A weld in which the top layer ends below the plane of the surrounding material in a butt joint, or beneath a plane of 45″ for a fillet weld. Although considered to be undesirable for many applications because of the reduced throat section and because of the increased possibility of hot cracking, properly fused concave fillet welds can be beneficial for resistance to fatigue in
service. The absence of mechanical discontinuities in these welds minimizes crack initiation due to concentrated residual stresses.