Published data, as well as data available in TWI on Charpy-fracture toughness correlations and specimen testing orientation of carbon manganese and low alloy steels was collected. Charpy-fracture toughness correlations for the steels cited above were analysed.
Correlations between Charpy energy and fracture toughness have been reviewed. Various upper-shelf Charpy-fracture toughness correlations were investigated during the project. However as it can be seen, table 1, the impact test values to which the older equations for the upper shelf apply tend to be lower than is generally assumed with modern steels. This is because major improvements in toughness have been made in steel manufacture, especially with regard to cleanliness, over the last half-century.
The correlations of Barsom et al (5), Ault et al (4) and Priest (6) have restricted range of applicability with respect to impact energy and yield strength and were not applicable to EDF steels.
The correlations are plotted in figure 1 using the steel yield strength at 330°C, where applicable. This comparison shows that for Charpy energies above approximately 30J, the Roberts and Newton (7) correlation as quoted in API RP 579-1 / ASME FFS-1 (1) provides the most conservative correlation. Table 1 shows all the upper shelf correlations with their limitations.
Data for Charpy impact and fracture toughness tests for different steels were then collected from the literature and TWI reports. The data represent the upper shelf Charpy energy and upper shelf fracture toughness.
The data were compared with fracture toughness estimates using the BS 7910 (2) and Wallin’s (8) correlations (both included in the 2013 edition of BS 7910) and the results are shown in Figure 2. The “valid” points indicated in the figure refer to Kmat values meeting the small scale yield criterion given in ASTM E1921 so the results should be relatively size independent.
Both upper shelf equations J.5 and J.6 given in the BS 7910:2013 (3) are shown to be conservative with respect to the experimental data in predicting upper shelf fracture toughness. These correlations were considered to be applicable to EDF carbon manganese and low alloy steels.
In the Charpy upper shelf region, a conservative relationship such as those proposed by Roberts-Newton or BS 7910:2005 Equation J.5 is appropriate for old, high sulphur steels. However for modern, low carbon, low sulphur steels the new BS 7910:2013 Equation J.6 (3, 8) is recommended.