A MICROMECHANICS MODEL OF CORROSION-FATIGUE CRACK-GROWTH IN STEELS

Principles of Microstructural Fracture Mechanics (MFM) are used to dev elop a model for the characterization of environment-assisted short fa tigue crack growth. Fatigue cracks are invariably initiated at corrosi on pits formed at inclusions, hence the analysis includes stress conce ntration effects at pits that lead to the propagation of fatigue crack s the rates of which are considered to be proportional to the crack ti p plastic displacement. This plasticity is constrained by microstructu ral barriers which are overcome in a non-aggressive environment at cri tical crack lengths only when the applied stress is higher than the fa tigue limit. However, the superposition of an aggressive environment a ssists fatigue damage via crack tip dissolution, enhancement of crack tip plastic deformation, the introduction of stress concentrations at pits and a reduction of the strength of the microstructural barrier. T hese environment effects are manifested in st drastic reduction of the fatigue limit and higher crack propagation rates. The model is compar ed with fatigue crack propagation data of a BS251A58 steel tested in r eversed torsion when submerged in a 0.6M NaCl solution.