Prediction of Fatigue Life Under Helicopter Loading Spectra for Safe Life and Damage Tolerant Design

    Published: Jan 1999

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    Variable amplitude loading spectra have been developed from service data gathered on a helicopter rotor head. Over 90% of the cycles have r values between 0.7 and 0.9. Variable amplitude fatigue crack growth tests have been performed using the modified rotorcraft spectra on CT specimens of 10-2-3 β-titanium alloy and a 7010 aluminium alloy. Samples were subjected to variations on the original loading spectrum where smaller load cycles had been progressively removed. Crack closure was monitored at intervals throughout selected load sequences. The results showed that the small range, high mean cycles originating in rotor vibrations contributed over 80% of the crack growth damage. In the titanium samples the contribution of the different parts of the spectrum could be predicted using a simple summation of constant amplitude crack growth increments, without load interaction effects. In the aluminium alloy, the situation was more complicated, with a reduced contribution of the small cycles. The implications for damage tolerance in helicopter components are discussed.


    Variable amplitude fatigue, omission levels, crack closure, damage tolerance

    Author Information:

    Irving, PE
    Professor and research student, Cranfield University, Cranfield, Beds,

    Buller, RG
    Professor and research student, Cranfield University, Cranfield, Beds,

    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP14980S

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