Hot flow forming(HFF)is a promising forming technology to manufacture thin-walled cylindrical part with longitudinal inner ribs(CPLIRs)made of magnesium(Mg)alloys,which has wide applications in the aerospace field.How...Hot flow forming(HFF)is a promising forming technology to manufacture thin-walled cylindrical part with longitudinal inner ribs(CPLIRs)made of magnesium(Mg)alloys,which has wide applications in the aerospace field.However,due to the thermo-mechanical coupling effect and the existence of stiffened structure,complex microstructure evolution and uneven microstructure occur easily at the cylindrical wall(CW)and inner rib(IR)of Mg alloy thin-walled CPLIRs during the HFF.In this paper,a modified cellular automaton(CA)model of Mg alloy considering the effects of deformation conditions on material parameters was developed using the artificial neural network(ANN)method.It is found that the ANN-modified CA model exhibits better predictability for the microstructure of hot deformation than the conventional CA model.Furthermore,the microstructure evolution of ZK61 alloy CPLIRs during the HFF was analyzed by coupling the modified CA model and finite element analysis(FEA).The results show that compared with the microstructure at the same layer of the IR,more refined grains and less sufficient DRX resulted from larger strain and strain rate occur at that of the CW;various differences of strain and strain rate in the wall-thickness exist between the CW and IR,which leads to the inhomogeneity of microstructure rising firstly and declining from the inside layer to outside layer;the obtained Hall-Petch relationship between the measured microhardness and predicted grain sizes at the CW and the IR indicates the reliability of the coupled FEA-CA simulation results.展开更多
Most tunnel projects are designed with cross-sectional loads,and the inhomogeneity of the longitudinal forces is ignored.In theory,such a support structure can resist large loads,but in practice,large deformation,conc...Most tunnel projects are designed with cross-sectional loads,and the inhomogeneity of the longitudinal forces is ignored.In theory,such a support structure can resist large loads,but in practice,large deformation,concrete cracking,steel frame distortion,and other phenomena often occur in tunnels under poor surrounding rock conditions.Hence,the longitudinal stability of the tunnel must be considered.In this study,the mechanism of longitudinal connecting ribs(LCRs)of tunnels was investigated through element tests,theoretical analyses,and numerical simulations,and the effect of the LCRs was evaluated experimentally.The applicability of the constitutive relations and boundary conditions of the numerical model was verified.The instability mode of the steel frame reflecting the longitudinal stress gradient of the tunnel was analyzed,and the longitudinal surrounding rock pressure and the verified numerical model were applied to analyze the LCR using the load structure method.The results indicate the following:(1)LCRs can effectively improve the ultimate bearing capacity and stability of a structure and reduce the area and degree of damage;(2)Two types of instability modes occur in tunnel steel frames,and the main factor is bending failure caused by the axial force;(3)The distance sensitivity of the LCR in the tunnel is higher than the stiffness sensitivity.For large deformations of tunnels,double rows of rebars with a spacing of less than 1.5 m should be used as longitudinal connections.展开更多
基金supported by the National Nat-ural Science Foundation of China(Grant Nos.51775194 and 52090043).
文摘Hot flow forming(HFF)is a promising forming technology to manufacture thin-walled cylindrical part with longitudinal inner ribs(CPLIRs)made of magnesium(Mg)alloys,which has wide applications in the aerospace field.However,due to the thermo-mechanical coupling effect and the existence of stiffened structure,complex microstructure evolution and uneven microstructure occur easily at the cylindrical wall(CW)and inner rib(IR)of Mg alloy thin-walled CPLIRs during the HFF.In this paper,a modified cellular automaton(CA)model of Mg alloy considering the effects of deformation conditions on material parameters was developed using the artificial neural network(ANN)method.It is found that the ANN-modified CA model exhibits better predictability for the microstructure of hot deformation than the conventional CA model.Furthermore,the microstructure evolution of ZK61 alloy CPLIRs during the HFF was analyzed by coupling the modified CA model and finite element analysis(FEA).The results show that compared with the microstructure at the same layer of the IR,more refined grains and less sufficient DRX resulted from larger strain and strain rate occur at that of the CW;various differences of strain and strain rate in the wall-thickness exist between the CW and IR,which leads to the inhomogeneity of microstructure rising firstly and declining from the inside layer to outside layer;the obtained Hall-Petch relationship between the measured microhardness and predicted grain sizes at the CW and the IR indicates the reliability of the coupled FEA-CA simulation results.
基金supported by the Major Project of Science and Technology Research and Development Plan of China Railway Corporation(2017G006-B)High-Speed Rail Joint-Fund Funded Projects(U1934213).
文摘Most tunnel projects are designed with cross-sectional loads,and the inhomogeneity of the longitudinal forces is ignored.In theory,such a support structure can resist large loads,but in practice,large deformation,concrete cracking,steel frame distortion,and other phenomena often occur in tunnels under poor surrounding rock conditions.Hence,the longitudinal stability of the tunnel must be considered.In this study,the mechanism of longitudinal connecting ribs(LCRs)of tunnels was investigated through element tests,theoretical analyses,and numerical simulations,and the effect of the LCRs was evaluated experimentally.The applicability of the constitutive relations and boundary conditions of the numerical model was verified.The instability mode of the steel frame reflecting the longitudinal stress gradient of the tunnel was analyzed,and the longitudinal surrounding rock pressure and the verified numerical model were applied to analyze the LCR using the load structure method.The results indicate the following:(1)LCRs can effectively improve the ultimate bearing capacity and stability of a structure and reduce the area and degree of damage;(2)Two types of instability modes occur in tunnel steel frames,and the main factor is bending failure caused by the axial force;(3)The distance sensitivity of the LCR in the tunnel is higher than the stiffness sensitivity.For large deformations of tunnels,double rows of rebars with a spacing of less than 1.5 m should be used as longitudinal connections.
