Page 429 - Practical Design Ships and Floating Structures

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The above techniques have proved to increase the computational efficiency significantly.

Figure 1 : Mesh topology employed for general models.
2 BASICMODEL

A basic model for fillet welding with the emphasis on the modelling of geometrical restraints was
established in Andersen (2000,2001a). The general aspects of the model may be described as follows:
The thermal and the mechanical analysis are sequentially coupled. The material is modelled as thermo-
elasto-plastic with temperature-dependent material properties. The microstructural effects are included
indirectly through the microstructural dependency on the cooling time and the austenisation peak
temperature. A small strain implicit formulation, which disregards the geometrical non-linearities ,is
applied to the mechanical analysis. Rate independent plasticity is assumed and modelled by use of the
von Mises criterion, the associated flow rule and kinematic hardening. Further, transformation induced
plasticity (TRIP) is accounted for.

The finite element applied is a version of the graded element developed by McDill et al. (1987). The 8-
26 noded isoparametric hexahedron is similar to the familiar 8-noded linear brick but the constraints
associated with mesh grading are embedded in the shape functions to ensure interelement
compatibility. The variable number of nodes and the interelement compatibility make the graded
element extremely efficient in mesh grading algorithms. An example of the mesh topology employed
is shown in Figure 1.
Considerable effort was placed on the modelling of the geometrical aspects. The web and the base
plate are allowed to move relatively to each other in welding, restrained at the beginning only by
contact and tack welds. As the filler elements are activated, the parts are locked relatively to each other
in the distorted configuration. The modelling of this phenomenon involves dynamic activation of fillet
elements, dynamic coupling of parts and modelling of contact between web and base plate. It was
shown that the modelling of these factors is essential for a physical correct prediction of the
deformation pattern.

3 DYNAMIC MESHING

The basic model establishes the basis for the dynamic remeshing scheme developed in Andersen (2000,
2001~). In dynamic meshing it is exploited that the thermal and the mechanical activity are localised in
the region of the heat source and the basic task is therefore to provide a dense mesh only where needed
and thus, reduce the number of DOFs. The graded element introduced in the basic model has already