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the Methodology of Tribo-Fatigue
Objectives and tasks Finally the fourth attribute of any scientific discipline is its objectives and tasks [28]. The task challenging the specialists in
the mechanics of fatigue fracture is to combat fatigue breakdowns. Prediction
and prevention of breakdowns is the apex of their achievements. Though fatigue
fracture of a structural element relates, as a rule, to hazards for personnel,
in practice its prevention is not always absolute. Economic considerations come into play and specialists of strength set
the problem of strength optimization formulated as
according to
which the probability of fatigue fracture F(s)
should be minimal providing the total cost С0 in the spheres of production and maintenance is
also minimized. This cost definitely depends on the parameters of distribution
of effective and limiting stresses to be achieved (
The major problem of tribologists is to combat wear. The consummation is the achievement of nearly wearless friction. They similarly formulate the problem of optimization as
since they have
to bear in mind the powerful economic factor;
The main problem of the specialists in tribo-fatigue is to control the processes of complex (wear-fatigue) damage in order to achieve optimum (and feasible) service life of a specific active system. Efforts are made to use wear and fatigue damage in the process of operation useful to extend durability of a unit. The essence is simple. Tribologists consider additional cyclic stresses in a friction pair as a damaging factor. Specialists in strength, in their turn, consider wear as the damaging factor of a structural element. Specialists in tribo-fatigue consider that friction, wear, fatigue are the phenomena which interact kinetically and may either intensify strongly the degradation of a material, or, on the contrary, to produce spontaneous and extended preservation of the load carrying capacity depending on conditions. To understand the conditions and mechanisms of evolution of these processes is to gain a simple key to control them. Thus, the specialists in tribo-fatigue should formulate and solve the problem of optimum control of a dynamic (active) system (III):
The content of the problem is the following. An active system (AS) is considered as an object under controlling. The problem of formulation of an optimum control program is set as the problem of optimization: F(s, p) Þ min, C0 Þ min, i.e. feasibility of calculations yield the parameter of optimization opt F. In operation a set {aj} of parameters aj of the state of the AS is measured. The results yield a current measure wå(t) of the complex wear-fatigue damage (WFD) which is a function of time t and particular measures of damage ws, wp, wch, wT induced by cyclic stresses (index s), contact pressure (index p), the processes of electrochemical corrosion (index ch), temperature (index T) in the zone of contact interactions between elements of the AS. The condition of the AS at any moment of its operation is estimated using the integral parameter Ft (s, p, wå(t)). Another objective is to correlate (compare) the optimum (opt F) and the current (Ft) values of the integral parameter F. The obtained disagreement DF of the parameters opt F and Ft serves to solve the problem of synthesizing dynamic or optimal control U = j (DF). The executive body (EB) makes management of the AS feasible. Comparison of problems I, II and III indicates that problem III is definitely more intricate, yet more practical: it implies effective management of reliability of the most essential active systems of a machine in operation based on the most significant criteria of performance and simultaneous reduction of the cost of labor, means and materials in production and maintenance.
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