STP436: Engineering Applications of Fractography

    Brothers, AJ
    Materials and Processes Laboratory, General Electric Co., Schenectady, N.Y.

    Yukawa, S
    Materials and Processes Laboratory, General Electric Co., Schenectady, N.Y.

    Pages: 17    Published: Jan 1968


    Abstract

    To date the primary use of microfractography has been qualitative. For this purpose the characteristic features of a specific fracture mechanism are compared with those of the service failure to be identified. However, the optimum use of fractography is that of both a quantitative and qualitative tool. This paper illustrates its usefulness in interpreting quantitatively the microfractographic features of various failure mechanisms, and suggests some currently speculative but promising areas for future work.

    The incidence of fatigue striations and their association with cyclic alternating loads provide the basis for quantitative interpretation of fatigue failures. It is shown that fractographic determination of microscopic crack growth rates, coupled with currently available analyses for macroscopic crack growth rates, permits one to estimate relative failure stress or strain level, approximate number of failure cycles, and, in some cases, the relative stress gradient and mode of loading. The general stress dependence of fatigue striations, if determined from component tests, can be used directly for estimating relative operating stress levels of service failed parts. However, the influence on crack growth rate of geometrical factors suggests caution in comparing service failures with fractographic features of laboratory specimens. For this general case the correlations of crack growth rate with Griffith-Irwin fracture mechanics concepts are recommended. Since these analyses consider both component and crack geometry and mode of stressing, they are more generally applicable to a service consideration.

    The high magnification used in electron microfractography and its use of replica techniques are shown to be useful in interpreting quantitatively the metallurgical features associated with creep microvoid formation. It is shown that fractography, in combination with standard electron diffraction techniques, permits one to isolate and identify the distribution, size, and content of specific metallurgical phases.

    Keywords:

    electron microscopy, fractography, fatigue (materials), cleavage fracture, creep rupture


    Paper ID: STP32006S

    Committee/Subcommittee: E08.03

    DOI: 10.1520/STP32006S


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