Shear band deformation and fatigue striations during three-point bending fatigue crack growth in brittle metallic glass

A universal energy-intensive micromechanism of periodic splitting-rupture (PSR) is revealed which proceeds at the front of the fatigue cracks in metallic materials, providing their steady growth, forming T-shaped crack tip and striated microrelief of the fracture surface. The PSR micromechanism is caused by a critical (prior to fracture) fragmentated structure formed in the area of the crack front where the material is subjected to multiple and increasing plastic deformation. This universal prefracture structure is a final stage of the evolution of the deformational structures emerged in front of the fatigue crack at the stage of stable crack growth in metallic materials with different initial structural states. This is responsible for universality of PSR micromechanism and fatigue striations. Fatigue striations are the traces of extending crack front with T-shaped tip formed during brittle transverse microsplitting along the overstressed boundaries of critical fragmentated structure. Based on 3D finite element modeling of the stress-strain state in front of the cracks with T-shaped tip, it is established that the value and the location of maximum of normalized in-plain stresses (acting in front of crack tip in the plane of crack along the normal to its front) are close or coincide for the cracks of different configuration and different types of tensile load under condition that splitting in the T-shaped crack tip is considerably less than the crack length. Taking into account the PSR micromechanism and asymptotic stress distribution in front of T-shaped crack tip the physically based mathematical model for steady fatigue crack growth is developed along with the techniques for prediction of steady fatigue crack growth in full-scale components under simple and complex loading cycles.

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Fatigue Striations and Fissures in 2024-T3 Aluminum Alloy

Materials Science Forum ◽

10.4028/www.scientific.net/msf.567-568.397 ◽

2007 ◽

Vol 567-568 ◽

pp. 397-400 ◽

Cited By ~ 1

Author(s):

A.J. McEvily ◽

Masahiro Endo ◽

S. Cho ◽

J. Kasivitamnuay ◽

Hisao Matsunaga

Keyword(s):

Aluminum Alloy ◽

Fatigue Crack ◽

Fatigue Crack Growth ◽

Fracture Surface ◽

Crack Growth ◽

Fatigue Striations ◽

Aluminum Alloy 2024 ◽

Low K

A study has been made of the striations and fissures developed in the aluminum alloy 2024-T3 during fatigue crack growth. Fissures were found to form on inclined facets. They were uniformly spaced as the result of a shielding process. Striation spacings were in accord with da/dN values at the higher levels of K investigated, but at low K levels striation spacings were larger than the corresponding da/dN values. The percentage of the fracture surface containing striations varied with the K level, ranging from less than 1 % at low K levels to 80 % at higher K levels. The reason for the discrepancy between the spacing of striations and the corresponding da/dN values is discussed.

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