| Magnesium alloy is the most common structural material in the density of the smallest metal materials,based on the advantages of low density,the application of magnesium alloy has been widely attention in various fields.With the magnesium alloy molding process research,the comprehensive mechanical properties of magnesium alloy has been further improved,magnesium alloy in transportation,medical,and aerospace and other fields more widely.The crystal structure of magnesium alloy is packed in hexagonal structure,the sliding system is less,the plasticity of the material,the fracture toughness is relatively poor,and the deformation process has obvious twin phenomenon.Therefore,it is of great significance to study the dynamic and static fracture behavior of magnesium alloy.By the advanced non-destructive testing means,that is,infrared thermal imaging technology and acoustic emission(AE),the energy dissipation of the solid material during the process of deformation of the material caused by the surface temperature changes and dislocation slip,twin and crack initiation phenomenon of the acoustic emission signal generated were studied.In this paper,the mechanical properties of the material can be analyzed by using the infrared temperature characteristic and the acoustic emission signal.The material and its welding structure can be monitored and analyzed in real time during the static load or dynamic load process.The research on the material structure in the service process The safety of the important significance.In this paper,the infrared temperature and acoustic emission characteristics of AZ31 B magnesium alloy fracture toughness test were studied by infrared thermal image and acoustic emission method.The dynamic and static fracture process and fracture behavior of magnesium alloy were analyzed.In addition,the axial stretching and three-point bending fatigue behavior of magnesium alloy were studied by infrared thermal imaging technique.(P-V)curve of the AZ31 B magnesium alloy fracture toughness test,the tensile curve of the specimen and the stress life curve(S-N)of the three-point bending fatigue test were obtained.The surface temperature change curve and the AE signal evolution curve of the fracture toughness test specimen were obtained by means of infrared and acoustic emission technique.In the process of tensile and fatigue testing,the temperature change curve of the sample during static axial stretching was obtained by infrared thermal imaging technique,and the temperature evolution curve of the tensile zone during the three-point bending fatigue test was obtained.The fracture toughness test is a typical static fracture process,accompanied by a relatively significant energy accumulation and release process.Therefore,the infrared and AE characteristics of the fracture toughness test of magnesium alloy were studied by using the infrared and acoustic emission methods mentioned above.The load-crack tip displacement(P-V)curve of the tensile toughness test of AZ31 B magnesium alloy was obtained.The curves of surface temperature change and AE signal evolution of fracture toughness test were obtained by infrared and AE techniques.The results show that the PV curve,T-t curve and cumulative data of AZ31 B magnesium alloy fracture toughness test.The ringing time curve(Ccum-t)also can be divided into three stages.The time domain boundary points of the first and second stage is coincided with the P-t curves,and the tensile twins play an important role in the fracture process.Before the crack initiation reaches the maximum load,obvious nonlinear deformation and work hardening can be seen on the load displacement curve.According to the fracture curve,microstructure evolution and EBSD analysis show that the tensile twin is mainly in the region before the HCP metal crack tip.In addition,studies have shown that tensile twins play a key role in reducing the fracture toughness of the specimen by reducing the propagation of cracks in the three-dimensional direction.The microstructures of AZ31 B alloy were analyzed by EBSD and SEM,and the characteristics of AE signal and microstructure were combined.Discuss the generation of AE sources,and conclude that the fracture of the specimen has different forms of damage mechanism at different stages.The damage mechanism in the three stages is as follows: the first stage PV curve is characterized by linear rise,deformation mainly in dislocation slip,due to the small amount of deformation,only produce a small amount of twin;the second stage curve for the nonlinear rise,At this stage in order to coordinate deformation,magnesium alloy produced a large number of twin;Finally,the sample instability stage,the load decreased,crack propagation quickly.In addition,the axial tensile and three-point bending fatigue behaviors of magnesium alloys were studied by infrared thermal imaging technique.The tensile curves of the specimens and the stress life curves(S-N)of the three-point bending fatigue test were obtained.In the AZ31 B quasi-static tensile test,the surface temperature of the sample was recorded in real time with infrared thermal imaging system.The relationship between the tensile curve and the temperature curve of AZ31 B magnesium alloy is discussed.The mechanism of the surface temperature change during the tensile test is analyzed.According to the thermoelastic effect,the temperature change of the specimen is proposed.The method of estimating the elastic limit of the magnesium alloy and the formula for predicting the stress concentration coefficient of the specimen during the temperature change rate.In order to study the dynamic fracture behavior of AZ31 B magnesium alloy,the three-point bending fatigue behavior of AZ31 B magnesium alloy was analyzed.During the fatigue process,the sample surface temperature change data were recorded in real time with an infrared thermal imager.The characteristics of heat generation and heat transfer are described,and the relationship between temperature and stress is analyzed.During the three-point bending fatigue test,the surface temperature of the test has gone through four stages:(Ⅰ)the initial temperature rise stage,(Ⅱ)the temperature drop stage,(Ⅲ)temperature stabilization stage,and(Ⅳ)Abruptfracture stage.The results show that the fracture mechanism of the specimen is quasi-dissociation fracture.In order to analyze the reason of temperature change,the twins change characteristics are analyzed.Based on the theory of fatigue failure accumulation,a theoretical formula for predicting the fatigue life of AZ31 B magnesium alloy was established.The fatigue strength obtained by Luong method is 128.97 MPa,the test result is 133.87 MPa,the two are very close,the prediction model is 137.33 MPa,which is almost the same as the test result.Finally,the fracture of three kinds of samples was analyzed by SEM.The grain change,twin phenomenon and texture change of the samples before and after the fracture of magnesium alloy were analyzed by metallographic analysis technique and EBSD technique. |
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