What is it about?

Among the main service life-enhancements for aircraft structures is the process of hole cold expansion. The standard application process follows several steps, including removal of any surface defects and reaming to final diameter. However eliminating all cracks and isolating the structure from aggressive environment may not always be possible. In this work we show the benefits and environmental interactions of hole cold expansion if pre-cracks exist, under galvanic corrosion.

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Why is it important?

The application of cold work hole expansion to fastener holes in AA 2024-T3 was found to provide benefits to the overall fatigue life even with cracks present at the hole and in an aggressive environment. Cold hole expansion created residual compressive stresses around fastener holes that slowed the crack growth rate, thus providing fatigue life benefits, such as an improved inspection interval, to the overall structure. The research also displayed signs of a location shift in critical stress intensity factor due to the corrosive environment, which an evaluation of the inspection area may be necessary in future research for the prevention of a secondary crack initiation.

Perspectives

Local investigation of the initiation site with Scanning Electron Microscope (SEM) is recommended. This research places emphasis on the effects of cold expansion and the resulting residual stress on the crack growth in a corrosive environment. As seen from the theoretical curves, corrosion has shown significant effects on the crack growth. Therefore, the electrochemistry aspect of the material should be examined along with the microstructure of the material. As the new crack starts, the specimen can be removed and observed under SEM in order to identify the cause of the new crack initiation. As observed by Nicolas et al. the probable cause could be a local corrosion around a cathodic precipitate

Alberto Mello
Embry-Riddle Aeronautical University

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This page is a summary of: Influence of Cold Expansion and Aggressive Environment on Crack Growth in AA2024-T3, AIAA Journal, March 2022, American Institute of Aeronautics and Astronautics (AIAA), DOI: 10.2514/1.j061090.
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