## 3 Results and Discussion
### Effect of Imperfections at High Stress Levels  
As already mentioned, the welded series presented several imperfections that can be considered as crack-like imperfections; there were fatigue beginnings at the weld root in the toe and in the undercuts, as well as in the underfill and pores. The fractured surfaces of all specimens at the stress level of 600 MPa were examined and it was found that the size, quantity and the location of imperfections are important to explain their fatigue lives, as described below with the specimens shown in Figure 9.  
In Figure 9a-d, the specimens of all series in the as-welded condition show, as a principal characteristic, that the fatigue failure occurred covering the entire width of the specimens and the fatigue lives were similar with the exception of specimens of the S2 series, which had a shorter life. The latter is due to the fact that unlike the other series, where the starting points of fatigue were observed in cracks that grew semi-elliptically, in the S2 series, the growth of this type of crack was not evident and there was practically only one crack front growing in depth along the width of the specimens. A similar failure mode was reported in [36] where by the high density of undercuts, one straight-fronted crack was the cause of the great reduction in fatigue strength. In Figure 9e,f, the factor that seems to influence the shorter life (29,910 cycles) of one of the two specimens of the S2 series in the bottom-side-removed condition is the location of the failure starts toward the corner and the principal fracture surface on the lateral side of the specimen. Meanwhile, although the specimen with the longest life (36,597 cycles), presents fatigue starts that cover almost the entire width of the specimen, it was possible to observe a small growth until it coalesced and then a single semi-elliptical crack grew to the fracture. This last fact marks the difference of this specimen with the others of lesser lives, see Figure 5.  
In Figure 9g-j, the fatigue lives of these specimens increased considerably in relation to the previous specimens presenting large and semi-elliptical fatigue cracks located centrally. The differences observed in the fatigue lives between the two specimens in the bottom-side-removed condition are: the size and number of fatigue starts. The specimen with the longest life (91,815 cycles) had a single large visible start while the one with the shortest life (75,849 cycles) had several starts that were smaller that coalesced. However, it can be seen that the width after coalescence is greater than the width of the start of specimen with the longest life and there is also a little difference in the final aspect ratios (depth/length). The increase in the fatigue lives of the specimens presented in Figure 9i,j and the differences between them are explained in a similar way to the previous specimens: they are due to the fact that the size of the fatigue starts decreased significantly, which, as seen in the photographs, are a bit more difficult to identify despite the higher magnification. For these specimens it should be clarified that although the elimination of imperfections on both sides of the weld beads was verified through the microscope, the small imperfections that appear in the photographs may correspond to subsurface imperfections that did not disappear through the grinding applied to the specimens. In the work of [20] this issue was also reported. Figure 9: Fractured surfaces of specimens at 600 MPa, from the shortest life (**a**) to the longest life (**I**). In the last specimens of the S5 series shown in Figure 9k,l, it is relevant that once the imperfections on both sides of the weld beads were removed, the fatigue failure corresponded to pores; surface pores located on the lateral side for the specimen with shorter life (56,343 cycles) and internal pores for the specimen with longer life (207,400 cycles).