STP982: Closure Behavior of Small Cracks Under High Strain Fatigue Histories

    McClung, RC
    Graduate research assistant and associate professorsenior research engineer, University of Illinois at Urbana—ChampaignSouthwest Research Institute, UrbanaSan Antonio, ILTX

    Sehitoglu, H
    Graduate research assistant and associate professorsenior research engineer, University of Illinois at Urbana—ChampaignSouthwest Research Institute, UrbanaSan Antonio, ILTX

    Pages: 21    Published: Jan 1988


    Abstract

    Specimens of a Society of Automotive Engineers (SAE) 1026 steel are subjected to constant amplitude and block loading fatigue histories. Nominal strain amplitudes range from 0.001 to 0.007. The behavior of small cracks (0.1 to 1.5 mm) initiated at very small (50 μm) electro-discharge machined edge notches is studied using a two-step replication technique that enables identification of the load level at which the crack tip first opens or closes. Replicas of the crack tip region are examined in the scanning electron microscope at magnifications up to ×4500. The normalized crack opening level is observed to decrease with increasing strain amplitude for constant amplitude histories. Results compare favorably with the modified Dugdale analysis of Newman. Crack closing levels also decrease with strain amplitude, and closing levels are significantly lower than opening levels at high strains. Correlations of constant amplitude crack growth rates based on estimates of the change in the J integral (ΔJ) or crack-tip opening displacement (ΔCTOD) are more successful when effective stress ranges are considered. Large strain excursions in simple block histories are observed to have a major impact on crack opening behavior during the smaller strain cycles. Typically the crack tip remains entirely open during subcycles immediately following a major cycle, causing significant crack growth acceleration. A simple model is used to correlate these variable amplitude crack growth rates.

    Keywords:

    fatigue (materials), low-cycle fatigue, high strains, crack propagation, small cracks, crack closure, scanning electron microscopy, J, integral, crack-tip opening displacement, variable amplitude cycling


    Paper ID: STP27216S

    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP27216S


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