STP945: Crack Opening Displacement Measurements on Small Cracks in Fatigue

    Larsen, JM
    Materials research engineer and materials engineer, Materials Laboratory Air Force Wright Aeronautical Laboratories, AFWAL/MLLN, Wright-Patterson Air Force Base, OH

    Jira, JR
    Materials research engineer and materials engineer, Materials Laboratory Air Force Wright Aeronautical Laboratories, AFWAL/MLLN, Wright-Patterson Air Force Base, OH

    Weerasooriya, T
    Research scientist, University of Dayton Research Institute, Dayton, OH

    Pages: 17    Published: Jan 1988


    Abstract

    A computerized interferometric displacement gage was used to monitor the crack mouth opening behavior of naturally initiated small fatigue cracks in the high-strength alloy Ti6Al-2Sn-4Zr-6Mo. Complete load versus crack opening displacement curves were obtained for naturally initiated semicircular cracks of surface length as small as 50 µm. For a crack of this size, the maximum crack mouth opening displacement was less than 0.5 µm, and the precision of the individual displacement measurements was approximately 0.01 μm. An excellent correlation between crack mouth opening compliance and surface crack length was demonstrated, and the data were shown to agree with an analytical expression for part-through surface cracks. A matrix of experiments was performed for a range of loading conditions, and crack growth data were plotted versus the nominal ΔK and ΔKeff, which was defined using crack closure measurements. Experimental measurements obtained for a small crack indicated that closure often develops discontinuously as the crack initially extends. This behavior was speculated to result from the formation of individual asperities on the fracture surface. In general, the interferometric method proved to be very effective and demonstrated a unique capability to monitor the behavior of very small fatigue cracks.

    Keywords:

    automated, crack propagation, fatigue (materials), fracture mechanics, mechanical properties, minicomputer, microcrack, short crack, small crack, test methods


    Paper ID: STP23282S

    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP23282S


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