Edward L. Colvin

Effect of Prior Corrosion on the S/N Fatigue Performance of Aluminum Sheet Alloys 2024-T3 and 2524-T3

Aviation industry demand for continuous safety improvement in the face of trends toward increasing service life of aircraft and cost control necessitates stronger prevention and control measures to avoid the likelihood ofstructural failures linked to widespread damage involving corrosion and fatigue. New materials with improved damage tolerance attributes can improve the margin of safety in the presence of widespread damage. An excellent example of one such material is new aluminum alloy 2524 (formerly C188) which has improved fracture toughness and fatigue crack growth resistance relative to incumbent alloy 2024. In this study, the effect of prior corrosion on the S/N fatigue performance of 1.60 and 3.17-mm thick 2524-T3 and 2024-T3 bare sheet was evaluated. The fatigue strength of 2524 was approximately 10% greater and the lifetime to failure 30 to 45% longer than that of the 2024. Two main factors are believed to have contributed to the better performance of 2524: a less damaging configuration of corrosion pits and its better fatigue crack growth resistance.

The Evolution of Plate and Extruded Products with High Strength and Fracture Toughness

From the first use of 2017-T74 on the Junkers F13, improvements have been made to plate and extruded products for applications requiring the highest attainable strength and adequate fracture toughness. One such application is the upper wing of large aircraft. The progression of these product improvements achieved through the development of alloys that include 7075-(T6 & T76), 7150-(T6 & T77) and 7055-(T77 & T79) and most recently 7255-(T77 & T79) is reviewed. The most current advancements include aluminum-copper-lithium, alloy 2055 plate and extruded products that can attain strength equivalent to that of 7055-T77 with higher modulus, similar fracture toughness and improved fatigue, fatigue crack growth and corrosion performance. The achievement of these properties is explained in terms of the several alloy design principles. The highly desired and balanced characteristics make these products ideal for upper wing applications.