Lack of Penetration (LOP)

“Lack of penetration” (LOP) is a welding flaw or defect which occurs when the surface of the metal to be joined has not been melted. Melting of the metal surface permits the weld bead filler metal (that metal added by the welding rod/wire) to melt and mix with the surface of the metal to be joined. A LOP flaw/defect will appear, many times as an almost straight line, where the melted weld bead merely lays on the “cold” metal surface without good adhesion or joining of the two metal surfaces (see LOF).