STP982

    The Effect of Test Frequency and Geometric Asperities on Crack Closure Mechanisms

    Published: Jan 1988


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    Abstract

    The effect of frequency, if any, on fatigue crack closure mechanisms is investigated for a series of Fe-Si alloys. Fatigue cracks were grown at 30, 3, 0.3, and 0.03 Hz. At the end of each of these, closure loads were measured using the point of deviation from linearity of the load versus crack opening displacement (COD) curve in the unloading direction. Oxide-induced closure effects were minimized by performing all experiments in nitrogen or hydrogen. An attempt was also made to isolate the effect of geometric asperities by comparing the results to those obtained from single crystal specimens, in which the cracks were grown in hydrogen gas to keep the crack surfaces as flat as possible on the {100} plane. For these specimens, closure loads of the order of 1 to 4.5 MPa · m1/2 were observed, and the closure effect was seen to arise from the longitudinal cleavage river-steps associated with fatigue crack propagation. The heights of these steps were estimated from scanning electron micrographs; they ranged from 1.5 to 7 μm. The closure loads in the polycrystalline specimens were of the order of 4.7 to 6.5 MPa · m1/2. The fracture surfaces are seen to be much rougher than in the single crystal specimens. The closure effect in these materials is shown to be largely geometry related for the single crystals, while for the polycrystals, both roughness and reverse slip are shown to be important.

    Keywords:

    closure, fatigue crack propagation, opening stress intensity, closure mechanisms, Fe-binary alloys, single crystals, frequency effects, geometric asperities, reverse slip


    Author Information:

    Sheth, JK
    Graduate student and professor, University of Minnesota, Minneapolis, MN

    Gerberich, WW
    Graduate student and professor, University of Minnesota, Minneapolis, MN


    Paper ID: STP27203S

    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP27203S


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