As a basic study on fatigue damage accumulation of structural members subjected to service load, fatigue tests under superposed stress and varying stress amplitude have been carried out in the past, and various fatigue life prediction methods have been proposed.
To investigate the applicability of six different cycle counting methods to more complicated stress pattern than the superposed stress, the in-plane bending tests were carried out on the specimens made of low carbon steel S10C under the combined superposed stress having such a pattern that the maximum stress amplitudes of two superposed stresses were varied stepwise or that the superposed stress and sine wave were mutually repeated. The cycle counting methods investigated were zero cross range pair mean method (ZRPM), range pair mean method (RNPM), peak method (PEAK), range pair method (RNPR) among others. The modified Goodman's method was used to evaluate the effect of mean stress on stress amplitude. The lives estimated by using the linear cumulative fatigue damage law were compared with the experimental lives, and the applicability of the cycle counting methods was examined.
The results obtained were as follows;
(1) For the superposed stress wave, the ratio of the experimental life (N) to the estimated life (Nes) was within the range of 1/3≤N/Nes≤3 except the PEAK method.
(2) For the combined superposed stress wave and the superposed-sine stress wave, the ratio of the experimental life (N) to the estimated life (Nes) was within the range of 1/3≤N/Nes≤3 except the PEAK and RNPR method. The PEAK method showed a conservative life against the experimental results, but RNPR method was not.
(3) The scatter band of the estimated-experimental life ratio by the ZRPM and RNPM method was smaller than that of the other 4 kinds of cycle counting methods.
