Weld bead geometry cannot, by its nature, be precisely defined. Parameters such as bead shape and toe radius vary from joint to joint even in well-controlled manufacturing operations (Taylor et al., [
]). In the notch stress intensity approach to the fatigue assessment of welded joints, the weld toe is modelled as a sharp V-notch and local stress distributions in plane configurations are given on the basis of the relevant mode I and mode II notch stress intensity factors (NSIFs). These factors quantify the magnitude of asymptotic stress distribution obeying Williams’ solution [
]. When the constancy of the weld toe angle is assured and the angle is large enough to make mode II contribution non-singular, the mode I NSIF can directly be used to describe the fatigue strength of fillet welded joints having different geometries (Lazzarin and Tovo [
], Lazzarin and Livieri [
]). As an example, Fig.1 summarised fatigue strength data related to fillet welded joints, with an angle at the weld toe equal to 135 degrees. In those welded joints, all fatigue failures originated at the weld toe.