Issue 50

L. Romanin et al., Frattura ed Integrità Strutturale, 50 (2019) 251-263; DOI: 10.3221/IGF-ESIS.50.21 258 The thermal model has been validated using data of the specimen named “Test 4” in which two thermocouples have been inserted and a macrograph of the middle cross section has been obtained (Fig. 3a). About 40000 3D linear elements were used. The mesh has been graded both in the transversal and in the longitudinal direction to optimize the number of elements. On the symmetry plane, along the thickness, 8 elements of around 0.3 mm have been used (Fig. 9, cross section). Moving far from the weld bead, gradually, only 4 elements along the thickness have been used. In the longitudinal direction, the mesh is finer at the start and end of the weld seam and coarser in the middle of the plate where thermal condition can be assumed quasi-stationary (Fig. 9, top view). For this reason, in the longitudinal direction, a parabolic bias has been imposed to have a finer mesh in the starting and ending regions because of the higher temperature gradients due to the start and stop of the weld. The elements size ranges from 0.17 mm (welding start/end) to 0.77 mm in the middle of the plate. Figure 9: Mesh of the numerical model Boundary Conditions Heat loss by radiation and conduction trough the clamps has been taken into account. Thermal radiation was simulated by using the Stefan-Boltzman law, while the heat loss by conduction by using an equivalent convection coefficient. As a matter of fact, because of the small dimensions of the specimen, the effect of the clamps is non negligible and the advantage of using the convection coefficient is to avoid the modelling of the contact resistance which would have required a phenomenological calibration and introduced another source of error. The value of the convection coefficient was empirically determined to be 10 W/m 2 , while the room temperature has been set to 25°C. The initial temperature has been set to 95°C, according to experimental data. This is due to the tack welding operation carried out before EBW. Heat Source During EBW, in the beam impingement zone, a portion of material evaporates creating a cavity containing metallic plasma because of the high temperature reached.