Application of the competing risks analysis to fracture toughness of silicon‐killed C‐Mn plate steels

Abstract

ABSTRACTThe available Mode I fracture toughness data measured in the brittle‐to‐ductile transition temperature region on C‐Mn silicon‐killed plate steels used in the construction of Magnox reactor pressure vessels have been examined with the view to obtain statistically based estimates of fracture toughness parameters for use in structural integrity assessments. Although fracture toughness data have been measured for several pressure vessel materials, in all cases, except one, the number of test results per station is small.A statistical analysis based on the maximum likelihood method has been performed on fracture toughness data for all plate steels to investigate the effect of test temperature and specimen thickness on pre‐cleavage ductile crack growth and the effect of all three on cleavage fracture toughness. For the purpose of the analyses, Weibull distributions have been assumed for pre‐cleavage ductile crack growth and cleavage fracture toughness with a threshold of zero. The equations derived for pre‐cleavage ductile crack growth and cleavage fracture toughness have been used to estimate their joint distribution.The methodology accommodates the unfailed (censored) data by use of survival statistics. The probabilities of cleavage fracture are evaluated by use of the method of competing risks. Equations for cleavage fracture toughness given for the plate steels are of the same form as already available for C‐Mn submerged arc weld metal.