13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- parts of the panel has been set up. In the riveted stringer panel, each group of mutually contacting surface are defined as an contact pair, then four contact pairs in the riveted stringer panel were built. In the welded panel, the constraint tie were set up to characterize the welded areas in panel and stringers, other contact areas were defined as contact pairs in same parameters with riveted stringer panel. Fig.1 Riveted stringer panel Fig.2 Stringer 2.2 Mesh Reasonable mesh of complex structure need appropriate partition (Fig.1 illustrates the partition of each part of this panel). In order to get a more accurate calculation result, the rivet must be meshed carefully. The areas around the rivet holes are critical areas where stress concentrate will emerge in tensile load, so these areas also need meticulous grid. Based on this principle, the panel were meshed carefully (shown in Fig.3). Mesh of the welded panel is easier because there is no rivet in welded model. 2.3 Loads The same load is applied to the two models. 200MPa tensile pressure is loaded along the x direction, in addition, a FORTRAN program of the SIGINI subroutine is made to input the residual stress profile to the finite element model of friction stir welded panel before tensile pressure is loaded. 2.4 Results Fig.4 shows the numerical simulation result of riveted stringer panel, the high stress areas appears around the rivet holes (especially the middle row) and the curved areas on both sides, the stress in stringers is lower compare with the panel. The maximum stress around the rivet holes reaches more than 600MPa (exceed the ultimate strength of the material) and the stress in the curved areas reaches from 400MPa to 500MPa (shown in Table.1), stress in other areas is much lower than these areas. So the rivets connection areas on the panel may be the first to be destroyed in proper tensile load. Fig.5 shows the numerical simulation result of welded panel, the high stress areas appears in the top of the welded areas and the curved areas on both sides, stringers are also the low stress areas. The maximum stress in the curved areas is nearly 400MPa, a little higher than 350MPa in the top of the welded areas, however, the tensile strength of the welded area is much lower than the base material (about 70% of the base material), and so the welded areas will be weak zone. After comparison of the results of two different models show in Table 1, under the same tensile load，stress profile in welded panel is uniform and much lower than it in riveted stringer panel, so we can predict that the welded panel could withstand the larger tensile load than riveted stringer panel.
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