Investigation On The Mechanical Property Test And Ballistic Resistance Of 6061-T651 Aluminum Alloy

At present,with the rapid development of aviation and aerospace industries,the demand for aluminum alloy is increasing and its dynamic mechanics performance requirements are getting tougher.At the same time,the aircraft suffered impact events with exotic objects(such as birds,hail,sand and metal et al.)frequently in recently years.In addition,the 6061-T651 aluminum alloy is mainly used in aircraft fuselage skin.Therefore,it is of practical significance for aircraft structure design and maintenance that investigate the mechanical properties of this aluminum alloy and the ballistic impact resistance against the exotic.In this context,many experts and scholars at home and abroad gradually began to pay attention to the research on the protective effect of aluminum alloy,which including the constitutive model and fracture criterion of aluminum alloy materials as well as the ballistic impact resistance against the exotic.However,there are relatively few investigates on the mechanical properties(constitutive model and fracture criterion)and ballistic resistance of6061-T651 aluminum alloy at home and abroad.In the present study,the mechanical performance and ballistic resistance test are conducted for the 6061-T651 aluminum alloy which is taken as the research object,and the corresponding numerical simulations are investigated.The present study selected 2 mm thick plate for ballistic resistance test and numerical research since the thickness of aircraft skin was between 2 mm and 5 mm.In addition,most scholars investigated on thin and medium thickness plates in the field related to ballistic impact,while there are little reports on the study of thick plate.Therefore,the present study conducted ballistic resistance test and numerical research where 20 mm thick plate was taken as research object.Firstly,the mechanical property of 6061-T651 aluminum alloy under quasi-static,high temperature and high strain rate are tested by universal material testing machine and Hopkinson pressure bar respectively.The mechanical property test results show that the flow stress has no significant change with the increase of plastic strain,so the material has no obvious strain strengthening effect for the 6061-T651 aluminum alloy material.However,the flow stress increases significantly with the increase of strain rate while decreases greatly with the increase of temperature,so the material has obvious strain rate hardening effect and temperature softening effect.The fracture strain of 6061-T651 aluminum alloy material decreases with the increase of stress triaxiality.And also,the fracture strain of material is related to the Lode angle parameter,especially the effect of theLode angle more significantly at the range of lower stress triaxiality.Basing on the test results of mechanical properties,the MJC model was obtained by modifying Johnson-Cook constitutive model and fracture criterion,and the parameters of MJC model and Modified Mohr-Coulomb(MMC)fracture criterion are calibrated.Subsequently,the ballistic resistance test of 6061-T651 aluminum alloy thin plates struck by different nose shapes of projectiles were conducted on an one-stage light gas gun,and the corresponding numerical simulation was conducted by using MJC constitutive model and either the MJC or MMC fracture criterion.It is found that Lode angle has different effects on different failure modes of target plates.Meanwhile,the results show that the effect of Lode angle in numerical prediction has the greatest influence when the target struck by blunt projectile,followed by hemispherical and spherical projectile,while the effect of ogival projectile is the least,which is related to the curvature radius of projectile nose shape,i.e.,the influence of Lode angle decreases with the increase of curvature radius.Finally,the ballistic impact test was conducted on 20 mm thick 6061-T651 aluminum alloy penetrate by blunt projectile at an one-stage light gas gun.Meanwhile,the numerical simulation corresponding to the test was conducted by using MJC constitutive model and either the MJC or ASCE fracture criterion.It is found that the results predicted by using the ASCE fracture criterion was obtained very well accordance with the test results(in 0.15%)comparing with the MJC fracture criterion.In addition,for the ductile metal,the effect of Lode angle decreases with the ductility increase of metal material under the action of blunt projectile penetration was found by the simulation result by using the Lode angle dependent fracture criterion.To sum up,the mechanical properties of 6061-T651 aluminum alloy materials are systematically investigated in this paper,and the ballistic impact properties of thin plates against projectiles with different nose shapes,the high speed penetration effects of thick plates against blunt projectiles,and the corresponding numerical simulation were studied.Investigation method of the hybrid experimental-numerical was adopted in the present study,and the application of the Lode angle dependent fracture criterion in the numerical prediction has a good reference and application value in the related filed of impact dynamic mechanical.The results have practical engineering implications for high-speed impacts,explosions,structural collisions,aerospace industries and other transportation fields.