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# ASME reference curve vs the Fracture Toughness Master Curve

Both the ASME reference curve and the fracture toughness master curve represent an idealised fracture toughness transition curve for ferritic steels, ie fracture toughness as a function of reference temperature.

Historically the ASME reference curves (see TWI FAQ: How do I use the ASME reference curve to estimate the fracture toughness of ferritic steels?) have been treated as representing absolute deterministic lower bound curves of fracture toughness. In reality, they represent deterministic lower bound curves to a specific set of data, which represent a certain probability range. The more recently developed statistical lower bound estimation method known as the 'Fracture Toughness Master curve' (See TWI FAQ: What is the fracture toughness master curve?), has been proposed as a candidate for a new lower bound reference curve concept.

In the ASME reference curve, the reference temperature RT NDT is the reference transition temperature derived from the drop-weight tear test (see TWI FAQ: What is the drop-weight test (or 'Pellini' test)?) or Charpy test [ASTM E208] . In the fracture toughness master curve, the reference temperature T0 is defined as the transition temperature for a median fracture toughness (50th percentile) of 100MPa √m in a 25mm thick specimen, which is derived from fracture toughness tests (ASTM E1921).

In order to be able to substitute the old ASME reference curves with lower bound curves based on the master curve concept, Kim Wallin (1999) reviewed the confidence level of the ASME reference curves by re-evaluating the ASME reference curve with the statistical method proposed in the fracture toughness master curve. The analysis reveals that the 5% lower bound fracture toughness master curve has the same inherent degree of safety as originally intended for the KIC -reference curve. Similarly, the 1% lower fracture toughness bound master curve corresponds to the KIR -reference curve.

Based on a large-scale fracture toughness testing programme for Japanese pressure vessel steels, the database was analysed using the fracture toughness master curve method and compared with the original database behind the ASME reference curve. The results confirm that the reference temperature T 0 used in the fracture toughness master curve is the superior indexing parameter for fracture toughness of ferritic steels compared to the RTNDT approach used in the ASME reference curve [K Toon 2003] .

## References

W He: 'TWI Frequently asked questions: What is the ASME reference curve?'

W He: 'TWI Frequently asked questions: What is the fracture toughness master curve?'

C Wiesner: 'TWI Frequently asked questions: What is the drop-weight test (or 'Pellini' test)?

ASTM E208: 'Standard Test Method for Conducting Drop-Weight Test to Determin Nil-Ductility Transition Temperature of Ferritic Steels'. Philadelphia, PA, 2006.

ASTM E 1921: 'Standard Test Method for Determination of Reference Temperature, T 0 , for Ferritic Steels in the Transition Range.' Philadelphia, PA, 2006.

K Wallin: 'Statistical re-evaluation of the ASME K IC and K IR fracture toughness reference curves', Nuclear Engineering and Design, Vol 193, 3, Oct 1999, 317-326.

K K Toon: 'Fracture Toughness Data Analysis Using the Master Curve Method'. In WRC Bulletin 486 Part 3, Nov 2003.

Henryk Pisarski: ' Developments to the Master Curve for application to welds', TWI members' report 831/2005