Influence of testing frequency on the fatigue behavior in the HCF/VHCF-range of the magnesium alloy AZ31

• Applicant: Professorin Dr.-Ing. Martina Zimmermann
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2020 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 452852691

Due to their high strength and low density, magnesium alloys are an attractive material, which is increasingly used in the automotive and aerospace industries. Since components in these application areas are often exposed to cyclical loading, fatigue behavior is of importance with regard to the possible uses and limitations of an alloy class. In wrought magnesium alloys, which, in contrast to cast alloys, are to a great extent free of coarse defects, the deformation behavior is primarily determined by the hexagonal lattice structure and hence plays a decisive role in the fatigue behavior. In addition to the dislocation glide, the formation of deformation twins is of particular importance . Both mechanisms can cause crack initiation. Which one of them dominates during fatigue damage accumulation depends on various factors, such as the load amplitude. At high load amplitudes, cracks tend to occur in twins, whereas at low amplitudes, crack initiation takes place in slip bands.In recent years, increasing research activities have dealt with the properties under cyclic loading and have investigated various influencing factors, such as the texture of the microstructure, the atmosphere and the influence of the test frequency. However, especially regarding likely frequency influences, there is a mixed picture regarding the fatigue behavior. So far, phenomenological descriptions of the results have primarily been found in the literature. Furthermore, rarely any results can be found on the influence of frequencies above 100 Hz. But especially in the course of the increased investigations regarding the VHCF range, corresponding testing machines were also developed, such as the ultrasonic fatigue testing technique, operating at a frequency of 20 kHz.In the proposed project, the influence of the test frequency on the fatigue behavior of the magnesium wrought alloy AZ31 (Mg-Al-Zn) in the HCF / VCHF range will be investigated at frequencies of 100 Hz and 20 kHz. A special focus of the investigation will be on the deformation mechanisms that contribute to crack initiation. As a further influencing factor on the fatigue behavior of AZ alloys, a pronounced transient behavior and a significant Bauschinger effect could be observed. These aspects are also addressed in connection with the displacement controlled ultrasonic testing system as part of the planned project.

• DFG Programme: Research Grants