Abstract
A non-standard circumferentially cracked round bar (CRB) specimen has been examined as a replacement for the standard fracture toughness compact tension (CT) specimen since 1981. The thickness of CT specimens corresponds to the circumferential direction without a free surface in CRB specimens. This allows the strain constraint around a crack tip in a CT specimen to be replicated by a small CRB specimen. Fracture toughness testing using small specimens is especially important for materials intended for fusion reactors. Several studies have been conducted on fracture toughness tests using CRB specimens for materials with low fracture toughness, and it has been shown that CRB specimens can substitute for CT specimens. However, data on materials with medium and high fracture toughness are scarce, preventing detailed validation. There are several issues to address, including the required specimen size, achieving strain constraint equivalent to plane strain, measuring crack length, and obtaining the J-R curve. In the author’s previous study, it was found that, if an adequately sized CRB specimen is used, materials with medium fracture toughness (100 - 200 kJ/m²) can also be evaluated. However, there are specific considerations in the experimental procedure, and applying the procedure to high fracture toughness materials is expected to be more challenging. In particular, necking of the round bar surface in materials with high toughness alters the stress concentration field around the circumferential crack tip. When a CRB specimen with an outer diameter of 10 mm is used, evaluating materials with fracture toughness greater than 200 kJ/m² becomes difficult. This report discusses the possibilities and limitations of fracture toughness testing for materials with medium and high toughness using CRB specimens.
