FITNESS FOR SERVICE is a term used by Wells [1] in the early 1960s to describe the actual safety or reliability of a structure at a given time. It takes account of the actual stresses, flaw sizes, material toughness, service conditions, etc. in a realistic manner so that the “fitness” for continued service of an existing structure can be established on a rational basis. It has also been referred to as common sense engineering [2] and serves as the basis for a decision as to the “life extension” of an existing structure.