۱٫ Introduction: NSIF-based local approaches

۲٫ The Peak Stress Method

۳٫ Defining a SED-based design stress using the PSM

۴٫ Structural steels joints: geometries and FE stress analyses according to the Peak Stress Method

۵٫ Assessment of weld toe and weld root fatigue failures

۶٫ Conclusions

References

In fatigue design of welded joints according to the notch stress intensity factor (NSIF) approach, the weld toe profile is assumed to be a sharp V-notch having tip radius equal to zero, while the root side is assumed to be a pre-crack in the structure. The Peak Stress Method (PSM) is an engineering, FE-oriented method to estimate the NSIFs starting from the singular linear elastic peak stresses calculated at the V-notch or crack tips by using a coarse FE mesh. The element type is kept constant and the average element size can be chosen arbitrarily within a given range. The method is used in conjunction with Ansys software. The FE meshes are claimed to be coarse in comparison to those necessary to evaluate the NSIFs from the local stress distributions. Two-dimensional as well as three dimensional FE analyses can be adopted to apply the method. By using the averaged Strain Energy Density (SED, which can be expressed as a function of the relevant NSIFs) as a fatigue strength criterion, a so-called equivalent peak stress is defined to assess either weld toe or weld root fatigue failures in conjunction with a properly calibrated design curve. After presenting the theoretical background of the method, the paper presents a review of applications of the PSM relevant to steel welded joints under uniaxial as well as multiaxial fatigue loadings. Because of the relatively coarse FE analyses required and simplicity of post-processing the calculated peak stresses, the PSM might be useful in the everyday design practice.

Introduction: NSIF-based local approaches

The Standards and Recommendations relevant to the fatigue design of steel welded joints [1,2] suggest structural engineers to perform the fatigue strength assessment following different procedures: those based on S-N curves by adopting the nominal, the structural hot-spot or the notch stresses, or that based on the Linear Elastic Fracture Mechanics (LEFM). These approaches have the advantage to be quite easy and rapid to apply, however, concerning the assessment capability, it is well known in the literature that local approaches, such as those based on Notch Stress Intensity Factors (NSIFs), provide the best level of accuracy. In the fatigue design of welded joints, the NSIF-based approaches assume both the weld toe and the weld root as sharp V-notches, having a notch tip radius  = ۰, according to a worst case hypothesis, and notch opening angle greater than zero (typically 135°) and equal to zero, respectively, as shown in Fig. 1 [3,7]. Then singular, linear elastic stress fields in the vicinity of the notch tip can be quantitatively described by means of the relevant NSIFs,.