Theoretical study on the strength of steel elements with notches under cyclic loadings

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This work consists of experimental and theoretical investigation. In the first part, fatigue tests were carried out for the structural steel specimen with notches. Various magnitude of prestraining was applied by overloads to the specimen before the test. It is confirmed that the prestraining affects the fatigue strength. As long as the prestraining is moderate, it works in favor of fatigue strength. When the magnitude of overloads is fixed, the higher stress concentration at notches results in larger fatigue strength. It is also true, however, fatigue strength degrades with too large prestraining, Observation of crack propagation revealed that the prestraining controls mainly the first stage of fatigue fracture, or the stage of crack initiation. In the second part, this phenomenon was studied theoretically and quantitatively by elasto-plastic analysis. Since very large deformations are inevitable around stress concentrator, the theory of elasto-plasticity at finite deformation was employed. It is assumed that plastic flow is governed by the J2 rule with combined isotropic-kinematic hardening. Governing differential equations were solved numerically by an FEM. It was numerically shown that prestraining causes the residual stresses in the neighborhood of stress concentrators. The elasto- plastic analysis indicated that the seemingly complex relation between the prestraining and the fatigue strength was easily understood in view of such residual stresses. Within the assumptions employed in this work, the residual stresses do not disappear with small cyclic loadings. Therefore, the method of fracture mechanics can be applied to the phenomenon without any difficulty.