Numerical Study On Damage Mechanisms And Protection Of Torsional Fretting Wear

As a fundamental mode of relative motion and deformation, torsional fretting takes place in many practical engineering applications, such as ball socket matching parts and rotary fastener widely used in vehicles, wheel axle matching parts, the human joint and artificial joint, etc. Wear caused by torsional fretting can lead to problems about mechanical components service safety and human health. With recent beginning of torsional fretting research, few correlative reports exist. Analytic solution about torsional fretting is limited. At present, the systemically experimental study of torsional fretting is made by the Tribology Research Institute of Southwest Jiaotong University. The operation and damage mechanism of torsional fretting wear has been established basing on two kinds of fretting maps. But the mechanical behavior and mechanism of torsional fretting are required to be analyzed. At the meantime, the research of the measure to slow down the fretting damage mainly focuses on tangential fretting while there is little report on resisting torsional fretting damage by the technology of surface engineering. Thus, analyse on the mechanical behavior of torsional fretting wear and protection not only has a great scientific significance to deepen the torsional fretting damage mechanism and to enrich the fretting tribology theory, but also offers important engineering guidance for anti-failure of torsional fretting wear in practical application.Based on finite element method of contact problem, numerical simulation of torsional fretting has been developed. The effect of pivotal fretting parameters on surface and sub-surface stress and strain distribution is investigated. Damage characteristic of torsional fretting has been revealed by mechanical analyse, and comparation between mechanical behavior and experimental phenomenon has been made. The isotropic variable coefficient of friction (COF), which is expressed in space and time, is introduced and implemented in the finite element code ABAQUS by the user subroutine FRIC. Then numerical simulation of torsional fretting was carried out. With the good consistency between simulation results of friction torque curves and experimental results, mechanical behavior of torsional fretting was further analyzed. Torsional fretting finite element analyse of MoS2coating has been performed to discuss the feasibility of the solid lubricant coating in resisting torsional fretting wear application,comparing with the mechanical behavior of the substrate. The main obtained conclusions are listed as follows:(1) There was good consistency between finite element results and experimental results.That is, contact surface deformation behavior and friction behavior simulated by finite element analyse agree with the practice. In the finite element analyse, the running region was partitioned according to T-θ curves shape and contact state transformation. Elliptic loops and sticking zone minishing with cycles indicated fretting run in the mixed slip regime.(2) For friction torque-angular displacement amplitude curves, there was good consistency between the simulated results and the experimental results under three different fretting running regimes.It indicated that the COF model was able to simulate the COF evolution and deformation behavior during the torsional fretting. The numerical simulation considering dynamic change of COF was much more reliable.(3) Surface wear was mainly dominated by surface plastic deformation and friction shear stress. Accordingly, the different distribution and evolution of plastic strain and friction shear stress under three different torsional fretting running regimes leaded to different wear damage degree and damage evolution.(4) Combining the SWT parameter with the critical plane approach, prediction analysis of contact surface crack initiation and propagation position in the mixed slip regime was carried out. Results indicated that sub-surface stress and strain state decided crack initiation and propagation, and cracks were expected to initiate and propagate at the stick-slip interface or near contact edge within the slip region. Prediction results agreed well with the experimental phenomenon.(5) Compared with LZ50steel, no mixed slip in the coating existed in the running regime for MoS2. Its gross slip running region removed to small angular displacement amplitude and high normal load. As a result, damage in the mixed slip regime was eliminated by the transformation of the fretting running regime. Because of surface low friction, MoS2coating was much easier to entering slip regime than the substrate, and surface relative motion amplitude was larger. But, friction torque value was in lower level, and plastic strain and surface friction shear stress were smaller. Consequently, surface damage caused by friction and wear was slighter than the the substrate material.