The Master Curve concept, developed originally for quasistatic loading condition, has proven to be useful for dynamic KJd tests conducted at a high loading rate. Assuming that only the value of T0 is rising (along with the material yield strength) with increasing loading rate, dK/dt, the dynamic fracture toughness, Kjd and associated T0 should, in principle, be esti­mated from the value of T0 measured by static tests. This dependence was empirically evaluated in 1997 by Wallin using a large dataset consisting of dynamic and static fracture toughness data measured for vari­ous structural steels (yield strength ranging from about 200 to nearly 1000 MPa). The Master Curve method was applied to both the static and dynamic loading rates.44 Based on these results, as well as an IAEA round-robin exercise (report to be published in the IAEA Report Series within the framework of the Technical Working Group on Life Management of Nuclear Power Plants), the Master Curve approach
appears to be fully applicable to dynamic fracture toughness measurements conducted in the ductile — to-brittle transition region.

4.14.5.2 T0 Versus Charpy V-notch Transition Temperatures

In the case of dynamic loading with notched speci­mens, the correspondence with the fracture tough­ness test and T0 is complicated due to several uncertainties associated with the Charpy V-notch impact test. First, the loading situation is very differ­ent in the dynamic loading of a notched specimen compared to the quasistatic loading of a fatigue pre­cracked specimen. Due to the differences in the load­ing conditions, the measured Charpy energy includes a significant proportion of both crack initiation and propagation, and often some energy associated with crack arrest; whereas, the quasistatic SE(B) test in the transition region characterizes mainly crack initiation conditions. Additionally, the inherent data scatter and curve fitting required to obtain Charpy V-notch parameters increase the uncertainty of estimating and correlating the transition temperatures.

Correlations between the Charpy V-notch tem­peratures T28j and T41j versus T0 are presented in Sattari-Far and Wallin.11 The correlations, which are based on data from over 200 pressure vessel steels, are currently being reassessed in more detail, but in applications where the direct estimation of T0 is not possible, the correlations can be used as indicated below (s is standard deviation):

T0 = T28J — 19 °C (s = 22 °C) [42]

T0 = T41J — 26 °C (s = 25 °C) [43]

4.14.2 Summary and Conclusions

The Master Curve methodology has been described as an advanced, direct technique of determining the fracture toughness of ferritic structural steels. The application of the methodology has increased during the last decade and spread worldwide extending beyond the initial applications associated with NPP surveillance and integrity assessment programs. Today, the methodology is well known and increas­ingly accepted by safety authorities as a standar­dized method for application in safety assessments. Methods based on conventional approaches, such as the Charpy V-notch test, are still widely used, and probably will be used in parallel into the foreseeable future. Once a sufficient amount of reference data have been measured using the Master Curve method, it will gain even further acceptance. Also, further understanding of the limits of applicability for differ­ent steels will be obtained. Over this transfer period, the correlations developed between the different methods should play a significant role, providing support for properly analyzing data and encouraging the use of the more advanced methods. Note that some correlations, like those proposed for estimating crack arrest toughness from Charpy V-notch tests have brought new applications for the instrumented Charpy test. The overall trend in fracture mechanics testing is toward characterization and methods which allow the use of small and/or moderate-size speci­mens simulating the true loading conditions and accounting for the expected micromechanisms of fracture. The Master Curve approach and its implementation in the ASTM E 1921 methodology have proven to be a valuable and powerful analysis tool for a wide variety of applications involving ferritic steels.