3-points bending fatigue tests and microfractographical observations were carried out on spheroidal graphite cast irons in the as-cast (FCD55), annealed (FCD45), or normalized (FCD70) condition. The main results obtained are summarized as follows.
(1) The fatigue strength of as-cast specimens increases with increasing pearlitic matrix and decreasing graphite particles.
(2) In the case of high loading repeated stress, the fatigue strength increases in order of annealed, as-cast, and normalized specimens. The S-N curves cross each other at about 6×105 cycles, and the order of the fatigue strength is reversed at the fatigue limit. It seems that these phenomena are due to the tensile residual stress originated from surface grind finishing of specimens. Their fatigue limits are in the range of 118N/mm2-157N/mm2.
(3) The macrostructure observations of fatigue fracture show that ductile fracture occurs in the case of high loading repeated stress, and brittle fracture in low loading stress.
(4) The initial point of crack mostly occurs from a graphite edge or non-metallic inclusion on the loading side of the maximum tensile stress. This result is the same as those other conventional ratary bending fatigue tests.
