damage occurs around an internal defect, observed for plane strain. According to our own observations and those in the literature [3] two main factors are operating in the Gigacycle regime: - The stress concentration due to parameter when the applied load is low. The inclusions in steels or in nickel base alloys porosity in cast aluminium or in powder metallurgy are concentrators. - The stress concentration due to the anisotropy of metals is another parameter. At low deformation, plasticity can appear only if the grain orientation and the grain size are in suitable for dislocation glide. A large grain can carbon steel, austenitic steel or titanium alloy. 2-Fish eye growth When the crack initiation site is in the eye” on the fracture surface, and the origin of the fatigue crack is an inclusion, a “ (micro structural homogeneity microscope (or naked eye), the fish fish-eye looks grey. In almost all cases, this fish area in the centre, inside of which the crack initiation site is located. Controversies exist on the origin of this dark area, which some authors have (ODA)” by Y.Murakami [4], “Fine Gra Bright Facet (GBF) by Shiozawa formation of ODAs is presumed to be micro coupled with internal hydrogen trapped by the size of an ODA exceeds the critical size for the intrinsic material fatigue limit in the absence of hydrogen, the crack grows w Sakai et al., the mechanism of formation of FGAs is caused by intensive polygonisation induced around the inclusion, followed by micro this fine granular area. By the way, The fractographic observations show zones - a dark area zone (so-called ODA) - a penny-shaped zone (short or elongated inclusion, super around the initiation site. - A zone with small radial ridges corresponding to the short to long crack transition internal defect, typically a cellular dislocation network in 3D According to our own observations and those in the literature two main factors are operating in the Gigacycle regime: The stress concentration due to metallurgical microstructure misfit becomes an important in cast aluminium or in powder metallurgy are among the most The stress concentration due to the anisotropy of metals is another parameter. At low plasticity can appear only if the grain orientation and the grain size are in . A large grain can become a critical location for Figure 1 fatigue crack initiation sites When the crack initiation site is in the interior, this leads to the formation of a “fish and the origin of the fatigue crack is an inclusion, a “ homogeneity), or porosity. At the macroscopic scale, under the optical microscope (or naked eye), the fish-eye area looks white, whereas the region outside of the eye looks grey. In almost all cases, this fish-eye appears virtually circular, with a dark which the crack initiation site is located. Controversies exist on the origin of this dark area, which some authors have variously named: “Optically D ], “Fine Granular Area (FGA)” by T. Sakai [5 t (GBF) by Shiozawa [6]...). According to Y. Murakami et al., the mechanism of formation of ODAs is presumed to be micro-scale fatigue fracture caused by cyclic stress coupled with internal hydrogen trapped by non-metallic inclusions. It is presumed that when absence of hydrogen, the crack grows without the assistance of hydrogen induced around the inclusion, followed by micro-debondings which can coalesce leading he way, no PSB has been reported for fish eye formation. The fractographic observations show zones (figure 2) in agreement with a mechanical model: called ODA) due to the initiation mechanisms. shaped zone (short crack growth). Whatever the crack initiation site (spherical or elongated inclusion, super-grain, pore), the fracture surface becomes circular dislocation network in 3D is According to our own observations and those in the literature [2] metallurgical microstructure misfit becomes an important parameter when the applied load is low. The inclusions in steels or in nickel base alloys or the most efficient stress on for initiation in low interior, this leads to the formation of a “fishand the origin of the fatigue crack is an inclusion, a “super grain” ), or porosity. At the macroscopic scale, under the optical eye area looks white, whereas the region outside of the circular, with a dark “Optically Dark Area nular Area (FGA)” by T. Sakai [5], and “Granular . Murakami et al., the mechanism of fracture caused by cyclic stress inclusions. It is presumed that when ithout the assistance of hydrogen. According to T. debondings which can coalesce leading to fish eye formation. mechanical model: due to the initiation mechanisms. [4] Whatever the crack initiation site (spherical grain, pore), the fracture surface becomes circular
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