Evaluation of the limiting state of the material under cyclic and contact loadings with boundary friction

Abstract

Optimum experimental design was made up and data on the limiting state of the material under combined cyclic and contact stresses in the oil medium were obtained. The approach to determining loading parameters that provide a maximum life of the tribosystem was proposed. The condition of simultaneous failure by fatigue and wear mechanisms was used.Introduction. Many machine components operate under the combined action of friction forces, contact and cyclic stresses in the oil medium. Each of these factors makes a certain contribution to the limiting state of the material and further failure of the unit. It was shown earlier [1] that crack nucleation in a cyclically loaded tribosystem element can be initiated by non-optimal contact and friction conditions or high cyclic loads, which prevent the onset of fracture through contact stresses and friction forces. At the same time, there is a certain optimum level of contact stresses and a corresponding level of friction forces whereby a cyclically loaded specimen exhibits the longest life. It may be suggested that under optimum contact and friction conditions and at a preset level of cyclic stress amplitudes, the limiting state of the material in the surface layer of the specimen, where crack nucleation occurs, comes simultaneously as a result of both factors, viz friction forces and contact stresses, on the one hand, and cyclic loads, on the other hand.Thus, the material exhibits equal response to active damage mechanisms. It was shown [2] that similar optimum conditions can also be determined experimentally for a cyclically loaded tribocouple when friction surfaces are modified with boronizing and carbovanadizing. As the level of cyclic loads, contact stresses and friction forces, and the composition of a lubricating medium change, the ratios of mechanical effects corresponding to a maximum life of a cyclically loaded tribocouple element (farther these conditions would be termed optimum) also change. In this connection, of interest is the development of approaches to evaluating the limiting state of the material and its life under the examined conditions based on a limited number of experimental data. The present communication poses such a problem since its solution is necessary for designing machine units and components operating under fatigue with friction.Procedure. Experimental investigations were performed according to a procedure [3] based on cyclic flexure tests with the revolution of a circular-section specimen having a neck concentrator. The latter is the pressing point of the counterbody for creating contact stresses and friction forces on the side of tensile stresses induced by flexure. The counterbody is a circular-section rod, thus, the cylinder-to-cylinder contact is formed. A lubricant is supplied to the contact zone.Material and Lubricant. A cyclically loaded specimen and a counterbody were prepared from 45 normalized medium-carbon steel with the following mechanical characteristics: ultimate strength σ u = 610 MPa, yield stress σ 0 2 410 . = MPa, Young...

Friction modelling of tribofilm performance in a bench tribometer for automotive engine lubricants

“…Graphs are presented in the system of Cartesian spatial coordinates and are constructed by calculated points [21]. Equation (10) was used to obtain the friction coefficient (represented on the z-axis) for calculated points represented on x-and y-axes, with the remaining variables taken as constants.…”

“…The computer programs Minitab and SPSS TM provided the friction response model in boundary and mixed lubrication conditions (equation ( 10)), and Minitab, Origin Pro TM , and MS Excel TM solvers were used to obtain the three-dimensional response surface plots. Graphs are presented in the system of Cartesian spatial coordinates and are constructed by calculated points [21]. Equation (10) was used to obtain the friction coefficient (represented on the z-axis) for calculated points represented on x-and y-axes, with the remaining variables taken as constants.…”