Analysis of specimen size conversion in the ductile to brittle transition region of ferritic steels

The ASTM E 1921-15 standard covers the determination of the Master Curve, a function used to characterize the fracture toughness of ferritic steels that experience onset of cleavage fracture in the transition region. In the standard, the specimen size effect on fracture toughness at cleavage (KJc) in the transition range is explained by the weakest-link theory, using a three-parameter Weibull distribution with shape (b) and threshold (Kmin) parameters fixed. When specimens from different sizes are tested, a KJc conversion is necessary. The effect of size conversion using either the equation given by the standard and a three parameter Weibull distribution with the parameters estimated from data sets are compared in this work. It was found that the distributions obtained from results converted to one-inch size did not adjust the data in the way the weakest link model would predict, meaning that the statistical effect due to the difference in the crack front volume would not be the only responsible for the difference in results for different specimen sizes. This was found using both the ASTM E1921 method and the three parameter Weibull distribution.