The Use Of Engineered Compressive Residual Stresses To Mitigate Stress Corrosion Cracking And Fatigue Failure In 300m Landing Gear Steel

300M steel combines ultra high tensile strength with good fracture toughness, making it an excellent candidate for aircraft landing gear applications. However, like most high strength steels, 300M is vulnerable to stress corrosion cracking (SCC) and FOD initiated high cycle fatigue (HCF) failure. The sensitivity of 300M to salt water corrosion fatigue and static SCC adversely impacts Navy and Marine aircraft readiness and maintenance costs. SCC and FOD initiated failures are well documented in both military and civilian high strength steel landing gear and structural components. Shot peening (SP) is commonly used to improve the fatigue performance of high strength steels, but provides only partial corrosion fatigue benefit, only very shallow damage tolerance, and no significant SCC improvement.
Surface treatment with low plasticity burnishing (LPB) has been applied to create a compressive residual stress distribution with a depth and magnitude designed to mitigate both SCC and FOD in 300M landing gear steel. The damage tolerance,
corrosion fatigue and static salt-water SCC performance of LPB treated 300M steel are compared with the SP currently used for landing gear and a low stress ground (LSG) baseline condition. LPB produced compression to a depth of 1.27 mm (0.050 in.), compared to 0.127 mm (0.005 in.) for the current SP treatment. The depth of LPB compression dramatically improved both the damage tolerance and corrosion fatigue strength. The 107 cycle fatigue strength for the LSG baseline condition decreased dramatically with salt-water exposure, from nominally 760 MPa (110 ksi) to 207 MPa (30 ksi). Exposure to salt water appears to eliminate the typical endurance limit behavior of the steel, causing the fatigue strength to continuously diminish with time and cyclic loading. The HCF and corrosion fatigue performance with a 0.5 mm (0.020 in.) deep notch was even worse. In contrast, LPB treated specimens with 0.5 mm (0.020 in.) deep surface damage showed a definite endurance limit of 1035 MPa (150 ksi) under corrosion fatigue conditions. Statically loaded salt-water SCC was eliminated entirely by LPB treatment. Shot peened 300M failed in just a few hours under static loading in salt water. LPB treated samples exhibited only general corrosion and survived 1500 hrs. at the same stress without failing.
Pitting and intergranular surface corrosion result in early crack initiation and cyclic SCC assisted growth when exposed to salt-water, dramatically decreasing fatigue performance. Despite the surface corrosion, the deep compressive surface residual stresses from LPB mitigate both the individual and synergistic effects of corrosion fatigue and pitting. The depth and magnitude of LPB compression, introduced without the cold working and surface damage generated by SP, maintains the surface layer exposed to the SCC environment in compression or below the SCC threshold even under tensile service loading, eliminating SCC as a failure mechanism.