TWI Industrial Member Report Summary 1021/2012
By D Zhou and M Warwick
Current methods for assessing the integrity of structures against fracture are mostly deterministic. Finite element analyses or analytical formulae are used to identify a single value of crack driving force for a particular flaw. This is then used to assess the structure based on standard procedures such as those described in BS 7910 (BSI, 2005) or R6 (EDF Energy Nuclear Generation Ltd, 2001-2011). Due to inherent variability in loads, material properties and geometric parameters, a probabilistic methodology is needed to evaluate the statistical distributions of fracture response and reliability of welded structures. Although BS 7910 and R6 propose two different procedures (a partial safety factor method in BS 7910 and a simplified probabilistic method in R6) to assess failure probabilities, these methods are subject to some limitations. For example, it is recognised that the procedures are mainly established for elastic global response, although their probabilistic fracture mechanics procedures can also be used to assess the probability of plastic collapse.
A full probabilistic fracture mechanics analysis using the finite element method is necessary for more accurate prediction of the probability of failure of flawed structures. Most probabilistic models focus on fracture of structures under elastic or small plastic deformation (Pisarski, 1997; Rahman, 1995). Probabilistic analysis for large plastic deformation and strain based design for pipelines has received only very limited attention to date (Sandvik et al, 2007).
The present work was carried out to develop a framework for investigation, through numerical modelling, of the probability of failure of structures containing flaws subjected to large plastic deformation. Deterministic analysis of the integrity of pipes containing flaws is well-established. A probabilistic analysis of such flaws will give confidence in the assessment result and also help to identify the most influential parameters.
- Develop a general procedure for probabilistic failure analysis with input from finite element analysis.
- Apply the procedure to probabilistic fracture analysis of pipes containing flaws under large deformation.