Measurement and Analysis of Slips at Metal-to-Metal Frictional Contact Interfaces

Lots of components in mechanical systems are connected by friction contacts, such as bolts joints, shafts and bushings, etc. When they are subjected to tangential loads, small slips may occur at the friction contact interfaces. Under multiple cyclic tangential loads, the small slips may just occur at the early stages and cease after some cycles, reaching a shakedown state. Sometimes the slips may be cumulative and cause a significant relative displacement between the contact parts, leading to failure of the connections. But this micro slip is difficult to measure directly, and people often can only measure the relative displacement between the contact parts. In this article, a triboelectric nanogenerator, which was of vertical contact-separation type was used to measure the slip in a metal flat-on-flat contact in the testing platform. In the triboelectric nanogenerator fabricated in this study, two triboelectric layers were fixed on the two test pieces separately at the contact interface and were in contact initially, and the open-circuit voltage generated by the separation of the triboelectric layers could be adopted to characterize the relative slip displacement of the contact surfaces, because the magnitude of the open-circuit voltage was proportional to the separation displacement of the triboelectric layers, which was the same as the slip displacement of the contact surfaces. The slip testing platform established in this study included the vibration exciter, the upper test piece, the lower test piece, the force application piece, the excitation force sensor, the triboelectric nanogenerator, the power amplifier, the Keithley 6514 electrometer and the 6500 data acquisition system. The linear relationship between the open circuit voltage of the triboelectric nanogenerator and the separation displacement of the triboelectric layers was obtained by calibration. Then, the accumulation of slips in the metal flat-on-flat contact were tested using the slip testing platform, in which the upper test piece subjected to the uniform normal pressure generated by the force application piece was in contact with the lower test piece, and the vibration exciter was pressed on the left side of the upper test piece to apply cyclic tangential loads. When the upper test piece slipped relative to the lower test piece, the triboelectric nanogenerator would generate an electrical signal due to separation of the triboelectric layers, which was collected by the 6514 electrometer and the 6500 data acquisition system and converted to the slip displacement of the upper test piece. Then, the finite element model of the flat-on-flat contact configuration in the experiment was developed and the dynamic analysis was implemented to simulate this process. The simulation results were in good agreement with the experimental results. Based on the finite element dynamic analysis, the mechanisms of accumulation and shakedown behaviors of slips at contact interfaces were analyzed. It was found that there existed an area on the interface where the tangential stress was less than the maximum friction stress during the second and subsequent load cycles when the slip was shakedown. Further numerical simulation results showed that the slip displacement was related to the magnitude and form of the load, as well as the deformation at the contact interface. As the normal load increased, the tangential load required for the same slip displacement of the upper specimen did not increase proportionally. This was because the increase in normal load leads to a decrease in the sticking area at both ends of the contact surface. By using the decreasing normal load along the surface of the upper specimen with the maximum value on the side where the tangential force was applied, the edge areas of the contact surfaces could be more closely fitted and slip displacement could be reduced.