A Study On The Uniaxial Ratcheting Effect And Low-Cycle Fatigue Behavior Of AZ91Magnesium Alloy

As a lightweight and high-performance metal material, magnesium alloy is widely used in automotive, aviation, aerospace, electronics and communications industries, and the excellent fatigue performance is required. However, magnesium alloy structural components inevitably subject to the asymmetric cyclic loadings during the actual service process, and then the ratcheting effect will occur, which severely influence their fatigue performance. So, the ratcheting effect and its effect on the fatigue behavior must be considered in the safety assessment and fatigue life estimation of magnesium components. In this dissertation, the uniaxial ratcheting effect and low-cycle fatigue characteristics of AZ91magnesium alloy are investigated. The failure mechanism under the combined function of the ratcheting effect and low-cycle fatigue is analyzed. The results and achievements of the dissertation are as follows:(1) The cyclic characteristics and the uniaxial ratcheting effect of AZ91magnesium alloy are investigated through asymmetrical cyclic loading experiments. The results indicate that magnesium alloy displays cyclic hardening characteristic for the first more cycles, and then shows cyclic softening at room temperature. The uniaxial ratcheting strain increases with the increasing of mean stress, stress amplitude, peak stress, peak/valley holding time. In addition, ratcheting effect is significantly influlenced by stress loading history. Besides, there is serious anisotropy property in rolling magnesium alloy plane, which leads to larger ratcheting strain produced in rolled direction than transverse direction.(2) Based on energy dissipation mechanism during fatigue failure process, the effects of various loading parameters on low-cycle fatigue behavior are studied. The results indicate that the low-cycle fatigue life decreases with the increasing of mean stress, stress amplitude and peak stress, while improves with the increasing of stress ratio. (3) Based on low-cycle fatigue life experimental results, a model for estimating the low-cycle fatigue life of AZ91magnesium alloy is established by correcting the fatigue strength coefficient and fatigue strength index of the Basquin model, which considers the influence of loading parameters on low-cycle fatigue life. The error dispersion between predicted values and experimental results is only23.77%, which indicates that the proposed model has high prediction accuracy.(4) The failure mechanism under the combined function of the ratcheting effect and low-cycle fatigue is analyzed through fatigue fracture morphology observation and optical microstructure analysis experiments. The results shows that at the beginning of the stress cycles, there is severe ratcheting effect, which results in the initiation of cracks at the twin boundary, dislocation pile-ups, etc; duiring the middle of the cyclic stress cycles, the ratcheting strain is the driving force of crack growth because of the ratcheting strain always develops in direction of the maximum ductility of the material; In the later of the cyclic stress cycles, due to the inner defects such as voids growth and crack propagation are accelerated severely, leading to more obvious internal stress concentration, which decreases the bearing capacity of material, so fracture failure occurs finally.