The dynamic photoelastic technique is employed to visualize and quantify the propagation properties of backward Lamb waves in a plate. Higher energy leakage of second-order symmetric backward wave mode S2b in contrast...The dynamic photoelastic technique is employed to visualize and quantify the propagation properties of backward Lamb waves in a plate. Higher energy leakage of second-order symmetric backward wave mode S2b in contrast to third-order anti-symmetric backward mode A3b is shown by the dispersion curve of a plate immersed in water, and then verified by experiments. To avoid the considerable high leakage, the plate is placed in air, both group and phase velocities of modes S2b and A3b in the glass plate are experimentally measured. In comparison with the theoretical values, less than 5% errors are found in experiments.展开更多
With the idea of the phononic crystals, a thin rectangular plate with two-dimensional periodic structure is designed. Flexural wave band structures of such a plate with infinite structure are calculated with the plane...With the idea of the phononic crystals, a thin rectangular plate with two-dimensional periodic structure is designed. Flexural wave band structures of such a plate with infinite structure are calculated with the plane-wave expansion （PWE） method, and directional band gaps are found in the ΓX direction. The acceleration frequency response in the ΓX direction of such a plate with finite structure is simulated with the finite element method and verified with a vibration experiment. The frequency ranges of sharp drops in the calculated and measured acceleration frequency response curves are in basic agreement with those in the band structures. Thin plate is a widely used component in the engineering structures. The existence of band gaps in such periodic structures gives a new idea in vibration control of thin plates.展开更多
We analyze the effect of second-harmonic generation（SHG） of primary Lamb wave propagation in a two-layered composite plate, and then investigate the influence of interfacial properties on the said effect at low freq...We analyze the effect of second-harmonic generation（SHG） of primary Lamb wave propagation in a two-layered composite plate, and then investigate the influence of interfacial properties on the said effect at low frequency. It is found that changes in the interfacial properties essentially affect the dispersion relation and then the maximum cumulative distance of the double-frequency Lamb wave generated. This will remarkably influence the efficiency of SHG. To overcome the complications arising from the inherent dispersion and multimode natures in analyzing the SHG effect of Lamb waves, the present work focuses on the analysis of the SHG effect of low-frequency dilatational Lamb wave propagation. Both the numerical analysis and finite element simulation indicate that the SHG effect of low-frequency dilatational Lamb wave propagation is found to be much more sensitive to changes in the interfacial properties than primary Lamb waves. The potential of using the SHG effect of low-frequency dilatational Lamb waves to characterize a minor change in the interfacial properties is analyzed.展开更多
Laser self-mixing interference（SMI） wave plate measurement method is a burgeoning technique for its simplicity and efficiency. But for the non-coated sample, the reflected light from the surface can seriously affect...Laser self-mixing interference（SMI） wave plate measurement method is a burgeoning technique for its simplicity and efficiency. But for the non-coated sample, the reflected light from the surface can seriously affect the measurement results.To analyze the reason theoretically, a self-consistent model for laser operation with a sub-external and an external cavity is established, and the sub-external cavity formed by the sample and a cavity mirror is proved to be the main error source.A synchronous tuning method is proposed to eliminate the sub-external cavity effect. Experiments are carried out on the synchronously tuning double external cavities self-mixing interference system, and the error of the system is in the range of -0.435°~0.387° compared with the ellipsometer. The research plays an important role in improving the performance and enlarging the application range of the laser self-mixing interference system.展开更多
In this work,two typical interfaces are established for the antisymmetric plate wave by introducing linear defect in between phononic crystals with opposite valley Hall phases.The evolution of projected curves is demo...In this work,two typical interfaces are established for the antisymmetric plate wave by introducing linear defect in between phononic crystals with opposite valley Hall phases.The evolution of projected curves is demonstrated as the defect width increases,which magnifies the occurrence of multiple edge states on both interfaces.The mapping of transmission coefficient is obtained upon the incidence of transversely symmetric and antisymmetric sources,respectively.It is shown that the first edge state contains only symmetric or antisymmetric component regardless the defect width modulation,while other edge states contain both components.When void cavity occurs on the interfaces,the mapping of transmission coefficient shows that the first edge state is topologically protected and its transverse symmetry or antisymmetry is almost unperturbed by the void cavity,while other edge states do not perform the same way.Our work provides useful guidance for the design of topological edge states,and bridges the topological edge states and linear defect modes on the same interface.展开更多
We investigate the three-dimensional （3D） scattering problem of an incident plane shear horizontal wave by a partly through-thickness hole in an isotropic plate, in which the Lamb wave modes are also included due to...We investigate the three-dimensional （3D） scattering problem of an incident plane shear horizontal wave by a partly through-thickness hole in an isotropic plate, in which the Lamb wave modes are also included due to the mode conversions by the scattering obstacle in the 3D problem. An analytical model is presented such that the wave fields are expanded in all of propagating and evanescent SH modes and Lamb modes, and the scattered far-fields of three fundamental guided wave modes are analyzed numerically for different sizes of the holes and frequencies. The numerical results are verified by comparing with those obtained by using the approximate Poisson/Mindlin plate model for small hole radius and low frequency. It is also found that the scattering patterns are different from those of the SO wave incidence. Our work is useful for quantitative evaluation of the plate-like structure by ultrasonic guided waves.展开更多
A simple and precise retardation measurement based on laser feedback is demonstrated. The measurement principle is based on polarization flipping induced by optical feedback from an external birefringence cavity. The ...A simple and precise retardation measurement based on laser feedback is demonstrated. The measurement principle is based on polarization flipping induced by optical feedback from an external birefringence cavity. The measured wave plate is located in the external cavity. When the length of the external cavity is tuned, the polarization states of laser will flip between two eigenstates, and the position of polarization flipping in one period of intensity modulation will vary with retardation of the wave plate. The duty ratio of two eigenstates is used to determine the retardation. Main advantages of the technique are that it is compact, low cost, fast and flexible. Especially, it is insensitive to a fluctuation of laser intensity and is suitable for on-line measurement. The experimental results have shown that the measurement uncertainty is better than 0.03° in the range 30°-150°.展开更多
This paper presents a novel parallel implementation technology for wave-based structural health monitoring (SHM) in laminated composite plates. The wavelet-based B-spline wavelet on he interval (BSWI) element is cons...This paper presents a novel parallel implementation technology for wave-based structural health monitoring (SHM) in laminated composite plates. The wavelet-based B-spline wavelet on he interval (BSWI) element is constructed according to Hamilton’s principle, and the element by element algorithm is parallelly executed on graphics processing unit (GPU) using compute unified device architecture (CUDA) to get the responses in full wave field accurately. By means of the Fourier spectral analysis method,the Mindlin plate theory is selected for wave modeling of laminated composite plates while the Kirchhoff plate theory predicts unreasonably phase and group velocities. Numerical examples involving wave propagation in laminated composite plates without and with crack are performed and discussed in detail. The parallel implementation on GPU is accelerated 146 times comparing with the same wave motion problem executed on central processing unit (CPU). The validity and accuracy of the proposed parallel implementation are also demonstrated by comparing with conventional finite element method (FEM) and the computation time has been reduced from hours to minutes. The damage size and location have been successfully determined according to wave propagation results based on delay-and-sum (DAS). The results show that the proposed parallel implementation of wavelet finite element method (WFEM) is very appropriate and efficient for wave-based SHM in laminated composite plates.展开更多
We report both experimentally and numerically that ultra-broadband asymmetric acoustic transmission is realized by a brass plate and a right triangle reflector immersed in water. This exotic phenomenon arises from the...We report both experimentally and numerically that ultra-broadband asymmetric acoustic transmission is realized by a brass plate and a right triangle reflector immersed in water. This exotic phenomenon arises from the asymmetric excitation of the leaky asymmetric zero-order Lamb mode in the brass plate induced by the incident angle of external bulk waves. The results show that the bandwidth of the asymmetric acoustic transmission could reach 2000 k Hz, and the positive transmitted wave is only a single acoustic beam. The device has the advantages of ultra-broadband, single transmitted beam,and simpler structure, which has great potential applications in ultrasonic devices.展开更多
The research aims at validating the ability of topological imaging to blind holes in isotropic plates using Lamb waves. Due to the defect is not symmetric around the mid- plane of the plate, the effect of Lamb mode co...The research aims at validating the ability of topological imaging to blind holes in isotropic plates using Lamb waves. Due to the defect is not symmetric around the mid- plane of the plate, the effect of Lamb mode conversion will have to be taken into account. The imaging method is based on two computations of ultrasonic fields, one forward and one adjoint, performed for the defect-free reference medium. The excited signal and scattered Lamb waves caused by the blind hole, are used as emitting sources to compute the forward problem and the adjoint problem, respectively. With the help of the finite element simulations, the natural refocusing process of the multimode Lamb waves at the defect location is visually demonstrated by the transient acoustic field snapshots at the different moments to strengthen the physical mechanism of the topological imaging method. The numerical results demonstrate that topological imaging has relatively stronger applicability to the blind hole in contrast to classical Delay And Sum （DAS） method and Time Reversal （TR） method. The topological imaging could handle complex Lamb wave signals containing mode conversions without the imaging quality being affected. The proposed imaging method presents a certain developing potential for detecting and imaging asymmetric defects in plate-like configurations using Lamb waves.展开更多
A spectroscopic method to determine thickness on chromatic polarization interferometry. With of quartz wave plate is presented. The method is based the polarization-resolved transmission spectrum （PRTS） curve, the p...A spectroscopic method to determine thickness on chromatic polarization interferometry. With of quartz wave plate is presented. The method is based the polarization-resolved transmission spectrum （PRTS） curve, the phase retardation of quartz wave plate can be determined at a wide spectral range from 200 to 2000 nm obviously. Through accurate judgment of extreme points of PRTS curve at long-wave band, the physical thickness of quartz wave plates can be obtained exactly. We give a measuring example and the error analysis. It is found that the measuring precision of thickness is mainly determined by the spectral resolution of spectrometer.展开更多
Hydrodynamic characteristics for two-dimensional flow around a waving plate are investigated. Under large Reynolds number approximation, the flow is assumed to be a combination of the outer potential flow and a thin v...Hydrodynamic characteristics for two-dimensional flow around a waving plate are investigated. Under large Reynolds number approximation, the flow is assumed to be a combination of the outer potential flow and a thin vortex layer, which consists of a boundary layer and a shed free shear layer. A nonlinear mathematical formulation for describing the outer unsteady potential flow coupled with an unsteady boundary layer equation for the inner viscous flow adjacent to the waving plate is developed. A semi-analytical method with a nonlinear Kutta condition imposed at the trailing edge is used to solve the velocity field of the outer flow and the evolution of wake vortex induced by a large-amplitude waving plate. The unsteady boundary layer equation is solved by extending Pohlhausen＇s method to its unsteady counterpart. The thrust and viscous drag coefficients, propulsive efficiency, and the pattern of wake vortex sheet are discussed.展开更多
Generally, a confocal Fabry-Perot interferometer is only able to detect the out-of-plane component of a displacement field; while the in-plane component often has the information about the material which cannot be fou...Generally, a confocal Fabry-Perot interferometer is only able to detect the out-of-plane component of a displacement field; while the in-plane component often has the information about the material which cannot be found in this out-of-plane component. In this paper, based on a confocal Fabry-Perot interferometer set-up for detecting the out-of-plane component of a laser generated acoustic field, a technique is developed to detect both the out-of-plane and in-plane displacement components simultaneously with a novel two-channel confocal Fabry-Perot interferometer.展开更多
in the acousto-ultrasonic technique, a broadband ultrasonic wave is injected onto the surface at one location of the tested plate with the help of a longitudinal transducer and a receiving transducer is coupled to the...in the acousto-ultrasonic technique, a broadband ultrasonic wave is injected onto the surface at one location of the tested plate with the help of a longitudinal transducer and a receiving transducer is coupled to the same surface at another location. Thus, understanding the propagation characteristics of the ultrasonic wave is essential for successful application of the technique. It has been found that the dominant acousto-ultrasonic waves produced experimentally in thin plate are multi-mode Lamb waves. The broadband generation and detection of Lamb waves have been performed in two aluminum plates with different thickness using the acousto-ultrasonic method.展开更多
The speed of the streaming flow in the liquid layer loaded on a thin plate, in which Lamb waves are propagating, was calculated. The results are good agreement with Moroney's experiment
The characteristics of Lamb wave propagating in a solid plate with rough surfaces are studied on the basis of small perturbation approximation. The Rayleigh-Lamb frequency equation expressed with SA matrix is presente...The characteristics of Lamb wave propagating in a solid plate with rough surfaces are studied on the basis of small perturbation approximation. The Rayleigh-Lamb frequency equation expressed with SA matrix is presented. The Rayleigh-Lamb frequency equation for a rough surface plate is different from that for a smooth surface plate, resulting in a small perturbation △k on Lamb wave vector k. The imaginary part of △k gives the attenuation caused by wave scattering. An experiment is designed to test our theoretical predications. By using wedge-shape pipes, different Lamb wave modes are excited. The signals at different positions are received and analyzed to get the dispersion curves and attenuations of different modes. The experimental results are compared with the theoretical predications.展开更多
The Lamb wave technique is used in the detection of internal defects in plates. The reflection of the Lamb wave at the free edge of the plate has been shown to be rather complicated. The reflected wave is usually comp...The Lamb wave technique is used in the detection of internal defects in plates. The reflection of the Lamb wave at the free edge of the plate has been shown to be rather complicated. The reflected wave is usually composed of infinite number of Lamb wave modes even in the case of incidence of one pure mode. The analysis of the reflection problem is complicated in nature as the orthogonality relations of the eigen modes of the Lamb wave concern both the stresses and displacements. We adopt the principle of the least squares as a new approach to simplify the analysis, in which the eigen-modes are taken as the trial function. It seems to be simple and effective. In most cases a good enough result can be obtained on using an 8- bit microcomputer. Here the coefficients of the various reflected modes are calculated, some interesting phenomena on the mode conversion are indicated and the conditions on the pure mode reflection are given.展开更多
The displacement and stress as well as the dispersion equation for the guided waves of antiplane shear (SH) modes in a bonded plate are presented explicitly. The spring model is used to account for the interface rigid...The displacement and stress as well as the dispersion equation for the guided waves of antiplane shear (SH) modes in a bonded plate are presented explicitly. The spring model is used to account for the interface rigidity between the layers of the bonded plate. It is found that the dispersion equation is composed of the terms pertinent with the welded and the cracked or slip boundary conditions with the compliance sT of the spring model as the sum-coefficients. Numerical results for an adhesive plate of aluminum and steel whose wave velocities are very close to each other of equal thickness indicate that the modes of odd order are more sensitive than the even ones to the variations of the bond quality, and so are the higher modes than the lower ones for this special layered plate. The phenomena of the cut-off frequency-shift and the phase velocity decrease resulting from the degradation of the bond quality are found to the modes of odd order, which may be used to evaluate the bond states of the adhesive plate. Numerical results for an adhesive plate of rubber and steel whose wave velocities are very different to each other of equal thickness are presented and discussed as well.展开更多
Based on elastic wave propagation theory, the dispersion equation for a thin anisotropic plate (such as commonly used Zinc okide in micro-transducers) bordered with liquid layers is derived. Higher symmetry crystals, ...Based on elastic wave propagation theory, the dispersion equation for a thin anisotropic plate (such as commonly used Zinc okide in micro-transducers) bordered with liquid layers is derived. Higher symmetry crystals, such as orthorhombic, tetragonal, cubic, isotropic, are included in this analysis as well. For the case of one liquid layer loading, numerical calcu- lations show that the phase velocity changes periodically with the thickness of the liquld layer. When the thickness 2d of the anisotropic plate is very small, mass sensing application of Ao mode Lamb wave is also discussed.展开更多
The suppression of polarization cross talk in lead zirconate titanate phase modulators as a key error source has been challenging for open-loop fiber optic gyroscopes(FOGs).We developed a polarization-diversity optica...The suppression of polarization cross talk in lead zirconate titanate phase modulators as a key error source has been challenging for open-loop fiber optic gyroscopes(FOGs).We developed a polarization-diversity optical frequency domain reflectometry(OFDR)to measure the distributed modulation polarization error in the modulator.The error contributes 8×10^(−6) rad to FOG’s bias instability.Using a UV-fabricated in-fiberλ/4 wave plate and polarization-mode converter with fiber taper technology,the modulation error has been suppressed by 15 dB in assembled FOGs.This approach reduced error with temperature from 25°/h to 0.7°/h,meeting the requirements of control-level gyroscopes with bias errors less than 1°/h.展开更多
基金Supported by the National Natural Science Foundation of China under Grant Nos 11374325 and 11427809
文摘The dynamic photoelastic technique is employed to visualize and quantify the propagation properties of backward Lamb waves in a plate. Higher energy leakage of second-order symmetric backward wave mode S2b in contrast to third-order anti-symmetric backward mode A3b is shown by the dispersion curve of a plate immersed in water, and then verified by experiments. To avoid the considerable high leakage, the plate is placed in air, both group and phase velocities of modes S2b and A3b in the glass plate are experimentally measured. In comparison with the theoretical values, less than 5% errors are found in experiments.
基金This project is supported by National Basic Research Program of China (973Program, No.51307).
文摘With the idea of the phononic crystals, a thin rectangular plate with two-dimensional periodic structure is designed. Flexural wave band structures of such a plate with infinite structure are calculated with the plane-wave expansion （PWE） method, and directional band gaps are found in the ΓX direction. The acceleration frequency response in the ΓX direction of such a plate with finite structure is simulated with the finite element method and verified with a vibration experiment. The frequency ranges of sharp drops in the calculated and measured acceleration frequency response curves are in basic agreement with those in the band structures. Thin plate is a widely used component in the engineering structures. The existence of band gaps in such periodic structures gives a new idea in vibration control of thin plates.
基金Supported by the National Natural Science Foundation of China under Grant Nos 11834008,11632004,11474361 and 11622430
文摘We analyze the effect of second-harmonic generation（SHG） of primary Lamb wave propagation in a two-layered composite plate, and then investigate the influence of interfacial properties on the said effect at low frequency. It is found that changes in the interfacial properties essentially affect the dispersion relation and then the maximum cumulative distance of the double-frequency Lamb wave generated. This will remarkably influence the efficiency of SHG. To overcome the complications arising from the inherent dispersion and multimode natures in analyzing the SHG effect of Lamb waves, the present work focuses on the analysis of the SHG effect of low-frequency dilatational Lamb wave propagation. Both the numerical analysis and finite element simulation indicate that the SHG effect of low-frequency dilatational Lamb wave propagation is found to be much more sensitive to changes in the interfacial properties than primary Lamb waves. The potential of using the SHG effect of low-frequency dilatational Lamb waves to characterize a minor change in the interfacial properties is analyzed.
文摘Laser self-mixing interference（SMI） wave plate measurement method is a burgeoning technique for its simplicity and efficiency. But for the non-coated sample, the reflected light from the surface can seriously affect the measurement results.To analyze the reason theoretically, a self-consistent model for laser operation with a sub-external and an external cavity is established, and the sub-external cavity formed by the sample and a cavity mirror is proved to be the main error source.A synchronous tuning method is proposed to eliminate the sub-external cavity effect. Experiments are carried out on the synchronously tuning double external cavities self-mixing interference system, and the error of the system is in the range of -0.435°~0.387° compared with the ellipsometer. The research plays an important role in improving the performance and enlarging the application range of the laser self-mixing interference system.
基金Shanghai Science and Technology Innovation Plan(Grant No.20ZR1462700)State Key Laboratory of ASIC&System(2020KF006)Jinfeng Zhao would like to acknowledge helpful discussions with Prof.Jie Ren from Tongji University.
文摘In this work,two typical interfaces are established for the antisymmetric plate wave by introducing linear defect in between phononic crystals with opposite valley Hall phases.The evolution of projected curves is demonstrated as the defect width increases,which magnifies the occurrence of multiple edge states on both interfaces.The mapping of transmission coefficient is obtained upon the incidence of transversely symmetric and antisymmetric sources,respectively.It is shown that the first edge state contains only symmetric or antisymmetric component regardless the defect width modulation,while other edge states contain both components.When void cavity occurs on the interfaces,the mapping of transmission coefficient shows that the first edge state is topologically protected and its transverse symmetry or antisymmetry is almost unperturbed by the void cavity,while other edge states do not perform the same way.Our work provides useful guidance for the design of topological edge states,and bridges the topological edge states and linear defect modes on the same interface.
基金Supported by the National Natural Science Foundation of China under Grant Nos 11474195,11274226,51478258 and 51405287
文摘We investigate the three-dimensional （3D） scattering problem of an incident plane shear horizontal wave by a partly through-thickness hole in an isotropic plate, in which the Lamb wave modes are also included due to the mode conversions by the scattering obstacle in the 3D problem. An analytical model is presented such that the wave fields are expanded in all of propagating and evanescent SH modes and Lamb modes, and the scattered far-fields of three fundamental guided wave modes are analyzed numerically for different sizes of the holes and frequencies. The numerical results are verified by comparing with those obtained by using the approximate Poisson/Mindlin plate model for small hole radius and low frequency. It is also found that the scattering patterns are different from those of the SO wave incidence. Our work is useful for quantitative evaluation of the plate-like structure by ultrasonic guided waves.
基金Supported by the National Natural Science Foundation of China under Grant No 60438010.
文摘A simple and precise retardation measurement based on laser feedback is demonstrated. The measurement principle is based on polarization flipping induced by optical feedback from an external birefringence cavity. The measured wave plate is located in the external cavity. When the length of the external cavity is tuned, the polarization states of laser will flip between two eigenstates, and the position of polarization flipping in one period of intensity modulation will vary with retardation of the wave plate. The duty ratio of two eigenstates is used to determine the retardation. Main advantages of the technique are that it is compact, low cost, fast and flexible. Especially, it is insensitive to a fluctuation of laser intensity and is suitable for on-line measurement. The experimental results have shown that the measurement uncertainty is better than 0.03° in the range 30°-150°.
基金supported by the National Natural Science Foundation of China (Grant Nos. 51421004 & 51405369)the National Key Basic Research Program of China (Grant No. 2015CB057400)+1 种基金the China Postdoctoral Science Foundation (Grant No. 2014M560766)the China Scholarship Council,and the Fundamental Research Funds for the Central Universities(Grant No. xjj2014107)
文摘This paper presents a novel parallel implementation technology for wave-based structural health monitoring (SHM) in laminated composite plates. The wavelet-based B-spline wavelet on he interval (BSWI) element is constructed according to Hamilton’s principle, and the element by element algorithm is parallelly executed on graphics processing unit (GPU) using compute unified device architecture (CUDA) to get the responses in full wave field accurately. By means of the Fourier spectral analysis method,the Mindlin plate theory is selected for wave modeling of laminated composite plates while the Kirchhoff plate theory predicts unreasonably phase and group velocities. Numerical examples involving wave propagation in laminated composite plates without and with crack are performed and discussed in detail. The parallel implementation on GPU is accelerated 146 times comparing with the same wave motion problem executed on central processing unit (CPU). The validity and accuracy of the proposed parallel implementation are also demonstrated by comparing with conventional finite element method (FEM) and the computation time has been reduced from hours to minutes. The damage size and location have been successfully determined according to wave propagation results based on delay-and-sum (DAS). The results show that the proposed parallel implementation of wavelet finite element method (WFEM) is very appropriate and efficient for wave-based SHM in laminated composite plates.
基金Project supported by the Major Program of the National Natural Science Foundation of China(Grant No.51239005)the National Natural Science Foundation of China(Grant No.11404147)+3 种基金the Natural Science Foundation of Jiangsu Province of China(Grant No.BK20140519)China Postdoctoral Science Foundation(Grant No.2015M571672)the Scientific Research Project for Graduate Students of Universities in Jiangsu Province,China(Grant No.CXZZ13 06)the Training Project of Young Backbone Teachers of Jiangsu University
文摘We report both experimentally and numerically that ultra-broadband asymmetric acoustic transmission is realized by a brass plate and a right triangle reflector immersed in water. This exotic phenomenon arises from the asymmetric excitation of the leaky asymmetric zero-order Lamb mode in the brass plate induced by the incident angle of external bulk waves. The results show that the bandwidth of the asymmetric acoustic transmission could reach 2000 k Hz, and the positive transmitted wave is only a single acoustic beam. The device has the advantages of ultra-broadband, single transmitted beam,and simpler structure, which has great potential applications in ultrasonic devices.
基金supported by the National Natural Science Foundation of China(11474195,11274226,61171145)
文摘The research aims at validating the ability of topological imaging to blind holes in isotropic plates using Lamb waves. Due to the defect is not symmetric around the mid- plane of the plate, the effect of Lamb mode conversion will have to be taken into account. The imaging method is based on two computations of ultrasonic fields, one forward and one adjoint, performed for the defect-free reference medium. The excited signal and scattered Lamb waves caused by the blind hole, are used as emitting sources to compute the forward problem and the adjoint problem, respectively. With the help of the finite element simulations, the natural refocusing process of the multimode Lamb waves at the defect location is visually demonstrated by the transient acoustic field snapshots at the different moments to strengthen the physical mechanism of the topological imaging method. The numerical results demonstrate that topological imaging has relatively stronger applicability to the blind hole in contrast to classical Delay And Sum （DAS） method and Time Reversal （TR） method. The topological imaging could handle complex Lamb wave signals containing mode conversions without the imaging quality being affected. The proposed imaging method presents a certain developing potential for detecting and imaging asymmetric defects in plate-like configurations using Lamb waves.
基金This work was supported by the National Ba sic Research Program of China (973 Program) under Grant No. 2006CB806000.
文摘A spectroscopic method to determine thickness on chromatic polarization interferometry. With of quartz wave plate is presented. The method is based the polarization-resolved transmission spectrum （PRTS） curve, the phase retardation of quartz wave plate can be determined at a wide spectral range from 200 to 2000 nm obviously. Through accurate judgment of extreme points of PRTS curve at long-wave band, the physical thickness of quartz wave plates can be obtained exactly. We give a measuring example and the error analysis. It is found that the measuring precision of thickness is mainly determined by the spectral resolution of spectrometer.
基金Project supported by the Innovation Project of the Chinese Academy of Sciences (Grant No. KJCX-SW-L04)the National Natural Science Foundation of China (Grant No.10332040)the Program of Hundred-Talent of the Chinese Academy of Sciences.
文摘Hydrodynamic characteristics for two-dimensional flow around a waving plate are investigated. Under large Reynolds number approximation, the flow is assumed to be a combination of the outer potential flow and a thin vortex layer, which consists of a boundary layer and a shed free shear layer. A nonlinear mathematical formulation for describing the outer unsteady potential flow coupled with an unsteady boundary layer equation for the inner viscous flow adjacent to the waving plate is developed. A semi-analytical method with a nonlinear Kutta condition imposed at the trailing edge is used to solve the velocity field of the outer flow and the evolution of wake vortex induced by a large-amplitude waving plate. The unsteady boundary layer equation is solved by extending Pohlhausen＇s method to its unsteady counterpart. The thrust and viscous drag coefficients, propulsive efficiency, and the pattern of wake vortex sheet are discussed.
基金This work was supported by the National Nature Science Foundation of China (No. 10174025 and No.10134020).
文摘Generally, a confocal Fabry-Perot interferometer is only able to detect the out-of-plane component of a displacement field; while the in-plane component often has the information about the material which cannot be found in this out-of-plane component. In this paper, based on a confocal Fabry-Perot interferometer set-up for detecting the out-of-plane component of a laser generated acoustic field, a technique is developed to detect both the out-of-plane and in-plane displacement components simultaneously with a novel two-channel confocal Fabry-Perot interferometer.
基金National Natural Science Foundation of China Thans-Century Training Programme Foundation for the Talents by the State Educat
文摘in the acousto-ultrasonic technique, a broadband ultrasonic wave is injected onto the surface at one location of the tested plate with the help of a longitudinal transducer and a receiving transducer is coupled to the same surface at another location. Thus, understanding the propagation characteristics of the ultrasonic wave is essential for successful application of the technique. It has been found that the dominant acousto-ultrasonic waves produced experimentally in thin plate are multi-mode Lamb waves. The broadband generation and detection of Lamb waves have been performed in two aluminum plates with different thickness using the acousto-ultrasonic method.
文摘The speed of the streaming flow in the liquid layer loaded on a thin plate, in which Lamb waves are propagating, was calculated. The results are good agreement with Moroney's experiment
基金This work was supported bly the National Natural Science Foundation of China(No.19774062).
文摘The characteristics of Lamb wave propagating in a solid plate with rough surfaces are studied on the basis of small perturbation approximation. The Rayleigh-Lamb frequency equation expressed with SA matrix is presented. The Rayleigh-Lamb frequency equation for a rough surface plate is different from that for a smooth surface plate, resulting in a small perturbation △k on Lamb wave vector k. The imaginary part of △k gives the attenuation caused by wave scattering. An experiment is designed to test our theoretical predications. By using wedge-shape pipes, different Lamb wave modes are excited. The signals at different positions are received and analyzed to get the dispersion curves and attenuations of different modes. The experimental results are compared with the theoretical predications.
文摘The Lamb wave technique is used in the detection of internal defects in plates. The reflection of the Lamb wave at the free edge of the plate has been shown to be rather complicated. The reflected wave is usually composed of infinite number of Lamb wave modes even in the case of incidence of one pure mode. The analysis of the reflection problem is complicated in nature as the orthogonality relations of the eigen modes of the Lamb wave concern both the stresses and displacements. We adopt the principle of the least squares as a new approach to simplify the analysis, in which the eigen-modes are taken as the trial function. It seems to be simple and effective. In most cases a good enough result can be obtained on using an 8- bit microcomputer. Here the coefficients of the various reflected modes are calculated, some interesting phenomena on the mode conversion are indicated and the conditions on the pure mode reflection are given.
基金National Natural Science Foundation of China! (No. 19974063).
文摘The displacement and stress as well as the dispersion equation for the guided waves of antiplane shear (SH) modes in a bonded plate are presented explicitly. The spring model is used to account for the interface rigidity between the layers of the bonded plate. It is found that the dispersion equation is composed of the terms pertinent with the welded and the cracked or slip boundary conditions with the compliance sT of the spring model as the sum-coefficients. Numerical results for an adhesive plate of aluminum and steel whose wave velocities are very close to each other of equal thickness indicate that the modes of odd order are more sensitive than the even ones to the variations of the bond quality, and so are the higher modes than the lower ones for this special layered plate. The phenomena of the cut-off frequency-shift and the phase velocity decrease resulting from the degradation of the bond quality are found to the modes of odd order, which may be used to evaluate the bond states of the adhesive plate. Numerical results for an adhesive plate of rubber and steel whose wave velocities are very different to each other of equal thickness are presented and discussed as well.
基金supported by the National Natural Science Foundation of China.
文摘Based on elastic wave propagation theory, the dispersion equation for a thin anisotropic plate (such as commonly used Zinc okide in micro-transducers) bordered with liquid layers is derived. Higher symmetry crystals, such as orthorhombic, tetragonal, cubic, isotropic, are included in this analysis as well. For the case of one liquid layer loading, numerical calcu- lations show that the phase velocity changes periodically with the thickness of the liquld layer. When the thickness 2d of the anisotropic plate is very small, mass sensing application of Ao mode Lamb wave is also discussed.
基金supported by the National Natural Science Foundation of China(Nos.61975166,62322510,and 62375223).
文摘The suppression of polarization cross talk in lead zirconate titanate phase modulators as a key error source has been challenging for open-loop fiber optic gyroscopes(FOGs).We developed a polarization-diversity optical frequency domain reflectometry(OFDR)to measure the distributed modulation polarization error in the modulator.The error contributes 8×10^(−6) rad to FOG’s bias instability.Using a UV-fabricated in-fiberλ/4 wave plate and polarization-mode converter with fiber taper technology,the modulation error has been suppressed by 15 dB in assembled FOGs.This approach reduced error with temperature from 25°/h to 0.7°/h,meeting the requirements of control-level gyroscopes with bias errors less than 1°/h.
