Investigation Into Friction-Induced Noise Under Metal Reciprocating Sliding Conditions

In the present dissertation, friction-induced noises emitted by a flat specimen rubbing against another flat specimen or a ball specimen under reciprocating sliding are studied. The detailed observation of the scars is conducted by means of a scanning electrical microscope (SEM), laser scanning microscope (LSM), optical microscope and profilometer. The dynamic state variables are analyzed in detail using the discrete wavelet transform. Moreover, a simulation of friction-induced vibration is also carried out using MATLAB/SIMULINK software.Firstly, the identification of the modal parameters of the friction testing system is performed. The correlation between the friction-induced noise and vibration under reciprocating sliding is verified. The result shows that analogous to those in unidirectional sliding conditions, the dominant frequencies of the friction-induced noise are very close to the dominant frequencies of the friction-induced vibration. It is found that coherence functions between these two signals are more than 0.75 - 0.8 at these dominant frequencies. Therefore, it is concluded that the friction-induced noise is emitted by the simultaneous vibration.Secondly, by means of the measurement of the vibration and associated noise, the area in the presence of the friction-induced noise is determined accurately. As a result, a comparative analysis on the topography of the scar surfaces with and without noise is performed in detail. The difference in topography between the two scars with and without noise is found clearly. It is also observed that at the same scar, the topography of the area with noise is different from that without noise. It is observed that the topography of the area with noise on a scar surface is characterized by larger surface roughness, plowing grooves, pits or adhesive joined asperities. While it is found that the topography of the scar after noise disappearance is characterized by a clear polished scar surface and a smaller surface roughness. According to the Suh's theory of friction genesis and based on the change in topography with and without noise, it is assumed that the there is dynamic components of friction force in the area where uneven plowing grooves and pits or adhesive joined asperities are generated. It is considered that when the dominant frequencies of these dynamics components are close to the natural frequencies of the friction system, the system will show a resonant behavior and emit associated noise. The assumption may be used to explain why the occurrence of squeal is always accompanied with a larger coefficient of friction. It is because in the area with a larger coefficient of friction the dynamic friction components due to plowing and7ivasperities deformation are strongest.By imposing the displacement of reciprocating sliding, an influence of the friction-velocity slope on squeal generation is tested. The result shows that a negative friction-velocity slope can not always cause squeal. It is often observed that the squeal can also occur in the area with a positive friction-velocity slope. Moreover, it is also observed that in the area with a severe stick-slip motion the squeal and associated vibration are very weak. That may suggest that the squeal is not due to stick-slip motion.The test result also displays that sometimes the normal vibration associated with squeal firstly occurs prior to the tangential vibration. The tangential vibration does not occur until the normal vibration gets stronger. This phenomenon shows that stick-slip motion and negative friction-velocity slope are not responsible for the present squeal. It is because these two generation mechanisms of squeal only govern the tangential vibrations.The measured tangential force, tangential inertial force and normal vibration acceleration are decomposed using a discrete wavelet transform. The relation among these three dynamics state variables is investigated. The result displays that there is always a dynamic friction component in the presence of squeal. There are two forma...