Subsurface crack initiation and propagation mechanisms in gigacycle fatigue

Abstract

In the very high cycle regime (N f > 10 7 cycles) cracks can nucleate on inclusions, "supergrains" and pores, which leads to fish-eye propagation around the defect. The initiation from an inclusion or other defect is almost equal to the total crack growth lifetime, perhaps much more than 99% of this lifetime in many cases. Integration of the Paris law allows one to predict the number of cycles to crack initiation. A cyclic plastic zone around the crack exists, and recording the surface temperature of the sample during the test may allow one to follow crack propagation and determine the number of cycles to crack initiation. A thermo-mechanical model has been developed. In this study several fish-eyes from various materials have been observed by scanning electron microscopy, and the fractographic results analyzed as they related to the mechanical and thermo-mechanical models.

Fatigue crack initiation behaviors throughout friction stir welded joints in AA7075-T6 in ultrasonic fatigue

“…It can be seen that the fracture surface at the NZ is more flat than that at the TMAZ because of the generation of fine and equiaxed recrystallized grains. At the center of the initiation zone, a particle can be detected with a length of 12 m, and this is very similar to the failure mechanism of high-strength steels in the VHCF regime [21][22][23], which the crack initiation mostly takes place at inclusions inside the specimens and is accompanied by the formation of a ring-like fracture surface, the so-called fish-eye.…”

Subsurface inclusion-induced crack nucleation and growth behaviors of high strength steels under very high cycle fatigue: Characterization and microstructure-based modeling

“…Although there is a consensus that the formation of FGA governs the VHCF property of steel, the unanimous understanding or conclusion has not yet been drawn. Some researchers point out that the fatigue process within the FGA should be interpreted as the crack nucleation process [3,10,17]. However, others argue that the fatigue progress within the FGA can be described by the crack growth with a very slow rate [15,[18][19][20].…”

“…From the viewpoint of safe life design, the novel VHCF methods are urgently needed to develop, especially for property characterization, damage assessment and life prediction. However, a number of peculiar VHCF properties, such as duplex or stepwise S-N properties [2][3][4][5][6], competing failure mechanisms [6][7][8][9], complex crack nucleation and growth behaviors [3,5,7,[9][10][11][12], etc., are not yet well understood. As an essential prerequisite for exploring VHCF methods, it is crucial to clarify the fundamental VHCF properties of materials.…”

Effect of carburizing treatment on the “fish eye” crack growth for a low alloyed chromium steel in very high cycle fatigue

“…As analyzed in previous section and reference [4], the crack starts from a defect with a size a int (initiation size, Fig. 9) after initiation cycles damaged (N i ).…”

“…The VHCF crack initiation mechanism was explained by Murakami [3]. The ''fish eye'' crack growth has been studied by Paris and Bathias [4,5] who gave a fracture mechanics approach to estimate the number of cycles of crack propagation.…”