Abstract
Setting process is often practiced on coil springs in order to improve their fatigue resistance and prevent their creep deflection. Torsional residual stresses are produced by this process, and it is generally understood that these stresses would play a role in improving the fatigue properties. In this experiment, round bar specimens of the spring steel SUP2 were used, and after being twisted by the torsional moment 25% beyond that corresponding to the yield point, they were subjected to the fatigue test in alternating torsion. The distribution of residual stresses was measured by the etching method, by measuring the angle of torsion during the etching process. Three stress levels were employed in repeated stressing and the number of stress cycles was made to be the same in each stress level. As a new attempt, we studied the fading of residual stresses under repeated stressing in successive two stress levels.
The results obtained are summariaed as follows:
(1) Residual stresses produced by plastic torsion are of the thermal stress type near the surface, being negative at the surface layers.
(2) Residual stresses subjected to repeated stressing fade noticeably in the first stage of fading and then gradually with the repetition of stress cycles. In the second stage of fading, the relation obtained between the ratio of surface residual stresses τr/τo, (τr is the current value and τo is the initial value of surface residual stress) and the logarithm of cycle ratio n/N, formed straight lines, and experimental formulas concerning the fading of residual stresses were established.
(3) In repeated stressing under successive two stress levels, the fading of residual stresses is larger in the case of descending stressing than in the case of ascending stressing, when the same numbers of stress cycles are given to each stress level, respectively. Hardness has also the same tendency as the residual stress.
