Evolution of real contact area during stick-slip movement observed by total reflection method**Project supported by the National Natural Science Foundation of China (Grant No. 11872033) and the Beijing Natural Science Foundation, China (Grant No. 3172017).

Abstract

We build an experiment system based on total reflection (TR) method to observe the evolution of real contact area of polymethyl methacrylate (PMMA) in the continual stick-slip movement. The bilateral friction is adopted to overcome the bending moment in the lateral friction movement. Besides some classical phenomena of stick-slip movement such as periodical slow increase of frictional force in sticking phase and a sudden drop when slipping, a special phenomenon that the contact area increases with the tangential force is observed, which was called junction growth by Tabor in 1959. Image processing methods are developed to observe the variation of the junction area. The results show that the center of the strongest contact region will keep sticking under the tangential force until the whole slipping, the strongest point undergoes three stages in one cycle, which are named as sticking stage, fretting stage, and cracking stage, respectively. The combined analysis reveals a physical process of stick-slip movement: the tangential force causes the increase of the real contact area, which reduces the pressure between the contact spots and finally leads to the slipping. Once slipping occurs, the real contact area drops to the original level resulting in the pressure increase to the original level, which makes the sticking happen again.