EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

For a long time it was well accepted, that steel structures can be exposed to service load spectra involving very high numbers of cycles (up to one billion of cycles and more) based on fatigue strength data limited to not more than 10 millions of cycles. Since several severe accidents and more recent experiments had shown that fatigue failure may occur at very high numbers of cycles exceeding 10 million, the testing procedures needed to be adapted. Today, testing facilities operating at frequencies up to 20,000Hz allow for reaching 1 billion of cycles during one day only. However, does this rapid testing represent the real-world fatigue conditions in an adequate way? VHCF damage sets in as very localized cyclic plasticity, the mechanism of which may change when the frequency is increased. Depending on the material's strength, local self heating or the strain-rate sensitivity of carbon steel lead to a change of the fatigue strength at high frequencies as compared to low frequencies. In addition to that, time-dependent reactions at persistant slip bands (PSBs) or small cracks with the environment (corrosive, laboratory or vacuum atmosphere) may alter the overall fatigue damage rates. For the example of different grades of carbon steel, the present paper summarizes the potential frequency effects during VHCF testing and discusses the respective microstructural mechanisms.