Corrosion fatigue short crack growth behaviour in a high strength steel

A frequent cause of the premature failure of structural components is Corrosion Fatigue Cracking. Historically corrosion fatigue studies have shown that this failure process depends strongly on the interactions between loading mode, metallurgical texture and electrochemical parameters. This has become a serious problem for concerns such as the nuclear, automobile, oil, gas, aerospace and marine industries. This research study was carried out using a quenched and tempered silico-manganese spring steel (DS 250A53). Smooth hour-glass shaped fatigue specimens were tested under fully reversed torsional loading in both laboratory air and aerated O.6M NaCI solution environments. Crack growth behaviour in both air and corrosion fatigue tests was monitored using a plastic replication technique. Intermittent air fatigue/corrosion fatigue tests were also conducted at sub-fatigue limit stress levels in an attempt to determine an environment-assisted critical (thresh-old) crack length necessary to cause subsequent air fatigue failure and therefore elucidate the mechanisms operative during the first stages of crack development and growth. To assist in this electrochemical experiments were performed to determine the corrosion characteristics for this metal-environment system. In air fatigue tests cracks initiated at non-metallic inclusions due to a strain incompatibility between the inclusion and the matrix. Air fatigue modelling was based on the observation that crack growth rate decreases as cracks approach microstructural barriers. In the present study it is suggested that the 4th prior austenite grain boundary was the major barrier to a growing crack. This regime of crack growth is described as the 8hort crack regime and may be represented by the following equation; After overcoming the major microstructural barrier crack growth rate increases with an increase in its length. This regime of crack growth is represented by long crack regime and may be quantified by the expression; (da/dN)la=Claa - Dt Corrosion fatigue crack initiation was associated with chemical pitting of these inclusions. Failure at stresses close to the 'in-air' fatigue limit was due to the coalescence of a small number of cracks. While at low stresses growth of individual cracks led to failure. Corrosion fatigue crack growth modelling suggested that a chemical driving force arising from chemical reactions was present in addition to the mechanical driving force of the applied stress. The presence of this additional force enabled a crack to continue its propagation at low stresses which would otherwise arrest under air fatigue conditions. Corrosion fatigue crack growth rate was calculated using the following superposition model. (da/dN)cf = (da/dN)a + (da/dN)e where (da/dN)a and (da/dN)e represent air fatigue crack growth rate and environmental crack growth rate respectively.