Master curve analysis of ductile to brittle transition region fracture toughness round robin data: The "EURO" fracture toughness curve

Brittle fracture in the ductile to brittle transition regime is connected with specimen size effects and - more importantly- tremendous scatter of fracture toughness, which the technical community is currently becoming increasingly aware of. The size effects have the consequence that fracture toughness data obtained from small laboratory specimens do not directly describe the fracture behaviour of real flawed structures. Intensive research has been conducted in the last decade in order to overcome these problems. Different approaches have been developed and proposed, one of the most promising being the master curve method, developed at VTT Manufacturing Technology. For validation purposes, a large nuclear grade pressure vessel forging 22NiMoCr37 (A508 Cl.2) has been extensively characterised with fracture toughness testing. The tests have been performed on standard geometry CT-specimens having thickness 12.5 mm, 25 mm, 50 mm and 100 mm. The a/W-ratio is close to 0.6 for all specimens. One set of specimens had 20% side-grooves. The obtained data consists of a total of 757 results fulfilling the ESIS-P2 test method validity requirements with respect to pre-fatigue crack shape and the ASTM E-1921 pre-fatigue load. The master curve statistical analysis method is meticuously applied on the data, in order to verify the validity of the method. Based on the analysis it can be concluded that the validity of all the assumptions in the master curve method is confirmed for this material.