A large base of KIC, KId and JIC (R-curve) fracture toughness data has been used to develop reference toughness curves. The most successful results were obtained when a sigmoidal function was fitted to data from which the heat-heat variation in both the temperature and fracture toughness had been reduced by referencing. Several referencing procedures have been studied, but the only one found to be ‘successful in this work was based upon the precracked instrumented Charpy V-notch test. The tanh function  
(K = toughness, T = temperature, and A, B, T0 and C are coefficients which give the best fit between curve and data) fitted to precracked instrumented Charpy V-notch test data provided suitable referencing quantities. Using the coefficients A and B to reference fracture toughness, and T0 and C to reference temperature, lower bound reference curves were developed. Weighted, nonlinear regression procedures were used to define lower bound reference toughness curves for each of three stress intensification rates. The lower bound was the statistical global tolerance bound to the referenced data. The reference curves can be readily used to define a lower bound relationship between fracture toughness and temperature for nuclear pressure vessel steel on the basis of a set of precracked instrumented Charpy V-notch tests.
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