Abstract
In order to investigate the effect of residual stress on the fatigue strength of nodular cast iron in use hardened by high frequency induction, rotating bending fatigue tests were made on the specimens drilled along the central axis to change the surface residual stress of the induction hardened FCD45 and FCD70. Some irregular shape graphite exists in nodular cast iron in use. In this examination, the irregular shape graphite was the initiation point of fatigue fracture.
When the applied stress (cyclic stress and residual stress) was under the yield stress of the matrix, the relation between the residual stress and the endurance limit was shown by the straight line of gradient -1.0. When the applied stress was over the yield stress of the matrix, the gradient of the relation line became low. But in this case, the residual compressive stress became smaller by cyclic loading. Accordingly, when the final residual stress was used in the relation between the residual stress and the endurance limit, the gradient of the relation line was also -1.0. This decreased value of the residual stress corresponded about 1:1 to the difference between the yield stress and the applied stress.
