The behaviour of short cracks under fatigue and corrosion fatigue conditions in structural steels

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High cycle fatigue tests under constant stress amplitude in fully reversed bending were carried out an air, plain sodium solutions and in sodium chloride solutions containing nitrite, using smooth specimens of carbon manganese structural steel (bs 4360-50d). the initiation and growth of short fatigue cracks on the surface of the specimen were studied using optical microscopy monitoring techniques îân both air and corrosive solutions. in some tests, the surfaces of specimens were etched at the end of the short crack regime to reveal, retrospectively, the growth behaviour of cracks in conjunction with the local microstructure. the fatigue strength decreased în aqueous environments compared to that in air. crack initiation in this material occurred predominantly at non-metallic inclusions. in air, cracks initiated from oxide inclusions whereas in aqueous environments, the initiation sites switched to sulphide inclusions through a pitting process. with respect to microstructure, cracks initiated in ferrite in both air and corrosive solutions except at low stresses (below the air fatigue limit) in plain sodium chloride where initiation occurred in pearlite. the effect of corrosion was also to increase crack growth rates as we11 as the density of crack initiation. however, growth enhancement was primarily observed an early stages of short crack growth particulariy during the crossing of the first few grains.