A systematic investigation of the effects of specimen size on the cleavage fracture toughness of a typical pressure vessel steel is reported. Size dependence arises both from: i) statistical effects, related to the volume of highly stressed material near the crack tip, that scales with the crack front length (B); and ii) constraint loss, primarily associated with the scale of plastic deformation compared to the uncracked ligament dimension (b). Previously, it has been difficult to quantify the individual contributions of statistical versus constraint loss to size effects. Thus, we developed a single variable database for a plate section from the Shoreham pressure vessel using a full matrix of bend specimens, with B from 8 to 254 mm and b from 3.2 to 25.4 mm, that were tested at a common set of conditions. The University of California Santa Barbara (UCSB) b-B database was analyzed using three-dimensional finite element simulations of the crack tip fields combined with a cleavage model calibrated to the local fracture properties of the Shoreham steel. This paper focuses on the possible significance of these results to the Master Curve Method Standard as formulated ASTM E 1921-97. The statistical scaling procedure used in E 1921-97 to treat variations in B was found to be reasonably consistent with the UCSB b-B database. However, constraint loss for bend specimens begins at a deformation level that is much lower than the censoring limit specified in E 1921-97. Unrecognized constraint loss leads to a non-conservative, negative bias in the evaluation of To, estimated to be typically on the order of a −10°C for pre-cracked Charpy specimens.

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