On the Fatigue of Nodular Cast Iron under Double Stress Levels

Cast iron is generally considered as notched material， because the graphite bears less load. This paper is the results of the experiments of nodular cast iron on the fatigue damage of double stress levels under rotary bending stress. There are many researches on fatigue test under double stress levels and it has been discussed whether they were applicable to Miner's linear damage law or not. By their results， it is clear that it depends on the definition of fatigue life，wheher or not Miner's linear damage law is substantiated. That is， if the repetitions of initiation of a micro crack is chosen as standard damage life， Miner's law is substantiated. But in the case of adopting the life to fracture as its standard， it is not substantiated， by the effect of stress dependence. Then， we were interested in the behavior of nodular cast iron as the isotropic notched materials， under these stress state. In our experiments， the surface layer of the fatigue specimens is removed by electro－polishing about 10 micron thick to take off the effect of work hardening of machining. As stress levels for double stress test，σ=25 and 28 kg／mm2 are adopted， and their fatigue lives are N＝35. 6×104 and l7. 5×104 respectively. The ratios of the number of the first stress repetitions to the fracture life are 0. 5 and 0. 8，and then the specimen is tested to fracture at the second stress levelσ2. From our results，the cumulative damage Σn／N is almost equal to unit， when σ1＞σ2，and therefore we could say that Miner's law is nearly substantiated. But when σ1＜σ2，Σn／N has values greater than unit. In later case the results show stress dependence and Miner's law is not substantiated. We discuss on these results. And following the check of crack propagation by the method of oxiding in the furnace，crack geometry（t/a；t＝crack depth， a＝crack length on the surface）is almost constant independently of the stress level until half area of cross section of specimen