Abstract
The effect of the interaction between fatigue and creep damages on rupture life and crack propagation has been investigated on SUS 304 and SUS 316 stainless steels at 650°C. It was shown that under higher nominal stress and at intermediate frequencies, the rupture life under cyclic load increased in total time to failure as compared with the creep rupture life under static load, and the sum of fatigue and creep damage fractions is more than one. These results showed that a linear superposition model was not applicable. But under lower nominal stress, the behavior of rupture life could be interpreted on the basis of a linear superposition model. From an engineering design viewpoint, the model was in the safety side within the range of these experiments.
In order to account for the behavior above mentioned under higher stress and at intermediate frequencies, crack propagation tests and observations of fractograph were made. These tests showed that the rate of crack propagation under cyclic load became slower than that under static load from the halfway through crack propagation. Fractographic studies indicated the fracture mode transition from intergranular creep fracture to transgranular fatigue fracture, corresponding to the change of crack propagation rate.
The reason of this behavior of lower crack propagation or fracture mode transition is that creep strain recovery decreases net deformation in one cycle, causing to depress creep fracture and promote fatigue fracture.
After all, it was found that creep strain recovery as well as fatigue and creep damages should be considered as the factors controlling the combined fatigue and creep fractures.
