An analysis of frequency effect in fatigue data of metal alloys with different crystal structures

In the last decades, ultrasonic fatigue testing has became an interesting and competitive technology to characterize the very high cycle fatigue regime (VHCF) (the region beyond the conventional fatigue limit of 10 millions of cycles), promoting the extension of the service life of engineering components. These testing systems can provide a considerable reduction on the testing time, since they can typically operate at high frequencies around 20 kHz (while conventional machines usually operate at frequencies between 10 Hz and 100 Hz ). However, a frequency effect, with higher or lower impact, can be observed in experimental fatigue data for certain materials, when they are tested at different frequencies, particularly at ultrasonic frequencies. As consequence, the applicability of ultrasonic experimental fatigue data can be compromised. This work intends to comprehend the frequency effect as well as to identify the main causes of it. Therefore, the characteristics of crystal structures are related to the fatigue behaviour observed in different metal alloys as a way to explain the strain rate sensitivity, and consequently, the frequency effect.