One of the purposes of the present study was to investigate the P-S-N
diagrams at stress levels around the ordinary endurance limit, and to discuss the distribution of fatigue lives under constant load. Another purpose was to investigate the effect of variation of stress amplitude by comparing the P-S-N
diagrams under the program load of two-stress repeated-block type with those under constant load. Carbon steel S50C and chromium-molybdenum steel SCM4 were used as the specimens and the tests were conducted under cantilever type rotating bending stress.
The major conclusions obtained are as follows:
curves do not reach a constant level but decrease even in the stress region below the ordinary endurance limit. And P-S-N
curves tend to bend toward the left at stress levels around the ordinary endurance limit.
diagrams are thought to be composed of three H-, F
-, and S
-curves, each of which represents the S-N
relation based on the different failure mechanism. H
-curve represents the S-N
relation at stress levels well above the endurance limit, F
-curve appears at stress levels around the ordinary endurance limit, and S
-curve at still lower stress levels.
(3) The effect of variation of stress amplitude on the fatigue life appears as the fall of the endurance limit, or the decrease of probability of survival at stress levels below the ordinary endurance limit.