Fitting small data sets in the lower ductile-to-brittle transition region and lower shelf of ferritic steels

The use of a 3-parameter Master Curve approach where the Weibull modulus is 4 and a minimum toughness value of 20 MPa m1/2 is now well accepted and standardised. The standard allows the use of a minimum of six specimens, tested at a single temperature, to produce the Master Curve. However, the examination of a small data set can lead to difficulties when trying to fit to this standard formula especially if the toughness is larger than 20 MPa m1/2. A small data set obtained from lower shelf testing of a ferritic pressure vessel steel is used to explore some of the issues related to the Weibull modulus and minimum toughness and how a user might try to fit such data. Finite element models are used to show that enhanced minimum toughness values larger than 20 MPa m1/2 are unlikely to have arisen from warm pre-stressing. The implications for the Master Curve of the different fitting options are also briefly explored.