PSI - Issue 47

Ilia Nikitin et al. / Procedia Structural Integrity 47 (2023) 617–622 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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After the initiation and early crack growth in the plane of the maximum shear stress a spontaneous bifurcation to crack propagation on the plane of maximum normal stress is observed. The crack path far from the initiation is still containing the zigzag shape regions where the crack propagation spontaneous change the plane. Macroscopically, the scenario of the VHCF torsion crack is similar to torsion crack growth under high cycle fatigue loading conditions. However, the study on crack initiation mechanisms at fracture pattern outlined a new feature of VHCF loading. It was found that there are two types of crack initiations under VHCF torsion: (1) surface, Fig.2-a, and subsurface, Fig.2-b. In the case of surface crack initiation, the fractur pattern shows a two regions with different morphology. At the initial stage the fracture surface is smooth without significant perturbation of the relief. Later a clear ‘manufactory roof’ fracture surface is forming. This manufactory roof patterns are the secondary crack tra cks. At the later stages of propagation crack starts branching and bifurcate to propagate on alternative 45-degree plane. As a result, we can see a series of inclined wings at the fracture surface. The manufactory roof pattern is typical for torsion crack in high cycle fatigue region as well in the VHCF range in the case of surface initiation. The second type of initiation is subsurface crack. The subsurface stage of crack propagation is outlined by red-dushed line on Fig.2-b. The fracture pattern does not show a typical ‘manufactory roof’ morphology. The subsurface stage of crack growth is clearly recognized as darker surface. The crack initiation may be found far (about 300 micrometers) from the specimen surface. The detailed analysis on the crack initiation site does not outline any significant feature of microstructure responsible for the crack origination. There are no nonmetallic inclusion, pores, porosity, or other strong defects of the microstructure. The crack initiation is associated with regular structure. Probably, an unfavorable arrangement of the lamellar microstructure played a critical role in crack origination. As was reported in Nikitin (2016-b) the coarse alpha-platelets has a higher micro-hardness compared to regular structure. Therefore, such structures could play a role of stress concentrator.

(а) (b) Fig. 2. (a) Fracture surface with (a) surface and (b) subsurface crack initiation.

The crack initiated in the bulk of material first developing as circular crack with further elongation into the ellipse crack. During the subsurface cack propagation it collapsed with the second surface crack. The surface crack is developing in orthogonal plane as the secondary cracks presented on Fig.2-a. The surface and subsurface crack forms the X-type macro crack at the specimen lateral surface, Fig.1-b. Such scenario of torsion fatigue crack growth is unusual for HCF region. The numerical simulation on spatial crack path shape was applied to qualify such scenario.