## 1 Introduction  
The fatigue behavior of steel welds has been widely studied, but there are still gaps in the knowledge of this complex subject due to several factors that affect the fatigue strength and even more so due to concerns related to thin thicknesses and unconventional welding processes. The weld quality, the residual stresses and the material, among others, are pointed out as the main factors that affect the fatigue strength [1, 2]. It has been shown that weld quality control [3], or the use of post-weld treatments [4, 5, 6, 7] in welded joints with high strength steels, increases the fatigue strength by reducing the severity of its inherent imperfections; this fact is recognized in design guides. However, when allowing the increase in fatigue assessment classes of welded joints (FAT), when the imperfections have been verified with nondestructive testing (NDT) or when an improvement method has been used [8], there is no differentiation in the increase according to the tensile strength of the materials. The foregoing has been explained in [9], due to the fact that the crack growth dominates the fatigue life of the welded joints and because the fatigue crack growth rate (FCGR) is insensitive to the mechanical strength of the material. However, as counterpart in [10], it was shown that when the weld quality is high, in high-strength steels for example, the fatigue strength is positively correlated to tensile strength. The general effect of notches and residual stresses on fatigue strength of welded joints is well known as their combined effect was analyzed in various studies. However, in almost all works, thick plates and conventional welding processes were used [11, 12, 13, 14]. Microstructural details such as grain size, phases and hardness also affect the fatigue behavior of the welded joints. The crack propagation generally occurs in the heat-affected zone (HAZ) and in the fusion zone (FZ) because the properties of these zones are usually different from base metal (BM). The use of local properties such as hardness and fatigue crack growth parameters can be useful to improve the analyses and predictions of fatigue strength [15; 16; 17]. The increase in the thickness of the plates in welded joints decreases the fatigue strength. For the same nominal stress, thicker welded plates have lower fatigue strength than thinner welded plates. In [18] it is explained that the above is due to the fact that the stress in the crack depends on the relationship between the crack depth and the thickness, this effect being greater in crack growth for thick plates than for thin plates. While in [9] it is pointed out that the effect of stress concentration on the weld toe is greater in a crack in a thick plate. A reduction factor is applied to fatigue strength if the thickness plates exceed 25 mm [8], however if the plates' thickness is lower, there is no rule for the fatigue strength increase. In [19] the increase in fatigue limit due to the decrease in the thickness of the plate was estimated for thicknesses less than 25 mm as a function of the initial crack size. This work highlights that for thicknesses less than 6 mm, the opposite effect occurs when the initial crack size is greater than 0.3 mm. The presence of defects acting as a fatigue pre-crack are the main reason to a substantial reduction in fatigue strength in welded joints. This determines that the initiation period can be neglected [8; 18] and that fracture mechanics seem to be the appropriate approach to determine the fatigue life [9]. In various works where the assessment and prediction of fatigue life are based in fracture mechanics, the basic procedure takes as base the Paris law and the stress intensity factor (SIF) applied to imperfections in all ranges of stress levels. However, due to the small size of cracks or low stress levels, the initiation stage is also considered [19; 20]. Additionally, refinements or variants to the Paris law are applied to predict or analyze the fatigue life of welded joints [21; 22].