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the Methodology of Tribo-Fatigue
Processes and phenomena Now let’s consider the third most essential attribute of any scientific discipline, viz. the processes and phenomena it studies. Surface damage is the basic process of degradation of the specimen and the counterspecimen under the effect of contact loading in the friction pair; it is studied by tribology (see Table 1). Volume (fatigue) fracture is the basic process of degradation of the structural element under the effect of alternating loading; it is studied by the mechanics of fatigue fracture. In case of friction processes (under the effect of contact loading) and the processes of cyclic deformation (under the effect of both contact and non-contact alternating loading) combined in the element of an active system, complex surface damage (due to any mechanism) is just an initial process of its degradation which evolves in time and inevitably ends in volume fracture; this type of complex surface damage and fracture is studied by tribo-fatigue (see Table 1) [21–25]. It is easy to discriminate between the surface damage and the volume fracture. It is harder to solve the following problem: what makes the surface damage of the specimen in a friction pair different from the complex surface damage of the element of an active system termed as wear-fatigue damage in tribo-fatigue. This difference should be established with one essential and obligatory provision: the contact loading should be the same whether it applies to an active system or to a similar friction pair. Application of accurate experimental methods of studies allows to investigate and understand the features of complex — wear-fatigue damage [26, 27]. Fig. 12 exemplifies it with the results of studies (method of the atom force microscopy) of the processes of cracking of steel 45 specimens in rolling friction (the left column of figures) and under wear-fatigue tests (the remaining figures) as a function of the level of contact pressure p0 and the amplitude of cyclic stresses sа. Figures (~35´35 mm2 in size) show the morphology of cracks typical for the relevant conditions of tests. The histogram shows the dependence of the critical depth h of the damaged layer on the level of cyclic stresses (under constant contact pressure р0 = 2130 MPa). These experimental data allow to conclude the following.
Figure 12 – Microtopography of surface damage in rolling friction (vertical column of images) and during wear-fatigue tests (remaining images) In case of pure rolling friction any higher contact pressure intensifies plastic deformation and hence deformed fragmentation of grains, it initiates the appearance of discrete pores and cracks followed by their coalescence into chains. The system of deformed grains, chains of pores and cracks is unidirectional and it is oriented in the direction of rolling. This process produces relatively large discrete pitting. Two types of wear, viz. delaminating and pitting, come to dominate. The critical depth of the damaged layer is estimated to be ~ 0.4 — 0.5 mm. Similar deformed fragmentation of grains and origination of pores and cracks are observed during wear-fatigue tests. But the process of damage changes significantly as the amplitude of cyclic stresses increases, the processes of origination of the second system of cracks accelerate and these cracks arrange transverse to the rolling direction. Hence, damage becomes dissipated and an almost regular grid of intersecting cracks and pores appears around finely dispersed particles (fragments of grains) of the material. The stronger the cyclic stresses are the denser is the grid of cracks and pores, the finer and thinner the separating particles become, the critical depth of the damaged layer reduces to 0.05 mm. It prevents the appearance of larger pittings, they are not observed under these conditions. Surface chipping is the dominating wear process in this case. It is characterized by the separation of fine dispersed particles from the working surface, these particles result from multiple microshearing of intersecting planes and fine shattering of grains. This mechanism of complex surface damage is termed the multiple microshearing dissipated effect (MMSD phenomenon) [5] in tribo-fatigue. The above results allow to establish additionally the following causes why wear-fatigue damage under definite conditions becomes less dangerous than damage by friction (under similar contact pressures), as it follows from the experimental data in Fig. 8. 1 Superposition of the fields of contact and bending stresses causes dissipation of a larger share of applied energy in a finer surface layer of the material and localization of the processes of cracking and wear in it. The energy of deformation is expended more on finer shattering of grain fragments and their separation in multitudes than on penetration of damage into the depth of the specimen material. 2 Wear of the surface layer damaged by a grid of cracks and pores exposes a new, relatively intact surface with a higher resistance to fracture. Hence, the appearance of relatively large pittings, which have dangerous microconcentrations of stresses at their bottom and a dangerous main crack is postponed in time or even entirely prevented (depending on the conditions of loading). 3 Approximately tenfold rejuvenation of the working surface by fragmentation, chipping and separation of metallic particles during wear-fatigue tests is required before damage reaches the depth comparable with that reached in rolling friction, providing contact pressure is the same in both these cases. Hence, wear-fatigue damage is a specific, peculiar type of surface damage of the basic element of the active system. Its feature under these conditions is surface chipping due to the MMSD phenomenon occurring on the intersecting planes of sliding. Its characteristic property is that, notwithstanding the fact that it is a damaging process, it is useful since it improves significantly the reliability and durability of the active system. The optimal combination of loading parameters sа and ра apparently yields such a state of the active system when its load carrying capacity is maintained spontaneously and during a long period of time (or it is automatically controled) by fine wear of the surface layer and its removal from the friction zone. To summarize it is worthwhile to note that the active system is a peculiar dynamic system and its behavior can and should be controlled (U) by adjusting the wear rate in a non-traditional manner (see the example in equation (10) and in Fig. (10)). Optimal control of the process of wear-fatigue damage is a fundamentally new and practically essential trend of studies evolving within the framework of tribo-fatigue and involving the methods of the theory of control. It is shown in Fig. 5 by the arrows indicating the relationship between these two scientific disciplines. In relation to the studies of the direct and back effects two circumstances are to be highlighted. First, the experimental results obtained
in tribo-fatigue and presented in Fig. 11 are unexpected from the traditional
point of view: a significant reduction of the fatigue limit (32% at contact
pressure 8.5 MPa) is observed when no physical wear of specimens occurs at all.
In other words, wear produces no damaging effect (the diameter of the specimen
does not reduce, no marks as concentrators of stresses appear on the friction
surface, no seizure occurs over the real contact spots, etc.), meanwhile fatigue
resistance drops drastically. It is explained by the effect of little studied
mechano-physical phenomena evolving in the steel-to-polymer friction zone. In
particular, products of tribo-destruction of polymer are known to act as
surfactants. They accumulate in the contact zone and stimulate the migration and
proliferation of dislocations on the metallic friction surface (the Rebinder
effect). This leads to the acceleration of its fatigue damage. Also, as the
contact pressure goes up and the time of tests extends, the mean temperature in
the friction zone increases (up to 70 °C
in the experimental conditions). It induces thermal activation of damaging chemi-physical
processes, hence the fatigue resistance of specimens reduces even more. A
theoretical analysis (from the standpoint of tribo-fatigue) corroborates these
regularities: the wear characteristic in formula (9) is absent, still a
significant variation of the magnitude s–1р
is predicted due to the effect of the parameters of
the mechano-physical state of the polymer
Second, the back effect is not known in tribology or in the mechanics of fatigue fracture; tribo-fatigue was the first to establish it [20]. According to the experimental data presented in Fig. 10, the durability of a MPS assessed basing on the wear criterion is in many respects determined by the back effect. Let us analyze Fig. 13 in order to answer the question why the wear of the polymeric counterspecimen is strongly intensified when cyclic stresses are induced in the contacting metallic counterspecimen. The specimen is shown as a rotating disk 1 with a smooth working surface, the counterspecimen is shown as a stationary single indentor 2. In common friction tests (Fig. 13, a) only contact load qr is effective, indentor 2 is statically bent (in the direction opposite to the direction of rotation w1) and the area of deformation on the working surface of a disk is a strip (the friction path). In case of wear-fatigue tests (Fig. 13, b) additional cyclic stresses (deformations) s = ± sZ are excited in the disk. Oscillations of the working surface of the disk in the direction z make the friction path on the surface zigzag shaped, the indentor experiences cyclic bending (in the direction z). The wear processes of the two elements naturally intensify in accordance with the magnitude of the cyclic stresses s = ±sZ. If the indentor is polymeric and the disk is steel, only the wear of the polymer as a weaker material is intensified.
Figure 13 – To the analysis of the back effect Hence, in the general case of wear-fatigue tests, the back effect has two manifestations: wear accelerates in both elements under the effect of cyclic stresses s excited in one of the elements of the active system. Analytically this effect for a MPS is described by equation (10).
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