Push-pull fatigue tests were carried out on two kinds of specimens of 0.33%C carbon steel subjected to a static high tensile load prior to stress cycling. The main objectives were to clarify the behaviour of cyclic plastic strain and to compare the results of the push-pull fatigue tests with those of the rotating bending fatigue tests reported previously. The test results were discussed on the basis of plastic strain change, hardness change, residual stress change and surface observations during fatigue tests.
The conclusions obtained are summarized as follows:
(1) The lower yield point pre-load reduces the push-pull fatigue strength. However, when the pre-load is above the yield point, the fatigue strength becomes higher. This increase of fatigue strength is most pronounced in the case of pre-load close to the tensile strength.
(2) The plastic strain range of the specimen preloaded above the yield point increases gradually up to its failure during stress cycling. On the other hand the specimen preloaded up to the lower yield point shows a similar cyclic plastic strain behaviour to that of the unprestrained specimen.
(3) The cyclic plastic strain becomes smaller with increasing pre-load. This shows that the work hardening caused by pre-load restrains the plastic deformation during stress cycling.
(4) Independently of the magnitudes of pre-loads, there is a good relation between the mean value of plastic strain range 2Δεp and the number of cycles to failure Nf, as given by the following equation:
2Δεp·Nf0.264=0.013
(5) Though the static slip lines generated by pre-loads are subjected to stress concentration during stress cycling, they do not grow to a main crack for failure.
