Atomistic modelling of fatigue crack growth and dislocation structuring in FCC crystals

Abstract

Fatigue damage in face-centred cubic crystals by dislocation substructuring and crack growth was computationally simulated at the atomic scale. Single-crystal copper specimens with approximately 200 000 atoms and an initial crack were subjected to fatigue loading with a constant strain amplitude of ϵ max =0.01 and a load ratio of R = ϵ min / ϵ max =0.75. Cyclic plastic deformation around the crack tip is the main influencing factor for the propagation mechanisms of nanocracks. The main crack-propagation mechanisms occurred either by void nucleation in the high-density region near the crack tip or by fatigue cleavage of the atomic bonds in the crack plane. Fatigue crack growth at grain boundaries was also studied. For high misorientation angle grain boundaries, the crack path deviated while moving from one grain to another. For low crystal misorientations, the crack did not experience any significant out-of-plane deviation. For a large crystal misorientation, voids were observed to nucleate at grain boundaries in front of the crack tip and link back with the main crack. During fatigue loading, dislocation substructures were observed to develop throughout the atomic lattices. Fatigue crack growth rates for nanocracks were computed and compared with growth rates published in the literature for microstructurally small cracks (micron range) and long cracks (millimetre range). The computed growth rates for nanocracks were comparable with those for small cracks at the same stress intensity ranges and they propagated below the threshold for long cracks.

An atomistic investigation into the nature of near threshold fatigue crack growth in aluminum alloys

“…When no blunting occurs, the crack usually closes in a reversible manner. The model predictions of crack growth along crystallographic planes (including the dominant planes) is consistent with previous atomistic modeling of magnisium [7] and copper [6].…”

“…One of the largest challenges is the limited spatial domain that is typical of atomistic models. Simulations having a small spatial domain can artificially influence the movement of dislocations away from the crack tip and ultimately bias crack tip behavior [1][2][3][4][5][6][7][8]. Discrete dislocation (DD) dynamics simulations are not generally plagued by this problem as they can accommodate a much larger spatial domain, while still explicitly modeling every dislocation.…”

Molecular-dynamic modeling of the fatigue fracture of metal nano-structure under the action of the external cyclic stress

“…Many works involve periodical boundary conditions. They may be positioned along the direction coinciding with the long edge of the sample and with the external loading direction [6,14], which permits the authors modelling the infinite long nano-wire, or the periodical boundary conditions are set in the directions perpendicular to the external loading action [7,11,12,15 20,22]; quite often, the authors apply the periodical boundary conditions in all three directions [16,17,19,21]. The works using free boundary conditions [7,18], are noteworthy; they analyzed either the effect of the sample thickness on the results [7], or the influence of the whole system size [18,24].…”

“…Such metals as copper [7,10,11,12,15,20,22], nickel [7,10,12,13,15,22], iron [14,18,19,21], aluminum [16], gold [22], alloys of TiAl [6], nano-scale Zr-Cu metal glasses [17] were analyzed. Many works involve periodical boundary conditions.…”

“…A number of works deal with the molecular-dynamic modeling of the damage and fracture of nano-size samples from metals and alloys under the cyclic loading; they also use the 3-stage different-scale approach [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Such metals as copper [7,10,11,12,15,20,22], nickel [7,10,12,13,15,22], iron [14,18,19,21], aluminum [16], gold [22], alloys of TiAl [6], nano-scale Zr-Cu metal glasses [17] were analyzed.…”