The equivalent medium theory of metamaterials provides a way to obtain their effective constitutive parameters.However,because of its non-reciprocity,the complexity of the electromagnetic coupling,and a metallic botto...The equivalent medium theory of metamaterials provides a way to obtain their effective constitutive parameters.However,because of its non-reciprocity,the complexity of the electromagnetic coupling,and a metallic bottom layer,it has been challenging to retrieve them from a metamaterial absorber.In this paper,we propose a method without any approximation to obtain them,in which the non-reciprocity and the strong electromagnetic coupling are included.Compared with the three methods such as symmetric metamaterial method,asymmetric metamaterial method and metasurface method,our method can reveal the metamaterial absorber’s electrical and magnetic resonance and show its electromagnetic coupling coefficients.To deal with a metamaterial absorber with a metallic bottom layer,four corners of the metallic bottom layer in the unit cell are removed,making it possible to retrieve the electromagnetic parameters.Surprisingly,these results show that the metamaterial absorber with a metallic bottom layer in our example operates in a negative refraction state at the half absorption frequencies,which helps further understand the absorbing mechanism of these metamaterial absorbers.展开更多
This paper is to determine the flow stress curve of 5049-O aluminium alloy by a tube hydraulic bulging test with fixed end-conditions. During this test, several tubular specimens are bulged under different internal pr...This paper is to determine the flow stress curve of 5049-O aluminium alloy by a tube hydraulic bulging test with fixed end-conditions. During this test, several tubular specimens are bulged under different internal pressures before their bursting, and the corresponding bulging height and wall thickness at the pole are measured. An inverse strategy is developed to determine the constitutive parameters of tubular materials based on experimental data, which combines the finite element method with gradient-based optimization techniques. In this scheme, the objective function is formulated with the sum of least squares of the error between numerical and experimental data, and finite difference approximation is used to calculate the gradient. The tubular material behavior is assumed to meet the von Mises yield criterion and Hollomon exponential hardening law. Then, constitutive parameters identification is performed by minimization of the objective function. In order to validate the performance of this framework, identified parameters are compared with those obtained by two types of theoretical models, and tensile tests are performed on specimens cut from the same tubes. The comparison shows that this inverse framework is robust and can achieve a more accurate parameter identification by eliminating mechanical and geometrical assumptions in classical theoretical analysis.展开更多
An artificially accelerated alternation of aging and seawater erosion test of rubber materials used in lead rubber bearing(LRB)was performed,mainly to study the time-varying laws of rubber materials mechanical propert...An artificially accelerated alternation of aging and seawater erosion test of rubber materials used in lead rubber bearing(LRB)was performed,mainly to study the time-varying laws of rubber materials mechanical properties.Time-varying laws of the Mooney–Rivlin and Neo-Hookean constitutive parameters of rubber materials under the alternation of aging and seawater erosion were also analyzed.Results indicate that the rubber material mechanical properties were significantly affected by alternation of aging and seawater erosion.Hardness and elongation stress increased exponentially with test time.And 120 days after the test,the hardness increased by 14%,the maximum percentage increase in stress of 124.76%occurred at 100%constant elongation and the minimum percentage increase in stress of 68.32%occurred at 300%constant elongation;Tensile strength and elongation at break decreased by 44.96%and 53.09%.Besides,constitutive parameters of Mooney–Rivlin and Neo-Hookean all changed greatly with test duration.Finally,time-varying laws of constitutive parameters were verified by comparing the simulated and experimental results of the lead rubber bearing’s stiffness.Research results are of great significance to the seismic performance research and life-cycle performance analysis of offshore traffic engineering such as cross-sea bridges and bridges in the marine environment.展开更多
The method of designing electromagnetic invisible cloaks is usually based on the form-invariance of Maxwell's equations in coordinate transformation.The exterior boundary of a cylindrical invisible cloak is unchan...The method of designing electromagnetic invisible cloaks is usually based on the form-invariance of Maxwell's equations in coordinate transformation.The exterior boundary of a cylindrical invisible cloak is unchanged and the interior boundary is extended from that of a point to that of a cylindrical region in coordination transformation.This transformation process makes perfect cloaks,but it causes singularity in the constitutive material parameters of cloaks.This singularity makes the cloaks impossible to realize in practice.In order to remove this singularity,this paper sets a small cylindrical region replacing a point in the space transformation.The cylindrical region is so small that it does not affect the invisibility effects,but it can remove the singularity for material parameters.Full wave simulations based on the finite element method were used to verify the designed cloaks.展开更多
This paper reports that a general method of designing invisible cloaks is using variant constitutive material parameters to realize the space transformation.A hollow region can be hidden after this transformation.It w...This paper reports that a general method of designing invisible cloaks is using variant constitutive material parameters to realize the space transformation.A hollow region can be hidden after this transformation.It was recently shown (Ma H,Qu S B,Xu Z and Wang J F 2009 Appl.Phys.Lett.94 103501) that when the original point moves to the boundary of a cloak,the cloak can be designed to be open.Based on this theory,we propose multi-window invisible cloaks which can conceal a group of objects.Full wave simulations for invisible cloaks with regular and irregular shapes verified this method.展开更多
The isotropic and anisotropic behaviors are considered as the important formats of the constitutive behaviors,and can also be called the global properties.To improve the identification ability of virtual fields method...The isotropic and anisotropic behaviors are considered as the important formats of the constitutive behaviors,and can also be called the global properties.To improve the identification ability of virtual fields method(VFM)when the global properties are unknown,this paper proposes the strain correlation method(SCM)to determine the global properties before the parameter identification using the VFM.Firstly,the basic principle of SCM is described in detail.Then,the feasibility and accuracy of SCM are verified through the numerical experiments based on the three-point bending configuration and the real experiment of polymethyl methacrylate(PMMA).The influence of the additive Gaussian white noise,local errors in the strain fields,and missing data at the specimen edges on the characterization results are evaluated.The results show that the SCM has good noise immunity and lower accuracy requirements for the strain fields.As an application,the mechanical properties of Ti-6A1-4V alloys fabricated by selective laser melting(SLM)are characterized by the SCM.The results show that the alloys are isotropic,and the isotropic VFM is utilized to determine the mechanical parameters.By using the SCM,the accuracy of identification results can be improved for the isotropic or bidirectional reinforced orthotropic materials when using VFM.展开更多
基金Project supported by the National Natural Science Foundation of China(Grant No.11872013).
文摘The equivalent medium theory of metamaterials provides a way to obtain their effective constitutive parameters.However,because of its non-reciprocity,the complexity of the electromagnetic coupling,and a metallic bottom layer,it has been challenging to retrieve them from a metamaterial absorber.In this paper,we propose a method without any approximation to obtain them,in which the non-reciprocity and the strong electromagnetic coupling are included.Compared with the three methods such as symmetric metamaterial method,asymmetric metamaterial method and metasurface method,our method can reveal the metamaterial absorber’s electrical and magnetic resonance and show its electromagnetic coupling coefficients.To deal with a metamaterial absorber with a metallic bottom layer,four corners of the metallic bottom layer in the unit cell are removed,making it possible to retrieve the electromagnetic parameters.Surprisingly,these results show that the metamaterial absorber with a metallic bottom layer in our example operates in a negative refraction state at the half absorption frequencies,which helps further understand the absorbing mechanism of these metamaterial absorbers.
基金the financial support from China Scholarship Council (CSC) (No. 201706080020)。
文摘This paper is to determine the flow stress curve of 5049-O aluminium alloy by a tube hydraulic bulging test with fixed end-conditions. During this test, several tubular specimens are bulged under different internal pressures before their bursting, and the corresponding bulging height and wall thickness at the pole are measured. An inverse strategy is developed to determine the constitutive parameters of tubular materials based on experimental data, which combines the finite element method with gradient-based optimization techniques. In this scheme, the objective function is formulated with the sum of least squares of the error between numerical and experimental data, and finite difference approximation is used to calculate the gradient. The tubular material behavior is assumed to meet the von Mises yield criterion and Hollomon exponential hardening law. Then, constitutive parameters identification is performed by minimization of the objective function. In order to validate the performance of this framework, identified parameters are compared with those obtained by two types of theoretical models, and tensile tests are performed on specimens cut from the same tubes. The comparison shows that this inverse framework is robust and can achieve a more accurate parameter identification by eliminating mechanical and geometrical assumptions in classical theoretical analysis.
基金This work was supported by the Programme for National Natural Science Foundation of China(52078150,51878196)National Key R&D Program of China(2019YFE0112500)2019 Guangzhou University Full-Time Graduate“Basic Innovation”Project(2019GDJC-D11).
文摘An artificially accelerated alternation of aging and seawater erosion test of rubber materials used in lead rubber bearing(LRB)was performed,mainly to study the time-varying laws of rubber materials mechanical properties.Time-varying laws of the Mooney–Rivlin and Neo-Hookean constitutive parameters of rubber materials under the alternation of aging and seawater erosion were also analyzed.Results indicate that the rubber material mechanical properties were significantly affected by alternation of aging and seawater erosion.Hardness and elongation stress increased exponentially with test time.And 120 days after the test,the hardness increased by 14%,the maximum percentage increase in stress of 124.76%occurred at 100%constant elongation and the minimum percentage increase in stress of 68.32%occurred at 300%constant elongation;Tensile strength and elongation at break decreased by 44.96%and 53.09%.Besides,constitutive parameters of Mooney–Rivlin and Neo-Hookean all changed greatly with test duration.Finally,time-varying laws of constitutive parameters were verified by comparing the simulated and experimental results of the lead rubber bearing’s stiffness.Research results are of great significance to the seismic performance research and life-cycle performance analysis of offshore traffic engineering such as cross-sea bridges and bridges in the marine environment.
基金Project supported partly by the National Natural Science Foundation of China (Grant Nos.50632030,10474077,and 60871027)partly by the National Basic Research Program of China (Grant No.2009CB613306)partly by the Natural Science Foundation of Shaanxi Province,China (Grant No.SJ08F01)
文摘The method of designing electromagnetic invisible cloaks is usually based on the form-invariance of Maxwell's equations in coordinate transformation.The exterior boundary of a cylindrical invisible cloak is unchanged and the interior boundary is extended from that of a point to that of a cylindrical region in coordination transformation.This transformation process makes perfect cloaks,but it causes singularity in the constitutive material parameters of cloaks.This singularity makes the cloaks impossible to realize in practice.In order to remove this singularity,this paper sets a small cylindrical region replacing a point in the space transformation.The cylindrical region is so small that it does not affect the invisibility effects,but it can remove the singularity for material parameters.Full wave simulations based on the finite element method were used to verify the designed cloaks.
基金Project supported partly by the National Natural Science Foundation of China (Grant Nos.50632030,10474077,and 60871027)partly by the National Basic Research Program of China (Grant No.2009CB613306)partly by the Natural Science Foundation of Shaanxi Province,China (Grant No.SJ08F01)
文摘This paper reports that a general method of designing invisible cloaks is using variant constitutive material parameters to realize the space transformation.A hollow region can be hidden after this transformation.It was recently shown (Ma H,Qu S B,Xu Z and Wang J F 2009 Appl.Phys.Lett.94 103501) that when the original point moves to the boundary of a cloak,the cloak can be designed to be open.Based on this theory,we propose multi-window invisible cloaks which can conceal a group of objects.Full wave simulations for invisible cloaks with regular and irregular shapes verified this method.
基金This research was financially supported by the National Key Research and Development Program of China(Grant 2017YFB1103900)the National Science and Technology Major Project(Grant 2017-VI-0003-0073)+1 种基金the National Natural Science Foundation of China(Grant 11672153)Hubei Provincial Major Program of Technological Innovation(Grant 2017AAA121).
文摘The isotropic and anisotropic behaviors are considered as the important formats of the constitutive behaviors,and can also be called the global properties.To improve the identification ability of virtual fields method(VFM)when the global properties are unknown,this paper proposes the strain correlation method(SCM)to determine the global properties before the parameter identification using the VFM.Firstly,the basic principle of SCM is described in detail.Then,the feasibility and accuracy of SCM are verified through the numerical experiments based on the three-point bending configuration and the real experiment of polymethyl methacrylate(PMMA).The influence of the additive Gaussian white noise,local errors in the strain fields,and missing data at the specimen edges on the characterization results are evaluated.The results show that the SCM has good noise immunity and lower accuracy requirements for the strain fields.As an application,the mechanical properties of Ti-6A1-4V alloys fabricated by selective laser melting(SLM)are characterized by the SCM.The results show that the alloys are isotropic,and the isotropic VFM is utilized to determine the mechanical parameters.By using the SCM,the accuracy of identification results can be improved for the isotropic or bidirectional reinforced orthotropic materials when using VFM.
