Acoustic/Thermal Synergistic Characterization Of Fatigue Damage Of AZ31B Magnesium Alloy And Its FSW Joints

Because of its excellent mechanical properties,magnesium alloy has become the first choice material for transportation and aerospace.Its welding structure often bears the alternating load in the use process.The research of its fatigue performance is very important for the safety of magnesium alloy structure.However,the traditional fatigue experiment is time-consuming and costly,which brings a lot of inconvenience for fatigue related research.Based on the phenomenon of energy dissipation in the process of metal deformation,this paper presents the acoustic / thermal co characterization of the fatigue damage behavior of AZ31 B magnesium alloy and its FSW joint,and the fatigue assessment and life prediction of the material are completed by the relationship between energy evolution and microstructure evolution.In this paper,from the point of view of energy dissipation,the energy dissipation of AZ31 B magnesium alloy and its FSW joint under static and dynamic load is characterized by two energy detection methods,i.e.infrared thermography and acoustic emission.At the same time,the macro deformation behavior and microstructure evolution in the experimental process are measured and analyzed.Through the comparative analysis of three groups of data,the deformation of magnesium alloy and its joint is revealed The essence and evolution of energy dissipation in the process,the energy stress level fatigue fracture assessment model,the static and dynamic mechanical properties data of magnesium alloy and its joints are obtained,and the rationality of the model is verified.The energy dissipation behavior of magnesium alloy and its welded joints under static load is different from that under cyclic load: under static load,the magnesium alloy and its welded joints undergo the process of elastic deformation,yield,hardening and instantaneous failure.The generation of energy dissipation phenomenon is related to the plastic deformation of materials,the rise of temperature and acoustic emission signal during the loading process The results show that the yield stage and the instant break stage are the main generation stages,and there is almost no obvious energy change in the elastic deformation stage and the hardening stage.Through microstructure analysis,it isfound that the microstructure evolution of magnesium alloy under static load is the result of dislocation movement and twinning,and the effective thermal energy and acoustic emission energy are only produced during plastic deformation.Magnesium alloy and its welded joints are accompanied by obvious energy dissipation behavior in the process of cyclic loading.There are three stages: at the initial stage of loading,the material produces obvious plastic strain,and releases a lot of heat and acoustic emission energy;at the second stage,the deformation tends to be stable,cyclic hardening occurs,and the energy dissipation remains at a low level;at the third stage,the crack is unstable,and the rapid expansion leads to the material loss This process produces a second large plastic deformation and releases a lot of energy,resulting in a second rise in temperature and a second accumulation of acoustic emission energy.The results of microstructure analysis show that the main plastic deformation of magnesium alloy and its joints is dislocation movement during cyclic loading,and twins only appear in the limited area of crack tip.The thermal energy evolution and acoustic emission energy evolution of magnesium alloy and its joints under cyclic load are in good agreement,which is in proportion to the applied load in stages.Based on the corresponding relationship between energy dissipation and load stability,a three line model is proposed to evaluate the fatigue damage of magnesium alloy and its joints.The prediction of fatigue life is in good agreement with the actual experimental results,and the prediction error is only 4%.Through the method of acoustic /thermal co characterization,the fatigue damage assessment can be completed by using the energy dissipation of materials in the first thousands of cycles of cyclic loading,which effectively reduces the experimental time and improves the efficiency of fatigue related research.