Figure 3 Finite element model.
Moreover, the simulation of the weld passes has been carried out according to the element birth and death technique. Such technique starts by the modelling of the whole weld bead (Figure 4A), by separating it in 124 groups of finite elements (64 for each weld pass), named in the following “components” (Figure 4B). Subsequently, all elements are deactivated by multiplying their properties by a severe reduction factor (e.g. 10-6) in a way to exclude them from the simulation. When the added material needs to be simulated in order to virtually perform the welding process, it is not actually added to the model, but it is progressively reactivated, component by component: material properties of the finite elements belonging to the weld seam return to their starting values, participating again to the evolution of the joint material.
The model has been developed by taking advantage of the symmetry of the joint geometry and of the use of the same material for all joint parts (weld bead included). Under this condition, it is possible to simulate the experimental test just by modelling one plate and a half seam.
Both thermal and mechanical analyses involve a first step, 1 · 10-4 s long, during which all “components” simulating the whole weld seam are removed. Then, 62 couples of load steps, corresponding to about 158.97 s, are alternatively set to simulate the first weld pass (Figure 4B – red elements). Each couple consists of a first step, 1 · 10-4 s long, which allows reactivating a single “component”; a second step, which allows simulating the thermal load on the previously reactivated “component” (the duration time, in this case, depends on the weld speed). Before simulating the second pass, a load step, 108 s long, is set to simulate the time dedicated to the first weld seam cleaning operations. Afterwards, other two groups of 62 load steps, corresponding to about 114.5 s are set to simulate the second pass (Figure 4B – blue elements) according to the same modelling technique of the first pass. Finally, a load step, 1886 s long, has been set to simulate the plate cooling phase up to a temperature of about 60 °C. The FE analysis time increment is automatically calculated by the software and a full Newton-Raphson method is used to obtain the incremental calculation.