The fatigue strength of iron casting is very sensitive to its microstructure, particularly the shape and distribution of graphite, which varies with the section size of casting. In the present study, the effect of section size on the fatigue strength was investigated in a rotating bending test, for two kinds of plain specimens cut from 20mm diameter cast round bars and 250mm cubic castings of flake graphite iron.
Cast iron products are frequently used with cast skins. Therefore, the effect of surface conditions on the fatigue strength was also investigated. A completely reversed plane bending fatigue test was made for the spheroidal graphite iron plate specimens with shot blasted cast skin, with machined surface and with machined and shot peened surface.
Furthermore, an attempt was made to examine the possibility of estimating the fatigue strength of the above-mentioned three kinds of iron castings by the method proposed previously. For this purpose, the reduction factor by graphite for the tensile yield strength was determined by the X-ray stress measurement technique and compared with that for the fatigue strength.
The results obtained are as follows:
(1) In the rotating bending test, the fatigue strength of 250mm cubic castings was about half of that of 20mm diameter cast round bars. The difference is mainly ascribed to the variations in shape and distribution of graphite.
(2) In the reversed bending test, the endurance limit of the specimens with shot blasted cast skin was about 30% larger than that of the specimens with machined surface. However, the endurance limit of the latter was increased to the level of the former by the shot peening treatment.
(3) The reduction factor by graphite for the tensile yield strength agreed mostly well with that for the fatigue strength, for the three kinds of cast irons investigated.