Fatigue crack growth behavior in gradient microstructure of hardened surface layer for an axle steel

This paper experimentally investigated the behavior of fatigue crack growth of an axle steel with a surface strengthened gradient microstructure layer. First, the microstructure, residual stress and mechanical properties were examined as a function of depth from surface. Then the fatigue crack growth rate was measured via three point bending fatigue testing. The results indicated that fatigue crack growth rate decelerated first and then accelerated with the increase of crack length within the gradient layer. Especially, the fatigue crack was arrested in the gradient layer under relatively low stress amplitude due to the increase of threshold value for crack growth within the range of 3 mm from surface. Based on these results, the parameters of Paris equation and the threshold value of stress intensity factor range within the gradient layer were determined and a curved surface was constructed to correlate crack growth rate with stress intensity factor range and the depth from surface. The effects of microstructure, residual stress and surface notch on the crack growth behavior were also discussed.