Periodic self-organising frictional process of granular cereal material

The paper analyses the phenomenon of friction of granular cereal materials, at very low sliding velocities. Wheat was used as a test material. Tests were carried out for sliding velocities (0.05, 0.1, 0.2, 0.5, 1, 5, 10, 20, 50 [mm/min]) and loads (14.22, 26.00, 43.65, 65.34 [N]). At low slip velocities, a regular periodic change of the friction force (stick-slip) is observed, which disappears as the velocity increases. For each load value, velocity values were determined for which this phenomenon disappears. This confirmed the significant influence of not only velocity but also load on the nature of periodic changes the friction force. Three characteristic phases of friction force changes were identified (I) regular periodic frictional process; (II) no-regular periodic frictional process; (III) no periodic frictional process. In addition, for phase (I) the profile of periodicity was described by mean amplitude (Ap) and mean periodical distance (Pf) of friction profile. The correlation between Ap, Pf with load and velocity was confirmed.