Yinghui Zeng

1 Introduction The 4340 steel is commonly used to manufacture key components such as aircraft landing gears, helicopter rotor shafts, and turbine shafts in turbojet engines that operate under demanding service conditions, owing to its excellent combination of mechanical properties [1-3]. These key components endure long-term service integrated in the high-end equipment under cyclic loading or alternating loading conditions, and consequently, fatigue fracture is their primary failure mode [4-8].