Under very high cycle fatigue (VHCF) loading, failure of high-strength steels is mostly initiated at internal imperfections, such as non-metallic inclusions. This is in contrast to classical low cycle fatigue (LCF) and high cycle fatigue (HCF), where cracks initiate mostly at the surface. Furthermore, there is a specific crack initiation mechanism observed in VHCF, whereby a so-called fine granular area (FGA) is formed in the immediate vicinity of the crack-initiating non-metallic inclusion. This FGA consists of a very fine-grained microstructure, which is produced by the localization of the cyclic stresses and thus a localized plastic deformation around the inclusions. The local grain refining is probably due to the formation of energetically favourable dislocation cells in the plastic zone around inclusions. Those cells build new grain boundaries during further cyclic loading and these smaller grains cause a reduced threshold value of the stress intensity factor for long crack growth leading to failure at lower stress amplitudes in VHCF than in HCF.
The aim of our project is to increase the VHCF-lifetime and the VHCF-strength. Therefore, it is our hypothesis that it is necessary to suppress the above mentioned grain refinement process by a stabilized dislocation structure. This stabilized dislocation structure might be realized by a thermomechanical treatment (TMT) process applied in the temperature range of maximum dynamic strain aging. During the TMT, particularly in the area of stress exaggeration, in the immediate vicinity of the non-metallic inclusions, the dislocation structures are modified, stabilized and, thereby, counteract grain refinement during VHCF loading.
In order to verify the effect of dynamic strain aging on the VHCF-fatigue strength ultrasonic tests were carried out up to the ultimate number of 109 cycles. Additionally, fracture surfaces and microstructure were investigated for thermomechanically treated and untreated specimens.