CHARPY ABSORBED ENERGY AND JIC 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 toughness 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 test. 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 austenitic 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 toughness 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.