PSI - Issue 36

Iakov Lyashenko et al. / Procedia Structural Integrity 36 (2022) 24–29

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Iakov Lyashenko, Vadym Borysiuk / Structural Integrity Procedia 00 (2021) 000 – 000

3. Experimental results Developed laboratory equipment, shown in Fig. 1, allows one to perform experiments with arbitrary indenter trajectories in normal and tangential directions. Results of one of such experiments are shown in Fig. 2. Here indenter was indented into the surface of jelly at depth D = 1 mm (time of the indentation phase is 10 s), after this indenter was moved at constant velocity V = 0.1 µ m/s in tangential direction. Fig. 2a shows dependencies of the normal F N and tangential F x contact forces on the tangential coordinate x of the indenter movement measured in the experiment. The “stiction spike” can be observed on both dependencies. The presence of such pike is a well -known phenomenon that can be explained by the strong interaction between two contacting surfaces – the magnitude of the tangential force, needed to start the motion of the indenter is larger than force needed to support stationary motion in dynamic mode. However, in Fig. 2a stiction spike is also present in the dependence of the normal force F N ( x ). This pike has a completely different nature. The reason for its appearance is the crack at the beginning of motion in the area where jelly undergoes tensile stresses caused by the indenter and its following opening during the movement. As the cracks opens, the contact area is not axial symmetric and covers all accessible area of the indenter. With sufficient distancing from the crack, contact area became axial symmetric again. After some time, both normal and tangential forces decrease. First reason for this decreasing is the heating of the jelly as in the beginning of the experiment temperature of the jelly is lower than the temperature of the laboratory environment (the jelly have to be relatively cold to ensure its flat surface). Another reason for this may be the fact that it is impossible to accurately expose the surface of the jelly layer absolutely parallel to the indenter trajectory. Moreover, total motion time equals to 5000 seconds (approximately 1 hour 24 minutes) during that time gelatin can became thinner because of the evaporation of the water, which also may lead to the decreasing of both force components.

Fig. 2. Results of the experiment concerning tangential movement of the steel sphere with radius R = 2.2 cm, that was indented into gelatin at depth D = 1 mm. (a) dependencies of the normal F N and tangential F x contact forces vs. tangential coordinate of the sphere x ; (b) the same as in the panel (a), but for smaller interval of the coordinate 26 mm < x < 28 mm; (c) friction coefficient calculated according to the formal definition µ = F x / F N vs. tangential coordinate of the sphere x ; (d) the same as in the panel (c), but for smaller interval of the coordinate 26 mm < x < 28 mm. Fig. 2b shows enlarged fragments of the dependencies, shown in Fig. 2a. As it can be seen from the figure, at small distances normal force remains constant, while stick-slip mode observes in tangential direction, where F x ( x ) dependence has a saw-like shape. This is typical situation for movement with adhesion. Fig. 2c shows the dependence of the friction coefficient calculated according to the formal definition µ = F x / F N on coordinate. Until the appearance of the pike in the F N ( x ) curve, static friction is observed in the system, as the calculated friction