THEORETICAL UNDERPINNING OF THE CONSTRAINT EFFECT BETWEEN DEEPLY CRACKED C(T)-AND SEN(B) SPECIMENS

摘要

In the standard test method for the determination of the reference temperature T_0 in the transition range, ASTM E 1921-03, the remark is given that different specimen types could lead to discrepancies in the calculated T_0 values. Especially C(T) and SEN(B) specimens indicate by experimental evidence that a 10°C to 15°C difference in T_0 has been observed. In the course of the European research project VOCALIST 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. 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 T_0. 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 T_0. 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 ΔT_0 = 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.