| Steel structure has been widely promoted because of its light weight,high strength,good seismic performance,beautiful building shape,short construction period,flexible functional space layout,green environmental protection and recyclability.But the uneven settlement of foundation and earthquake will damage the steel structure.The steel will have different degree of strain ageing in a period of time after damage.The steel will harden and become brittle after strain ageing.If the initial stress-strain constitutive model is still used to reinforce and repair the structure after strain ageing,it will easily lead to the problems of poor seismic performance and easy cracking.In this paper,strain aging test and butt weld test of Q460 C and Q345 B steels are carried out.The constitutive relationship of Q460 C steel material test is fitted by modified Ramborg-Osgood model.The welding residual stress and monotonic tensile simulation of Q460 C steel weld specimens are carried out by finite element method.The main conclusions of this paper are as follows:(1)Twenty-six specimens of Q460 C steel and Q345 B steel were fabricated respectively.The mechanical properties of the two steels(mainly Q460 C steel)under the influence of strain aging were analyzed,and the effects of strain aging on the mechanical properties of the two steels were compared.The constitutive relationship of Q460 C steel was fitted by the modified Ramborg-Osgood model.The results show that:(1)the strain hardening phenomenon of steel after pre-straining is obvious,and the strain hardening phenomenon of the specimen can be restored in the range of 6h-12 h.(2)Age hardening occurs in steel after aging.The stress-strain curves of specimens differ slightly between different aging stages,and the hardening degree of steel after aging hardening is lower than that of strain hardening.(3)Compared with specimens H1-1,the yield strength,ultimate strength,ultimate strain and fracture strain of specimens H4-5increased by 27.9%,3%,80.3% and 53.9% respectively under 7% pre-strain and 30-day aging.With the increase of strain aging,the strength of Q460 C steel increases,the ductility decreases greatly,and the possibility of brittle failure of the structure increases.(4)Strain aging increases the yield-strength ratio of steel,the yield-strength ratio of specimen H1-1 is about 0.80,the yield-strength ratio under 2% pre-strain is about 0.85,the yield-strength ratio under 4.5%pre-strain is about 0.95,and the yield-strength ratio under 7% pre-strain is close to 1.0.With theincrease of strain aging,the brittleness of steel increases.The elastic modulus of steel decreases after strain aging.Under 7% pre-strain and 30 days aging,the elastic modulus of specimen H4-5is 136.2 GPa.The modified Ramboerg-Osgood model is in good agreement with the experimental results,and can accurately fit the constitutive relationship of Q460 C high strength steel after strain aging.Contrast the effect of strain aging on Q460 C steel and Q345 B steel,the effect of strain aging on the strength of Q345 B steel is more significant,and the ductility of Q460 C steel is more significant.(2)Seven weld joint specimens were made for Q460 C steel and Q345 B steel respectively.The mechanical properties of the butt weld joint specimens of the two steels under the influence of strain aging(mainly Q460 C steel)were analyzed,and the effects of strain aging on the mechanical properties of the two steels were compared.The results show that:(1)under 1.5%pre-strain and 30-day aging,the yield strength and ultimate strength of specimens HW4-3increase by 35.1% and 5.1% respectively,the yield-strength ratio is 0.988,and the ultimate strain decrease by 47.0%;under 1.5% pre-strain and 30-day aging,the yield strength and ultimate strength increase by 48.7% and 3.4% respectively,and the ultimate strain decrease by 95.6%.(2)The effect of strain aging on the yield strength of welded joint specimens of Q345 B steel is more significant than that of Q460 C steel,and the effect on the ultimate strength and yield strength ratio is basically the same.The effect on the ultimate strain,fracture strain and elastic modulus of welded joint specimens of Q460 C steel is more significant than that of Q345 B steel.The ultimate stress of welded joint of Q345 B steel decreases while that of welded joint of Q460 C steel increases after aging for 2 days.(3)there are two failure modes of butt weld connection specimens,one is necking fracture at the base metal,the other is tearing at the weld.The failure position of the specimen without strain aging is at the base metal,and after strain aging,the failure position of the specimen is at the weld joint.After strain aging,there will be some microcracks in the weld.In the second stage of tensile test,the weld is more likely to be damaged.(4)Both pre-strain and aging can affect the mechanical properties of weld joint specimens,making weld joint specimens harder and brittle,but the effect of pre-strain on weld joint specimens is more significant.(3)The welding residual stress and monotonic tension of Q460 C steel weld joint specimens were simulated by ANSYS finite element method.The following conclusions are drawn:(1)From the temperature distribution of the model,it can be seen that the temperature at the weld is obviously higher than that at the base metal.(2)Residual stress concentrates at the weld seam.Residual stress has a significant influence on the weld seam and heat affected zone,but has a little influence on the area far from the weld.(3)In the finite element simulation results of weld specimens,the failure mode,yield strength and ultimate strength of the specimens are in good agreement with the experimental results,and the accuracy of the experimental and finite element simulation results is verified. |
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