R curve obtained by a round bar with a circumferential crack and its effectiveness for a fracture toughness test (For a material with medium fracture toughness)

Abstract

A non-standard circumferentially cracked round bar (CRB) specimen has been examined for replacement of the standard fracture toughness CT specimen since 1981. In the case of a material with low fracture toughness, the CRB specimen can be substituted for the CT specimen. However, when it is applied to those with medium or high fracture toughness, there are many problems: the specimen size required, the strain constraint equivalent to plane strain, the measurement of crack length and the procedure to obtain the J-R curve. In this investigation, fracture toughness testswere conducted on a medium fracture toughness material using the CRB specimens. The normalization method was tested for the first time on CRB specimens and was considered adequate to measure its J–R curves. The CRB specimens with about (the ratio of ligament to radius) b/R0=0.3 showed the better strain constraint and almost the same fracture surface as the CT specimen within Δa=0.2mm: blunting, initial crack growth and the grown crack blunting. On the other hand, after Δa=0.2mm, the latter half of the stable crack growth region showed a difference in fracture surfaces between two specimens: the void growth was dominant, or the shear dimple was dominant. Therefore, for fracture toughness indexing of CRB specimens, J0.1 is preferable to J0.2 which is typically used for the standard CT specimens.

Keywords

• fracture toughness
• j-r curve
• j-integral
• circumferentially cracked round bar (crb)
• fracture surface
• strain constraint