STRUCTURAL COLLAPSE, THE FAILURE ASSESSMENT DIAGRAM AND OFF-THE-SHELFOPERATION

The criteria for determining whether ferritic material exhibits fully ductile behaviour are generally based on the fracture toughness vs tem perature relationship determined from standard laboratory test pieces (e.g. Charpy V Impact tests or fracture toughness specimens). This rel ationship defines a ductile-brittle transition region. When fully duct ile, microvoid coalescence behaviour is experienced, and fracture toug hness is described as being on the ''upper shelf''. At ''off the shelf '' temperatures brittle, cleavage fracture is experienced. On the lowe r shelf the material is entirely brittle, exhibiting 100% crystallinit y on the fracture face. As the temperature increases, initiation of te aring by microvoid coalescence occurs and some stable tearing can occu r prior to the cleavage event. Material toughness increases with tempe rature until the upper shelf condition is achieved. The characteristic s of fracture toughness tests in terms of the toughness level exhibite d and the extent of ductile tearing experienced have been used as a gu ide to whether the structural application (e.g. a pressure vessel) wil l behave in a brittle or a ductile manner. This paper reports on a fea sibility study where various worked examples have been performed to ex amine the concept of using a ''cut off'' on the failure assessment dia gram, determined from the conditions required to cause plastic collaps e of a pressure vessel, as a criterion for defining effective ''ductil e'' operation. Fracture assessment procedures (R6 revisions 2 and 3 an d PD6493 levels 2 and 3) have been utilised to determine the influence on pressure vessel performance of the behaviour of fracture toughness test specimens. The procedure of plotting a structural collapse ''cut off'' on a failure assessment diagram enables the assessment of wheth er a particular flaw geometry would result in gross deformation of the structure at failure. The use of this procedure provides an unambiguo us demarcation between ''fracture dominated'' and ''collapse controlle d'' conditions. This procedure facilitates judgements on the level of toughness necessary to ensure ductile operation and whether a ''tearin g plus toughness'' requirement is necessary. It is recommended that co nsideration be given to including structural collapse into fracture as sessment procedures carried out using R6 revision 3 or PD 6493:1991 pr ocedures in order to determine the conditions when enhanced toughness no longer influences structural performance (i.e. when effective ''upp er shelf'' conditions are attained).