An overview of using the minimum of three equivalent (MOTE) in the assessment of fracture toughness behaviour in the context of BS 7910

Abstract

In this work, fracture toughness values used in engineering critical assessment (ECA) based on the minimum of three equivalent (MOTE) concept within the British Standard (BS) 7910 framework has been examined. The MOTE concept, statistical analyses, and the implications of selecting fracture toughness values using MOTE on ECA outcome are discussed. BS 7910 specifies that the lowest test result should be used when 3 to 5 results are available, the second lowest for 6 to 10 and third lowest for 11 to 15. However, this approach assumes that the data is homogeneous, and the MOTE concept is only valid for minimum of three results when scatter is limited.

Four fracture toughness distributions were defined in this study, with characteristic values estimated using a $\ln$-normal distribution and Monte Carlo simulations. The findings suggest that the $\ln$-normal approach offers more reliable fracture toughness estimates, and ten test results are generally adequate. However, for high-integrity applications, it is recommended to use at least twenty-five test results to estimate the characteristic fracture toughness based on a 5th percentile with 75 % confidence.