Treatment of reliability in fracture assessments using BS 7910 Annex K

Abstract This paper presents the background to the development of Annex K (‘Probabilistic assessment’) of BS 7910:2013, in particular the use of Partial Safety Factors (PSFs) in fracture assessment. These were originally derived using Probabilistic Fracture Mechanics (PFM) calculations, and in principle allow the user to estimate the probability of failure ( P f ) associated with a deterministic fracture assessment simply by dividing (or multiplying) the inputs by a series of PSFs given in look-up tables. Calculations carried out using modern PFM software have shown that the PSFs given in Annex K are appropriate only for a narrow range of fracture scenarios, reflecting the conditions used to derive the PSFs in the first place – namely, thick-section plates with low fracture toughness and large through-thickness flaws. The use of Annex K tends to over-estimate P f for other situations, and the use of direct PFM calculations is preferable where possible. The implications for the next revision of BS 7910 are discussed.