With LIMIT 2021 we introduced a method called ‘Section’ for the assessment of welded structures modelled with volume meshes. The primary application of this method is the strength assessment of fillet welds and partial penetration welds with increased probability of failure starting at the weld root.

In these cases, the eccentricity of the weld and the supporting effect of the surrounding material plays an important role and has a significant impact on the fatigue life. Using the modelling technique shown in the image below, the stiffness of the welded joint is captured with sufficient accuracy, and the loading of the weld throat can be analysed using the section method of LIMIT.

According to most design codes, the fatigue assessment of welds is based on three stress components relative to the weld orientation:

• Loading longitudinal/parallel to weld orientation
• Loading perpendicular/lateral to weld
• Shear loading

Procedure in LIMIT

For fillet welds and partial penetration welds the stresses due to loading in perpendicular direction are in most cases the critical ones with respect to fatigue. The figure below illustrates the linearized stress distribution, also denoted as structural stress in the weld throat. The perpendicular loading of the weld throat can be defined in terms of two section forces n and q and the bending moment m. The forces and the moment shown in the figure are usually calculated for the interface of the weld and the horizontal continuous sheet.

In the direct approach of LIMIT the section forces n and q and the bending moment m are calculated directly in the weld throat of the global model using the section method of LIMIT. These values are then used to scale and superpose results from pre-calculated 2D notch stress models.

LIMIT will then search for the maximum stress range within the rounded toe or root point, see below. Depending on the radius different FAT-classes are used for the assessment, which is repeated along the weld for each section element.

Application

The procedure is explained in detail in LIMIT documentation. The library of pre-calculated 2D effective notch stress models are included in the standard package. The application is simple. Once a specific weld type ich chosen, LIMIT will plot the outline of the 2D slice in the FE model. This way the correct positioning and orientation is guaranteed.

The final result is then visualized on the section elements. This way the same accuracy of a 3D effective notch stress approch can be achieved with much less modelling effort and computation time.