Finite-deformation analysis of the crack-tip fields under cyclic loading

The plane-strain crack subjected to mode I cyclic loading under small scale yielding was analysed. The influence of the load range, load ratio and overload on the near-tip deformation-, stress- and strain-fields was studied. Although the near-tip zones of appreciable cyclic plastic flow for all loading regimes matched closely one another, when scaled with (Delta K/sigma(Y))(2), the activities of plastic flow within them manifested dependence on K-max and K-min, as well as on overload. Cyclic trajectories of the crack-tip opening displacement (CTOD) converged to stable self-similar loops of the sizes proportional to Delta K-2 and positions in CTOD-K plane dependent on the maximum K along the whole loading route, including an overload. Computed near-tip deformation evidenced plastic crack advance, this way visualising of the Laird-Smith concept of fatigue cracking. This crack growth by blunting-resharpening accelerated with rising Delta K and was halted by an overload. Crack closure upon unloading had no place. The affinities were revealed between computed near-tip stress-strain variables and the experimental trends of the fatigue crack growth rate, such as its dependence on K-max and K-min (or Delta K and K-max), and retardation by overload. Thus, the effects of loading parameters on fatigue cracking, hitherto associated with crack closure, are attributable to the stress-strain fields in front of it as the direct drives of the key fatigue constituents - damage accumulation and bond breaking. (C) 2009 Elsevier Ltd. All rights reserved.