The “chessboard” distribution of normal and tangential tensile and compressive stresses on the “nanostructured surface layer — substrate” interface is revealed for metal materials (Fig. 1).
The “chessboard” model of the conjugation between nanostructured surface layer and substrate (“+” - compressive stress,“-” - tensile stress)and channeling propagation of localizedshear.
A stochastic two-level model of the chessboard stress distribution on the interface is worked out. The simulation results correlate well with appropriate experimental data.
In tension of metal materials with nanostructured surface layers one can observe the effects of strain channeling in the nanostructured surface layer and fracture of the specimen as a whole. This is related to shear localization and subsequent propagation of the main crack along “chessboard squares” with normal tensile stresses (Fig. 2).
On the stage of uniform specimen elongation in the nanostructured surface layer localized deformation bands evolve in the conjugate directions of maximum tangential stresses τ
. On the prefracture stage a localized deformation macroband in the form of an extended neck propagates in the conjugate τ
directions zigzagging along the whole specimen length. On the stage of localized neck formation one can see the development of two macrobands self-consistent by the scheme of a dipole or a cross (Fig. 3).
We have measured experimentally the evolution of main plastic shear in localized deformation macrobands in the neck and rotational deformation modes associated with localized shears. With non-compensated rotational deformation modes, the main crack is generated in two localized deformation macrobands in the neck and then the specimen fails.
Methods of governing the channeling effects of plastic flow and fracture of the material with nanostructured surface layers are proposed.