This paper introduces our experience of using miniaturized Charpy type SE(B) specimen in obtaining fracture toughness Jc of a material in the ductile to brittle transition temperature (DBTT) region. 0.55% carbon steel JIS S55C, whose tensile to yield stress ratio σTS/σYS was equal to 1.8 was chosen as a material to simulate a degraded (embrittled) material in the DBTT region.
Focus was placed on whether the modified Ritchie–Knott–Rice (RKR) failure criterion which predicts the onset of cleavage fracture when the crack opening stress measured at 4 times the crack-tip opening displacement σ22d exceeds this σ22c can be applied to miniaturized specimen that does not meet the ASTM E1921 requirement of M = (W-a)σYS/Jc ≥ 30. In concrete, the points 1) whether it could predict whether cleavage fracture will occur or not and 2) whether the σ22c in the case cleavage occurs is identical with that of full sized specimens, were focused. Charpy type SE(B) specimens of Width W × thickness B of 2 × 2 mm, 3 × 3 mm and 10 × 10 mm, whose M were predicted to be smaller than 30, were chosen as miniaturized specimens and 25 × 25 mm were chosen as full sized specimen. Results showed that the modified RKR criterion could appropriately predict cleavage fracture naturally for 25 × 25 mm specimens and 10 × 10 mm specimens, though M < 30 for this type specimen. The modified RKR criterion could also predict that cleavage fracture will not occur for 2 × 2 mm specimens. σ22c obtained from 25 × 25 and 10 × 10 mm specimens showed small difference, indicating that Jc obtained from 10 × 10 mm specimens can be used to transfer Jc obtained by 25 × 25 mm specimens.