Velocity Dependence during the Stick-Slip Process in the Surface Friction of Fibrous Polymers

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Abstract
Examination of chart tracings representing frictional force during the slip phase of the stick‐slip process, in fibers of two polymer types, revealed that these tracings do not have the sinusoidal character associated with a constant coefficient of kinetic friction. Considerations of the dynamics of the system of fiber surface and measuring instrument led to the suggestion that a velocity‐dependent term be introduced into the equation of motion of the slider. The new force represented by this term was assumed to be directly proportional to the velocity of the slider. Curves for the displacement of the slider as a function of time, based on solutions of the modified equation of motion, for both the underdamped and overdamped cases, were found to be in excellent agreement with typical examples of the experimentally observed curves for the two fiber types. An expression is obtained for the coefficient of friction, which predicts the well‐known experimental fact that, for most materials, the kinetic coefficient is less than the static.