The mass normalization of the displacement and strain mode shapes in a strain experimental modal analysis using the mass-change strategy

The classic experimental modal analysis (EMA) is a well-known procedure for determining the modal parameters. The less frequently used strain EMA is based on a response measurement using strain sensors. The results of a strain EMA are the modal parameters, where in addition to the displacement mode shapes the strain mode shapes are also identified. The strain EMA can be used for an experimental investigation of a stress–strain distribution without the need to build a dynamical model. It can also be used to determine the modal parameters when, during modal testing, a motion sensor cannot be used and so a strain sensor is used instead. The displacement and strain mode shapes that are determined with the strain EMA are not mass normalized (scaled with respect to the orthogonality properties of the mass-normalized modal matrix), and therefore some dynamical properties of the system cannot be obtained. The mass normalization can be made with the classic EMA, which requires the use of a motion sensor. In this research a new approach to the mass normalization in the strain EMA, without using a motion sensor, is presented. It is based on the recently introduced mass-change structural modification method, which is used for the mass normalization in an operational modal analysis. This method was modified in such a way that it can be used for the mass normalization in the strain EMA. The mass-normalized displacement and strain mode shapes were obtained using a combination of the proposed approach and the strain EMA. The proposed approach was validated on real structures (beam and plate).     

Mixed mode I/II fracture investigation of Perspex based on the averaged strain energy density criterion

In this work, some recent mixed mode I/II fracture toughness results obtained from Perspex (or polymethylmethacrylate (PMMA)) with four simple cracked specimens subjected to the conventional three-point bend loading are reanalysed based on local energy concept. Although all the mentioned samples have been tested under the same and similar mode mixities, different fracture toughness envelopes were obtained for mixed mode I/II fracture of PMMA. The averaged strain energy density (SED) criterion has been applied in the past for different types of notched specimens (including U, V, O and keyhole notches). It is shown that the mixed mode tensile-in plane shear fracture toughness data obtained from the semicircular and triangular crack type specimens are successfully predicted for sharp cracked PMMA samples using the SED criterion.     

On-line detection of fatigue cracks using an automatic mode tracking technique

Experimental fatigue tests usually require large testing times. In addition to the resulting increased time-to-market, the large fatigue test time also implies that any structural health monitoring technique that is used should be automatic. When using the modal parameters as damage indicators, an important amount of user interaction is still needed to separate physical poles from computational ones. In this paper, an experimental framework will be developed to automatically track the health of the structure on-line with the performance of fatigue tests. The modal parameters are tracked using a combination of the maximum likelihood estimator and an auto-regressive model. Since confidence levels on the modal parameter are available it is possible to detect if damage is present. In addition, the quasi-static stiffness with computed confidence levels is also used as a damage indicator. The proposed techniques are demonstrated on a steel beam with a propagating fatigue crack.     

Mode II brittle fracture assessment using an energy based criterion

In this paper the minimum strain energy density criterion is modified to predict the values of mode II fracture toughness reported in the literature for several brittle and quasi-brittle materials. The experimental results are all related to mode II fracture tests performed on the semicircular bend specimen. The modified mode II fracture criterion takes into account the effect of T-stress (in addition to the singular terms of stresses/strains) when calculating the strain energy density factor at a very small critical distance from the crack tip. It is shown that the proposed criterion provides significantly better predictions for mode II fracture toughness compared with the classical minimum strain energy density criterion.     

Frequency-domain modal delay measurement for higher-order mode fiber based on stretched pulse interference

We propose and demonstrate a novel modal delay measurement technique for a higher-order mode fiber (HOF) based on optical frequency-domain reflectometry (OFDR) using an extremely simple, entirely passive, and ultrafast wavelength sweeping mechanism, namely, dispersion-induced optical pulse stretching. We obtained a high temporal resolution of approximately 1.12 ps, which was sufficient for discerning the four excited modes in an HOF with a length of only approximately 5 m. The results from our measurements were very consistent with those obtained by using a traditional time-domain measurement method and a conventional OFDR measurement based on a tunable CW laser. Our proposed technique can be also easily adapted to perform conventional time-domain modal delay measurements for very long HOFs.     

An empirical approach to clustering based on atomic energies

We present an empirical scheme that relates the Ω parameter, a quantity important in the context of clustering, to atomic p-energies. This scheme allows the determination of hitherto experimentally unknown Ω values by extrapolating from the known ones.     

Control of modulated vibration using an enhanced adaptive filtering algorithm based on model-based approach

Conventional adaptive filtering algorithms, typically limited to the control of single or multiple sinusoids, are not appropriate to control modulated vibrations, especially in the presence of rich side band structures. To overcome this deficiency, a new control algorithm is proposed that introduces a feedback loop with the model predictive sliding mode control (MPSMC) in the adaptive filtering system. Several amplitude and frequency modulation cases are first computationally studied, and conventional and proposed methods are comparatively evaluated in terms of estimation error, performance in time and frequency domains, stability, and uncertainty in the reference signal. To experimentally validate the proposed algorithm, an active strut (with longitudinal vibrations) is constructed. Overall, the proposed adaptive algorithm yields superior reductions at the main frequencies and at side bands; also, good attenuation is found on a broadband basis.     

All-fiber modal interferometer based on an up-taper-core-offset structure for curvature sensing

A high-sensitivity curvature sensor based on an up-taper-core-offset structure is proposed and demonstrated in this paper. Here two specially designed cascaded up-tapers with maximum diameters of 247 μm and 251 μm, respectively, are used as a cladding mode exciting component. The excited cladding modes will propagate in the cladding and re-couplers with the core mode at the core-offset jointing point. When the curvature is changed, the dip wavelength of the sensor will shift to a blue wavelength and an average curvature sensitivity of more than-12.5 nm/m-1is achieved within the measured curvature intervals.     

A modal test method using sound pressure transducers based on vibro-acoustic reciprocity

A modal test method that uses sound pressure transducers at fixed locations and an impact hammer roving over a test structure is developed in this work. Since sound pressure transducers are used, the current method deals with a coupled structural–acoustic system. Based on the vibro-acoustic reciprocity, the method is equivalent to one, where acoustic excitations at fixed locations are given and the resulting acceleration of the test structure is measured. The current method can eliminate mass loading due to use of accelerometers, which can destroy existence of repeated or close natural frequencies of a symmetric structure. It can also avoid effects of a nodal line of a mode and an inactive area of a local mode, and measure all the out-of-plane modes within a frequency range of interest, including global and local ones. The coupling between the structure and the acoustic field in a structural–acoustic system introduces asymmetry in the model formulation. An equivalent state space formulation is used for a damped structural–acoustic system and the associated eigenvalue problem is derived. The biorthonormality relations between the left and right eigenvectors and the relations between the structural and acoustic components in the left and right eigenvectors are proved. The frequency response functions associated with the current method are derived and their physical meanings are explained. The guidelines for using the current method, including the types of structures that are suitable for the method, the positions of the sound pressure transducers, and the orientation of the test structure relative to the transducers, are provided. Modal tests were carried out on an automotive disk brake using the traditional and current methods, where multiple accelerometers and microphones were used to measure its dynamic responses induced by impacts, respectively. The differences between the measured natural frequencies using the current method and those from the finite element model of the disk brake are less than 3 percent for the first 18 elastic modes, and the modal assurance criterion values of the associated mode shapes are all above 90 percent. The current method was also used to measure the natural frequencies, damping ratios, and mode shapes of a light circuit board.     

Optical flowmeter using a modal interferometer based on a single nonadiabatic fiber taper

A novel in-fiber modal interferometer is presented that is based on a nonadiabatic biconical fused taper that couples light between the cladding and the core, combined with the Fresnel reflection at the fiber end. It is observed that the returned light from this fiber structure shows a channeled spectrum similar to that of a two-wave Michelson interferometer. The application of this device as a fiber optic flowmeter sensor is demonstrated.     

In-fiber modal interferometer based on dual-concentric-core photonic crystal fiber and its strain, temperature and refractive index characteristics

An in-fiber modal interferometer is demonstrated by splicing a section of homemade dual-concentric-core photonic crystal fiber (DCCPCF) and two segments of single-mode fibers (SMFs) with collapsing the air holes in the splice regions. By analyzing the transmission spectra and the spatial frequency spectra of the interferometers, it's observed that the main interfering modes are the high-order ring modes, which are different from the previously reported interferometers using the single-, few- or multi-mode fibers. The influences of the lengths of DCCPCF and the first collapsed region on the interferometers were investigated experimentally. The strain, temperature and refractive index characteristics of the interferometer were analyzed.     

基于背景最佳滤波尺度的红外图像复杂度评价准则

提出一种基于背景最佳滤波尺度的红外图像复杂度评价准则来解决传统方法评价背景效果较差的问题. 同时, 这种方法还可以为红外图像滤波提供最佳高通滤波尺度信息, 从而对红外图像进行性能最佳滤波. 首先, 生成高斯仿真目标并与红外图像进行融合, 获得包含仿真目标及真实红外背景的图像. 然后, 在不同高斯滤波尺度下对图像滤波, 并计算滤波后仿真目标的信噪比. 最后, 取滤波后目标信噪比最大时的滤波尺度作为背景最佳滤波尺度, 使用该尺度可评价红外图像的复杂度. 另外, 本文还使用数学模型推导了红外图像最佳滤波尺度, 得出最佳滤波尺度的数学表达式. 大量实验表明: 1) 本文推导的最佳滤波尺度数学表达式与实验曲线吻合. 2) 这种方法在评价红外图像复杂度方面比传统的基于信息熵的方法效果要好很多. 并且这种方法获取的红外背景复杂度为滤波最佳尺度, 可以直接利用这项指标对图像进行最佳滤波从而更好地检测弱小目标. 3) 仿真目标尺度越大, 最佳滤波尺度也会相应增大. 因此, 在评价图像复杂度时, 应使用相同尺度的仿真目标, 不同图像之间才具备可比性. 同时, 最佳滤波尺度与仿真目标的强度无关. 4) 本文算法使用的滤波器宜用高斯及Butterworth高通滤波器实现. 5) 本文提出的方法不仅可以有效分析红外视频的复杂度, 并且可以通过复杂度的变化分析图像内容的突变.     

An enhanced non-uniformity correction algorithm for IRFPA based on neural network

Influenced by detector materials’ non-uniformity, growth and etching techniques, etc., every detector’s responsivity of infrared focal plane arrays (IRFPA) is different, which results in non-uniformity of IRFPA. And non-uniformity of IRFPA generates fixed pattern noises (FPN) that are superposed on infrared image. And it may degrade the infrared image quality, which greatly limits the application of IRFPA. Non-uniformity correction (NUC) is an important technique for IRFPA. The traditional non-uniformity correction algorithm based on neural network and its modified algorithms are analyzed in this paper. And a new improved non-uniformity correction algorithm based on neural network is proposed in this paper. In this algorithm, the desired image is estimated by using three successive images in an infrared sequence. And blurring effect caused by motion is avoided by applying implicit motion detection and edge detection. So the estimation image is closer to real image than the estimation image estimated by other algorithms, which results in fast convergence speed of correction parameters. A comparison is made to these algorithms in this paper. And experimental results show that the algorithm proposed in this paper can correct the non-uniformity of IRFPA effectively and it prevails over other algorithms based on neural network.     

Microcavity confinement based on an anomalous zero group-velocity waveguide mode

We propose and demonstrate a mechanism for small-modal-volume high-Q cavities based on an anomalous uniform waveguide mode that has zero group velocity at a nonzero wave vector. In a short piece of a uniform waveguide with a specially designed cross section, light is confined longitudinally by small group-velocity propagation and transversely by a reflective cladding. The quality factor Q is greatly enhanced by the small group velocity for a set of cavity lengths that are separated by approximately pi/k0, where k0 is the longitudinal wave vector for which the group velocity is zero.     

基于运动控制技术的编码器自动检测系统

为了改进编码器误差检测方法,提高检测效率,基于对光电轴角编码器误差检测现状的分析,设计了一种编码器误差自动检测系统。介绍了自动检测系统的工作原理,系统的硬件设计和软件设计。利用运动和全闭环控制技术,以24位高精度增量式光电轴角编码器为反馈元件,该自动检测系统可自动完成对编码器测试点的定位、数据获取和误差数据分析,定位精度为2″,可以检测18位以下各类光电轴角编码器的误差,检测效率是标准检测装置的6倍。该实验结果验证了方案的可行性。     

Nonlinear finite element model updating of an infilled frame based on identified time-varying modal parameters during an earthquake

A model updating methodology is proposed for calibration of nonlinear finite element (FE) models simulating the behavior of real-world complex civil structures subjected to seismic excitations. In the proposed methodology, parameters of hysteretic material models assigned to elements (or substructures) of a nonlinear FE model are updated by minimizing an objective function. The objective function used in this study is the misfit between the experimentally identified time-varying modal parameters of the structure and those of the FE model at selected time instances along the response time history. The time-varying modal parameters are estimated using the deterministic–stochastic subspace identification method which is an input–output system identification approach. The performance of the proposed updating method is evaluated through numerical and experimental applications on a large-scale three-story reinforced concrete frame with masonry infills. The test structure was subjected to seismic base excitations of increasing amplitude at a large outdoor shake-table. A nonlinear FE model of the test structure has been calibrated to match the time-varying modal parameters of the test structure identified from measured data during a seismic base excitation. The accuracy of the proposed nonlinear FE model updating procedure is quantified in numerical and experimental applications using different error metrics. The calibrated models predict the exact simulated response very accurately in the numerical application, while the updated models match the measured response reasonably well in the experimental application.     

基于酒精与磁流体填充的单模-空芯-单模光纤结构温度磁场双参数传感器

提出了一种基于空芯光纤模间干涉原理的环境温度和磁场双参数传感器,为了使光入射进空芯光纤壁中,将空芯光纤与单模光纤错位熔接,传感部分用毛细玻璃管封装,空芯光纤内外分别填充酒精和磁流体.除了光纤材料的热光效应和热膨胀效应外,环境温度变化会引起两种溶液折射率的变化,而磁场变化仅引起空芯光纤外的磁流体折射率变化.理论计算可知空芯光纤壁中可支持多个模式传输并相互干涉,各模式传输相位对内外溶液折射率变化灵敏程度不同.因此,干涉谱中两个含有不同模式成分的波谷,即波谷1和波谷2,它们的漂移可以作为指示信号,通过建立敏感矩阵可同时解调出周围环境温度与磁场的变化.实验中,在28—58℃范围内,温度传感灵敏度可达-468 pm/℃;在0—169 Oe范围内磁场传感灵敏度可达82 pm/Oe.该传感器具有高灵敏度与高机械强度,并且能够实现温度与磁场的同时测量,有效消除了温度波动对磁场测量信号的干扰.     

基于零磁通电流传感的自动电阻电桥设计

因商用直流电流比较仪(DCC)精度高、结构复杂、成本高昂,基于商用零磁通电流传感器基础上研制了一套直流比较仪自动电阻电桥,以降低成本、满足电阻测量精度要求较低应用场合的需求。围绕零磁通电流传感器的原、副边线圈安匝平衡机制下,其原、副边电流比例一定的特点,设计了自动电阻电桥电路。采用基于DSP的嵌入式数字电路实现系统控制和算法,使用自制且经过校准的24位AD采集电路读取电桥中电阻两端的压差信号,经实时处理及屏幕显示,最终得到被测电阻(Rx)和标准电阻值(Rs)的比例值。采用经校准过的标准电阻(Rs)对该电阻电桥100Ω：10Ω和10Ω：1Ω的比例进行校准,实验结果表明：该电阻电桥测量的比例准确度可达10-6 量级,测量的相对标准差为10-6 ~10-7 量级。     

Traffic dynamics based on an efficient routing strategy on scale free networks

In this paper, we propose a new routing strategy on the basis of the so-called next-nearest-neighbor search strategy by introducing a preferential delivering exponent α. It is found that by tuning the parameter α, the scale-free network capacity measured by the order parameter is considerably enhanced compared to the normal next-nearest-neighbor strategy. Traffic dynamics both near and far away from the critical generating rate R_c are discussed, and it is found that the behavior of 1/f-like noise of the load time series not only depends on the generating rate R but also on the parameter α. We also investigate R_c as functions of C (capacity of nodes), m (connectivity density) and N (network size). Due to the low cost of acquiring next-nearest-neighbor information and the strongly improved network capacity, our strategy may be useful for the protocol designing of modern communication networks.