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

镁合金具有高强度、低密度等优点，是一种极具吸引力的材料，在汽车、航空航天等领域得到了越来越广泛的应用。由于这些应用领域中的部件经常暴露在周期性载荷下，因此疲劳性能对于合金类别的可能用途和限制非常重要。与铸造合金相比，变形镁合金在很大程度上没有粗大缺陷，变形行为主要由六方晶格结构决定，因此在疲劳行为中起决定性作用。除位错滑移外，形变孪晶的形成尤为重要。这两种机制都会导致裂纹萌生。在疲劳损伤累积过程中，它们中的哪一个起主导作用取决于各种因素，如载荷幅度。在高载荷下，裂纹倾向于孪晶，而在低载荷下，裂纹发生在滑移带上。近年来，越来越多的研究活动涉及循环载荷下的性能，并研究了各种影响因素，如微观组织的织构，气氛和试验频率的影响。然而，特别是关于可能的频率影响，关于疲劳行为的情况好坏参半。到目前为止，对结果的现象学描述主要是在文献中找到的。此外，关于100赫兹以上频率的影响，很少能找到任何结果。特别是随着对真空热流变液范围研究的不断深入，相应的试验设备也被开发出来，如工作频率为20 kHz的超声波疲劳试验技术。在拟议的项目中，将研究在100 Hz和20 kHz频率下，试验频率对镁变形合金AZ31(Mg-Al-Zn)在高压氢/真空热流变液范围内疲劳行为的影响。调查的一个特别重点将是促成裂纹萌生的变形机制。作为对AZ合金疲劳行为的进一步影响因素，可以观察到显著的瞬时行为和显著的包辛格效应。作为计划项目的一部分，还结合位移控制超声波检测系统讨论了这些方面。

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.