which are analyzed herein from the perspective of moments that lead to buckling. Various parameters of system configuration, geometry, and graphical analyses based on theoretical calculations of buckling potential are considered. Distribution of temperature as a system parameter is given particular importance. It is our belief that these three contributions each provide further understanding of their respective domains while their results and their implications provide bases from which future research can be based to further a more unified understanding.
Bendawi, Ali, "Thermomechanical Stabilities in Automotive Brakes & Clutches Systems" (2015). Electronic Theses and Dissertations. 1061.
Received from ProQuest