| Magnesium alloys possess many excellent properties, such as low density,high specific strength, high stiffness and recyclability, which make magnesiumalloys very attractive as structural materials in a wide variety of applications,such as aerospace industry, car industry and marine industry. As a new type ofstructural material, its wide application can not be separated from the weldingmethod. When the components are subjected to cyclic loading, the fatigueproblems of welded joint usually become the bottleneck of the total performance.So the fatigue behavior and fracture mechanism of magnesium alloys weldedjoint is urgently needed to be studied.In this paper, temperature evolutions of the magnesium alloy which hadbeen jointed by electron beam welding in tensile tests, fatigue tests and fatiguecrack growth tests were analyzed by infrared thermography. And theoreticalmodels were formulated to explain the mechanisms of heat generations, respectively. The fatigue limit and the threshold value KTfor unstable fatiguecrack growth were predicted by infrared thermography. Finally, combined withthe fracture morphology, the fracture mechanisms were analyzed.Result shows that four stages of superficial temperature evolution wereobserved during the tensile tests: an initial temperature decrease in whichthermoelastic effect is in dominant status, a temperature increase in whichinelastic effect is in dominant status, an abrupt temperature increase wheresevere strain-hardening effect appears and a temperature drop after the failure.The temperature evolution undergoes five stages during a fatigue test: aninitial temperature increase in which inelastic effect is in dominant status, atemperature decrease in which heat conduction effect is in dominant status, atemperature equilibrium in which dynamic equilibrium is achieved betweeninelastic effect and heat conduction effect, an abrupt temperature increase wherefatigue cracks initiate and final drop.Four superficial temperature variation stages of crack tip in weld wereobserved during the fatigue crack growth test: an initial temperature increase(rapid crack growth, da/dN=9.05×10-14(ΔK)9.13), a temperature equilibrium(stable crack growth, da/dN=1.01×10-7(ΔK)2.49), an abrupt temperature increase(unstable crack growth, da/dN=3.38×10-16(ΔK)10.17) and a temperature drop afterfailure.The fatigue limit and the threshold value KTfor unstable fatigue crackgrowth predicted by infrared thermography were83MPa and17.73MPa m1/2, which agrees well with the conventional experimental results.Infrared thermography can qualitatively identify the evolution of fatiguedamage in real time. The evolution of color and hot spot zone on the specimensurface can be used as an effective tool to assess the fatigue damage inspecimens during fatigue tests.Three different regimes were found during the fatigue crack growth.,fracture morphology shows a cleavage fracture characterized by a river patternin regime Ⅰ, The predominant fracture mode is quasi-cleavage fracture inregime Ⅲ, both fracture modes coexist in regime Ⅱ. |
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