Locations of cleavage initiation sites in miniaturized C(T) specimens and their relation to mechanical field quantities

• Cleavage fracture initiation sites of MCT and standard-sized CT specimens are located around mid-thickness. • Significant loss of constraint near side surfaces of MCT specimens is observed as fracture load is approached. • Initiation site locations are aligned with region of high constraint where saturation of triaxiality is observed. • Fracture behavior of MCT specimens is comparable to standard-sized specimens. The miniaturized C(T) specimen is a promising geometry to be used in future surveillance programs for the long-term operation of second-generation nuclear reactors. In multiple studies, the geometry was shown to provide Master Curve reference temperature values comparable to standard-sized C(T) specimens. International testing standards such as ASTM E1921 additionally require cleavage fracture to occur under high constraint conditions. Due to its reduced thickness, constraint loss is observed in the miniaturized C(T) specimen at high crack tip loads. In order to validate the miniaturized C(T) geometry, it must be ensured that cleavage initiation still occurs under similar conditions as with standard-sized C(T) specimens. In this study, the locations of cleavage initiation sites in miniaturized C(T) specimens of SA-508 Cl.3 reactor pressure vessel steel are determined by means of scanning electron microscopy. Applying finite-element-analysis, the mechanical fields in front of the crack tip, namely the maximum principal stress, stress triaxiality and equivalent plastic strain, are obtained. By relating the locations of the initiation sites to the mechanical field quantities, the conditions for cleavage fracture are deduced. Particular attention is paid to the stress triaxiality, which is used as a measure of crack tip constraint. The mechanical fields of miniaturized specimens are compared to standard-sized geometries to evaluate similarities and differences in the fracture behavior based on the load level. It is shown that cleavage fracture initiation in miniaturized specimens remains located within a region of high crack tip constraint, even when significant macroscopic plastic deformation occurs prior to failure. Consequently, the conditions for cleavage initiation are found to be similar in miniaturized and standard-sized C(T) geometries.