STP924

    Change of Dislocation Structures and Macroscopic Conditions from Initial State to Fatigue Crack Nucleation

    Published: Jan 1988


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    Abstract

    Dislocation structures in fatigue were investigated with special consideration given to the compatibility of the macroscopic observation by an optical microscope and the microscopic observation by a transmission electron microscope (TEM). First, previous works are reviewed from this point of view, that is, the compatability of macro- and microscopic observations. It is postulated that this viewpoint is very important for both the elucidation of fatigue mechanism and the construction of crack nucleation models. Second, the experiments are planned and carried out, and the results are discussed in this direction. Copper polycrystalline sheets with a thickness of 0.1 mm are used for the preparation of specimens for fatigue testing. The specimen contains an artificial hole of 200 μm diameter. The fatigue processes at the edge of the hole were observed successively with an optical microscope. In the early stage of the fatigue test, slipbands initiated at the hole edge and subsequently cracks nucleated after stress cycles of N ≅ 8 × 105. The foil specimens for TEM observation were prepared from the sheet specimens that were tested for various numbers of cycles, say, N = 2 × 105, 4 × 105, and 8 × 105. Every effort was made to show the correspondence between the site of TEM observation and that of macroscopic observation. Since it was very difficult to thin the edge of a hole sufficiently, only a few specimens were available for TEM observation though more than 100 sheet specimens were tested. The first part of the paper poses five fundamental questions; especially considered in the direction described above are (1) the relationship between vein structures and persistent slipbands (PSBs), (2) the relationship between crack nucleation and PSBs, and (3) dislocation structure at the stress concentration or stress gradient.

    Conclusions may be summarized as follow: (1) Vein structures and ladder structures in PSBs are closely related. (2) In the fatigued copper polycrystal specimens with no hole, PSBs with ladder structure can develop in certain grains of the specimen. (3) At the edge of a hole of the specimens which contain a hole, however, PSBs with ladder structure are unlikely to be formed. (4) The crack nucleation sites at the edge of a hole are PSBs without ladder structure.

    Keywords:

    fatigue, dislocation, crack nucleation, vein structure, persistent slipbands, ladder structure, copper, small artificial hole, stress concentration, observation by optical microscope, observation by transmission electron microscope


    Author Information:

    Murakami, Y
    Professor and research associate, Kyushu University, Fukuoka,

    Mura, T
    Professor, The Technological Institute, Northwestern University, Evanston, IL

    Kobayashi, M
    Professor and research associate, Kyushu University, Fukuoka,


    Paper ID: STP23209S

    Committee/Subcommittee: E08.03

    DOI: 10.1520/STP23209S


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