Fatigue behaviour of reinforcing steel in the very-high-cycle-fatigue regime N ≥ 10 millions

The fatigue behaviour of reinforcing steels in the very-high-cycle-fatigue regime is almost unexplored. Especially wind turbines (on- and offshore) are exposed to these high numbers of cycles caused by wind and tide. 80 reinforcing steels with two different diameters (10 and 16 mm) were tested on three different stress ranges for investigating into this topic. All experiments ran in a high frequency pulsator with a frequency of about 100 Hz, some of them reached up to 100 million numbers of cycles. The surface quality of the specimen with a larger diameter had a great influence on the fatigue behaviour. These specimen failed early at areas with surface damages caused by the rolling or straightening process. The thinner specimen reached the given number of cycles without any failure. A great increase of the failure type "breakage" could be detected with increasing stress range. This was independent of the specimen diameter. A greater decrease of the survival probability was detected for specimen with a larger diameter. The surface of run-out specimen was examined by a scanning electron microscope. Many cracks have been detected especially at areas with high notch stresses like rib feet or notches caused by the straightening process.