Two specific concerns that could affect safety limits for aging aircraft are the effects of corrosion damage and widespread fatigue damage (WFD) on structural integrity. A common joint in fuselage structure is the riveted lap joint, which overlaps two fuselage skin panels. This design creates complex loading conditions that require various analysis methods for accomplishing a durability and damage tolerance (DaDT) analysis. Under an Air Force research project, Boeing evaluated the capabilities of several advanced analysis tools for assessing the effects of corrosion and WFD on the structural integrity of riveted lap joints. These existing structural analysis tools included:
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element code, FRANC2D/L, for determining stress distributions and stress intensity factors of cracked structure
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growth analysis code, AFGROW, for estimating fatigue crack growth life
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element codes, FRANC2D/L and FRANC3D/ STAGS, for predicting residual strength
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analysis code, PROF, for determining the probability of fracture.
To evaluate the analysis tools, three case studies were conducted on structural items having features common to a typical lap joint. The first case study investigated flat, unstiffened panels with corrosion and open holes containing multiple cracks. The second case study investigated built-up lap joint coupon specimens with and without corrosion. The third case study investigated analysis methods for assessing corrosion damage and WFD on a full-scale fuselage skin panel with a lap joint.
The tools demonstrated the capabilities to perform DaDT analyses on (1) structural components with multiple cracks, (2) thin structural components with multiple layers of material, and (3) large structural shell components such as fuselage panels. Analysis results of the first two case studies showed good agreement between predicted and experimental test results. The analysis tools could account for material thinning and multiple cracks, but limitations in some of the tools prevented a complete evaluation to account for corrosion pillowing.