Significance of crack tip plasticity to early notch fatigue crack growth

This work demonstrates the capability of the extents of both monotonic and cyclic crack tip plasticity in correlating experimental fatigue crack growth (FCG) rates from notches. Experimental results previously published by the authors on early mode I stage II FCG rates from different single edge U-shaped notches in low carbon steel plates were utilised. The plates had different stress concentration factors and had been tested near their fatigue limits at a zero stress ratio and at constant amplitude uniaxial stresses. Eleven tests were numerically simulated with a previously developed in-house two-dimensional cyclic elastic-plastic finite element programme. The cyclic plastic deformation accommodated at the tip of a physical short through-thickness crack artificially advancing from the root of each tested notch was analysed. When the tip of a crack was advancing within the affected zone of the simulated notch, transition behaviour from the notch-affected physical short crack regime to the relatively long crack regime was recognised. The extents of crack tip opening displacements and plastically deformed zones were the subject of that behaviour. Equivalent edge crack lengths were proposed on the assumptions of equal stress intensity factors and monotonic and cyclic crack tip plastically deformed zones. A length parameter devised from the behaviour of short crack tip plasticity showed its capability to correlate early FCG rates from notches. The proposed parameter started its behaviour with a relatively high extent at zero crack length, decreased to a minimum value when the tip of the propagating crack approached the cyclic elastic-plastic boundary before it increased to match the relatively long crack behaviour. Comparison with experimentally observed early FCG in the simulated tests showed an agreement. (C) 2003 Elsevier Ltd. All rights reserved.