| Magnesium alloys exhibit excellent properties such as light weight andhigh strength. As a structural material, it has been widely used and it has a broadapplication prospect in the field of industry. As close-packed hexagonal materialwith mere three slip systems, magnesium alloys possess low ductility and mightfracture in brittle manner especially under high stress concentration, which mayeasily lead to the occurrence of disastrous accidents in engineering structureapplication. Fracture toughness, a fundamental evaluation index of crackpropagation resistance, is important in designing, reliability and securityevaluation of structural components under ultimate load. At the same time, therapid development of welding technology broadened the scope of application ofmetallic materials. Therefore, relevant research about fracture failure ofmagnesium alloys and its welded joint will provide necessary theoreticalbasis for its potential application.In this paper, three point bending plane strain fracture toughness test ofAZ31B magnesium alloy and its electron beam welded joint was carried out on aelectronic universal testing machine, the ability of crack propagation resistancefor the base metal and welded joint were compared. The infrared thermal imagerwas used to monitor and record the surface temperature evolution of AZ31Bmagnesium alloy, and the energy dissipation at the crack tip in the quasi-staticfracture process was investigated. At the same time, the influence factors for fracture toughness of AZ31B magnesium alloy was studied, such as thematerial extrusion direction, sample size, loading rate and pre-crack processingmethod, and the fracture morphology and fracture mechanism were analyzed.The fracture toughness of20mm thick AZ31B magnesium alloy sheet withnotch direction parallel to the extrusion direction and perpendicular tothe extrusion direction are15.33~16.07MPa·m1/2and14.52~16.80MPa·m1/2,respectively. And the fracture toughness of20mm thick AZ31B magnesiumelectron beam welded joint is11.04~12.29MPa·m1/2. It can be concluded fromthe result above that the effect of extrusion direction on20mm thick AZ31Bmagnesium alloy is not obvious. The fracture toughness of the welded joint islower than that of the base metal. And under the same conditions, the base metalexhibits higher crack propagation resistance than welded joint.Fracture toughness of AZ31B magnesium alloy subjected to quasi-staticloading was investigated by infrared thermography. The results showed thattemperature evolution around the crack propagation path during fracture can bedivided into three stages: initial steady stage, monotonic increase stage and finalsteady stage. The temperature evolution was formed by the comprehensiveeffect of the thermoelastic effect, plastic dissipation around the crack tip andheat transfer. The energy resulting in temperature rise came from the energydissipation in the crack-tip plastic zone. The temperature increase at thebeginning of stage II is nearly corresponding to the initiation of unstable crackpropagation. And based on this phenomenon, a method applying infraredthermography to estimate fracture toughness of AZ31B magnesium alloy wasproposed. The critical stress intensity factor calculated through infraredthermography is estimated to be16.37~17.11MPa·m1/2. Compared withstandard method, the error is lower than8.50%. Fracture toughness was closelyrelated to the temperature evolution, the larger the fracture toughness, the higherthe temperature increase. The fracture toughness of fatigue pre-cracked10mm thick AZ31Bmagnesium alloy in a non-standard three point bending configuration with a5mm/min loading rate is12.35~13.06MPa·m1/2, the average value is12.71MPa·m1/2, which is lower than that of20mm thick AZ31B magnesium alloysheet. The fracture toughness of fatigue pre-cracked10mm thick AZ31Bmagnesium alloy in a non-standard three point bending configuration with a1mm/min loading rate is10.86MPa·m1/2, which is lower than that of the samespecimen with a5mm/min loading rate. The fracture toughness of wire cuttingpre-cracked10mm thick AZ31B magnesium alloy in a non-standard three pointbending configuration with a1mm/min loading rate is11.49~13.62MPa·m1/2,the average value is12.56MPa·m1/2, which is116%of the value of the10mmthick fatigue pre-cracked specimen.After the fracture toughness test, a certain degree of plastic deformationwas produced near the crack surface. The fracture of AZ31B magnesium alloy inquasi-static fracture toughness test is in ductile-brittle mixed mode. The brittlefracture proportion varies with the value of fracture toughness, the lower thefracture toughness value, the more obvious characteristics of brittle fracture. |
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