Research On Torsional Fretting Wear

According to the directions of relative motions, the fretting can be defined as four basic modes under a contact configuration of ball-on-flat:i.e. tangential, radial, rotational and torsional fretting. Presently, the absolute majority of researches have been focused on tangential fretting mode. The torsional fretting can be defined as a relative angular motion which was induced by reciprocating torsion in an oscillatory vibratory environment. It occurred in many rotary components, such as ball and socket joints, ball valves, center plates of vehicles, and so on. Although the torsion fretting phenomena are very common, much little related research focuses on it up to now. Study on torsional fretting not only has science significance of exploring the unknown to deepen and enrich the fundamental theory of fretting, but also have important guidance to palliate fretting damages in industry.Based on a rotary device with low speed and high precision and a six dimensional sensor (forces and torques of x, y and z directions), a new torsional fretting tester has been developed in this study to factually simulate the case of torsional fretting under a contact of ball-on-flat. This tester presents high coaxiality and a wide range of angular displacement amplitude, and its test results showed a better comparable characteristics and repeatability. To layout a controlled atmosphere system, the experimental simulation of various atmospheres and relative humidity environments can be realized.Torsional fretting tests for several typical metallic materials, polymers and natural active cartilage of bovine have been carried out under different load levels, angular displacement amplitudes and number of cycles. Dynamic analyses in combination with the microscopic examinations through profilometer, optical microscope, scanning electrical microscope (SEM), energy dispersive spectroscope (EDX), x-ray photoelectron spectroscope (XPS) and Hematoxylin-Eosin staining method, have been performed to study the running and damage mechanisms of the torsional fretting in detail. Using the finite element method (FEM) to establish the ball/contact model of torsional fretting. The comparison analysis of numerical simulation results and the experimental results were done. The main research works and obtained conclusions in this dissertation are as follows:(1) The running and damage mechanisms of torsional fretting for the metallic materialsFor Fe-C alloys (industrial pure iron,1020 steel and LZ50 steel) and 7075 aluminum alloy, the running and damage behaviors of torsional fretting have been studied systematically. Three kinetics characteristics curves (the friction torque vs. angular displacement amplitude curves, T-0 curves) for the torsional fretting:i.e. the linear, elliptic and parallelogram loops, were summarized. The research revealed that the kinetics characteristics of torsional fretting can be characterized by the T-0 curves, torque curves as function of time, friction dissipated energy (Ed), average deformation rigidity (k) and ratio of sticking zone (i). The results showed that the torsional fretting run in the partial slip regime (PSR), mixed fretting regime (MFR) and slip regime (SR) one by one with the increase of the angular displacement amplitudes. The running condition fretting maps (RCFMs) and material response fretting maps (MRFMs) of torsional fretting for different materials were established, respectively. It was found that the MFR of torsional fretting presented a particularity different from other fretting modes, i.e. the sticking zone of the MFR reduced until it disappeared with the increase of the number of cycles. The research results also showed that the torsional fretting was strongly dependent upon the normal loads, angular displacement amplitudes, number of cycles and material properties. A physical model of torsional fretting wear has been set up. The damage features for the metallic materials in the different fretting regimes are outlined as follows:(a) In the PSR:The wear scar appeared in shape of annularity, and its centre was stuck with free damage. The micro-slip occurred in the ring at the contact boundary zone, corresponding with slight wear. The wear mechanisms mainly were abrasive wear and slight oxidative wear. In this regime, fretting white layer has been formed.(b) In the MFR:The sticking zone gradually reduced until it disappeared with the increase of the number of cycles, while the contact state transformed from the partial slip to the gross slip. The profile of the wear scar of the MFR presented type of "W". The wear took place in the micro-slip zone, which was the result of combined action by the abrasive wear, oxidative wear and delamination. For the Fe-C alloys, the tendency of detachment of material by the mechanism delamination increased with the increase of the carbon content. The lateral cracks initiated at the interface between the white layer of fretting and the plastic deformation zone, and the detachment of particles was the result induced by the encounter of the lateral cracks and vertical cracks. At the cross-section, some titled cracks propagated to the base alloy were observed. It indicated that the growth rate of the local fatigue crack was more than the wear rate in the MFR.(c) In the SR:The wear occurred at the whole contact zone, and the profile of wear scar presented typical "U" type, corresponding with severe damage. Same to the MFR, the wear mechanisms mainly were abrasive wear, oxidative wear and delamination in the SR. No titled crack can be observed at the cross-section, which indicated that the local wear rate was more than the growth rate of the fatigue crack in this regime.(2) The oxidative behaviors of contact interfaces of torsional frettingIn the controlled atmosphere environment, he behaviors of the running and damage mechanism of torsional fretting for metallic materials have been studied under varied atmosphere (industrial pure nitrogen, pure oxygen and open air) and relative humidity (10%RH,60%RH and 90%RH). The tribo-oxidation mechanism was analyzed particularly. The research results showed that:(a) The MFR and SR shifted to the direction of low angular displacement amplitude with the increase of the oxygen content of atmosphere. In nitrogen, the wear mechanisms of torsional fretting were abrasive wear and delamination. For the atmosphere contained oxygen (industrial pure oxygen and open air), the oxidative wear was also included.(b) The friction torques and wear were reduced obviously by higher relative humidity. With the increase of the relative humidity, the range of MFR reduced, and the MFR and SR shifted to the direction of small angular displacement amplitude.(c) The XPS results indicated that the oxidative reaction during the friction process was promoted by water in moist atmosphere. The product of oxidation played a role as alike solid lubricant to alleviate wear. The slip between the contact interfaces was enhanced by the tribo-oxidation. Oxidative debris formed from the tribo-oxidation was difficult to remove out of the contact area, and was good to reduce wear.(3) The influence of varied counter-pairs of polymersThe running and damage behaviors of torsional fretting under varied counter-pairs for the PMMA and UHMWPE have been investigated, respectively. It found that the characteristics of torsional fretting were different with the variation of counter-pairs. The research results showed that:(a) The running rules of torsional fretting for the PMMA and UHMWPE were consistent with that of metallic materials. The effect of normal loads and angular displacement amplitude on the behaviors of the running and damage of torsional fretting was discussed in detail.(b) To compare the counter-pair of PMMA flat against GCr15 steel ball, the MFR and SR were shifted to the direction of high angular displacement amplitude by the counter-pair of PMMA flat against PMMA ball. Under the same test conditions, the depths and widths of the wear scars of the latter (PMMA/PMMA) were much higher than those of the former (PMMA/GCrl5), i.e. much severer wear occurred. For the PMMA/GCr15 counter-pair, the wear was controlled by the fatigue wear mechanism which induced the crazes. However, for the PMMA/PMMA counter-pair, the wear was controlled by the abrasive wear mechanism which generated the ploughs.(c) To compare the counter-pair of UHMWPE flat against TC4 titanium alloy ball, the MFR and SR were shifted to the direction of low angular displacement amplitude by the counter-pair of UHMWPE flat against Al2O3 ceramic ball. Under the same test conditions, the depths and widths of the wear scars of the latter (UHMWPE/Al2O3) were much higher than those of the former (UHMWPE/TC4). However, the surface damage of UHMWPE/TC4 counter-pair appeared a much severer damage, which was controlled by the wear mechanisms of fatigue and abrasive wear. And for the UHMWPE/Al2O3 counter-pair, the wear mechanism of torsional fretting was mainly abrasive wear.(4) The running and damage behaviors of torsional fretting for the natural active cartilage of bovineThe torsional fretting behaviors of the natural active cartilage of bovine against Al2O3 ceramic ball in the Ringer's solution have been studied. It was found that the cartilage presented different kinetics characteristics from the metallic materials and polymers. For example, the relative slip between the contact interfaces was enhanced with the increase of the normal loads; the evolution of friction torques appeared a different process from other materials. The special kinetics characteristics of the cartilage were probably related to the lubrication action induced by the extrudate come from the cartilage. SEM observation combined with the analysis of HE staining method indicated that the wear mechanism of torsional fretting of the cartilage was mainly fatigue wear. A damage physical model of the cartilage was set up.(5) The finite element analyses of the contact of torsional frettingA finite element model for torsional fretting under a ball-on-flat contact configuration was built up by using ANSYS software. In the range of elastic deformation, the effect of the number of cycles, angular displacement amplitudes and friction coefficients of contact interfaces on the contact stress distribution and the motion state of sticking/slip were analyzed in detail. In a word, the results of numerical simulation were well consistent with the experimental results.