Threshold fatigue crack extension characteristics of a low alloy steel: The influence of environment and microstructure

The present investigation is aimed at examining the influence of environment and microstructure on the fatigue crack extension behaviour of a low alloy steel. Significant environmental effects were recorded under low R-Ratio testing but not under high R-Ratio conditions, viz., the ΔK_th values recorded in salt solution were significantly larger than those for a dry air environment and a constant value of 10 MPa√m was recorded irrespective of microstructure (yield strength). Such effects were attributed to oxide induced closure effects.

Above threshold, environmental assisted cracking behaviour in the salt solutions was the result of the appearance of transgranular cleavage facets on the fatigue surfaces. The extent of this environmentally enhanced cracking was simply related to the extent of cleavage failure.