Interferometric flaw detection with an increased sensitivity of measurements in study of reflecting composite periodic structures

Methods that allow an interferometric comparison of the elementary structures of a reflecting composite periodic structure and a reference structure are described. Specific features of the realization of these methods by interferometric correlators are considered in the case of studying specular and diffusely reflecting composite structures with an increase in the sensitivity of measurements. Experimental results of testing these methods in application to the study of plane metal surfaces with cylindrical depressions are presented. Interference patterns with a twofold increase in the measurement sensitivity are obtained.     

Band gap behaviours of periodic magnetoelectroelastic composite structures with Kagome lattices

In this paper, the elastic wave propagation in periodic cylinder magnetoelectroelastic composite structures is studied using the plane wave expansion method. The band structure characteristics of magnetoelectroelastic rods embedded in polymer matrix and the reverse case are investigated taking the electric, magnetic and mechanical coupling effects into account. The generalised eigenvalue equation is derived to analyse the in-plane and out-of-plane modes, respectively. The numerical calculations for both the cases with Kagome lattices are performed. The relation between the gap widths and filling fractions are discussed in detail. The effects of the magnetoelectricity on the band structures and widths of band gaps are analysed. The band gap characteristics are illustrated further and the results will be helpful to design such kind of composite structures.     

Calculation of the flaw image in γ-ray flaw detection

A perturbation-theory method is described for calculating the image of a small flaw. Results calculated for flaws of several simple shapes are reported.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 111–114, January, 1971.The authors thank V. G. Bagrov for deriving Eq. (8).     

A novel sensitivity-based method for damage detection of structures under unknown periodic excitations

This paper presents a technique for damage detection in structures under unknown periodic excitations using the transient displacement response. The method is capable of identifying the damage parameters without finding the input excitations. We first define the concept of displacement space as a linear space in which each point represents displacements of structure under an excitation and initial condition. Roughly speaking, the method is based on the fact that structural displacements under free and forced vibrations are associated with two parallel subspaces in the displacement space. Considering this novel geometrical viewpoint, an equation called kernel parallelization equation (KPE) is derived for damage detection under unknown periodic excitations and a sensitivity-based algorithm for solving KPE is proposed accordingly. The method is evaluated via three case studies under periodic excitations, which confirm the efficiency of the proposed method.     

Photonic pseudogaps for periodic dielectric structures

We consider the problems of existence and structure of gaps (pseudogaps) in the spectra associated with Maxwell equations and equations that govern the propagation of acoustic waves in periodic two-component media. The dielectric constant is assumed to be real and positive, and the value of = _b on the background is supposed to be essentially larger than the value of = _a on the embedded component. We prove the existence of pseudogaps in the spectra of the relevant operators. In particular, we give an accurate treatment of the term pseudogap. We also show that if the contrast _b / _a approaches infinity, then the bands of the spectrum shrink to a discrete set which can be identified with the set of eigenvalues of a Neumann-type boundary value problem and thus can be effectively calculated.     

Unstable periodic orbit detection for ODEs with periodic forcing

The Davidchack–Lai iterative scheme for the complete detection of unstable periodic orbits (UPOs) in maps is applied to a second order, nonlinear ODE with periodic forcing. The modifications to the scheme required to apply it to ODEs are detailed before the results for a particular example, the varactor equation, are given.     

A novel optical method without phase unwrapping for subsurface flaw detection

This paper presents a novel optical method without phase unwrapping for subsurface flaw detection, based on the computer-vision reflection grating technique. In the setup, a computer-generated fringe pattern displayed on a computer-monitor is placed in front of the test object surface and forms a mirror image behind the reflective surface. The fringe pattern is perturbed according to the surface slope distribution. Unlike the current fringe pattern analysis techniques, which require one/several images, the analysis algorithm of the present method involves a sequence of fringe patterns having multiple frequencies to deduce the ‘pixel-slope’ information which is directly related to the surface slope of the test object. The novel method offers independent analysis for each pixel of the data array, and thus the problem of phase discontinuity in phase unwrapping can be eliminated. In this paper, experimental demonstration of the novel optical method to subsurface flaw detection is presented.     

一种用于缺陷超声检测的相位标准差分离谱新算法

在粗晶材料超声无损检测中,分离谱处理技术可用来降低材料内部的相干噪声、但分离谱处理对滤波器阵列参数较敏感、造成性能不稳,且至今对滤波信号有效利用的解释尚不明了.本文从理论上讨论了超声检测信号滤波输出的相位统计特性,据此提出了一种基于相位标准差的分离谱新算法,用计算机模拟及实验数据对该算法的世能进行了测试并与常用的极小值分离谱算法作了对比。结果证实,相位标难差的分离谱算法有好的信噪比增强能力,且对滤波器数不敏感.     

Dynamic analysis of periodic structures

The natural vibration analysis of a periodic structure with repeated identical substructures may be simplified by using some symmetrical properties of the substructure dynamic matrices, resulting in a set of linear difference equations in the displacements. These equations are readily solved for cyclic symmetric systems, simply supported systems and infinite systems. The order of the overall frequency equations is at most equal to one half of the total number of degrees of freedom retained for a single substructure regardless of the number of substructures in the system. With these natural modes, the system with general boundary conditions at end stations is analyzed by a fast converging method.     

A new set-up for pulsed digital shearography applied to defect detection in composite structures

An optical set-up based on the combination of new CCD technology, a ruby laser and a Mach-Zehnder shear interferometer is presented for defect detection in composites. Transient loads are used to excite defects in a sandwich structure and image-processing routines allow improving the detection resolution. Phase calculation is obtained by FFT algorithms applied to interference patterns with a spatial carrier in the primary fringes. Independent control of the shear and the frequency of the spatial carrier can be obtained with this set-up. The experimental implementation of the set-up and the results obtained with it are presented and discussed.     

High-sensitivity piezoelectric tube sensor for shear-force detection in scanning near-field optical microscopy

An easy-to-implement non-optical shear-force detection setup for tip–sample distance regulation in scanning near-field optical microscopy is demonstrated. The detection method is based on attaching the near-field probe to a piezoelectric tube resulting in excellent mechanical contact between tip and detector. The main advantages of the method are good signal-to-background contrast and thus potential for high sensitivity. The method is demonstrated by obtaining approach curves of silicon surfaces. The suitability for optical experiments is further shown by measuring the near-field intensity distribution of the emission of a semiconductor laser.     

Study on localization of plane elastic waves in disordered periodic 2-2 piezoelectric composite structures

Considering the effect of mechanic-electric coupling, the propagation and localization of plane elastic waves in disordered periodic layered piezoelectric composite structures are studied. The transfer matrix between two consecutive unit cells is obtained by means of the continuity conditions and the expression of the localization factors in disordered periodic structures is presented by regarding the variables of mechanical and electrical fields as the elements of state vectors. As examples, numerical results of localization factors are presented and discussed. It can be seen from the results that ordered periodic structures possess the properties of frequency passbands and stopbands and the phenomenon of wave localization in disordered periodic structures is observed, and the larger the coefficient of variation is, the larger the localization factor or the stronger the degree of wave localization is. The characters of wave propagation and localization are very different for different sorts of piezocomposites or different structural sizes, and even for same sorts of piezocomposites and same structural sizes the characters of wave propagation and localization are also very different for different non-dimensional wavenumbers. We may design different piezocomposites or adjust the structural sizes to control the characters of wave propagation and localization.     

Construction of Arnold tongue structures for coupled periodic oscillators

Arnold tongue structures generated due to the mutual entrainment of two periodic oscillators are studied experimentally and numerically. This mutual entrainment is provoked due to the mutual (bidirectional) coupling between the two oscillators. In experiments, this bidirectional coupling is achieved by immersing a pair of anodes (oscillators) in a common electrolytic solution. A voltage mismatch between these anodes renders the time period of the uncoupled oscillators non-identical. Moreover, the coupling strength between the two oscillators is uniquely determined by the Euclidean distance separating them. Systematically varying the distance between these two anodes as a function of their voltage mismatch, phase locked domains were located. Subsequently, Arnold tongue structures were constructed in the experiments. Numerical simulations, using a model for electrochemical corrosion, corroborate our experimental findings.     

Enhancing sensitivity of a contactless method for measuring profiles and revealing macrodefects in composite periodic structures of the transmission type

Various interference techniques for comparing individual periodic structures incorporated into a transmission-type composite are suggested. All of the techniques are based on the method of interference patterns resulting from superposition of waves diffracted by the elementary periodic structures. It is shown that the sensitivity of measurements can be significantly enhanced and the width and direction of reference fringes can be arbitrarily adjusted. An interference-moire technique allowing a comparative study of elementary periodic structures incorporated into the composite is considered. Efficiency of the technique is confirmed experimentally.     

Homogenization of periodic bi-isotropic composite materials

In this paper, we present a new method for homogenizing the bi-periodic materials with bi-isotropic components phases. The presented method is a numerical method based on the finite element method to compute the local electromagnetic properties. The homogenized constitutive parameters are expressed as a function of the macroscopic electromagnetic properties which are obtained from the local properties. The obtained results are compared to Unfolding Finite Element Method and Maxwell–Garnett formulas.