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Chapter 4 DIRECT AND BACK EFFECTS 4.4 Effect of interaction conditions.
Any practical analysis of the processes of mechanical fatigue, the processes of friction and wear are based on the main idea that the characteristics of the above processes are affected (usually damaging) by numerous factors, all their variety being classified into four groups, viz. factors of design, metallurgy, production process and operation (see p. 1.6). The effect of a factor, for example, on changes in the fatigue limit is, as a rule, unidirectional, i.e. it is the same qualitatively irrespective of the variations of the parameter characterizing this factor. Yet, the experience of operation and tests of active systems as specific objects has revealed that mechanical fatigue, on the one hand, and friction (with wear), on the other hand, cannot be considered as the factors affecting, being complex phenomena, strength and durability, in a definite and independent manner. These phenomena evolve concurrently in a single zone of the active system within a complex dangerous volume and dialectically interact. The result of such interactions may in principle be of double kind: (1) accelerated development of damage because softening becomes dominating and leads to sharp loss of durability by the active system; (2) on the opposite, delayed development of damage due to dominating hardening strongly increases its durability. While result (1) seems trivial (clear and explainable, see, for example, p. 1.6), result (2) needs proof and explanation: load growth, from the standpoint of the mechanics of deformation and fracture, always leads to the corresponding loss of the bearing capacity and durability of the material. So,
we proceed from the fact that it is sufficient and effective to analyze the
governing factors (the factor analysis) to estimate the bearing capacity and
durability of a component of a design or a
friction couple, while interactions between phenomena should be analyzed
dialectically to estimate the bearing capacity and durability of active systems
(the phenomena analysis).
1. What is the direct effect? Is it possible to study the regularities of the direct effect from the standpoint of tribology? Corroborate your view. 2.
What is the back
effect? Is it possible to study the regularities of
the back effect from the standpoint of mechanical fatigue? Corroborate your view. 3. Formulate the main experimentally established regularities of the direct effect during mechano-sliding fatigue of the metal-to-polymer active system. 4. Formulate the main theoretically predictable regularities of the direct effect during mechano-sliding fatigue of the metal-to-polymer active system. 5. Describe the role of thermofluctuation stresses in the development of wear-fatigue damage of the polymer during mechano-sliding fatigue. How is a singe thermofluctuation stress calculated? 6. What is the principal difference between the direct effect during mechano-sliding fatigue of the metal-to-polymer and that of metal-to-metal active systems? 7. Formulate basic experimentally established and theoretically predictable regularities of the direct effect during mechano-sliding fatigue of the metal-to-metal active systems. 8. What is the basic experimentally established regularity of the back effect in the metal-to-polymer active system during mechano-sliding fatigue. 9. Formulate basic theoretically predictable regularities of the back effect in the metal-to-polymer active system during mechano-sliding fatigue 10. Explain (from the standpoint of the mechanics of deformation) why the wear of the polymeric counterbody strongly intensifies when cyclic stresses are excited in the conjugated metallic body under pressure? 11. Do you know two manifestations of the back effect during mechano-sliding fatigue of the metal-to-metal active systems? Describe them. 12. Do you discriminate the notions wear-fatigue damage and fatigue wear? What do they have in common? What is their principal difference? What is its essence? 13. How do the basic regularities of the direct and back effect change in response to the level of contact and off-contact loads? 14. What do you think about the role of physical wear in the active system, whether it is positive or negative? Or in any other way? Corroborate your view. 15. What will the result be if two damaging phenomena, like friction (including wear with friction) and mechanical fatigue develop in one and the same zone of the active system’s components? Does the result depend on the conditions of loading of the active system? If yes, how? 16. Describe the basic regularities of the direct effect in the metal-to-metal active system during mechano-rolling fatigue if the specimen is made from soft steel and the roller from high-strength steel. According to what criterion is the limiting state reached? What damages are attendant? 17. Describe the basic regularities of the back effect in the metal-to-metal active system during mechano-rolling fatigue if the specimen is made from soft steel and the roller from high-strength steel. According to what criterion is the limiting state reached? What damages are attendant? 18. When are undulatory residual surface damages in the metal-to-metal active system possible? Can you indicate the causes of their appearance? 19. What is the direct effect factor? What numerical values may it have during (à) mechano-rolling fatigue? (b) mechano-sliding fatigue? 20. What is the back effect factor? What numerical values may it have during (à) mechano-rolling fatigue? (b) mechano-sliding fatigue? 21. Can you explain why the active system may have stronger durability than a similar friction pair (when contact loads are the same)? How can an “additional” cyclic load in the active system lead to stronger and not to weaker durability? 22. The popular idea is that when the energy of deformation excited in the deformable solid body augments, its durability (bearing capacity) diminishes correspondingly. Is this idea always true in respect of active systems? If not, why? What is the role of interactions between damages due to contact and off-contact loads? 23. Describe the wear of the metal-to-metal active system by crushing during mechano-rolling fatigue. When does it occur? 24. What is the scattered effect of multiple microshearing? What are its symptoms? What is its role in the appearance of wear-fatigue damage? 25. How is the governing parameter of the wear-fatigue damage calculated? What are its numerical values? Can they help identify the effect, direct or back, that occurs in these conditions? 26. How is it possible to determine the critical value of the wear-fatigue damage governing parameter and what does it imply? 27. After you have got an idea about the regularities of the direct and back effects is it clear now what tribo-fatigue is? In what way is it different from tribology, fracture fatigue mechanics, other disciplines studying the problems of strength, surface and volume fracture (strength of materials, machinery, structural mechanics)? 28. What is the principal difference of two-parametric diagrams of the limiting state of objects during mechanical fatigue and during friction from the diagram of limiting states of the active system during mechano-rolling fatigue? 29. What phenomenon is called the translimiting state during mechano-rolling fatigue of the soft steel / high-strong steel (roller) active system? Describe this state. 30. What are possible causes (and mechanisms) of appearance of residual undulatory damage along the rolling path? 31.
Do you
discriminate between the processes of addition and interaction between damages
due to contact and off-contact loads? What results of these two processes are
possible? Can the addition be considered as a particular case of interaction
between damages? 32. What coefficients of anisotropy of wear-fatigue damage do you know? How is it possible to calculate them using the results of relevant measurements? 33. Describe the relation between the coefficient of asymmetry and the coefficient of irregularity of wear-fatigue damage and the level of cyclic stresses during mechano-rolling fatigue. 34. What do you know about the differences between the processes of wear of active system components if friction occurs either in the zone of compression or in the zone of tension? 35. Compare the wear processes in the friction pair and in the similar active system. What regularities can you outline? How does the pattern of friction in the zone of tension and in the zone of compression affect wear? What stresses – compressive or tensile – are more hazardous?
1. Carry out the following experimental study (if you have learned to determine the characteristics of surface roughness in some other discipline). à) Test a metallic specimen for fatigue at stresses s > s–1 during 10–15 minutes. b) Test a metal-to-polymer friction pair at pressure pa > pf during the same time.
ñ) Test a metal-to-polymer active
system with the same loading parameters assumed in Every time obtain a profilogram of the working surface before and after tests. Use it to determine basic characteristics of the surface purity. Then make an exhaustive comparison: (1) how the condition of the surface changes after each of three tests? 2) if the surface characteristics are different depending on the type of tests? if the difference is only quantitative or qualitative? You must realize that to establish qualitative difference is more essential than quantitative. Note that the roughness of surfaces of both tested bodies should be measured and analyzed. 2. Your results may be incorporated into your presentation at the students’ conference. 3. Using PC make a graphic analysis of functions (4.1) and (4.4). Compare the obtained graphs. Does you analysis contain at least one conclusion that is not described in the manual? 4. Using PC make a graphic analysis of functions (4.5) and (4.7). Does your analysis contain any conclusions not described in the manual? If yes, do you intend to carry on research? 5. Functions (4.9) and (4.10) look identical but their content is different, of course. Find out one common feature of these two functions when predicting both the direct and back effects. Will this feature still exist if tests for mechano-rolling fatigue are carried out during rolling friction with slip and not just during pure rolling friction? Corroborate you view. 6. Fig. 4.6 shows the diagram of limiting states of the active system during mechano-rolling fatigue. In your view, what will a similar diagram look like during mechano-sliding fatigue? Try to make one, for example, for a metal-to-metal (or metal-to-polymer) active system. Do not imagine it is an easy task. |
