Welded Connections in Plated Structures

General

Assessment of high cycle fatigue in marine structures is based on linear elastic structural finite element analysis (FEA). Assessment of low cycle fatigue can be based on nonlinear FEA; see also Section 3.1.3. Some basic knowledge about finite elements is recommended in order to prepare finite element models that are representative of the physical behavior of different structural details. Before starting to model a detail, it is necessary to have a clear view of what the outcome of the analysis should be and of how the analysis results should be used, together with S-N data, for assessment of calculated fatigue life. In principle, three different types of finite element models can be prepared for fatigue assessment:

1. Model for calculation of membrane stresses, to be used together with S-N curves for nominal stress for calculation of fatigue damage.

2. Model for calculation of structural stress or hot spot that represents the stress due to the considered geometric detail, which is entered into a hot spot stress S-N curve for calculation of fatigue damage.

3. Model that accounts for the considered geometric detail, including the weld toe, where the calculated notch stress is entered into a notch S-N curve for calculation of fatigue damage.

A rather coarse finite element model may be appropriate for analysis of membrane stresses in plated structures. The elements used should represent a linear membrane stress distribution within each element. This is achieved by using eight-node isoparametric shell elements and four-node shell elements with internal degrees of freedom. The ability to represent bending stress over the thickness is less important for calculation of membrane stresses. This means that if only the membrane stress is to be derived from the analysis, this model cannot be used for fatigue assessment of plated structures that are subjected to significant dynamic lateral pressure. This limitation is also considered a drawback to using the nominal stress approach for such loading conditions and is one reason why structural stress methods were introduced into fatigue design of marine structures during the early 1990s. Structural stress methods are also called hot spot stress methods, and the local geometry of a detail is accounted for in the stress calculation, in addition to plate bending due to lateral pressure. The finite element model here needs to represent linear stress behavior through the plate thickness.