Persistent slip band

Fig. 8.60 Schematic diagrams showing common surface profiles produced during fatigue (a) coarse slip and crack initiation adjacent to grain boundaries (b) extrusions and intrusions (c) coarse slip within a persistent slip band (after Lynch Y...

Jenkins, P. J. (1990) Scanning acoustic microscopy of persistent slip bands. In EMAG-MICRO 89 (ed. H. Y. Elder and P. J. Goodhew), Inst. Phys. Conf. Ser. 98, pp. 153-6. Institute of Physics, Bristol. [282]... 

Precrack cyclic deformation that includes the formation of persistent slip bands, formation of extrusions and intrusions. 

In the first group, emerging persistent slip bands (PSBs) are preferentially attacked by dissolution. This preferential attack leads to mechanical instability of the free surface and the generation of new and larger PSBs, followed by localized corrosion attack, resulting in crack initiation. Under passive conditions, the relative rates of periodic rupture and reformation of the passive film control the extent to which corrosion reduces fatigue resistance. When bulk oxide films are present on a surface, rupture of the films by PSBs leads to preferential dissolution of the fresh metal that is produced.102... 

Hills and Nowell [S] propose that the contact stress state (assuming a ball-on-flat configuration) involves a maximum shear stress oriented at 45° to the sinface which produces slip and eventual persistent slip bands. Cracks are then generated in these bands. 

A. Hunsche and P. Neumann Quantitative measurement of persistent slip band profiles and crack initiation. Acta metallurgica, 34(2) 207-217, 1986. 

Well-documented studies have been performed to compare the fatigue behavior in air and under vacuum at low or moderate temperature of copper (Wang et al., 1984 Bayerlein and Mughrabi, 1992) and austenitic stainless steels (Gerland et al., 1988 Mendez et al., 1993). As an example. Fig. 5-12 shows the marked effect of an air environment at room temperature, even for a corrosion-resistant alloy. High cumulative plastic strain amplitudes can be reached under vacuum. The oxygen partial pressure controls the nature of the surface oxide and localization of the crack initiation process in persistent slip bands formed by cyclic straining. 

It is well known that shp bands, twins, interphases, grain boundaries, and constituent particles are classical sites for crack initiation. Moreover, persistent slip band... 

Fatigue crack nuclei that precede macro crack formation originate in persistent slip bands well before the final failure. It has been said in 1.4.1 that they start to appear on the surface of the material as soon as the hardening/softening process saturate and, in fact, it has been also said that if the cyclic hardening were periodically removed by annealing the specimen subjected to a fatigue test, the life of the specimen would increase enormously to almost become infinite. Saturation... 

Fig. 1.55 Appearance of persistent slip bands broken line) in a specimen of mild steel 0.09 % C tested in reci nocating bending. Wohler curve is also shown full line) [78]...

Fig. 1.57 Schematic of event progression that goes from a pore formation into persistent slip bands in some superficial grains, b growth and coalescence of submicroscopic damage with MSC formation, c grain boundary break-through by MSC and macro crack formation...

In the absence of inclusions or second-phase particles damage may develop either in persistent slip bands on some surface grains (see Sect. 1.5) or in slips activated by a micro-discontinuity such as surface roughness, as seen in the previous section. 

Therefore, the competition to create the first sub- damage is between these two factors persistent slip bands and slips activated by surface roughness. Persistent slip bands formation in just one or few surface grains is related to metallurgical variability while surface roughness depends on metal working conditions. The presence, non-eliminable, of inclusions and second-phase particles introduces a third element of competition related, like persistent slip bands, to metallurgical variability. 

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