Volume 20, Issue 4 (July 1992)
Charpy Absorbed Energy and JIc as Measures of Cryogenic Fracture Toughness
In this paper, we present experimental comparisons between the critical energy line integral (JIc) and Charpy absorbed energy (Cv) at 4 and 77 K for materials ranging from fully austenitic steels to ferritic steels. At 4 K the correlation between JIc and Cv is weak, indicating that Cv is a poor indicator of static fracture roughness at this temperature. At 77 K, a good correlation exists between JIc and Cv. A good correlation is also observed between JIc at 77 K and Cv 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 Cv between 4 and 77 K and the disparity in fracture modes between Charpy and fracture mechanics specimens. For c stainless steels, Cv changed little from 4 to 77 K whereas JIc increased significantly. For ferritic steels, Cv increased in proportion to JIc 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 Cv-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.