You may see, from time to time, on a drawing a double-fillet weld consisting of a large weld on the outside, called the 'structural' weld, and a smaller 'seal' weld on the inside. Apparently some engineer back in the mists of time assumed that nature would decide to route all of the load through the structural fillet weld and would leave the smaller seal weld to just fill in a crack while unmolested by the load. This assumption seems to have been based on wishful thinking rather than logic or mathematics. Apparently many other engineers then copied this approach without giving it a moments thought. Well in reality, as FEA will easily show you, nature has it's own notion of how to distribute stress, unencumbered by such cute ideas and naming conventions and it will decide to overload that seal weld and it will crack. Of course you might think this should have been obvious from the start - now that i've told you - but such myths die hard in engineering. Please do not make cute assumptions about fillet welds in the future. If in doubt then feel free to model it!