Fatigue crack propagation tests of a low alloy steel (JIS SNCM 439) tempered at 200°C and 600°C were conducted both in air and in 3.5% NaCl solution. The residual stress near the fatigue fracture surface was measured by the X-ray diffraction method. The results obtained are summarized as follows:
(1) The residual stress measured on the fracture surface was tension both in air-fatigue and corrosion fatigue. The tensile residual stress increased with the maximum stress intensity factor Kmax in the case of the material tempered at 200°C, while it had a maximum value at about Kmax=30MPa√m in the case of the material tempered at 600°C. When compared at the same Kmax value, the residual stress was lower for a lower stress ratio and in corrosion fatigue.
(2) The distribution of the residual stress beneath the fatigue fracture surface was able to be decomposed into two components: the tensile residual stress in the vicinity of the fracture surface caused by monotonic tensile plastic deformation, and the compressive residual stress in the vicinity of the fracture surface caused by stress relief due to roughness and by compressive plastic deformation.
(3) The maximum depth of the plastic zone was evaluated on the basis of the residual stress distribution. The depth ωy is related to Kmax by the following equation:
ωy=α(Kmax/σY)2
where σY is the yield strength obtained in tension tests. α is 0.19 for air fatigue and 0.06 for corrosion fatigue. The small value of α in corrosion fatigue suggests the hardening of the material in the plastic zone due to the environmental effect.
