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Development of a Mathematical Model to Predict Cracking in Corroded Aircraft Structures

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Final rept. 10 Jun 1997-10 Jun 1998

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There is increasing concern about the possible detrimental effects of corrosion on the structural integrity of fuselage lap joints. Corrosion in lap joints can lead to a decrease in strength because of loss in skin thickness, early fatigue crack initiation caused by the formation of stress risers, and increased fatigue crack growth rates. The mode of corrosion in lap joints has generally been considered to be uniform loss of material. Based on the concept of general thickness loss and the formation of voluminous corrosion products as a result of exfoliation corrosion, models to predict the stress distribution and fatigue crack initiation sites were previously developed. These models indicate that the combination of loss in skin thickness and the build up of voluminous corrosion products inside the lap joint will lead to high stresses in the joint where fatigue cracks are likely to initiate. However, these calculations are not based on the actual morphology of corrosion in the lap joint. In this paper, detailed metallography of a KC-135 lap-joint section describes the complex nature of corrosion on the contact or facing surface, with barely detectable corrosion penetrating deep into the skin. A finite element model was developed based on the actual corrosion morphology of the lap joint. The finite element program ABAQUS was used to model the strainstress distribution in a corroded lap-joint section. The corrosion was simulated by decreasing the skin thickness and applying a uniform pressure to represent the build up of corrosion by-products. A small hemisphere was introduced to simulate the localized intergranular corrosion. The results of the finite element analysis demonstrated that even a small hemispherical indent superimposed on uniform type corrosion near a fastener hole resulted in significant increase in elastic strain such that early fatigue crack initiation could be anticipated.

Subject Categories:

  • Transport Aircraft
  • Properties of Metals and Alloys
  • Numerical Mathematics
  • Mechanics

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