This paper considers the mechanical interaction due to surface roughness and examines the surface theories using the classical definition of coefficient of friction: the tangential-to-normal load ratio. The postulation for maximum static friction is used to experimentally evaluate the contact models. For this purpose, a pin-on-disk test apparatus is employed with the capability of measuring tangential and normal forces for a frictional contact. The tests involve pairs of disks and specimens, that is, steel-on-steel and aluminum-on-aluminum contacts. In each case, profilometer measurements are performed on the disk and the Greenwood and Williamson parameters, are determined. Using the parameters, the theoretical estimates of normal and tangential loads are obtained. The theoretical values of tangential-to-normal contact load ratios are compared with those obtained from measurements for various applied normal loads. The tests utilizing a pin-on-disk apparatus showed a partial agreement between the experimentally obtained load ratios and the predicted upper limit confidence interval using the theoretical elastic and elastic-plastic contact. The result suggested that the elastic-plastic formulations provide better predictions of load ratios than the elastic contact formulations.
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