The Master Curve transition temperature T0 for quasistatic loading conditions is statistically precise and measurable following testing standards such as ASTM E1921. Extension of this same type of approach to dynamic loading (KId and Kjd) and crack arrest (KIa) situations seems logical, and empir­ical studies with a large variety of structural steels have confirmed a similar relationship. In both cases, the material property (KJd and KIa) can generally be

described with the same temperature dependence as the quasistatic initiation fracture toughness, but with an increased value of T0. The correlations between T0 and other related toughness parameters are discussed next.

4.14.5.1 Crack Arrest Reference Temperature TKia

For an integrity assessment of real structures, it is often necessary to have information not only on the initiation fracture toughness but also on the crack arrest toughness, KIa. A definition of the reference curve for crack arrest toughness is given in the ASME Code, Appendix A of Section XI. The para­meters describing crack initiation, including the Master Curve T0 and the related parameter RTT0, cannot be used to directly describe the crack arrest toughness.

Associated with the development of the Master Curve concept, studies have concluded that it is possible to develop correlations describing the rela­tionship between the crack initiation and arrest toughness.43 These studies have focused on clarify­ing which elements of the Master Curve approach should be modified for assessing crack arrest; and finding possible correlations between initiation and arrest parameters. Due to different mechanisms and differences in factors controlling fracture initiation and arrest events (e. g., the local properties are crucial for crack initiation, but not so critical for crack arrest), the weakest link theory applied in the Master Curve approach is not directly suitable for crack arrest.

The analyses of nine well-defined crack arrest datasets, consisting of various pressure vessel base and weld metals (including those used to construct the ASME reference curve), confirm that43:

• No statistical size adjustment should be made to crack arrest data.

• The scatter seems to be material independent, but lower than the scatter for crack initiation.

• KIa data follow the same Master Curve tempera­ture dependence as Kjc.

Crack arrest data can thus, in general, be described following the same Master Curve approach, using the reference temperature TKIa to characterize the tem­perature corresponding to the crack arrest toughness at a level of 100 MPa Vm, consistent with the crack initiation transition temperature T0.

To clarify if a reasonable correlation exists between T0 and TKla, a total of 54 datasets, for
which both crack initiation and arrest data were available, have been analyzed with the Master Curve concept.43 The result shows an exponentially decreasing trend for TKla — T0 as a function of T0, but the standard deviation of this correlation is high. Taking into account the observed scatter, a rough estimate for the maximum TKIa can be obtained from the value of T0 at a confidence level of 85% from:

TkIs = T0 + AT + 19 °C [40]

where AT is obtained from the quasistatic T0 as follows:

Tq + 273 sys

136.3 °C + 683.3MPa where sys is the material yield strength. Equation [41] is recommended only for steels where the nickel content is less than 1%.

The crack arrest toughness (TKla) can also be assessed from instrumented Charpy V-notch data using the correlation developed between TKIa and the temperature corresponding to the crack arrest force of 4 kN. The method has been used for assessing the crack arrest toughness of irradiated RPV steels from the existing Charpy V-notch data. This correla­tion and its application are described in Wallin.43