Overview of the European FITNET Fitness-for-Service Procedure

Abstract

The European FITNET consortium was convened in 2002 with the remit of preparing a procedure for fitness-for-service (FFS) evaluation of flawed engineering components. The procedure is intended to be used by a broad range of industries across Europe, and can be used at any stage in the life of an engineering component, eg design, fabrication, operation, failure analysis or life extension. This paper presents an overview of the structure of the procedure. There are four main modules, each covering a particular failure or damage mechanism: fracture, fatigue, creep and corrosion (including environmentally-assisted cracking). These are linked by the use of a common terminology and a single set of reference compendia (annexes), eg for stress intensity, plastic collapse and residual stress, so that a particular flaw can be rapidly analysed for more than one failure mechanism. The FITNET fracture assessment procedures in particular represent a significant advance compared with current published FFS procedures such as API 579-1/ASME FFS-1 and BS 7910. There is a hierarchy of different approaches, designated Options 0 to 5, the choice between them depending on the quality of information (in particular, materials property data) available to the user. This could range from Charpy and tensile data only (Option 0) through to the constraint-dependence of fracture toughness (Option 5). Other Options allow crack driving force to be calculated directly from FEA (Option 4), or permit weld metal strength mismatch to be taken into account (Option 2). The fatigue analysis module likewise contains several alternative approaches, termed Routes. Some (Routes 1–3) are based on the concept of a nominally flaw-free structure, whilst Route 4 is based on cycle-by-cycle integration of the Paris law, and Route 5 addresses non-planar flaws. FITNET also set itself the goal of providing training in FFS techniques, both through a series of seminars held during the project (2002–2006) and through provision of lasting training material (slides, tutorials, case studies and a validation document), which are now publicly available.