STP605

    Microstructural Aspects of Fracture Toughness

    Published: Jan 1976


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    Abstract

    For most technical materials the dominant mechanism resisting crack extension is plastic deformation. Continuum mechanics analysis shows that fracture toughness, in addition to depending on Young's modulus, flow stress, strain hardening exponent, and yield strain, should be nearly proportional to the effective fracture ductility obtained for the stress state characteristic for the region ahead of the crack; plane stress or plane strain. The original equation for plane-strain fracture toughness-equibiaxial ductility is refined to include the effects of strain hardening. Such a correlation has been experimentally confirmed for steels; KIc was found to be proportional to the effective equibiaxial ductility. A model for the thickness effect on Kc has been developed on the basis of these observations and is in fair agreement with experimental results.

    The dominant microstructural events that control ductility, and therefore fracture toughness, are void nucleation, void growth, and void coalescence. Void nucleation at an inclusion—matrix interface is governed by the value of the interface strength—flow stress difference and is, consequently, temperature sensitive. Models for void growth mechanisms show the void coalesence strain to be a strong function of the nuclei density but rather insensitive to temperature. Qualitative relationships are presented which give some insight into the microstructural causes for ductility and fracture toughness transitions (or their absence in face-centered-cubic materials) with temperature, and can serve for the development of new high-toughness materials.

    Keywords:

    fracture properties, plastic deformation, toughness, ductility, voids, mechanical properties, nucleation


    Author Information:

    Weiss, V
    Professor, research fellow on academic leave from Daido Steel Co., Ltd., Nagoya, Japan, and graduate research assistant, Syracuse University, Syracuse, N.Y.

    Kasai, Y
    Professor, research fellow on academic leave from Daido Steel Co., Ltd., Nagoya, Japan, and graduate research assistant, Syracuse University, Syracuse, N.Y.

    Sieradzki, K
    Professor, research fellow on academic leave from Daido Steel Co., Ltd., Nagoya, Japan, and graduate research assistant, Syracuse University, Syracuse, N.Y.


    Paper ID: STP27798S

    Committee/Subcommittee: E08.08

    DOI: 10.1520/STP27798S


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