United States Department of Transportation
i
Generation of spectra and stress histories for fatigue and damage tolerance analysis of fuselage repairs
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1991-10-01
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[PDF-1.62 MB]
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Edition:Final report; March 1990-May 1991
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Abstract:This report describes a simplified procedure for the development of stress histories
for use in the analysis of aircraft repairs.- Although repairs of all components of
the airframe are of interest, this report concentrates on stress histories for
fuselage skin repair, description of typical fuselage loadings is provided, and
basic fuselage stress histories are described. A method for development of an
exceedance diagram for analysis of fuselage skin repairs is detailed. Subsequently, a
methodology for generating detailed stress histories is reviewed. Some of the key
features are 1) the inclusion of a range of flights of different severities, 2) the
inclusion of deterministic loads where they occur, e.g., ground-air-ground cycles, 3)
the use of a near-optimum number of stress levels (10-16 positive and negative), 4)
the combination of positive and negative excursions of equal frequency, and 5)
matching of the total number of flights and cycles with the total exceedance diagram.
Two methods of estimating fuselage skin stresses are presented, the first based on
static equilibrium requirements and the second based on a limit load analysis.
A comparison of the proposed history generation scheme with that of an airframe
manufacturer for the KC-135 is also presented. The predicted fatigue crack growth
patterns for a hypothetical through crack at a fastener hole are compared for the two
history generation schemes at three areas within a fuselage. Predicted crack growth
lives are within a factor of 1.5 for two of the three cases. For the third case
(which is predicted to be the least severe by both techniques), the proposed scheme
results in substantially longer crack growth life predictions. The probable reasons
for these differences are discussed.
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