Fatigue and Fracture Mechanics Analysis of Compression Loaded Aircraft Structure

    Published: Jan 1986

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    During depot maintenance of a U.S. Air Force F-15 fighter aircraft, cracks were discovered in the wing upper spar cap at the inboard end of the spar. Since the upper spar cap is a compression member during normal flight, these cracks were an unanticipated phenomenon which required extensive fatigue and fracture mechanics analyses. The tensile stresses resulting from maneuvers and landing were not considered sufficient to cause cracking. Investigation indicated that the local stresses around holes in the compression-loaded spar cap exceeded compression yield, and, upon unloading, residual tensile stresses were induced at the holes. To determine this cause and the exact nature of service loadings, analytical and experimental studies were performed. A large, detailed finite element model was constructed to determine the local loads in the area of fastener holes. A comprehensive program of element specimens subjected to spectrum loading was also conducted to simulate the in-service cracking and determine the effectiveness of various repair configurations. Crack initiation and crack growth characteristics were obtained from the tests.

    The number of flight hours to crack initiation at the side of the critical fastener hole was predicted. Crack growth analyses were made using a contact stress model computer program, modified to take into account the residual stresses adjacent to the fastener hole. The results of this crack growth analysis are being used to guide Air Force planning for in-service modifications of aircraft.


    crack propagation, fatigue (materials), design, stress analysis, plastic properties, mathematical prediction, residual stress

    Author Information:

    Rich, DL
    Unit chief and branch chief, Technology, McDonnell Aircraft Company, McDonnell Douglas Corporation, St. Louis, MO

    Pinckert, RE
    Unit chief and branch chief, Technology, McDonnell Aircraft Company, McDonnell Douglas Corporation, St. Louis, MO

    Christian, TF
    Aerospace engineer, Warner Robins Air Logistics Center, United States Air Force, Robins AFB, GA

    Committee/Subcommittee: E08.08

    DOI: 10.1520/STP19336S

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