Originally Posted by sorebutt
the major difference between the two is the size of the area where the 2 pieces of the joint.
In the case of a lug, the tube and the lug have a large area where they overlap, so stresses on the joint are spread through this area.
In the case of a weld, the the width of the joint, and the small overlap of the 2 tubes (basically 2 thin edges) make it more susceptible to stress fractures. Also, the act of welding itself with its high heat melts part of the tubes and changes the molecular structure of the steel (some time can be fixed with heat treatment).
Without raw data it is hard to argue if the increase in metal overlap of a lug offsets the strenght advantage of a weld coupled with increased metal at the welded joint due to butting, for bending type cyclic load. Only experiments can tell the truth.
Secondly, it stands to reason that when *pulling* forces (as opposed to bending) are applied a weld could very well be stronger than a lug. In case of pulling the only thing holding a lug is the joint between steel and bronze which is getting sheared. In case of a weld the forces are tensile (not shearing) and hence the joint is much stronger.