| As one of the lightest structural materials, magnesium and its alloys arewidely used in aerospace, transportation and other fields for its high specificstrength/stiffness ratio, recyclable. When it comes to practical application,magnesium alloys and their welded components inevitably subject to dynamicloading, especially subject the cyclic loading. According to the research, morethan80%of the the total fracture is due to fatigue failure. Therefore, the studyof the mechanism of fatigue fracture of magnesium alloy and service evaluationtheory has theoretical and practical value. The commonfatigue assessment islimited by the sample shape, amount of tests and so all. Infrared thermographybased on the energy theory is a method for the material fatigue assessment. Thefatigue life can be predicted by using single sample in theory. Currently usingInfrared thermography to predict the fatigue life is mainly used in steel material,this article study on fatigue behavior of magnesium alloys and its fatigueevaluation theory, which has important theoretical and practical significance.This paper studied the temperature evolution in the process of the stretchingand high cycle fatigue loading of AZ31B magnesium alloy based on energy conversion law. The elastic limit and structural stress concentration factor ofmagnesium is calculated using of temperature change on it and based onthermo-elastic effects. Based on irreversible deformation and microstructureevolution of materials, high cycle fatigue heat production mechanism ofmagnesium alloy were studied. Using the deformation and evolution oftemperature analyzed the evolution of the crack tip plastic zone of magnesiumalloy and the relationship between the crack growth rate and temperatureevolution rate of the tip. And applied the infrared thermal imaging method tofatigue assessment of welded joints of magnesium alloy. Mainly findings are asfollows:Based on thermo-elastic effect, stress is inversely proportional to the changeof temperature in the stretching process of magnesium alloy. When the stressexceeds the linear deformation, the temperature evolution trend changes. Usingthe stress corresponding to the point of the linear change of temperature, theelastic limit of the material can be measured. It measured the stressconcentration factor of different structures of the samples using of temperaturevariation when loaded in the elastic range and the ratioKt TmaxTvof thetemperature at the stress concentration and temperature values at the uniformstructure, the results obtained with the theoretical calculations related to lessthan10%.Fatigue failure is a process of cumulative damage with the dissipation ofenergy, Most of the plastic strain energy is converted into heat, and displayed in the form of variations in temperature. Unlike the temperature evolution offatigue of steel and other materials, When cyclic loading is above the fatiguelimit (σmaxabove110MPa), the temperature evolution mainly undergoes5stages:initial increase(phaseI), steep reduction (phaseII), steady state(phaseIII), abruptincrease(phaseIV), and final drop(phaseV). when the cyclic loading is belowthe fatigue limit (σmax110-25MPa) the temperature evolution mainly among theinitial temperature. With the increase of loading, temperature has little scope toincrease and then reach a balance. Fatigue loading maximum stress σmaxand thechange of temperature has the linear relationship. when the cyclic loading σmaxis below22.5Mpa, the temperature evolution of the specimen can be dividedinto two parts, the initial temperature decline stage and the steady stage later.The deformation of magnesium alloys under cyclic loading is awork-hardening process. The Initial deformation rate caused the temperatureincreasing of the specimen surface. The deformation rate reached balance as thecycles reach to104. The temperature is steady following drop to a certain value.During the rapid deformation stage (stageI), the dominant micrdeformation ofthe magnesium alloys is twinning. While dislocation glide is based during theequilibrium deformation stage (stage III). The mechanism of heat generationand temperature evolution during the high-cycle fatigue loading is studied withthe plastic damage model. Results show that, high-cycle fatigue damage of themagnesium alloy is a nonlinear deformation process. The macro cyclichysteresis loop gradually stabilized, and the deformation rate is reduced. The heat production process of the magnesium alloys during the fatigue test issimulated with the ABAQUS finite element software plastic damage model andthe thermal coupling unit. The temperature evolution curve corresponded to thetest results.Infrared thermography is used in magnesium alloy to study temperatureevolution in the proces of fatigue crack propagations. The temperature evolutionis related to the tip plastic deformation rate. With the increasing of crackexpansion rate, the tip temperature changes in a process of slow heating andrapid heating. In the steady expansion phase, there is no significant change intemperature. In the rapid expansion phase, the temperature rises linearly.Temperature evolution rate of magnesium alloy has the same regularity with thecrack growth rate. It can establish a linear relationshipdad C0K nTusing theratio da/dT of crack length and temperature change and the stress intensityfactor ΔK, calculate the crack tip plastic zone using the crack tip temperaturechanges of unit crack propagation when stress and crack length is known.The deformation and heat generation were affected during fatigue crackinitiation and generation. There are {0002}<21—1—0> and {0002}<101—0> basetextures in AZ31B magnesium alloy. The Schmid factor is zero in bothdirections. Base testures are hard orientation in both diredtions that lessdeformation would be made. The Schmid factors of non-base textures weredifferent in the two directions that mED>mTD; The deformation in ED easier thanTD specimen. Therefore, the temperature in the initial loading is higher in ED than that of TD.Infrared thermography is used to predict the fatigue properties ofmagnesium alloy and its welded joint and fatigue limit of107using three-linemethod, the slope method and area method respectively. The remaining life ofmagnesium alloys can be calculated through the area surrounded by thetemperature curve and the axes. And it can obtain the S-N curve of magnesiumalloy using infrared thermal imaging method. |
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