Vibration-induced nanoscale friction modulation on piezoelectric materials

Mechanical vibration, as an alternative of application of solid/liquid lubricants, has been an effective means to modulate friction at the macroscale. Recently, atomic force microscopy (AFM) experiments and model simulations also suggest a similar vibration-induced friction reduction effect for nanoscale contact interfaces, although an additional external vibration source is typically needed to excite the system. Here, by introducing a piezoelectric thin film along the contact interface, we demonstrate that friction measured by a conductive AFM probe can be significantly reduced (more than 70%) when an alternating current (AC) voltage is applied. Such real-time friction modulation is achieved owing to the localized nanoscale vibration originating from the intrinsic inverse piezoelectric effect, and is applicable for various material combinations. Assisted by analysis with the Prandtl—Tomlinson (P—T) friction model, our experimental results suggest that there exists an approximately linear correlation between the vibrational amplitude and the relative factor for perturbation of sliding energy corrugation. This work offers a viable strategy for realizing active friction modulation for small-scale interfaces without the need of additional vibration source or global excitation that may adversely impact device functionalities.

Friction Characteristics Analysis of Symmetric Aluminum Alloy Parts in Warm Forming Process

There are many typical symmetric large plastic deformation problems in aluminum alloy stamping. Warm stamping technology can improve the formability of materials and obtain parts with high-dimensional accuracy. Friction behavior in the stamping process is significant for the forming quality. An accurate friction coefficient is helpful in improving the prediction accuracy of forming defects. It is hard to obtain a unified and precise friction model through simple experiments due to the complicated contact conditions. To explore the effect of friction behavior on the forming quality, warm friction experiments of the AA6061 aluminum alloy and P20 steel with different process parameters were carried out using a high-temperature friction tester CFT-I (Equipment Type), including temperatures, the interface load, and sliding speeds. The variation curves of the friction coefficient with various parameters were obtained and analyzed. The results show that the friction coefficient increases with temperature and decreases with the sliding speed and load. Then, the influences of process parameters on the surface morphology of the samples after friction were observed by an optical microscope; adhesive wear occurred when the temperature increased, and the surface scratch increased and deepened with the increase in the load. Finally, the friction coefficient models of the speed and load were established by analyzing the data with Original software. Compared with the experimental and the finite element analysis results of the symmetrical part, the errors of the velocity friction model in thickness and springback angle are less than 4% and 5%, respectively. The mistakes of the load friction model are less than 6% and 7%, respectively. The accuracy of the two friction models is higher than that of the constant friction coefficient. Therefore, those coefficient models can effectively improve the simulation accuracy of finite element software.

Sensitivity Analysis of Holdback Bar Release Load during Catapult-Assisted Takeoff of Carrier-Based Aircraft

The release load of holdback bar will affect the safety of catapult-assisted takeoff of carrier-based aircraft, and the accurate control of releasing the load will ensure success. The magnitude and the control accuracy of release load are important parameters which impact the takeoff performance, therefore unstable release load and insufficient release precision are the main factors affecting the takeoff safety. In this paper, mechanical models of the carrier-based aircraft in the catapult-assisted takeoff tensioning state and gliding state after release are established based on multi-body dynamics, contact mechanics and tribological theory, and the influence of the release load of the holdback bar on the catapult-assisted takeoff performance is analyzed. Furthermore, a kinetic model of the holdback bar device is established, and the kinetic characteristics of the release process of the holdback bar are studied. Based on the kinetic model and friction model of the holdback bar, the influencing factors of the sensitivity of the holdback bar release load are analyzed and the structural parameters are optimized. The results show that the released load decreases slowly with the increase of the contact surface angle of the holdback bar structure and increases rapidly when that angle reaches the critical value; besides, the release load increases slowly with the increase of the friction coefficient of the contact surface and increases faster when the critical friction coefficient is reached.

Effect of novel continuous friction model on nonlinear dynamics of the mechanisms with clearance joint

A general methodology for the dynamic modelling and analysis of planar multi-body systems with a continuous friction model in joint clearance is presented. Joint clearance is the critical factor that influences the dynamic response and the performance of mechanisms for high-speed application. In light of recent developments in the joint clearance studies, the number of contact force models has been introduced with ignoring friction continuity. The selection of an appropriate continuous friction model is still challenging and essential, which requires further development. Therefore, a perfect continuous friction model, including the Stribeck effect, static, dynamic and viscous friction terms, is proposed and validated. Investigating the dynamic modelling and analysis of double rocker four-bar linkage mechanisms with frictional revolute clearance joints is presented to investigate friction models' effect when surfaces collide with a non-zero tangential velocity. Unlike the smooth crank input mechanism, a double rocker four-bar linkage mechanism is analysed as a challenging problem in the impact mode. Resolving this concern, the novel friction model avoids discontinuity at zero velocity considering the accurate static friction zone. The results reveal that the novel friction model, compared with the piecewise friction model, is more effective in reflecting the mechanical systems' dynamic behaviour. In order to grasp the nonlinear characteristics of the high-speed four-bar linkage mechanism with our model in joint clearance, the Poincaré portrait, and Fast Fourier transformation plot are employed. It is proved that chaos exists in the dynamic response with the influence of the restitution coefficients and kinetic coefficient of friction.

Investigation on friction-induced judder of vehicle driveline based on a nonlinear friction model

Due to friction characteristics of clutch, the driveline is prone to cause a judder during vehicle starting, and then to cause the vehicle body to vibrate, which affects driving quality. In order to analyze the judder phenomenon, a nonlinear numerical friction model based on the Gaussian friction model is established in this paper. For the driveline of a front-wheel-drive vehicle, a five-degree-of-freedom (5DOF) lumped parameter model including a nonlinear friction element is established. The complex mode of the driveline during the clutch in slip condition is calculated. The key parameters affecting the driveline stability are analyzed. The self-excited judder and pressure-induced judder of the driveline are numerically simulated, and the corresponding causes are analyzed. The nonlinear friction torque of the clutch is also calculated. Furthermore, the effects of the key parameters such as the torsional stiffness and damping of the clutch and drive shaft suppressing the self-excited judder and pressure-induced judder are numerically studied respectively. Compared with the widely used Karnopp friction model, the nonlinear numerical friction model established in this paper comprehensively includes the stribeck effect in slip and the friction torque characteristics in stick. The phenomena of the judder and stick-slip of the driveline during vehicle starting are more accurately simulated. The simulation results are in good agreement with the experimental results, which verify the accuracy and effectiveness of the dynamic model including the nonlinear friction element established in this paper.