Mechanistic Features of Short Fatigue Crack Growth Kinetics for High Strength Steels in Sea Water

The importance of fatigue to the integrity of offshore structures is well documented. Also, it has been demonstrated that much of the service life of members and components such as tendons and risers is comprised of an extension of cracks from initial surface defects to a size of several millimeters. At the same time, the growth kinetics of such short cracks has been shown to be more rapid than those of long cracks; however, it is upon the latter that most historical studies have focused. In the present paper, the results of scanning electron microscope fractographic analyses performed upon five high strength steels fatigued in air and seawater are presented. These revealed fracture surface morphology distinctions that were a unique function of material, environment (air versus seawater), potential, and crack length, and that the enhanced fatigue crack growth rate in the short crack regime was relatable to these morphological features. Of particular importance were (1) the development of secondary cracks as a precursor for the short crack to long crack growth rate kinetics transition, and (2) a change in fracture mode, either from quasicleavage (QC) to microvoid coalescence (MVC) or from intergranular to QC or MVC with increasing crack length. The results are discussed within the context of (1) alloy development for applications where a significant portion of the fatigue life transpires while cracks are relatively short such that the enhanced growth rate kinetics apply; and (2) materials selection and fatigue design of riser and tendon systems for deep water offshore structures.