Lack of side fusion
A failure of the weld metal to fuse properly with the side walls of the joint preparation. It forms a planar discontinuity that severely impairs performance under bending or tensile forces.
Lack of fusion at the root means the weld metal hasn't properly fused with the base material at the flank. That's a planar separation that seriously weakens the part under load – it can lead to cracks or, in the worst case, fracture. When welding, make sure you hit the flanks properly and that the arc or heat really pulls into the edges. Otherwise, you'll quickly get cold spots where nothing bonds. This happens especially with heavy cross-sections or when you're going at high speed. So: prepare the flanks cleanly and direct the heat exactly where it needs to go.