Fracture toughness measurements in the transition region using sub-size precracked charpy and cylindrical bar specimens

The objective of the study presented here is the validation of direct fracture toughness measurements using two small-size geometries, which can be obtained from machining broken Charpy-V specimens: the miniaturized precracked Charpy-V specimen (MPCCv); the miniaturized Cracked Round Bar (MCRB). Fracture toughness results have been obtained in the ductile-to-brittle transition regime for two well-characterized reactor pressure vessel steels, and compared using the Master Curve analysis to reference data measured using bigger samples of the more conventional C(T) geometry. To cope with specimen size limits established by test standards to avoid loss of constraint, two strategies were chosen: testing at very low temperatures, thus approaching lower shelf conditions, correcting fracture toughness for loss of constraint; loss of constraint was assessed by in depth finite element calculation. Sub-size results have been found in agreement with reference data, although in case of tough materials tests have to be performed at very low temperatures. From this point of view, the MCRB specimen offers the advantage that an analytical correction for the loss of constraint is already available.