A methodology for the reliability-based fracture assessment of structural components containing cracks
Fracture failure in piping systems is a common problem in the process industries, and the thesis describes the research that has been undertaken by the author in modelling and analysing a particular example of such a system, namely an expansion loop subjected to pressure and temperature loading. The work has been used to investigate the difficulties that arise in linking non-linear fracture mechanics analysis with reliability computations. The thesis includes a critical review of both linear and non-linear fracture mechanics and a separate review of advanced structural reliability methods. An improved response surface method has been proposed in this research to conduct a reliability assessment where the structural behaviour is modelled and the J-integral is computed using finite element software. Of particular interest has been the development of a methodology that is able to handle the non-linearities inherent in the material stress strain curve. Although it was possible to develop a response surface to achieve a good fit in the region of FROM/SORM design point, it led to instability in the FORM/SORM implementation. The research then focussed on the use of directional simulation as a solution technique for this class of problems. The material used in the construction of the expansion loop was AISI-321 stainless steel and to model this correctly in the reliability-based assessment the full range stress-strain behaviour was required in FE analysis. The material was tested in the laboratory and a new piecewise continuous stress-strain model has been developed which involves a linear-elastic part and two subsequent non-linear parts both of the Ramsberg-Osgood type. For the purposes of reliability assessment a new stochastic stress-strain model has been developed taking into account uncertainties in yield strength and in operating temperatures, together with the associated yield strength – temperature dependency. An integral part of the research has been a study of the J-integral profile along the crack front using 3-D FE analysis, for a number of CT (Compact Tension), beam and CCT (Central Crack Tension) specimens, with a range of thickness. This has brought new findings which are of importance in both deterministic and probabilistic assessment.