抄録
A fatigue process consists of crack initiation and crack propagation. So the authors have tried to estimate the fatigue life to failure, Nf, as the sum of the crack initiation life Ni and the crack propagation life Np. The authors also assumed that the effect of the mean stress on the fatigue damage is different in each of these two processes. Then, the actual service load patterns have been analysed by applying the range-pair-mean count method as follows. Each stress range pair σa with mean stress level σm, that is S(σa, σm), is translated into a stress amplitude σt by taking σti=σa+mσm, where m=0.4σw/σs, in the crack initiation, and σtp=σa+kσm for σa>|σm|, k=0.6∼1.0, or σtp=(1+k)(σa+hσm)/(1+h) for σm>σa>0, where h=0∼0.5, in the crack propagation process. Then the fatigue damage FN' which is produced in the N'th stress cycle, is expressed as
FN'=eAσt+D·e-γ(N'-1)
where A, D and γ are constants. These constants, which are given as Ai, Di and γi for the crack initiation and Ap, Dp and γp for the crack propagation, can be determined from the fatigue test results.
The fatigue tests on annealed low carbon steels under plane bending condition were conducted. The surfaces of the test pieces were observed occasionally with an optical microscope and by the replica method, and the micro-cracks were detected. The fatigue tests under varying mean stresses in stepwise were also conducted. Those test results were analysed by means of the method mentioned above, and the estimated values had a good agreement with the test results. It was also clarified that the fatigue damage is affected by the stress waveforms and their repeating speeds.
