Charpy Absorbed Energy and J_Ic as Measures of Cryogenic Fracture Toughness

In this paper, we present experimental comparisons between the critical energy line integral (J_Ic) and Charpy absorbed energy (C_v) at 4 and 77 K for materials ranging from fully austenitic steels to ferritic steels. At 4 K the correlation between J_Ic and C_v is weak, indicating that C_v is a poor indicator of static fracture roughness at this temperature. At 77 K, a good correlation exists between J_Ic and C_v. A good correlation is also observed between J_Ic at 77 K and C_v at 4 K. The results are explained by the large temperature rise during the Charpy rest. Further evidence of the temperature rise is the marginal change in C_v between 4 and 77 K and the disparity in fracture modes between Charpy and fracture mechanics specimens. For c stainless steels, C_v changed little from 4 to 77 K whereas J_Ic increased significantly. For ferritic steels, C_v increased in proportion to J_Ic from 4 to 77 K. Especially in steels with low nickel contents, fracture surfaces of Charpy specimens revealed higher ductility than those of fracture mechanics test specimens. The results qualitatively support the predicted temperature rises to 130 and 150 K for crack initiation during Charpy tests at 4 and 77 K, respectively. Due to a wide variation in roughness response to temperature rise, the C_v-based regulatory criteria developed for one group of alloys will have no validity when applied to another group. Therefore the Charpy test near absolute zero should not be regarded as a measure of the static fracture resistance. Alternative simplified methods of cryogenic fracture toughness are suggested.