The relation between friction force and relative speed during the slip-phase of a stick-slip cycle

In order to better understand the stick-slip phenomenon, several attempts have been performed in the past to measure the friction force-relative speed relation during the slip portion of a stick-slip cycle. In literature, two different shapes for this relation are reported: clockwise loops and counter-clockwise loops. In this paper, it is shown that both shapes can be obtained by changing the tangential stiffness of the test rig. Likewise, it is shown that the counter-clockwise friction-speed loop does not really occur, but that it results from neglecting the velocity variation (by vibration) of the most rigidly connected sliding specimen. Finally, a sufficiently large difference between the stiffness of both tangential sliding specimen-frame connections is recommended for accurate friction measurements during stick-slip. (C) 1998 Elsevier Science S.A. All rights reserved.