Flaw

A flaw in a metal is an undesirable alteration of the metal structure. Flaws can be internal, i.e., inside the metal object or located on an external surface. Internal flaws are not visually observable. These flaws may be porosity (Holes), inclusions (extra included non-metals/oxides), laminations (external surfaces into the interior of the metal), etc. The most troublesome flaw is a crack. Cracks usually (but not always) initiate at a free external surface and grow or propagate into the metal structure. However, cracks can be of insufficient size to create a problem and as such are classified as flaws (not defects).