STP827

    Some New Fractographic Features in the Fatigue of High-Strength Aerospace Alloys

    Published: Jan 1984


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    Abstract

    The object of this research was to further a systematic fractographic study of fatigue in aerospace materials to facilitate failure investigations of actual components. Smooth and notched laboratory specimens were fatigue tested in pulsating tension and in alternating stress at a frequency of 1 to 2 Hz and at stresses that gave failure in 103 to 105 cycles. Fractures were examined by stereoscopic and scanning electron microscopes.

    A ridge structure that had been previously observed in aluminum alloys that emanates from fatigue origins but precedes the formation of striations was observed in ultra-high-strength steels. The implications of such ridges in the mechanism of fatigue cracking are discussed. Not all the steels studied showed fatigue striations, and failure mechanisms not specific to fatigue were frequently observed.

    Such nonconformity of fractographic features increased with the tensile strength of the steels. Secondary cracking was sometimes found to be a more reliable method of identifying fatigue failure in some of these steels. Intergranular cracking at prior austenite grain boundaries was also frequently observed, although its formation appeared, on occasion, to be related to the attainment of a minimum level of stress intensity. Hydrogen from the environment also probably played a role in such intergranular cracking, as indicated by distinct hairlines on the fracture surface. Intergranular crack surfaces also sometimes showed unexpected crystallographic features.

    For the aluminum alloys, further quantitative relationships have been established between the total number of striations in the development of the crack and the number of fatigue cycles in the entire fatigue test. This enables the fatigue stress to be estimated where unknown.

    Counting striations in the steels was found to be very difficult and unreliable. However, trends in 4340 steel were found to be akin to those in aluminum alloys. Ridge formation in the aluminum alloys did not appear to be related to specimen geometry.

    Keywords:

    fractography, fatigue, high-strength low-alloy steels, aluminum alloys, aerospace


    Author Information:

    Cina, B
    Chief Metallurgist, Senior Metallurgist, and Senior Physicist, Materials and Process Engineering, Engineering Division, Israel Aircraft Industries Ltd., Lod,

    Eldror, I
    Chief Metallurgist, Senior Metallurgist, and Senior Physicist, Materials and Process Engineering, Engineering Division, Israel Aircraft Industries Ltd., Lod,

    Kaatz, T
    Chief Metallurgist, Senior Metallurgist, and Senior Physicist, Materials and Process Engineering, Engineering Division, Israel Aircraft Industries Ltd., Lod,


    Paper ID: STP37115S

    Committee/Subcommittee: E08.08

    DOI: 10.1520/STP37115S


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