Comparison of Mechanical Suppression by Shot Peening and Low Plasticity Burnishing to Mitigate SCC and Corrosion Fatigue Failures in 300M Landing Gear Steel

摘要

300M steel is widely used in aircraft landing gear because of its unique combination of strength and fracture toughness, but is vulnerable to foreign object damage (FOD), corrosion fatigue, and stress corrosion cracking (SCC) failures with potentially catastrophic consequences. The fatigue, corrosion fatigue in salt water, and SCC performance of LPB processed 300M steel was compared with shot peened (SP) and low stress ground (LSG) conditions. LPB, with and without simulated FOD, produced deep residual compression that dramatically improved both the HCF and corrosion fatigue strength. The fatigue strength of LSG and SP treated surfaces was drastically reduced by salt and FOD exposure with no discernible endurance limit for corrosion fatigue conditions. SCC testing of LPB treated landing gear sections at 1030 to 2270 MPa (150 to 180 ksi) static loads was terminated after 1500 hrs without failure, compared to failure in as little as 13 hours without treatment. Mechanistically, the deep compressive surface residual stresses from LPB treatment mitigated both the individual and synergistic effects of corrosion fatigue and FOD. LPB also reduced the surface stress well below the SCC threshold for 300M, even under high tensile applied stresses, effectively suppressing the SCC failure mechanism.