Adaptive beamforming for array imaging of plate structures using lamb waves

Lamb waves are considered a promising tool for the monitoring of plate structures. Large areas of plate structures can be monitored using active arrays employing beamforming techniques. Dispersion and multiple propagating modes are issues that need to be addressed when working with Lamb waves. Previous work has mainly focused on standard delay-and-sum (DAS) beamforming while reducing the effects of multiple modes through frequency selectivity and transducer design. This paper presents a minimum variance distortionless response (MVDR) approach for Lamb waves using a uniform rectangular array (URA) and a single transmitter. Theoretically calculated dispersion curves are used to compensate for dispersion. The combination of the MVDR approach and the two-dimensional array improves the suppression of interfering Lamb modes. The proposed approach is evaluated on simulated and experimental data and compared with the standard DAS beamformer. It is shown that the MVDR algorithm performs better in terms of higher resolution and better side lobe and mode suppression capabilities. Known issues of the MVDR approach, such as signal cancellation in highly correlated environments and poor robustness, are addressed using methods that have proven effective for the purpose in other fields of active imaging.     

复合共鸣腔Janus宽带换能器

提出了一种复合共鸣腔Janus换能器设计思想.在Janus-Helmholtz换能器基础上,增加Janus-Hammer Bell的圆环谐振模态,利用换能器三种工作模态的互相耦合,有效拓宽了换能器的工作频带.同时换能器整体采用溢流设计,实现内外静水压力平衡,适用于深水工作.文章首先介绍了复合共鸣腔的工作原理,给出了圆环...     

Band gaps of lamb waves in one- dimensional piezoelectric composite plates: effect of substrate and boundary conditions

We theoretically study the band structures of Lamb waves in one-dimensional phononic crystal plates consisting of piezoelectric ceramics placed periodically in epoxy with epoxy or piezoelectric ceramic substrate by the virtual plane wave expansion method. The dependences of the widths and starting frequencies of first band gaps (FBG) on the substrate's thickness, the filling fraction, and the lattice spacing are calculated for different materials of substrate under different electric boundary conditions, i.e., short circuit (SC) and open circuit (OC). The FBG width decreases gradually as the substrate's thickness increases and the FBG starting frequency increases progressively as the thickness increases on the whole. The FBG widths and starting frequencies with SC are always larger than with OC. Our research shows that it is possible to control the width and starting frequency of the FBG in the engineering according to need by choosing suitable values of the substrate?s thickness, the filling fraction, and the lattice spacing.     

The excitation and detection of lamb waves with planar coil electromagnetic acoustic transducers

Planar coil electromagnetic acoustic transducers (EMATs) are investigated for the excitation and detection of Lamb waves in nonferromagnetic metallic wave-guides. Such EMATs are attractive for certain applications due to their omni-directional sensitivity to wave modes with predominantly in-plane surface displacement, such as the So Lamb wave mode. A model is developed that enables the modal content of the radiated Lamb wave field from a transmitting EMAT to be calculated, and the output voltage from a receiving EMAT to be predicted when a Lamb wave mode is incident on it. The predictions from this model are compared with experimental data obtained from 12 different EMATs tested on a 5-mm thick aluminum plate, and good agreement is obtained. The model then is used to analyze the different effects that contribute to the overall Lamb wave modal sensitivity of an EMAT. The relationship between coil geometry and wavelength is examined.     

Numerical analysis of lamb wave generation in piezoelectric composite IDT

An equivalent, single-layer model for Lamb wave generation by interdigital transducer (IDT) on composite host structures is developed. The additional complexities generally encountered while launching the surface acoustic wave (SAW) on composite structure, such as the coupling between the Lamb wave modes, the complicated nature of the electromechanical actuation etc. are considered. The model of infinite IDT is extended to deal with the finite IDT with edge discontinuities. The effect of electromechanical actuation on the wavelength shifts with respect to the passive case is investigated. The problem of electrically driven instability within the IDT is analyzed. Numerical results are reported by considering a model of the IDT integrated with the host structure, which shows that there are significant deviations from the conventional design estimates while launching a targeted mode. The proposed approach may enable one to obtain new designs in material systems and geometry that avoid mode-mixing, or to introduce it by choice.     

Numerical analysis on the interaction of guided Lamb waves with a local elastic stiffness reduction in quasi-isotropic composite plate structures

This paper investigates how Lamb waves respond to the presence of material degradation in a plate-like structure using a series of finite element analyses. To facilitate this study, the propagation of these guided waves was interpreted with the dispersion characteristics and displacement profiles were analysed in the frequency and wave number domain. The results show that the material degradation simulated by a local stiffness reduction which leads to changes in the dispersive characteristic of the propagating waves has made the Lamb waves technique become an effective tool to assess the material degradation.     

Laser-generated elastic waves in carbon-epoxy composite

New contactless techniques using laser-generated ultrasound have been applied to the inspection of composite materials. Transient elastic waves were launched thermoelastically in half cylindrical composite samples using a long pulse dye laser or a Q-switched Nd-YAG laser. The waves were detected with piezoelectric transducers or with an optical heterodyne interferometer. The measurements have been carried out on two different lay up design composites: carbon/epoxy: undirectional and cross-ply 0 degrees /45 degrees /90 degrees /-45 degrees . Quasi-longitudinal, quasi-shear, and shear bulk waves and head waves are clearly discerned in the stacking of a large number of waveforms. Velocities of the different types of waves simultaneously generated are compared to the phase and group velocities computed using Christoffel equations and an hexagonal model. It is shown with this point-source measurement technique that the wavefront arrival times agree with the energy velocities rather than with the phase velocities. A pronounced anisotropy is observed in the amplitudes of the wave arrivals. Angular directivity patterns of quasi-longitudinal, quasi-shear, and transverse bulk waves are plotted.     

Complex waves in nonreciprocal guiding structures

Features of complex waves excited in nonreciprocal guiding structures are considered in the case of a round shielded waveguide with a longitudinally magnetized axial ferrite core.     

Influence of damaged area on lamb wave propagation in composite plate

The influence of a specified damage on transient propagation of Lamb wave in a composite laminated plate is studied by finite element analysis. The finite element formulation is developed for the laminated plate with embedded or surface-bonded piezoelectric layers. A higher order laminate model is used to describe the displacement field of both composite laminate and piezoelectric layer. The damaged area is modeled by a localized loss of stiffness and quantified by a degradation coefficient . Piezoelectric materials act as both actuators and sensors for generating and receiving Lamb waves. Numerical results show that the waveform, wave peaks and the arrival time of the transmitted Lamb wave are distinctly correlated with quantified degradation coefficient of the damaged area, which are helpful to damage detection for a composite laminated plate in a new way.Tel (Res).: 86-29-88242204, Tel (Off).: 86-29-88213623-8026     

不确定条件下复合材料结构的全局灵敏度分析

基于不确定条件下结构的全局灵敏度分析理论,研究了输入变量的不确定性对复合材料结构输出响应量方差和失效概率的影响。考虑材料力学性能、铺设角、铺层厚度及加载载荷的不确定性,利用基于方差和基于失效概率的全局灵敏度分析方法,对复合材料结构输出位移和强度比的不确定性来源进行分析,得到输入变量的全局灵敏度排序结果。对复合材料工字梁结构进行算例分析,验证了所得排序结果的有效性,为工程实际中复合材料结构稳定性优化设计提供了一定的指导。     

Numerical model for composite structures with experimental confirmation

The paper briefly presents a numerical model for the simulation of composite structures. The main structure is modeled with two‐dimensional plane finite elements. The composite surface is modeled with two‐dimensional interface elements for the continuous connection simulation and modified beam elements for the discrete connection simulation. The applied material model’s primary purpose is the simulation of reinforced concrete structures. It includes the most important nonlinear effects of reinforced concrete behavior: yielding in compression and opening and propagation of cracks in tension, with tensile and shear stiffness of cracked concrete, as well as the nonlinear behavior of reinforced steel. It also includes nonlinear behavior of the composite surface and the connection elements. The model was confirmed in experimental tests of composite concrete Omnia slabs, which are in common usage. The achieved test results were compared with the results obtained through the developed numerical model.     

Strengthening of historic masonry structures with composite materials

This paper deals with the applications of unidirectional fibre-reinforced polymer tendons for the reversible strengthening of masonry monuments. The tendons, anchored to the masonry only at the ends, are circumferentially applied on the external face of the structure and posttensioned to provide horizontal confinement. The relevant properties of fibre-reinforced polymer materials and prestressing systems are summarised; in addition, the concepts for their application, including anchorage, to masonry structures are developed, and a general design procedure is presented. The effectiveness of the strengthening technique is established both analytically, for structures with simple geometries, and numerically, for a real three-dimensional structure with openings, based on the finite element method. The effects of temperature changes on the tendons and the masonry are shown to be negligible. It is concluded that the effectiveness of the proposed method in the consolidation of historic masonry structures is quite satisfactory, especially when the strengthening elements are made of carbon fibre-reinforced polymer.     

Buckling of thin-walled composite structures with intermediate stiffeners

The interactive buckling of prismatic, thin-walled composite columns with open sections, reinforced with intermediate stiffeners and with edge reinforcements, has been considered. The columns are assumed to be simply supported. The nonlinear problem has been solved with the Koiter’s asymptotic theory within the first order approximation. The asymptotic theory of the first order nonlinear approximation allows for simultaneous evaluation of the effect of imperfections and interactions of various modes of buckling on the behaviour of thin-walled structures. This evaluation can be only the lower bound estimation of the load carrying capacity. Detailed calculations have been made for several cases of columns.     

Pyramidal lattice sandwich structures with hollow composite trusses

Pyramidal lattice sandwich structures with hollow composite trusses were fabricated using a thermal expansion molding approach. Composite lattice structures with three relative densities were fabricated with two fiber architectures and the out-of-plane compression properties were measured and compared. Lattice cores with a fraction of carbon fibers circumferentially wound around the hollow trusses (Variant 2) exhibited superior mechanical properties compared with similar structures comprised of unidirectional fibers (Variant 1). The out-of-plane compressive properties of composite pyramidal lattice structures in Variant 2 were well-matched by analytical predictions. Unusual strain hardening behavior was observed in the plateau region for Variant 2, and the energy absorption capabilities were measured and compared with the similarly constructed silicone rubber–core truss pyramidal lattice structures (Variant 3). The energy absorption per unit mass of selected hollow truss composite lattice structures reported here surpassed that of both hybrid truss counterparts (Variant 3) and hollow truss metallic lattice structures.     

Controlling the formation of DNA origami structures with external signals

Degradable Newkome-type and polylysine dendrons functionalized with spermine surface units are used to control the formation of DNA origami structures. The intact dendrons form polyelectrolyte complexes with the scaffold strands, therefore blocking the origami formation. Degradation of the dendron with an optical trigger or chemical reduction leads to the release of the DNA scaffold and efficient formation of the desired origami structure. These results provide new insights towards realizing responsive materials with DNA origami.     

Measurement of the instantaneous frequency of broadband signals in a short observation interval

A high-speed method of estimating the instantaneous frequency of broadband signals with slowly varying amplitude and phase using a short sample in real time is proposed, and results of numerical modeling of the measurement procedure are presented. The speed of response and noise immunity of the method is compared with the existing method, based on the theory of signal “splitting.” It is shown that in both methods it is possible to use recurrence relations, which considerably reduce the measurement time. __________ Translated from Izmeritel’naya Tekhnika, No. 2, pp. 50–54, February, 2008.     

Damage detection in T-joint composite structures

The use of composite structures in engineering applications has proliferated over the past few decades. This is mainly due to their distinct advantages of high structural performance, high corrosion resistance, and high strength/weight ratio. They are however prone to fibre breakage, matrix cracking and delaminations which are often invisible. Although there are systems to detect such damage, the characterisation of the damage is often much more difficult to achieve. A study is presented of the strain distribution of a GFRP T-joint structure under tensile pull-out loads and the determination of the presence and the extent of disbonds. Finite element analysis (FEA) has been conducted by placing delaminations of different sizes at various locations along the structure. The FEA results are also validated experimentally. The resulting strain distribution from the FEA is pre-processed by a method developed called the damage relativity assessment technique (DRAT). Artificial neural networks (ANNs) were used to determine the extent of damage. A real-time system has been developed which detects the presence, location and extent of damage from the longitudinal strains obtained from a set of sensors placed on the surface of the structure. The system developed is also independent of the magnitude of load acting on the structure.     

Moisture diffusion in composite sandwich structures

The mechanical properties of polymer core materials in sandwich structures are often degraded by moisture that is absorbed during storage. To date, there is no reliable model to predict the amount of moisture that is present in these sandwich core materials. A multi-layer diffusion model applicable to these sandwich structures is described in this report. Inputs to this model are: (1) diffusivities of core and face sheet materials as functions of temperature, (2) moisture saturation data as a function of relative humidity, and (3) sandwich structure exposure history. The output is a prediction of the amounts of moisture in the core material and face sheets as a function of time.

In order to validate this model, moisture diffusion experiments were performed on a sandwich material consisting of graphite–epoxy face sheets and a core of Rohacell^® polymethacrylimide 200WF foam. Samples of this material were dried, and then hydrated at either 32 °C or 65 °C at either 83% or 100% relative humidity. The face sheets were separated from the core and each component was weighed, dried, and weighed again in order to determine the moisture distribution in the sandwich structure. The results were then compared with the model predictions.     

Low-cost implementation of broadband microwave receivers in Ka-band using multiport structures

The authors introduce a new class of the microwave and millimeter wave multiport receivers. These receivers are designed based on a new five-port structure and offer a wide bandwidth. The low-cost implementation is the other advantage of this structure. The numerical and microwave simulations have been conducted to investigate the characteristics of the receiver. In addition, the new five-port receiver has been constructed and measured. The analytical and measurement results are in good agreement with each other. This indicates that the structure can effectively be used as a high-performance and low-cost wideband receiver for broadband applications. As a practical application, the European and North-American local multipoint distribution services band can be covered using this new architecture.     

Propagation of impact-induced stress waves in composite plates

A theoretical investigation is carried out of the response of a laminated plate to a surface impact. The plate consists of four layers of a uni-directional fiber composite in a symmetric cross-ply configuration. The composite material is modeled as a transversely isotropic, homogeneous, elastic continuum. The four layers are assumed to be of finite depth, infinite lateral extent, and perfectly bonded to each other. The surface impact has been modeled by three different conditions, namely, a delta function impulse, a square pulse, and a half-sine pulse. The variation with depth of the normal component of stress is examined as a step toward gaining insight into the through thickness response of the plate.