| The low cycle fatigue test of ductile cast iron was studied for the purpose of promoting the qualitative confidence for its importance. The main results obtained are as follows: 1. The stabilization of stress amplitude under the cyclic strain was faster when the strain level was higher or the strain rate was slower. 2. The cyclic strain grave rise to hardening in ferritic matrix but did not cause hardening or softening in pearlitic matrix. And the cyclic stress-strain curves obtained from companion specimens were in accord with that from incremental step test results. 3. The fatigue lives of the materials depended on Ce and Ke according to matrices, and depended on Cp and Kp according to strain rates when its matrix was same. Especially, Cp dependence of fatigue life was much greater than Kp. Here Ce and Ke are the coefficient and the exponent of cyclic elastic strain, and Cp and Kp are the coefficient and the exponent of cyclic plastic strain. 4. The equations for the fatigue life of the material were as follows. ⅰ) When its strain rate was 10＾(-2)/sec ε_ta = (0.239-0.263)/_Nf (0.653-0.698) + (0.003 -0.005)/_Nf (0.036 -0.073) ⅱ) When its strain rate was 17＾(-3)/sec ε_ta = (0.166 -0.172)_Nf (0.626-0.674) + (0.003 -0006)/_Nf (0.057-0.097) 5. Cyclic hardending exponent, n` of the annealed specimens ranged from 0.059 to 0.118, and the normalized specimens, 0.116 to 0.183. 6. Calculated fatigue strength was in fair agreement with experimental points within low cycle range. 7. Under the static stress, the crack initiation of ferritic matrix was caused by closing and linking up between nodules by plastic deformation in the stress direction, and the crack initiation of normalized specimens was attributed to the stress concentration in soft ferritic shell around the nodule due to a hard pearlitic matrix. And fatigue cracks were mainly due to notch effect of nodules. |
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