American Petroleum Institute

Growth Rate of Short Fatigue Cracks as Relevant to Higher Strength Steels for Offshore Structures (Year-Two)
 N REPORT 91-48

Annotation

It is generally accepted that design for adequate fatigue resistance is critical to integrity of offshore structures. In the case of threaded and welded, ground connectors fatigue life may be dominated by crack nucleation and growth to a size of several millimeters. It has now become recognized that growth rate for small fatigue cracks can exceed that of macrocracks by a factor of ten or more. Also, no data presently exist for such cracks in seawater, particularly under conditions of concurrent cathodic protection. Of further concern is that little information regarding fatigue crack growth rate is available for either short or macrocracks in new higher strength steels, including those strengthened by thermomechanical control processing (TMCP), which are presently being considered for deep water structure service.

The specific research addressed herein constitutes the second of a planned three year activity. During the initial year techniques were developed to accurately monitor development of fatigue cracks in the 0.1 to several millimeters regime, and data were obtained in air for several TMCP grade steels. For the present investigation period (Year-Two) it is proposed to extend these experiments to natural seawater under freely corroding conditions. Three of the TMCP grade steels for which short crack growth rate data exist in air will be investigated with emphasis on conditions relevant to offshore structures (high mean stress and low cyclic frequency). All test materials and instrumentation are presently on hand, and so all requested funds would be directed specifically at data development. The results from this project should provide material property information which is critical to design of deep water tendons, risers and related components and structures.

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