Torsional Tribological Behavior Of Monomer Casting Nylon Under Dynamic Normal Load

Under an engineering background of railway freight car bogie, the paper studied torsion tribological properties of the center plate material(monomer casting nylon composite) with a plane-on-plane torsional tester under different load amplitude and load frequency. By improving the pressure load device made it satisfy the requirement of experiments, realizing the accurate environmental simulation of torsional wear under dynamic normal load. The effects of load amplitude and load frequency on torsional wear behavior of MC nylon were comparatively researched. Contact mechanics behavior of Von Mises stress and distribution, friction stress and contact status of the MC nylon composites was calculated with ANSYS finite element software. The main conclusions were listed as follows:1 T-θ curve of MC nylon composites under static normal load exhibited the same shape of parallelogram,and contact area kept a gross slip; all T-θ curves transformed repeatedly between elliptic and quasi-parallelogram under dynamic normal load,corresponding that torsional regime changed from a partial slip to a gross slip, and the torsional regime trended to run in a gross slip with the increase of load amplitude.Friction torque gradually increased before reaching a steady state under static load,while friction torque increased then remained fluctuations within small range under dynamic load. The central zone of the worn surface was mainly characterized by adhesion wear, which became much more serious under higher load amplitude. In the peripheral annulus of the torsional contact area, the wear mechanisms were fatigue wear and accompanied by plastic deformation; contact interface was subjected to stronger alternating stress as load amplitude increased, which resulted in serious wear.Calculated by ANSYS, maximum Von Mises stress of the position did not change during running under static load; under dynamic normal load, maximum Von Mises stress from the edge zone gradually moved to the contact center area, and maximum Von Mises stress existed a big saltation in the edge zone, making the crack produce,which was consistent with experiment results of the crack produced in the edge area;contact status results was agreement with the test results.2 All T-θ curves converted between the elliptic and parallelogram as load frequency and torsion frequency kept in noncoincidence, which indicated the torsional regime transformed between a partial slip and a gross slip. Under higher load frequency, T-θ curves mainly exhibited parallelogram, and torsional regime remaineda gross slip. Friction torque gradually increased first and then reached a steady state with the increase of running cycles, and higher friction torque appeared under higher load frequency, which corresponded to higher maximum friction dissipation. Under high load frequency, the strain of the material surface quickened, material toughness and yield strength decrease, fatigue crack produce more easily in the surface and the material fall off easily, which made mass loss of MC nylon keep higher, leading to serious wear. By simulation,results showed that contact status kept dynamic change,which was consistent with torsional regime of test results. In a partial slip, with load frequency increasing, the adhesive area reduced and sliding area increased, maximum friction stress appeared the position of stick-slip border and this position was the most easy to crack initiation. In a gross slip, the friction stress distribution and its values was different, which led to the different wear; simultaneously the friction stress moved toward the contact centre zone with the increase of load frequency,making the central region worn seriously, producing more serious wear under high load frequency,which implied that the test results was consistent with the simulation results.3 When load frequency and torsion frequency was coincidence, with the increase of coincidence ratio, T-θ curves were likely to present parallelogram, and the torsional regime was a gross slip; steady-state torque kept a linear growth relationship to the coincidence ratio. Mass loss of MC nylon exhibited a positive correlation with coincidence ratio. Frequency coincidence could induce higher frequencies, interface strain sped up, the friction stress changed severely under higher frequencies, and contact interface tangential stress altered more intensely, which made substrate surface of MC nylon collapse more seriously and the interface produced the crack easily, causing to severe fatigue wear. Through ANSYS simulation, the contact was prone to be in a gross slip with higher coincidence ratio, which was consistent with the torsional regime; maximum Von Mises stress moved toward the contact centre zone, and maximum friction stress raised with the increase of coincidence ratio, which was 0.11MPa、0.15MPa、0.17 MPa,respectively, leading to seriously wear.4 Test results and simulation results have good consistency, which could explain the torsion regime of the change phenomenon in experimental results, so that we can understand further deeply torsion regime under dynamic normal load.