Mechanisms and modeling of subsurface fatigue cracking in metals

In terms of a synergetic system at its sequentially increased scale levels, evolution of the fracture-behavior patterns in various cyclically loaded metallic alloys is analyzed together with the alternatives of subsurface initiation of fatigue cracking. When free of the non-homogeneities like lamination sites, inclusions, etc., subsurface cracks arise due to the loss of plastic stability at the micro- or nanometer-scale level, i.e., in the local flat areas up to 500 nm in depth, normal to the load axis. Two mechanisms are controlling the formation of such a region, which is due to the instability of rotational plastic flow and fracture of the material in the state of three-dimensional compression and twisting; thereby, an even facet or a nano-structured zone forms, the latter comprising tiny particles of irregular, ellipsoid and/or spherical shapes. On further cycling, the fracture surface develops on the particle boundaries. The data of numerous investigations are shown to confirm the validity of the above-proposed models on the subsurface nanostructures in metal.