i think i have it figured out. i didnt want to try to explain it, so i drew a couple of pics. green's the welded on plate, red's the HAZ of the frame, tans the frame. the blue line could represent the area of any type of stress (i.e.- bending, tension, compression, shear) with the fishmouth you have re-enforcment from the plate welded on and non-HAZ metal in the frame. with the straight weld you can see the incidence of stress is completely in the HAZ. thanks for getting me thinking guys. if anybody knows the reason and this isnt it, let me know.
dang it, now i gotta go in there and replace that section of my frame rail.