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# How do I use the Fracture Toughness Master Curve to estimate the fracture toughness of ferritic steels?

The fracture toughness master curve is a statistical model based on the micro-mechanisms of cleavage fracture of ferritic steels (See TWI FAQ: What is the fracture toughness master curve?). The basic premise behind the approach is illustrated schematically in Figure 1, showing the fracture toughness distribution for various cumulative probabilities (F) that failure of the specimen will occur at or before K JC.

Fig.1. ( [T L Anderson 2005] ): Fracture toughness master curve.
Taken from E 1921-03, 'Standard Test Method for Determination of Reference Temperature, T 0 , for Ferritic Steels in the Transition Range.' American Society for Testing and Materials, Philadelphia, PA, 2003.

The fracture toughness master curve allows the fracture toughness corresponding to a particular temperature or section thickness to be estimated from data generated at a different temperature and/or section thickness, by reference to an indexing temperature, T 0 , In addition, it allows calculation of the likely variation in fracture toughness for a particular thickness and temperature ' and 'What is the difference between the ASME reference curve and the fracture toughness transition curve'). This approach is described in ASME Code cases N-629 and N-631 for the applications of nuclear pressure vessels (note that the form of the transition curve for temperatures above RT T0 follows the ASME rather than the Master Curve equations, although they are broadly similar).

Through an empirical correlation between T 0 and Charpy impact test values T 27J (or T 40J ), combined with the Master Curve model, it is possible to estimate the fracture toughness of ferritic steels in the lower transition region, based on Charpy energy. This is the basis of one of the fracture toughness estimation routes given in BS7910.

Similar as above item 2, API 579-1/ASME FFS-1 gives the correlation between T 0 and Charpy impact test values T 27J inthe combination of the facture toughness Master Curve. However it is noted that this approach might produce non-conservative estimates of the fracture toughness in cases concern with weld metals and HAZs, components subjected to mis-match constraint at welds or cold worked material and high strength steels.