Research On The Running And Damage Mechanisms Of Dual-Rotary Fretting Under Dry Condition

The fretting wear can be divided into four basic modes for the ball-on-flat contact configuration:i.e., tangential, radial, torsional and rotational fretting. Currently, the vast majority of researches have been focusing on the tangential fretting mode, but the actual fretting phenomena are very complex, which are often induced by the combination of two or more basic running modes. However, the dual fretting has been reported rarely and the dual-rotary fretting combined by the torsional and rotational fretting modes has been reported much more rarely up to now. Thus, the research on the dual-rotary fretting is not only a significant to realize and understand the mechanisms of complex fretting damages, but also can provide theoretical guidance for mitigating fretting damage in many fields.Based on a low-speed reciprocating rotary motor system, an adjustable inclining device mounted the motor, and a high-precision6-D forces/torques sensor, a new dual-rotary fretting test rig has been invented and developed in this study, which can really simulate the dual-rotary fretting under a ball-on-flat contact. Consequently, the test results showed better comparability and reproducibility.The dual-rotary fretting (DRF) tests have been carried out by utilizing several typical materials (LZ50steel,7075aluminum alloy and PMMA) and two lubrication media (VG46oil and pure water) under various tilt angles, angular displacement amplitudes, normal load levels and fretting cycles. Base on the dynamic behavior analyses in combination with the optical microscope (OM), scanning Electrical microscope (SEM), energy dispersive spectroscope (EDX), x-ray photoelectron spectroscope (XPS), triboindenter in-situ nanomechanical test system and surface profilo-meter of2D/3D dual modes, and also combined with the examinations of friction vibration and noise signs and simulations of finite element, the running and damage mechanisms of the dual-rotary fretting have been performed in detail.The main accomplished research works and obtained results are as follows:(Ⅰ) The running and damage mechanisms of dual-rotary fretting for various materialsFor LZ50medium carbon steel,7075aluminum alloy and PMMA, the running and damage behaviors of DRF have been investigated systematically. Based on great deal of the experimental results, three basic types of kinetics curves (the tangential force versus angular displacement amplitude curve. Ft-θ curve) of DRF have been outlined:i.e., the linear, elliptic and parallelogram loops. The running condition fretting maps (RCFM) of DRF have been set up. And the tests results indicated that the fretting running regimes and damage behaviors of DRF were strongly dependent upon the tilt angle, angular displacement amplitude, number of cycles, normal load and material properties. The results showed that the curves of Ft/Fn and friction dissipated energy reflected the fretting damage characteristics and the domination of fretting components to some extent. The DRF damages all appeared asymmetric damage morphologies, and presented easily to destroy than that of the simple modes of torsional or rotational fretting. From the point of view of surface damages, the delamination of DRF was much obvious to occur under medium title angles, and the maximal depth and the aspect ratio of the wear scar of DRF under same angular displacement amplitude were higher than that of the simple modes of torsional or rotational fretting. In addition, a physical model of surface damage process of DRF has been built up in this study.(Ⅱ) The competition mechanisms between local fatigue and wear of the dual-rotary fretting(1) The cross-section analyses of LZ50steel indicated that the distribution of white layer (TTS, tribologically transformed structures) was obviously influenced by the fretting components, the transverse cracks of subsurface were easy to form at the interfaces between the TTS and plastic deformation zone. The evolution of the TTS has a close relation to the formation of fatigue cracks and the fatigue detachment mechanism during the wear process.(2) The cross-section analyses of7075aluminum alloy indicated that the damages in varied fretting regimes entirely appeared different features. In the partial slip regime (PSR), the fatigue cracks very easily initiated and mainly propagated parallel to the surface under the action of DRF. And the local fatigue was main damage formal in this regime. In the mixed fretting regime (MFR). the propagation of fatigue cracks was very complicated with different characteristics. When the fretting damage was mainly controlled by the torsional component, the fatigue cracks mainly distributed below the annular wear zone and presented the severe bifurcation phenomenon. However, when the fretting damage was mainly controlled by rotational component, the fatigue cracks mainly distributed at the two sides of the sticking zone of contact center, and the propagation direction was relatively single with a symmetrical feature. In the MFR, the rate of fatigue crack propagation was higher than that of the material wear, and the fretting damage presented much obvious local fatigue than the local wear. In the slip regime (SR), the wear scars exhibited "U" shape deep pits, and the rate of wear was higher than the rate of crack propagation, when no crack distinctly propagated to the substrate was form.(3) It found that there were two kinds of local convexity to form under the condition of DFR. And the formation mechanisms of those two kinds of convexity have been revealed. It founded that the formation of the fatigue cracks has a close relation to the local convex profile in contact area. Additionally, the physical models for the formation of local convex profile controlled by the two fretting components and fatigue crack were built up, respectively.(Ⅲ) The friction vibration and noise analyses of rotary friction and wearIn order to establish the internal relations between the damage mechanisms and the frictional vibration/noise signals, and further reveal the damage mechanisms of DRF, the frictional vibration/noise signal examinations and analyses of the rotary friction and wear for the PMMA have been carried out. The research results showed as follows:(1) Under the partial slip regime, the fretting has emitted no evident detectable signal of frictional vibration/noise due to the relative motion of the contact interfaces mainly coordinated by the elastic deformation. However, under the gross slip status, it founded that the damage behaviors of materials was closely related to the frictional vibration/noise signals. For instance, the extent of surface damage have a close relation to the event number of the frictional vibration/noise signals; local convexity profile enhanced the tangential friction vibration; the formation of fatigue cracks was effective to inhibit the tangential vibration to some extent:abrasive wear has a remarkable enhancement effect both on the noise and the tangential vibration signals. In addition, through the wavelet analysis, it indicated that the signals of vibration and noise presented different evolution with the change of the damage mechanisms.(2) It found that the signals of frictional vibration and noise also demonstrated entirely different features under the status of fretting and reciprocating sliding. It mainly presented as: the event number changed with the number of cycles:the changes of the amplitude and distribution of the frictional vibration/noise signals. It showed that the frictional vibration and noise signals can act as the important reference basis for the judgment of fretting and reciprocating sliding field. Besides, it founded that the independent linear relationships between the accumulated number of events and the wear volume under fretting and sliding wear were presented, respectively. The wear volume for the dissipated number of events wear volume per unit under the sliding condition was obviously higher than that of under the fretting condition.