This paper presents the results of fatigue crack growth and fatigue fracture toughness studies of a high-pressure vessel steel with particular emphasis on the influence of heat treatment, low temperatures, plastic prestraining, the stress ratio and specimen dimensions.It has been shown that steels in an embrittled state, caused primarily by thermal treatment and lowtemperatures, exhibit unstable fatigue crack growth which is characterized by alternate crack jumps (cleavage zones) and zones of fatigue crack growth. The fatigue fracture toughness, which corresponds to the first crack jump, and final fracture can be appreciably lower (i.e. up to 50%) than the static fracture toughness under plane strain conditions at the corresponding temperature.An analysis has been performed of unstable and stable fatigue crack growth and a model of unstable crack propagation is proposed which accounts for the observed experimental behaviour. NOMENCLATURE a,, of= initial, and final, crack length B = specimen thickness d = crack jump (cleavage zone) length da/dN = fatigue crack growth rate ep = preliminary plastic strain F = specimen cross-sectional area after plastic prestraining F, = specimen original cross-section K,,, K,,, = minimum, and maximum, stress intensity factor (SIF) KO =initial maximum SIF KO, = SIF corresponding to crack opening Kth = A&/( 1 -R) = threshold SIF K , = dynamic fracture toughness K,,, K , = static plane-strain, and plane-stress, fracture toughness Kit, KE = fatigue fracture toughness corresponding to the first, and final, crack jump Pmi,, P, , , = minimum, and maximum, load PM = maximum load under static loading R = stress ratio (R = Kmin/K,.,) T = temperature W = specimen width Aa = fatigue crack extension between two sequential cleavage zones AK = K,, -Kmin = SIF range AK,,, = range of the threshold SIF = 0.2% offset yield stress AK,,,,, = K,,, -KO, = effective range of the threshold SIF o, = ultimate strength (I/ = reduction of area 6 = relative elongation SIF = stress intensity factor
