The Master S-N Curve Approach to Fatigue Evaluation of Offshore and Marine Structures

Abstract

As reported in the last OMAE conference (Dong, 2003), a robust structural stress method has been developed and validated for fatigue evaluation of ship structures through a major joint industry project. The structural stress method not only provides a consistent method for characterizing stress concentration effects on fatigue in different joint types and loading modes, but also offers a rapid estimation procedure for stress intensity factors for arbitrary joint geometries and loading modes in fracture mechanics context. As a result, a master S-N curve approach has been recently developed by using the mesh-insensitive structural stress parameter and its direct linkage to fracture mechanics principles. The master S-N curve is described by an equivalent structural stress range parameter which provides a single parameter description of stress concentration effects, thickness effects, and loading mode effects on fatigue in welded joints. A massive amount of S-N data since 1947, encompassing drastically different joint types, plate thickness, and loading modes have been used to validate the effectiveness of the master S-N curve approach. With the master S-N curve method, plate joints in ship structures, tubular joints in offshore structures, as well as pipe joints for riser applications can be collapsed into a singe curve, referred to as the master S-N curve. This paper provides the detailed theoretical development, application examples, and validation results. The applications for the master S-N curve approach will be illustrated by using various offshore/marine examples.