API REPORT 92-48
Growth Rate of Short Fatigue Cracks as Relevant to Higher Strength Steels for Offshore Structures
|Publication Date:||1 January 1994|
This report presents the results of a three year research program the overall objective of which was to characterize the fatigue properties of short Cracks (depth less than several millimeters) in relatively new, higher strength steels, including ones fabricated by thermomechanical control processing (TMCP). Enhanced understanding of the behavior of short fatigue cracks is necessary for effective design and materials selection of deep water petroleum production components such as tendons and risers. The experiments focused upon measurement of the fatigue crack growth rate (FCGR) of short cracks for five high strength steels (yield stress 370-570 MPa, alternately 54-83 ksi) in air and natural seawater, both freely corroding and with various levels of cathodic protection, and with loading conditions realistically simulating what occurs on offshore structures. The results indicated enhanced FCGR for short cracks compared to macrocracks by 3-20 times in air and 2-6 in seawater. The optimum potential for minimizing growth rate of short cracks was −950 mV (saturated calomel electrode reference). Crack length and stress intensity range at which FCGR transitioned from short to long crack behavior was determined for the different steels and test conditions. The mechanism by which FCGR of short cracks was enhanced was disclosed, thereby raising the possibility of development of new alloys with greater fatigue resistance. From analysis of the data it was possible to directly calculate the fatigue life with inclusion of the cycles associated with crack initiation, short crack growth and macrocrack growth. Of particular significance was the finding that a threshold stress intensity range (ΔKth) existed below which short cracks did not grow. At a stress ratio (R) of 0.5 the value for this threshold was 3.0 MPa(m)½ freely corroding and 5-8 MPa(m)½ at typical levels of cathodic protection, both of which are less than the corresponding values for long cracks. These observations indicate a critical need to incorporate short crack behavior into fatigue design and that consideration of macrocracks alone, as has been done historically, is unconservative. Alternately, a fracture mechanics based design approach utilizing a design curve extrapolated from the power law (Paris or Region II) regime is overly conservative. These and related findings are discussed within the context of offshore structure integrity.