Fatigue dislocation structure

G.P. Potimiche et al Atomistic modelling of fatigue crack growth and dislocation structuring in FCC crystals. Proc. Roy. Soc. London 462, 3707-3731 (2006)... 

Examples of dislocation structures after fatigue deformation appear in a series of microstructures (Figs. 7.55-7.59) obtained by TEM. Subramanian [27] claims that these dislocation substructures are similar to that of unidirectionaUy-stressed MgO. The micrographs presented below are of single-crystal magnesia which underwent a large number of cycles (in the millions) of low strain amplitude. The maximum strain was about 0.1 % per cycle. The characteristics of the dislocation structure in MgO, having a rock-salt structure, is as follows ... 

Kida, S., Itoh, T., Sakane, M., Ohnami, M., Socie, D.F. Dislocation structure and nonproportional hardening of type 304 stainless steel. Fatigue Fract. Eng. Mater. Stmct. 20, 1375-1386 (1997)... 

The structure of a fatigued nickel-based superalloy is presented in Figure 17.2. The deforming dislocations already begin to move after six cycles of the alternating load. 

In the case of intensive repetitive actions, the facilitation of plastic deformation in the surface layer may at some point result in the opposite effect, namely, an additional strength increase due to the accelerated accumulation of distortions in the metal structure. Direct observations by electron microscopy, conducted by Kostetskiy et al., indicated a significant increase in the dislocation density in the surface layer. Under the appropriate conditions (temperature, stress, velocity, etc.), such a peculiar sample training may be used in the improvement of the structure and the mechanical properties of the surface layer. However, this already corresponds to the adsorption-induced fatigue region, studied in detail by Karpenko et al. These studies showed that at a certain level of stress the adsorption-caused acceleration of defect accumulation within the surface layer may lead to the premature development of cracks and partial failure after a certain number of cycles (cyclic fatigue). 

The process of fatigue cracks origin results from the formation of critical dislocation clusters in front of structural barriers. 

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