A novel test apparatus to study the mechanism of harmonic normal force on fretting wear

Fretting wear significantly affects the hysteresis behavior of the contact surface and leads to joint degradation due to material removal. Although numerous tests have been conducted to investigate fretting wear under constant normal force, the applied mechanical load on the contract surface more or less deviates from that in the operational condition. The normal force is typically time-varying due to the coupling of tangential and normal vibration, rather than remaining constant. The time-varying normal force can be regarded as the superposition of harmonics for steady-state vibration which is widely encountered in engineering. In this paper, we investigate how would the harmonic component of the normal force affect the evolution of interface morphology and contact parameters with fretting wear. To do that, we design a novel fretting wear test apparatus that generates the stable harmonic normal force. The normal force is controlled by a closed-loop control system, ensuring its consistency throughout the entire wear test. In addition, we develop various subsystems to realize different functions like support, constraint and measurement, enabling the acquisition of hysteresis loops under different normal forces. The critical performances of the test apparatus (e.g. force transfer, closed-loop control and repeatability) are also demonstrated. Results indicate that the harmonic normal force can significantly influence the hysteresis loop and the energy dissipation distribution on the contact surface. In comparison to the constant normal force, the worn surface has a higher degree of non-uniformity, characterized by alternating pits and prominences. Furthermore, the friction coefficient exhibits an outstanding upward trend after reaching a steady state, while the tangential contact stiffness continuously weakens as fretting wear progresses. These unique evolution characteristics could be attributed to the non-uniform energy dissipation caused by the harmonic normal force acting across the fretting space. This viewpoint can be partly validated through observing the interesting hysteresis loops, which are captured by the test apparatus.