Prevention of Corrosion Related Failure of Aircraft Aluminum Using an Engineered Residual Stress Field

The effect of corrosion damage on the high cycle fatigue (HCF) performance of AA7075-T6 was evaluated. Corrosion fatigue specimens were processed using either a conventional 6-8A, 200% shot peening (SP) treatment or low plasticity burnishing (LPB) a CNC controlled process capable of being performed robotically ‘on wing’ as well as with conventional CNC machine tools.

Stress vs. life corrosion fatigue curves were established for each process, additionally specimens were tested at a fixed stress for 75% of the expected life and then subsequently re-processed to evaluate the benefits of each process when used as a repair treatment. Residual stress measurements were performed to quantify the magnitude and maximum depth of compression from each process. Pitting depth and anodic polarization studies were performed for SP and LPB.

 LPB  imparted a depth of compression 7X greater than SP without the severe cold working produced by SP. The corrosion fatigue life for LPB specimens was increased by greater than an order of magnitude over SP and when used as a repair process LPB improved the life by 500%.  Pit depths were measured for each treatment as a function of time and were found to asymptotically approach a maximum depth dependent upon the surface treatment used. Anodic polarization testing revealed a shift in the open circuit potential (OCP) of nominally 0.1V between SP and LPB treated specimens. The LPB specimens were found to be nobler, having a lower OCP than the SP specimens. Corrosion damage exceeding the depth of compression on a component serve as the nucleation point(s) for corrosion induced fatigue cracking. A deep, stable layer of compression can successfully mitigate the damage and greatly increase corrosion fatigue life.

Keywords: Corrosion Fatigue, Pitting, Stress Corrosion Cracking (SCC), High Cycle Fatigue (HCF), Low Plasticity Burnishing (LPB), Shot Peening (SP) Compressive Residual Stress.