Fillet Welded Joints

A typical fillet welded joint is illustrated. It connects two components, one of which is conveniently regarded as the loaded member - as all loads on it are known - the other is the support or reaction member. Clearly the loads are transmitted through the joint before being absorbed in the support.

A run may be three -dimensional however the majority of practical runs are two -dimensional and lie in a weld plane like the cantilever's joint here. We consider only such two -dimensional runs, the centroids of which must also lie in the weld plane. It is convenient to erect a Cartesian system at the centroid G, and to designate the x-y plane as the weld plane as shown at ( a) below.



In general the resultant load on the joint is a force F = [ Fx Fy Fz ]' through the centroid of the linear run, together with a moment M = [ Mx My Mz ]' whose components are given by the RH Rule, ( b). For the cantilever above, this resultant would be found by moving the sole force to act through the centroid, and introducing the moment corresponding to the force multiplied by the distance transverse to the force's line of action between the point of load application and the centroid.

This load is equilibrated by a force distributed along the length L of the run as indicated in ( c). By virtue of the stresses in the weld, each element of run δL contributes an elemental force q.δL towards equilibrium. q is a force intensity ; it is a vector force -per -unit -length and except in simple cases varies in magnitude and direction around the run.

Conceptually, force intensity is not too different from stress, which is a force -per -unit -area, that is δF = q δL = σ δA. Force intensity is also similar to the bending moment in a beam: both are stress resultants - of stresses in the weld throat and in the beam's cross-section respectively - and both vary in general along a linear path - the weld run and the beam axis.

For the majority of beams the bending moment is easily found in terms of the loads using statics. In the case of a fillet weld however, correlating the intensity q with the load F, M is less straightforward since the arrangement is statically indeterminate.

Two techniques for this correlation (having the same theoretical foundation) are presented below. The first traditional approach is based on recasting the building block stress equations for bending etc. in terms of force intensity rather than of stress. This approach though simple has limitations which in some situations requires the more general second technique, the unified approach.

Source: http://www.mech.uwa.edu.au/DANotes/welds/fillets/fillets.html