Influence of the intensity and duration of cyclic loading on the tribotechnical characteristics of steel

Abstract

We analyze the results of evaluation of the influence of cyclic loading on the wear rate of 45 steel under conditions of unilateral sliding in contact with the counterbody made of the same steel. It is shown that the presence of both low and high levels of cyclic loads relative to the fatigue limit leads to a decrease in the tribotechnical characteristics of the investigated steel. In author's opinion, this is explained by the deterioration of the conditions of contact sliding caused by the fatigue damage to the surface layer. The periods of initial hardening, crack initiation, and its growth typical of "pure" fatigue are also present in the dependences of the wear rate on the number of loading cycles in the case of superposition of fatigue and friction.Introduction. The reliability and durability of machines are caused by the preservation (in time) of the bulk and surface strength of materials subjected to the action of loads and environments. The processes of bulk fracture run as a result of accumulation of structural defects in the material followed by the initiation and propagation of cracks, which leads to the macroscopic fracture of an element or of the entire structure. The processes of surface fracture have a similar character but, in the presence of contact and friction, it is necessary to study a great number of additional microscopic events of fracture promoting a gradual decrease in the volume of the material (wear).The application of the concepts and methods of the physics of metals made it possible to develop the theories of plasticity, brittle and ductile fracture, fatigue and creep and obtain new results in the theory of internal friction and rheology. The insufficient knowledge of the physics of surface strength of materials does not enable us to give reliable predictions and interpret the processes of damage and fracture in the surface layers of materials. In the presence of friction, this is explained by the following objective reasons: principal difficulties encountered in the selection of the object of fracture and direct observations of this object, the fact that processes running in the material are thermodynamically nonequilibrium and, hence, the necessity of investigation of their kinetics, and the influence of the environment in all stages of the deformation and fracture processes.In numerous cases, friction units of machines and mechanisms (crankshafts, shafts, teeth of gears, etc.) operate under the combined action of cyclic bulk loading and surface loading caused by friction, which makes the analysis of the processes running in the surface layer much more complicated. The wear resistance of the material strongly depends on the strength of its surface layer. The characteristics of fatigue resistance of the materials also noticeably depend on various parameters of the surface layer. Thus, the interaction of cyclic and friction loading is of significant interest both from the viewpoint of evaluation of durability and fracture criteria of materials and from the viewpoint of investigation ...

Some Regularities of Life Time and Damage for the New Structural Material MoNiCa during Sliding Friction and Mechano-Sliding Fatigue

“…It is seen that the wear rate during tests under conditions of mechano-sliding fatigue in the entire range of contact load FN is lower than when tested under sliding friction conditions. Those under experimental conditions, cyclic bending stresses have a positive effect on the durability of the tribo-fatigue system, which is consistent with the known test results of other tribo-fatigue systems [14,15]. Fig.…”