Fatigue life is a consequence of multistage processes. At the beginning of a fatigue experiment the cyclic deformation behavior is dominated by basic dislocation processes, like strain hardening and softening. During further cycling localization of plastic deformation takes place. Depending on the microstructures strongly localized plastic deformation may lead to trans- or inter-granular crack initiation. From these crack nuclei larger fatigue cracks can develop, which finally lead to macroscopic fatigue failure.
In order to investigate the influence of different microstructures on relevant deformation and damage mechanisms in the HCF and VHCF-regimes three plain carbon steels: C15E (SAE 1017), C45E (SAE 1045) and C60E (SAE 1064) with different ferrite to pearlite ratios where investigated. C60E consists of a pearlite matrix with embedded ferrite grains, C45E has a microstructure with approximately the same amounts of ferrite and pearlite phases and C15E consists of a ferrite matrix with isolated pearlite grains. The varying pearlite/ferrite ratios lead to different deformation behaviours and fatigue lives. C15E and C45E show late fatigue failure after more than 107 cycles, whereas such late fatigue failure is absent in C60E. To monitor fatigue damage a new method was developed where the dissipated energy per fatigue pulse is calculated from the generator power input. Using these data significant changes in the cyclic deformation behaviour can be monitored for the whole fatigue life. Most interestingly, the data of the dissipated energy also enable to clearly determine at a very early stage whether there will be (late) fatigue failure or not.
In addition, different heat treatments were applied to C15E to change the ferrite pearlite microstructure to a bainite microstructure. The changes in the microstructure trigger slip planarity and consequently the crack initiation sites switch from grain/phase boundaries to the grain interior.