Residual Stresses Produced by Plastic Tension in Notched Plate Specimens and Fatigue Strength

Abstract

Plate specimens of a medium carbon steel (0.48%C) with sharp V notches on both edges were stretched so that the nominal tensile stress at the minimum cross section would have the values of 0.8σ₈ and σ₈, respectively. As the result of this plastic tension, residual compressive stresses were introduced around the notch root. These residual stresses were measured by the X-ray diffraction method, and their changes due to alternating stressing in reversed bending were investigated. In the fatigue tests, the specimens with residual stresses were found to have higher fatigue limits and also prolonged fatigue lives over those without residual stresses, and these improvements were larger in proportion to the values of residual stresses. The growth of fatigue cracks was also investigated, and it was clarified that residual compressive stresses have an effect of suppressing the propagation of fatigue cracks. Applying the theory of threshold stress to spread the fatigue cracks proposed by T. Isibasi, and based on the change in residual stresses and on the crack growth curves, the contribution of residual stresses to the fatigue strength was calculated.