Evaluation of Fatigue Strength of Ductile Cast Iron Composed of Ferrite-pearlitic Structures with As-cast Surfaces (Effect of Surface Roughness, Microstructure Transition Layers, Defects and Residual Stress on Fatigue Strength)

Abstract

In order to elucidate the complex effects of surface roughness, the transition of microstructures from the surface to the interior, defects and residual stress, tension-compression fatigue tests have been conducted by using shot blasted ferrite-pearlite ductile cast irons with as-cast surfaces. Regardless of the applied stress levels, the fracture origins of the specimens were mostly situated at a defect near an as-cast surface, slag, or pinhole at N f < 10 7. At a relatively lower stress amplitude, some fatigue fractures such as shrinkage occurred from an inner defect far from the surface, mostly at N f > 10 7. The area parameter model was applied to the quantitative evaluation of the fatigue limit. The equivalent hardness considering the transitional layers and the effective defect size with the interaction between the surface roughness and a defect were defined. Moreover, the relief of the residual stress during the fatigue tests was detected. The mean value of the relieved residual stress acting on a defect was employed for the quantitative evaluation. By using these parameters, the complex effects of ductile cast iron have been successfully evaluated for practical use.

Effect of Hardness and Stress Ratio on Threshold Stress Intensity Factor Ranges for Small Cracks and Long Cracks in Spheroidal Cast Irons

“…In particular, it is difficult to predict fatigue limit of spheroidal cast irons with cast surfaces due to the complex effects of surface roughness, transitional layers, defects, and residual stress. To solve this problem, we have conducted plane-bending and tension-compression fatigue tests (1) (2) by using specimens with cast surfaces whose residual stress is relieved by annealing in a vacuum. We have also quantitatively evaluated the effects of the above factors excluding the residual stress on the fatigue limit by using the area parameter model proposed by Murakami et al (3)- (5) .…”