In the standard test method for the determination of the reference temperature T0 in the transition range, ASTM E 1921-03 [1], the remark is given that different specimen types could lead to discrepancies in the calculated T0 values. Especially C(T) and SEN(B) specimens indicate by experimental evidence that a 10 °C to 15 °C difference in T0 has been observed. In the course of the European research project VOCALIST [2] a ferritic RPV steel has been investigated by conducting numerous fracture toughness experiments as well as intensive numerical studies. A local approach model based on the Weibull stress has been developed and calibrated for this material [3]. For the calculation of the constraint effect between SEN(B) and C(T) specimens with a crack to ligament ratio of approx. 0.5 the model has been applied to predict the constraint effects on fracture toughness and the resulting theoretical difference in the reference temperature T0. For this purpose the according specimens have been calculated by several finite element models and a reference solution in the small scale yielding space allowed for the calculation of the “constraint free” reference transition temperature T0. By means of theoretical constraint functions derived from the Weibull stress model, the difference for each specimen compared to the reference solution could be calculated. From the results a theoretical difference of ΔT0 = 10°C between SEN(B) (lower value) and C(T) specimens (higher value) caused by the different crack tip constraint has been obtained. This value confirms the experimental observations.

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