International cooperative interlaboratory test programs on elastic-plastic fracture toughness in the transition region have been conducted under the 129th Committee, Japan Society for the Promotion of Science (JSPS), and the Materials Properties Council (MPC). The objective of the tests is to obtain basic information regarding the measurement of lower-bound fracture toughness of steels using small specimens. The test program used ASTM A508 Class 3 steel. The 19 laboratories participated in these programs. Tests were carried out with 25.4-mm (1-in.) thick compact tension (1T-CT) specimens at three temperatures, -100°C (T1), -75°C (T2), and -50°C (T3). In addition to the interlaboratory tests, the effect of strain rate on small-specimen fracture toughness behavior was also investigated.
The Japanese test results provided some information to standardize a method for measuring the lower-bound fracture toughness of steels. Small-specimen fracture toughness KJc values indicated a wide scatter, especially in the higher temperature or in the lower strain rate or both. The causes of the scatter of KJc values typically depend on the stable crack growth, Δa0, the distance from stable crack front to trigger point, X, and the distance from fatigue crack front to trigger point, Δa0 + X. No effect of grain size and material cleanliness and slight effect of sampling location of the specimens and difference of the testing conditions between laboratories were observed. The KJc value at Δa0 = 0, X = 0, and Δa0 + X = 0 was defined as a lower-bound fracture toughness KJci, because additional study using A470 Ni-Cr-Mo-V rotor steel showed that the KJci value coincides well with the lower bound of large-specimen fracture toughness KIc per the ASTM Standard Test Method for Plane-Strain Fracture Toughness of Metallic Materials (E 399). Test results also indicated that the KJci value corresponds to 3% fracture probability toughness of the steel.
Based on these test results, the guidelines on the measurement of Δa0, X and Δa0 + X were established, and the tentative test procedure for the determination of the lower-bound fracture toughness using a fractographic data analysis has been developed by the JSPS 129th Committee.