Effect of Loading Rate and Constraint on Dynamic Ductile Fracture Toughness of P91 Steel

摘要

The dynamic ductile fracture toughness (J_(0.2d) and J_d-R curve) of P91 steel is an important parameter for design against failure of fast reactor sub-assembly wrappers under accidental loading conditions. Instrumented impact testing of pre-cracked Charpy specimens is well suited for this purpose due to their small size and resultant material economy for service-exposed materials. While the effect of impact loading rates on plastic deformation properties is well known, their effect on dynamic ductile fracture toughness is relatively less studied. Also, change in in-plane constraint is known to affect ductile fracture toughness under quasi-static loading conditions. Hence, a study was undertaken to evaluate the effect of loading rate and in-plane constraint on the dynamic J-R curves. The main challenge in this is determination of current crack length (a), which is required to evaluate J-integral. Though a few methodologies have been proposed in the literature for this purpose, there is no standard method as yet. The lowest impact loading rate has been chosen so as to ensure 'low-blow' conditions where the specimen is not fully broken. This enabled the usage of 'normalization' method of ASTM E 1820 standard to determine 'a' for this loading rate. Further for the higher loading rate full-blow tests, the data from low-blow test specimens have been used as reference in the 'Compliance Ratio' key curve method to determine 'a.' The dynamic J-R curves have been obtained from the ASTM E 1820 formulation of J-integral using as input the determined 'a' and load-displacement data. Dynamic J-R curves are found to be independent of loading rate in the range between 2.15 to 5 m/s and 1.5 to 5 m/s for short cracks (a/W ≈ 0.3) and long cracks (a/W ≈ 0.5), respectively. Short cracks (a/W ≈ 0.3) display higher J-R curves than those for long cracks (a/W ≈ 0.5) beyond a crack extension of 0.3 mm. The validity of the dynamic J-R curves obtained has also been explained.