A Simplified FE Model of the Four-Ball Tester for Evaluating Stress and Strain Distributions

This paper presents a simplified isothermal FE model of the four-ball tribotester in order to analyze stress and strain distributions, in mixt or boundary lubrication regime, for a range of loads and friction coefficients and the transition from a normal mild regime (acceptable load, very small permissible local plastic strain) to a severe regime (identifying maximum von Mises stress and plastic strain). The model consists of only two balls, in their actual position, the load being applied to the rotating ball and supported by the second, stationary ball. The equivalence of model loading is done by the displacement (approach) between the rotating ball and the stationary ball, identical to that obtained from tests. The constitutive model of ball material is based on experimental results reported in literature for the same steel grade. Research limitations are related to the mixt regime simulation (here characterized by a friction coefficient and not including a fluid film, the values experimentally obtained for the friction coefficient in mixt regime being in the selected range of the simulation). The effect of roughness is neglected by considering smooth bodies. The simulation was run for three values of friction coefficient (0.0, 0.08 and 0.1, respectively), pointing out its influence on stress and strain distributions. The model highlights the ellipsoidal shape of the indentation on the stationary ball, due to friction and particular sliding. It was found that strain and stress are induced from the first rotation of the tribosystem, if the system is statically loaded before starting sliding (as it happens in the actual test) and differences between strain and stress distributions are obvious for cases with friction (friction coefficient was considered constant) and the case without friction.