## 1 Introduction  
Fatigue failure of welded joints is a highly localized phenomenon which normally comprises crack initiation and propagation near weld-toe, while design guidelines given in the different codes roughly incorporate local stresses. These structural design guidelines typically consider nominal stresses which are based on S-N curves derived from large fatigue test data [1]. Fatigue strength is assessed by selecting an appropriate SN curve for the specific weld detail. This strength normally represents a 97.7% survival probability for the respected weld detail [2]. In \"global or nominal stress approaches\", the strength assessment is made directly from the applied loads or through the nominal stresses in a critical cross-section. The shortcoming of this design approach is that it only takes into account the global behaviour of the complex welded details, where nominal stress cannot be defined implicitly. Therefore, the local stress approaches should be applied for a better understanding of stresses in these fatigue prone welded joints. To overcome these problems, different methods have been developed based on local stress approaches for the fatigue life assessment of welded structures [1; 3; 4; 5; 6]. The application of these methods depends upon the field of engineering and also varies within research groups in different parts of the world. In \"local approaches\" the strength assessment is made from local strains or stresses in the specified weld detail. This can either be done by the local strain measurement from the strain gauges or by the local stress calculation using the finite element analyses (FEA) method. However, there are several complications associated with the local stress approaches due to inhomogeneous material, welding imperfections, and residual stresses present in weld detail [7,8]. These anomalies are normally found in the areas near to the weld-toe and the weld-root. Moreover, local approaches are very sensitive to weld profile or geometric parameters, because stresses near weld vary non-linearly and exact weld geometry is not known before the welding. Therefore, it is difficult to accurately predict the stresses near the weld-toe or the weld-root, as weld profile depends upon welding method, welding material, and plate thickness. Consequently, these local approaches may lead to inaccurate results due to irregularity in the weld geometry. Another approach that provides a link between these two (global and local) stress approaches and less demanding than local stress approaches is the \"structural hot-spot stress approach\". This approach can be used to measure strain by the application of strain gauges to the specified reference points or to calculate stresses from the FEA of the specified weld detail. This approach is computationally less demanding as compared to local stress approach, because it only takes into account the stress concentration effect produced by the geometrical constraint at the expected crack initiation point while excluding the non-linear stress peak caused by the sharp notch at the weld-toe. By using this approach, we can measure the fatigue strength of a welded joint without considering the weld profile itself, which saves a great deal of time and effort. Moreover, the exact dimensions of the weld are not known to us before its fabrication, and there are many irregularities associated with the weld profile which directly influence the local stresses, but this approach is less affected by the weld profile or the local notch effect, because it only considers the stress raising effects caused by weld geometry. There are various fatigue assessment methods, and there are numerous fatigue-related problems. We cannot generalize a specific method for all the fatigue-related problems in the industry, because it restricts any further development in these local stress approaches. However, some methods are well recognized for certain weld details.