Simulations of fatigue crack growth by blunting–re-sharpening: Plasticity induced crack closure vs. alternative controlling variables

Abstract

Finite-deformation elastoplastic analysis of plane-strain crack subjected to mode I cyclic loading with constant amplitude at various load ranges and ratios, as well as with solitary over- and under-load cycles, is presented. The Laird-Smith mechanism of crack advance by cyclic blunting and re-sharpening, which transfers material from the crack tip towards its flanks, is visualised. In the present modelling, crack closure has never been detected. As well, the supposed origin of plasticity induced crack closure (PICC), which is habitually attributed to filling-in a crack with material stretched out of the crack plane behind the tip, is ruled out. Nevertheless, the rate of simulated fatigue crack growth (FCG) by blunting–re-sharpening reproduces the key experimental trends concerning the effects of ΔK and single over-/under-load peaks. Calculated compliance curves are bent in spite of the absence of PICC, which raises the doubt about their trustworthiness as the means of the crack closure detection and assessment. This way, performed modelling manifests ambiguities concerning PICC as the universal intrinsic factor able to control FCG ubiquitously. On the other hand, having PICC absent, calculated near-tip stress–strain responses, being driving forces of fatigue damage and crack advance by bond-breaking, manifest affinities with the experimental FCG trends without intervention of PICC, too. This implicates both independent parameters of crack-tip cyclic loading under small scale yielding, such as the couple Kmax and Kmin, or Kmax and ΔK, or other equivalent one, as the indispensable variables that drive FCG directly without mediation of PICC.