Calculation of ultrasonic transducer signal-to-noise ratios usingthe KLM model

This paper describes a method for adding thermal and amplifier noise to a KLM model for a transducer element. The model is used to compare the magnitudes of various noise sources in a 5 MHz array element typical of those used for linear array imaging with and without an amplifier. Fundamental signal-to-noise ratio (SNR) issues of importance to array and amplifier designers are explored, including the effect on SNR of effective dielectric constant of the piezoelectric material, individual element size, changing the number of elements, and adding an amplifier to an element before and after a cable. SNR is considered both for the case in which the acoustic output is limited by the maximum rarefactive pressure which is considered safe for a particular application (Mechanical Index limitation) and the case in which acoustic output is limited by the maximum transmit voltage than can he delivered by the imaging system or tolerated by the transducer. It is shown that the SNR performance depends on many controllable parameters and that significant improvements in SNR can be achieved through proper design. The implications for 1.5-D and 2-D array elements are discussed     

An analytical assessment of the influence of skin imperfections on the indentation collapse mechanism in composite sandwich beams

The influence of skin imperfections, in the form of delamination damage or thickness variations, on the indentation collapse mechanism in composite sandwich beams with compressive yielding cores is studied using the models of non-prismatic beam and beam-column resting on a nonlinear Winkler foundation. Upper and lower threshold solutions are derived for the indentation response and collapse load and the transition between the two limits is defined as a function of size, magnitude and position of the imperfections. In beams where global bending effects are not negligible, the collapse load is limited from above by the indentation collapse load of beams with rigid-plastic cores and the face wrinkling collapse load of beams with elastic cores; the transition between the two limits is controlled by material/structure properties and the magnitude of the imperfections. Characteristic lengths, which depend on material and geometrical properties, define the minimum size of the imperfections with the strongest effect on the solution and the minimum distance between load and imperfections with no effect on the solution.     

Magneto-optical disk drive technology using multiple fiber-coupled flying optical heads. Part II. Laser noise considerations

A magneto-optical data storage system utilizing single-mode fiber is capable of providing high signal-to-noise ratio (SNR) recording if laser noise sources are properly managed. In particular, mode partition noise (MPN) associated with use of a Fabry-Perot laser diode can be a significant problem in a fiber-based system. The various mechanisms leading to MPN as well as to laser phase noise are discussed in the context of a system constructed with polarization-maintaining fiber. The primary noise mechanisms include spurious fiber-endface reflections and errors in the quarter-wave plate on the recording head. An understanding of these effects is essential for fabrication of a fiber-based recording system with suitable SNR performance.     

An analytical assessment of the influence of skin imperfections on the indentation collapse mechanism in composite sandwich beams

The influence of skin imperfections, in the form of delamination damage or thickness variations, on the indentation collapse mechanism in composite sandwich beams with compressive yielding cores is studied using the models of non-prismatic beam and beam-column resting on a nonlinear Winkler foundation. Upper and lower threshold solutions are derived for the indentation response and collapse load and the transition between the two limits is defined as a function of size, magnitude and position of the imperfections. In beams where global bending effects are not negligible, the collapse load is limited from above by the indentation collapse load of beams with rigid-plastic cores and the face wrinkling collapse load of beams with elastic cores; the transition between the two limits is controlled by material/structure properties and the magnitude of the imperfections. Characteristic lengths, which depend on material and geometrical properties, define the minimum size of the imperfections with the strongest effect on the solution and the minimum distance between load and imperfections with no effect on the solution.     

Phase and amplitude disturbances in direct digital frequency synthesizers

The problem of the spurious phase and amplitude disturbances in direct digital frequency synthesizers (DDFS) at the digital-to-analog (DAC) output is discussed. Such a situation is met when a commercial synthesizer lacks the output filter (e.g., the whole set of the so-called Number-controlled oscillators) or is supplied, for some reasons, with a lower clock frequency. Authors of recent books on DDFS mention this phenomenon but lack to provide the explanation. The problem is dealt with in detail in this paper.     

一类声子晶体角梁的振动带隙研究

针对声子晶体直梁的低维窄带减振特性的不足,提出了一类声子晶体角梁。采用传递矩阵法对角梁进行了理论分析和数值求解,并进行了有限元仿真验证。分析结果表明：声子晶体角梁能够通过纵向振动和弯曲振动的转化使得组成梁的两种带隙同时得以发挥减振作用,从而有效地实现了宽频多维减振;声子晶体角梁的构造角度对其减振性能有明显的影响,90度角梁减振能力明显优于45度和135度情况;当加载角由0度向90度变化时,角梁内的振动由纵向振动为主逐渐过渡到弯曲振动占主导地位,弯曲带隙内的衰减也随之更加显著,反之亦然;扭簧能够加强角梁的“弹簧-振子”效应,不仅可以增加总振动带隙的宽度和衰减量,而且提高了低频区减振能力。     

APS星跟踪器探测灵敏度研究

APS星跟踪器探测灵敏度同APS噪声源、填充因子、量化效应、频谱响应和恒星的光辐射特性有关,其中,APS噪声和APS对恒星辐射的响应特性是主要影响因素。提出了一种计算APS探测极限星等的简单方法。该方法根据最佳检测原则,由信噪比阈值和APS噪声值得到APS探测信号值,再通过典型APS对0星等恒星的辐射响应求出APS相应的探测极限星等。在信噪比阈值为5的情况下,得到某类APS的探测极限为6.31星等。     

The imperfect linearly viscoelastic column

Columns made of a linear three-element model material and with initial lateral deflections, referred to as ‘imperfections’, are analyzed by the dynamic and static approaches. Both analyses provide approximately the same time-deflection response for the structure. It is shown that imperfections may be regarded as a special class of disturbances which do not affect the value of the viscoelastic critical load. However, the influence of such ‘permanent’ disturbances on the rate of deformation is much more severe than that due to disturbances acting on a perfect column. Consequently, any possible safe operational period, related to the so-called ‘critical time,’ for the imperfect structure is much shorter than a corresponding period obtained from an analysis of the perfect column.     

Aluminum buffer rods for ultrasonic monitoring at elevated temperatures

Aluminum (Al) buffer rods, having circular, square, and rectangular cross-section shapes with and without cladding, were experimentally investigated to achieve high performance in ultrasound for industrial process monitoring. Focus was devoted to the signal-to-noise ratio (SNR) aspect in pulse-echo mode because high SNR can make many practical ultrasonic monitoring applications feasible. If the size of the rod cross section can be properly selected, spurious echoes induced in the rod having finite dimensions--due to mode conversion, wave reverberation, and diffraction--do not overlap the desired echoes during pulse-echo measurements. Thus, a sufficient SNR may be obtained using nonclad rods for certain applications, which is of low cost. Clad buffer rods, having the above three shapes and consisting of an Al core and a stainless steel cladding, also were manufactured through thermal spray method. These clad Al rods achieved a SNR of more than 40 dB and had better SNR than clad steel rods. It was shown that the rod with a larger cross-section area exhibited a wider ultrasonic beam. The Al rods are handy in real-time, nonintrusive and nondestructive ultrasonic monitoring.     

抗电磁干扰压电夹层优化设计

利用电磁干扰原理,对应用于航空结构健康监测的压电夹层信号易受到串扰、辐射干扰的情况进行了研究,得到若干耦合影响因子。通过合理设置这些影响因子达到了降低干扰的目的。在此基础上研制了抗电磁干扰压电夹层。实验表明,所设计压电夹层有着很好的抗串扰、辐射干扰能力,能够很好地实现损伤检测:在相同测试条件下,优化后的压电夹层信号串扰量较普通压电夹层下降一个数量级;对固定频率谐波干扰的信噪比提高了3.16倍。     

Elimination of spurious static modes in meshes of hybrid compatible triangular and tetrahedral finite elements

In the assumed displacement, or primal, hybrid finite element method, the requirements of continuity of displacements across the sides are regarded as constraints, imposed using Lagrange multipliers. In this paper, such a formulation for linear elasticity, in which the polynomial approximation functions are not associated with nodes, is presented. Elements with any number of sides may be easily used to create meshes with irregular vertices, when performing a non‐uniform h‐refinement. Meshes of non‐uniform degree may be easily created, when performing an hp‐refinement. The occurrence of spurious static modes in meshes of triangular elements, when compatibility is strongly enforced, is discussed. An algorithm for the automatic selection, based on the topology of a mesh of triangular elements, of the sides in which to decrease the degree of the approximation functions, in order to eliminate all these spurious modes and preserve compatibility, is presented. A similar discussion is presented for the occurrence of spurious static modes in meshes of tetrahedral elements. An algorithm, based on heuristic criteria, that succeeded in eliminating these spurious modes and preserving compatibility in all the meshes of tetrahedral elements of uniform degree that were tested, is also presented. Copyright © 2001 John Wiley & Sons, Ltd.     

Analysis of Outliers with Adjusted Residuals

Many statistical procedures designed to guard against the occurrence of outliers or spurious observations in normal theory are based upon examining the magnitude of the residuals. A major difficulty involved is caused by the fact that the residuals are correlated. It is shown that one way to avoid such difficulty is to adjust the residuals using information from an auxiliary experiment so that the adjusted residuals become uncorrelated. For the problem of making inferences about the unknown mean of a normal population N(μ, σ²) with known σ², this leads to a set of estimation procedures by which the observation(s) associated with the largest adjusted residual(s) in magnitude will be excluded. Certain properties of the procedures are discussed and exact numerical results are given for the cases of one and two spurious observations. Generalization to the case of unknown variance and to the general linear model is also given.     

Chequerboard effects on spurious currents in the lattice Boltzmann equation for two-phase flows

Spurious currents near an interface between different phases are a common undesirable feature of the lattice Boltzmann equation (LBE) method for two-phase systems. In this paper, we show that the spurious currents of a kinetic theory-based LBE have a significant dependence on the parity of the grid number of the underlying lattice, which can be attributed to the chequerboard effect. A technique that uses a Lax-Wendroff streaming is proposed to overcome this anomaly, and its performance is verified numerically.     

On the Enhancement of the SNR of Repetitive Signals by Digital Averaging

A simplified technique to improve the signal-to-noise ratio (SNR) of repetitive signals by digital averaging is presented. The technique uses a threshold detector and a memory instead of the analog-to-digital converter and memory, which is the common practice. It is shown that if the SNR of the measurements is small and the number of averaged measurements is large then the SNR of the resulting signal is smaller but comparable to that obtained by the usual procedure. The nonlinear distortion introduced by the process is also discussed.     

Sensibility of shells to buckling disturbances in connection with parameters of critical loading spectrum

This paper considers the critical loading density in the vicinity of the first eigenvalue for the stability problems of both a structurally orthotropic longitudinally-compressed cylindrical shell and a spherical panel. For structurally orthotropic shells it is shown that the critical loading spectrum in the above-mentioned problems can begin with the condensation point only when definite relations between structurally orthotropic parameters take place. In connection with the presence of eigenvalue condensation points in the stability problems it becomes expedient to obtain approximate solutions for nonhomogeneous problems where the right-hand part of the equation describes disturbances. The linear systems of such a type show a strong selectivity as regards to initial deflections and to small external disturbances. The experimental results were treated. The process of the additional shell deflection development was analyzed. It was established by experiment that the shells with more rigid longitudinal stiffeners are less sensitive to initial imperfections and disturbances that explains a good agreement of critical loadings of these imperfect shells with Euler loadings. The number of equivalent forms for the shells with longitudinally stiffened, transversely stiffened and non-stiffened shells has the following order, respectively: 0(1), 0(10), 0(100).     

Robust dynamical decoupling

Quantum computers, which process information encoded in quantum mechanical systems, hold the potential to solve some of the hardest computational problems. A substantial obstacle for the further development of quantum computers is the fact that the lifetime of quantum information is usually too short to allow practical computation. A promising method for increasing the lifetime, known as dynamical decoupling (DD), consists of applying a periodic series of inversion pulses to the quantum bits. In the present review, we give an overview of this technique and compare different pulse sequences proposed earlier. We show that pulse imperfections, which are always present in experimental implementations, limit the performance of DD. The loss of coherence due to the accumulation of pulse errors can even exceed the perturbation from the environment. This effect can be largely eliminated by a judicious design of pulses and sequences. The corresponding sequences are largely immune to pulse imperfections and provide an increase of the coherence time of the system by several orders of magnitude.     

The effect of geometric imperfections on the buckling behaviour of composite laminated cylinders under external hydrostatic pressure

Using a new FE modelling procedure for the accurate representation of a geometrically imperfect cylinder, the present study investigates the effect of the initial imperfection magnitude on the cylinder buckling load, when it is loaded by external hydrostatic pressure. The buckling behaviour of a carbon/epoxy filament wound cylinder is initially studied and the modelling procedure is validated through a comparison between calculated and experimental results. Various FE models are developed and evaluated, with increasing degree of complexity. The method is then applied to a cylinder with different end supports, to assess how the boundary conditions together with the imperfections affect the buckling behaviour. Finally, the effect of imperfection magnitude is investigated, leading to the calculation of knockdown factors as low as 0.6.     

Compressive failure mechanism and buckling analysis of the graded hierarchical bamboo structure

Bamboo is a unique unidirectional biocomposite, which consists of vascular bundles (VBs) as the reinforcement and parenchyma cells (PCs) as the matrix. The non-uniform distribution of VBs embedded in the matrix makes bamboo a functionally graded material. In order to investigate the compressive behavior of bamboo as a function of its components, compression tests were performed on specific bamboo samples with different VB volume fraction (V _vb), collected from different positions in the culm wall. The results show that both compressive strength and modulus increased linearly with V _vb, while the plastic deformation of samples in the compression decreased with increasing V _vb. This indicates that VBs dominate the compressive strength and modulus of bamboo, while the ductile performance of the bamboo is determined by the foam-like PCs. Scanning electron microscope observation indicated that VBs buckling were the main cause of failure. The Euler theory was applied to investigate the buckling behavior of bamboo blocks, showing that theoretical buckling strength when only considering the effect of VBs was much lower than with the test results, with the theoretical buckling strength being closer to test results when considering both the effect of VBs and PCs. It is therefore necessary to consider the contribution of PCs to resist buckling in bamboo, as their foam-like structure can effectively prevent the large-scale buckling of VBs.     

考虑节点偏差、杆件缺陷与偏心的单层三向柱面网壳稳定性研究

单层柱面网壳属于缺陷敏感型结构.为了理清节点偏差、杆件缺陷与杆件偏心对其稳定承载能力的影响程度:提出了能同时考虑三种缺陷的力学模型,并详细阐述了各种缺陷的实现方法;深入研究了三种缺陷及其耦合作用对柱面网壳稳定承载能力的影响规律;提出了确定三种缺陷最不利组合形式的方法.主要得出如下结论:柱面网壳稳定承载能力对节点偏差最为...     

纯介质光子晶体非线性效应研究进展

如何在低阈值小尺度(毫瓦或皮焦量级、微米以下)情况下激发非线性光学效应是近年来光学领域研究的重要课题。该研究最直接的应用需求就是光子集成芯片,这是未来实现超高速、大容量信息网络体系的基础。光子晶体具有类似于半导体能带的光子禁带(PBG),被誉为“光子半导体”,为人们提供了一种新颖而又实用的操纵光子的物理手段,使低阈值、可集成非线性效应产生成为可能。越来越多的非线性效应在光子晶体中已经被发现,例如光子晶体慢光、带隙孤子、电磁感应透明、二次谐波产生、光学双稳态等,本文将着重对可用于光子集成器件开发的光子晶体非线性效应研究领域的一些主要成果和进展进行总结,介绍其相关应用并对光子晶体非线性效应研究作出展望。