PSI - Issue 28

Stepan Major et al. / Procedia Structural Integrity 28 (2020) 561–576 Stepan Major/ Structural Integrity Procedia 00 (2019) 000–000

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Fig. 9. Spatial orientation of surface elements at the macroscopic level, with length of elementary square side a 99 = 99 μm: (a) loading ratio L R = 0.23; (b) loading ratio L R = 0.5; (c) loading ratio L R = 0.73.

The results for nitrided specimens are shown on the Fig. 10. For the case of nitrided samples, a comparative analysis for square network (again characterized by side length a 05 and a 99 ) is not presented here, as the behavior is comparable to the previous case of virgin specimen. This type of sample is characterized by a sharp and significant increase inclination, due to this fact, the values of the measured angles are also higher than in previous case (in case virgin specimen). The surface of the nitrided layer samples was complexly covered with a square mesh with the length of the edge a 99 = 99 μm. The improved mechanical properties, especially a high strength of the nitride layer and high compressive residual stresses introduced within the diffusion zone are the main causes of an improvement of fatigue strength. Since such a layer hinders the dislocation motion, the predominant failure mechanism in the case high cycle fatigue is subsurface fatigue crack growth, see Sirin et al. (2008) or Limodin and Verreman (2006). These cracks initiate on the internal inclusions within the core region and propagate in a near vacuum by forming so-called fish-eye cracks, see Fig. 11 (a). The fish-eye cracks look bright to the naked eye or in the microscope, whereas the region outside the fish eye crack seems to by gray. The difference in color is probably caused is probably caused by different micromechanism. The bright morphology of the fish-eye crack is produced by cyclic contact of the mating fracture surfaces in the absence of atmospheric effects. When the crack front approaches the low-toughness surface layer, a