STP1391

    Laboratory Techniques for Service History Estimations of High Strength Fastener Failures

    Published: Jan 2000


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    Abstract

    Two fastener failures have been analyzed to determine both failure mode and service loading history. The analyses were conducted through careful fractographic investigation and laboratory simulation to assess the integrity of the assembly design and to evaluate the role of unintended environmental effects in these failures. The first case involves Monel K-500 bolts that failed due to fatigue loading, terminated by overload fracture. The fatigue mode was intergranular (IG) at initiation, and then transitioned to classical transgranular fatigue prior to overload failure. Intergranular cracking in Monel K-500 is usually associated with environmentally assisted cracking (EAC). However, simulated service testing of several bolts under both fatigue and slow strain rate loading revealed that IG fatigue cracking could occur at low applied ΔK in air. Transgranular fatigue is associated with higher ΔK levels. Therefore, EAC was not the root cause of these failures.

    The second case examines the fatigue failure of several IN625 studs. The stud stresses are estimated from fatigue striation measurements using a representative fatigue crack growth behavior and driving force equation. The inferred applied stress is greater than the material yield strength, which is indicative of a basic deficiency in the original joint design. Laboratory testing verifies the relative accuracy of this stress estimation method for both simple and complex loading histories. The accuracy can be most significantly improved through fatigue crack growth testing of the actual failed material under relevant service conditions.

    Keywords:

    IN625, K-500, high strength fastener materials, failure analysis, environmentally assisted cracking, fatigue


    Author Information:

    Gaudett, M
    Materials engineer, Warfare Center, Bethesda, MD

    Tregoning, R
    Mechanical engineer, Warfare Center, Bethesda, MD

    Focht, E
    Materials engineer, Warfare Center, Bethesda, MD

    Zhang, XJ
    Materials engineer, Warfare Center, Bethesda, MD

    Aylor, D
    Corrosion engineer, Warfare Center, Bethesda, MD


    Paper ID: STP13522S

    Committee/Subcommittee: E08.09

    DOI: 10.1520/STP13522S


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