Safe Life Conversion of Aircraft Aluminum Structures via Low Plasticity Burnishing for Mitigation of Corrosion Related Failures

Corrosion induced cracking, pitting and the resultant failure of high strength aircraft aluminums is one of the most costly and potentially catastrophic materials problems affecting the modern aircraft fleet. Increased inspections, maintenance, and repair due to corrosion of aging aircraft adversely affect fleet readiness, personal safety and increase cost of operation.

Surface enhancement processes such as shot peening (SP) have conventionally been used to produce a compressive layer of residual stress on the surface of components to improve fatigue life and corrosion resistance thereby extending the useful service life of components. The compression produced by a typical shot peening process persists in depth only through the first few thousandths of an inch. Corrosion pits, cracks or other damage that exceed the depth of compression afforded by shot peening serve as the nucleation point(s) for corrosion induced fatigue cracking and other failure mechanisms. It is shown that there is a maximum pit depth threshold for aircraft aluminums which is dependent on the surface treatment used. Low plasticity burnishing (LPB) has been developed as a modern surface enhancement process, with characteristic low cold working, which imparts a deep layer of stable compression into the surface of a component. The depth of compression from LPB greatly exceeds the maximum corrosion pit depth in the studied materials therefore preventing failure from pitting or cracking and ensuring safe-life operation.

The corrosion fatigue, pitting, and stress corrosion cracking damage performance of several aircraft aluminum alloys are shown for SP and LPB treated test specimens. Both surface treatments were also evaluated as repair treatments for pre-damaged specimens. In all cases the LPB treatment provided greater corrosion fatigue resistance, and improved corrosion damage tolerance compared to the SP treated specimens.