Chapter 4 DIRECT AND BACK EFFECTS

4.1 General notions

4.2 Mechano-sliding fatigue.

4.2.1 Direct effect.

4.2.2 Back effect.

4.3 Mechano-rolling fatigue

4.3.1 Direct and back effects

4.3.2 Translimiting state

4.4 Effect of interaction conditions.

Self-test questions

Tasks for students' research

4.4. Effect of interaction conditions 

Since the conditions of damage interactions from contact and off-contact loading are highly diversified (see p. 2.5), it should be expected that the regularities of the direct and back effect may change correspondingly.

Really, let us study, for example, the results of tests of the metal-to-polymer active system for mechano-sliding fatigue. If the friction process evolves in the zone of tension of the cyclically bending specimen (cf. Fig. 4.1, b), growth of contact pressure reduces its resistance to fatigue. On the contrary, the fatigue limit follows the rise of contact loading F_N if friction occurs in the zone of compression (Fig. 4.12, а). Measurements of wear in relation to the effect of cyclic stresses show (Fig. 4.12, b) that the wear process is more intensive in the zone of tension (s = +330 MPa) than in the zone of compression (s = –330 MPa), i. e. the cyclical tensile stresses intensify  wear  stronger  than  the  cyclic compressive  stresses of  the  same level at

these testing conditions. As a rule, the polymer in the friction couple wears less (s = 0) than it wears during mechano-sliding fatigue in the corresponding active system (s = ± 300 MPa).

Now let us weigh the results of tests for mechano-sliding fatigue of metal-to-metal active systems.

Fig. 4.13 shows the results of wear-fatigue tests of the steel 45 / steel 45 system (without lubrication) within a broad range of variations of cyclic stresses s < s_–1 = 320 MPa. Wear regularities were different from those of the metal-to-polymer active system (compare Fig. 4.12, b and 4.13). A specific feature of the results of tests of the metal-to-metal system during oxidation wear is that cyclic stresses intensify wear in the zone of compression and its rise is up to 40% (in the test conditions), meanwhile it slows down in the zone of tension (and reduces to 32.5%) compared with the wear in the friction couple (when s = 0). It is because the friction surface in the zone of tension is coated with oxides that protect it against fracture (the effect of Roscoe, see p. 1.4.3). The friction surface in the zone of compression shows just traces of oxides and its fracture naturally intensifies in this case.

Fig. 4.13 – Effect of cyclic compressive (1) and tensile (2) stresses on wear of steel 45 specimens (V T Sharai)

Fig. 4.14 demonstrates the role of lubrication in ensuring the durability of the active system.

Fig. 4.14 – Effect of pressure during friction with lubricating material on fatigue durability of steel 45 specimens at s_а = 400 MPa: 1 - oil МС-20 + Р; 2 - oil МС-20 (without additive); 3 - oil МС-20 + ДФ (I G Nosovsky, et al.)

For example, oil МС-20 with various additives does not affect practically the fatigue durability of the specimens (at p_a = 0). Yet, during wear-fatigue tests the ratio N(p_a) has a bell-shaped pattern. The durability during such tests and within a broad range of variations of contact pressure is much (nearly 3.5 times) higher than during common fatigue tests (when p_a = 0). The higher the load the stronger the durability is; this is the main regularity in this case. Meanwhile the maximum durability is practically the same with all three lubricants, yet it is reached at strongly different pressures. The range of pressures within which the maximum durability is maintained depends on the additive type: it somewhat reduces with the oil МС-20 + Р and strongly increases with the oil МС-20 + ДФ versus the case when the oil МС-20 is used without additives.

If the metal-to-metal active system is tested by tough loading (when the deformation range e is assigned instead of the range of stresses s) in the low-cycle region, the durability (Fig. 4.15) during wear-fatigue tests (when F_N > 0) is less than in case of common fatigue (F_N = 0) just at relatively small deformation; all three fatigue curves practically merge at e » 0.5...0.6 %.

Fig. 4.15 – Results of tests for low-cycle fatigue of steel 30ХГСА / hard alloy Р6М5
(Zh M Blednova, A N Shauro)

Note in conclusion that the experiments described in this Chapter can be divided into two groups based on the author’s formulation: (1) the data that resulted from the factor analysis (cf. Fig. 4.3, 4.13, 4.14), and (2) the data that resulted from the phenomena analysis (see, for example, Fig. 4.1, 4.4, 4.6–4.8 etc.). It has taken several decades that the data that resulted from the factor analysis were interpreted on the phenomena analysis, and therefore it has become possible to conceive them as fundamental for tribo-fatigue to come into being. Moreover, extensive experimental results from studies of fretting fatigue and mechano-corrosion fatigue accomplished during the last decades on the basis of the factor analysis can and should be similarly interpreted on the basis of the phenomena analysis.

Modern ideas (and methods) of physical mesomechanics of materials will definitely add to knowledge of new regularities of wear-fatigue damage. Deformation carriers principally different from dislocations are considered at the mesolevel, they are three-dimensional structural elements (mesovolumes), translation-rotation motion of which leads to the appearance of deformational dissipative mesostructures in the loaded material. The nature of the latter too (the type, dimensions of components, kinetics of appearance and subsequent development) governs wear-fatigue damage in many respects. Fig. 4.16 shows the pattern of  vectors of displacements in the mesovolume  during  fretting fatigue  obtained  for  the  first  time.

                                                                        ¬ s ®            0.1 mm

Fig. 4.16 – Field of vectors of displacements ahead of the front of fatigue crack front on friction surface (alloy Д16АТ, N = 5.5×10⁴ cycles)
(V E Panin, V S Pleshanov, V V Kibitkin)

Three stages of wear-fatigue damage have been identified at the mesolevel: (1) appearance of stochastically distributed zones of plastic shear and centers of fretting damage on contacting surfaces; (2) nucleation and quasibrittle growth of fatigue cracks activated by fretting-processes; (3) brittle-plastic growth of cracks preceded by the appearance of the deformation small-domain mesosubstructure with discrete disorientations ahead of the front of the main crack (cf. Fig. 4.16). No systematic studies in the sphere of mesomechanics of wear-fatigue damage have yet been accomplished.