STP1171: Size Limits for Brittle Fracture Toughness of Bend Specimens

    Faucher, B
    Research scientist and program coordinator, Metals Technology Laboratories, CANMET, Energy, Mines and Resources, Ottawa,

    Tyson, WR
    Research scientist and program coordinator, Metals Technology Laboratories, CANMET, Energy, Mines and Resources, Ottawa,

    Pages: 12    Published: Jan 1993


    Abstract

    Fracture toughness data obtained previously from three-point-bend 10-mm, 20-mm, and 50-mm-thick side-grooved specimens and from 10-mm-thick plain-sided specimens are reanalyzed with respect to constraint. The test temperatures were low enough for the grade of steel used to ensure brittle fracture without any initiation of ductile tearing; nevertheless, extensive crack-tip blunting was observed at the highest temperatures, resulting in a highly nonlinear load-deformation behavior. Fracture toughness was evaluated from the J-integral value at the onset of brittle fracture, Jc. An equivalent stress intensity factor KJ was then obtained from the relation Kj2 = E'JC. The elastic stress intensity factor, Kc, is shown to be equal to Kj for specimen thicknesses down to half the size requirements of ASTM E 399. The macroscopic constraint, defined from the load-carrying capacity of the specimens, is independent of thickness for side-grooved specimens over the range of thicknesses studied in this work; however, it decreases slightly for plain-sided specimens. The results indicate that even when the validity limits for plane-strain fracture toughness evaluation as specified in ASTM E 399 are largely exceeded, it is still possible to measure plane-strain toughness of brittle materials using the J-integral. The results suggest that the validity limits of ASTM E 813 for measurement of cleavage fracture toughness should be increased by a factor of about two.

    Keywords:

    brittle fracture, constraint, side grooves, thickness effects, ferritic steel, fracture toughness


    Paper ID: STP18034S

    Committee/Subcommittee: E08.08

    DOI: 10.1520/STP18034S


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