Fatigue crack propagation tests of mild steel (JIS SM41B) and high strength steel (JIS SNCM439) tempered at 600°C and 200°C were conducted under stress ratios of 0.1 and 0.5. The distribution of the half-value breadth of X-ray diffraction profiles beneath the fracture surface was measured. In mild steel and 600°C tempered steel, the half-value breadth measured near the fracture surface was larger than the initial value measured before the fatigue test. On the other hand, the half-value breadth decreased near the fracture surface in 200°C tempered steel. From the distribution, the depth of the maximum plastic zone ω was determined. It is related to the maximum stress intensity factor Kmax by the relation: ω=α(Kmax/σY)2(σY=the monotonic yield strength), where α=0.09 for mild steel and 0.22 for tempered steels. The difference in α between the materials was used to evaluate the actual yield strength of the material in the plastic zone around the fatigue crack tip.
