Using cross correlations of turbulent flow-induced ambient vibrations to estimate the structural impulse response. Application to structural health monitoring

It has been demonstrated theoretically and experimentally that an estimate of the impulse response (or Green's function) between two receivers can be obtained from the cross correlation of diffuse wave fields at these two receivers in various environments and frequency ranges: ultrasonics, civil engineering, underwater acoustics, and seismology. This result provides a means for structural monitoring using ambient structure-borne noise only, without the use of active sources. This paper presents experimental results obtained from flow-induced random vibration data recorded by pairs of accelerometers mounted within a flat plate or hydrofoil in the test section of the U.S. Navy's William B. Morgan Large Cavitation Channel. The experiments were conducted at high Reynolds number (Re > 50 million) with the primary excitation source being turbulent boundary layer pressure fluctuations on the upper and lower surfaces of the plate or foil. Identical deterministic time signatures emerge from the noise cross-correlation function computed via robust and simple processing of noise measured on different days by a pair of passive sensors. These time signatures are used to determine and/or monitor the structural response of the test models from a few hundred to a few thousand Hertz.     

Estimation of structural system parameters from stationary and non-stationary ambient vibrations: An exploratory-confirmatory analysis

The natural frequency and damping parameters of a building structure are estimated from a long ambient vibration record that shows considerable non-stationarity. The long record is segmented into 57 approximately independent one minute duration stationary time series segments. Each segment is low pass filtered to reject unwanted higher frequency modes and is analyzed by a 2SLS (two stage least squares) time domain parametric model procedure. The scatter diagrams of the estimates of the natural frequency and damping parameters exhibit considerable variability. Estimates of the natural frequency and damping parameters and coefficient of variation expressions of their reliability are obtained by an exploratory-confirmatory data analysis of those 57 vibration time series. A procedure that can obtain the structural parameter estimates with the reliability that is available from stationary analysis from a long and not necessarily stationary record is indicated.     

Dual-probe laser interferometer for structural health monitoring

In this note we present the development of a dual-probe laser interferometer that uses the filtering properties of a polarized beamsplitter to enable two independent (uncoupled) detection probes. The robustness of this system is demonstrated by making broadband, noncontact, high fidelity measurements of Lamb waves in an aluminum plate.     

A structural health monitoring strategy using cepstral features

A statistical pattern recognition based damage detection algorithm is proposed. The algorithm is developed according to the training and testing scheme, typical of pattern recognition applications. The original contribution of the work is given by the use of an adaptation of Mel-Frequency Cepstral Coefficients as damage sensitive features, as their compactness and de-correlation characteristics make them particularly suited for statistical pattern recognition applications. At the same time, the ease of extraction, which requires minimal user expertise, represents an important advantage over other more popular features, and makes the cepstral features particularly convenient for implementation into automatic structural health monitoring routines. The damage detection algorithm employs the squared Mahalanobis distance to solve the Structural Health Monitoring assignment. The method is validated by using both simulated and experimental data, and the performance of said features is compared to that of Auto-Regressive (AR) coefficients, which have been largely used to solve the task of structural damage detection. The experimental data were measured on a steel frame, which behave nonlinearly in its damaged configuration, at the Los Alamos National Laboratory. Results demonstrate that the proposed approach may be conveniently used in real-life applications, since cepstral features outperform AR coefficients when dealing with experimental data modeled to mimic the operational and environmental variability.     

Laryngeal vibrations: a comparison between high-speed filming and glottographic techniques

This study was designed to compare information on laryngeal vibrations obtained by high-speed filming, photoglottography (PGG), and electroglottography (ECG). Simultaneous glottographic signals and high-speed films were obtained from two subjects producing steady phonation. Measurements of glottal width were made at three points along the glottis in the anterior--posterior dimension and aligned with the other records. Results indicate that PGG and film measurements give essentially the same information for peak glottal opening and glottal closure. The EGG signal appears to reliably indicate vocal-fold contact. Together, PGG and EGG may provide much of the information obtained from high-speed filming as well as potentially detect horizontal phase differences during opening and closing.     

动载体成像模糊的振动被动控制技术

动载体成像模糊的振动被动控制技术是解决无人机在高空飞行时,振动加速度为5g(20 500Hz)条件下引起的成像模糊问题。介绍了动载体成像模糊的机理、动载体成像模糊的容限。为了提高动载体光电系统的动态分辨能力,提出了解决动载体成像模糊的措施———振动被动控制技术。分析了振动被动控制的基本原理和设计原则,给出一些相关的典型设计实例。     

Equilibrium point damage prognosis models for structural health monitoring

Most structural dynamic systems are of high order; however, they often exhibit phenomena that can be dealt with effectively using low order models. This paper presents a method for describing certain kinds of damage evolution in mechanical systems. The method relies on a simple principle that as damage evolves in a structural dynamic system, the damage indicator (i.e., diagnostic feature) behaves like a stable quasi-stationary equilibrium point in a subsidiary non-linear bifurcating system within the so-called damage center manifold. It is shown that just as linear normal modes govern the behavior of linear structures with idealized damping, so too do non-linear normal forms govern the evolution of damage within structures in many instances. The method is justified with citations from the literature on certain types of mechanical failure and then applied in an experimental case involving reversible damage in a bolted fastener. Off-line experiments on a rotorcraft fuselage show that the evolution of damage is sensitive to both temporal and spatial bifurcation parameters. A diagnostic sensing strategy whereby output-only transmissibility features are used to decrease the order of high order structural dynamic measurements is also described.     

兰姆波结构健康监测中的概率损伤成像

应用概率成像方法对兰姆波结构健康监测中板的损伤进行识别。根据兰姆波损伤散射信号的传播时间以及传感器网络中一对发射-接收传感器的空间位置来确定一个椭圆轨迹,该轨迹显示了损伤可能出现的位置。将监测区域均匀网格化,计算各网格节点到椭圆轨迹的最短距离,将此距离映射为损伤出现的概率。采用灰度等级对所有网格节点处的概率值进行量化,则由每一个发射接收传感器对就确定了一个灰度图像。为了突出损伤,应用图像融合技术对传感器网络中所有发射接收传感器对所构成的灰度图像进行融合。对铝板中横穿孔损伤的实验结果显示了该方法能够有效地确定损伤的位置和范围,有助于推动兰姆波结构健康监测的实用化。     

Passive geoacoustic inversion with a single hydrophone using broadband ship noise

An inversion scheme is proposed, relying upon the inversion of the noise of a moving ship measured on a single distant hydrophone. The spectrogram of the measurements exhibits striations which depend on waveguide parameters. The periodic behavior of striations versus range are used to estimate the differences of radial wavenumber between couples of propagative modes at a given frequency. These wavenumber differences are stacked for several frequencies to form the relative dispersion curves. Such relative dispersion curves can be synthesized using a propagation model feeded with a bottom geoacoustic model. Inversion is performed by looking for the bottom properties that optimize the fit between measured and predicted relative dispersion curves. The inversion scheme is tested on simulated data. The conclusions are twofold: (1) a minimum 6 dB signal to noise ratio is required to obtained an unbiased estimate of compressional sound speed in the bottom with a 3 m s(-1) standard deviation; however, even with low signal to noise ratio, the estimation error remains bounded and (2) in the case of a multi-layer bottom, the scheme produces a single depth-average compressional sound speed. The inversion scheme is applied on experimental data. The results are fully consistent with a core sample measured around the receiving hydrophone.     

Robust methods of inclusive outlier analysis for structural health monitoring

The key novel element of this work is the introduction of robust multivariate statistical methods into the structural health monitoring (SHM) field through use of the minimum covariance determinant estimator (MCD) and the minimum volume enclosing ellipsoid (MVEE). In this paper, robust outlier statistics are investigated, focussed mainly on a high level estimation of the “masking effect” of inclusive outliers, not only for determining the presence or absence of novelty-something that is of fundamental interest but also to examine the normal condition set under the suspicion that it may already include multiple abnormalities. By identifying and detecting variability at an early stage, the prospects of achieving good generalisation and establishing a correct normal condition classifier may be increased. It is critical to highlight that there is no a priori division between the damaged and the undamaged condition data when the algorithms are implemented, offering a significant advantage over other methodologies. In summary, this paper introduces a new scheme for SHM by exploiting robust multivariate outlier statistics in order to investigate if the selected features are free from multiple outliers before such features can be selected for either supervised or unsupervised analysis.     

Temperature effects in ultrasonic Lamb wave structural health monitoring systems

There is a need to better understand the effect of temperature changes on the response of ultrasonic guided-wave pitch-catch systems used for structural health monitoring. A model is proposed to account for all relevant temperature-dependent parameters of a pitch-catch system on an isotropic plate, including the actuator-plate and plate-sensor interactions through shear-lag behavior, the piezoelectric and dielectric permittivity properties of the transducers, and the Lamb wave dispersion properties of the substrate plate. The model is used to predict the S(0) and A(0) response spectra in aluminum plates for the temperature range of -40-+60 degrees C, which accounts for normal aircraft operations. The transducers examined are monolithic PZT-5A [PZT denotes Pb(Zr-Ti)O3] patches and flexible macrofiber composite type P1 patches. The study shows substantial changes in Lamb wave amplitude response caused solely by temperature excursions. It is also shown that, for the transducers considered, the response amplitude changes follow two opposite trends below and above ambient temperature (20 degrees C), respectively. These results can provide a basis for the compensation of temperature effects in guided-wave damage detection systems.     

Efficient temperature compensation strategies for guided wave structural health monitoring

The application of temperature compensation strategies is important when using a guided wave structural health monitoring system. It has been shown by different authors that the influence of changing environmental and operational conditions, especially temperature, limits performance. This paper quantitatively describes two different methods to compensate for the temperature effect, namely optimal baseline selection (OBS) and baseline signal stretch (BSS). The effect of temperature separation between baseline time-traces in OBS and the parameters used in the BSS method are investigated. A combined strategy that uses both OBS and BSS is considered. Theoretical results are compared, using data from two independent long-term experiments, which use predominantly A₀ mode and S₀ mode data respectively. These confirm that the performance of OBS and BSS quantitatively agrees with predictions and also demonstrate that the combination of OBS and BSS is a robust practical solution to temperature compensation.     

Comparison of scale factor estimators for ultrasonic temperature monitoring: Application to structural health monitoring

In Structural Health Monitoring (SHM) of materials, estimating the effects of environmental and operational conditions such as temperature is important. Indeed, temperature changes induce modifications of the mechanical properties of the material and therefore cause a dilation of the acoustic signals characterized by a scale factor. This paper described four scale factor estimators able to monitor changes in temperature: The short-time cross-correlation (STXC) method, the stretching method (STRE), the Minimum Variance Based Estimator method (MVBE) and the Scale Transform Based Estimator method (STBE). The first two methods have already been assessed in the literature while the latter two have been specifically developed for this study. First, closed-form for the Cramer-Rao bound on the estimates of the scale factor, from a simplified deterministic signal, are derived and simplified expressions are given. Then, a statistical evaluation of the quality of estimates is conducted through Monte-Carlo simulations using synthetic signals, based on a model taking into account the influence of temperature. A raw estimate of the computational complexity of signal processing methods also completes this evaluation phase. Finally, the experimental validation of estimation methods is conducted on an aluminum plate subjected to temperatures variations in a controlled thermal environment. The temperature estimates are then faced with an analytical model describing the material behavior.     

Structural health monitoring of fixed offshore structures using the bicoherence function of ambient vibration measurements

This paper investigates the use of the bicoherence function of measured structural acceleration to provide automatic early detection of damage in an offshore structure. A detailed simulation model is developed, including realistic wave loading, and is used to assess the performance of the method. The method is shown through analytical and numerical analysis to be insensitive to typical operating parameter variations and to variations in wave excitation force. It is demonstrated that very small changes in stiffness of individual structural members are detectable from measurements of global structural motion.     

Passive acoustic survey of Yangtze finless porpoises using a cargo ship as a moving platform

In order to periodically investigate the population and distribution of the Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis) in its main distribution range in the Yangtze River, a passive acoustic system deployed on a cargo ship as a moving platform, rather than a dedicated research ship, was developed. A stereo acoustic event data-logger (A-tag) was installed on the cargo ship to passively detect phonating animals. In three surveys carried out in the Yangtze River from Wuhan to Shanghai, an average of 6059 clicks in each survey and 284 porpoises in total were acoustically detected along an 1100-km stretch. The animals were detected frequently in most of the survey range except two "gap sections" with 40 and 60 km lengths, respectively, where no animals were detected in all three surveys. Detected group sizes of the animals in each 120-s time window were not significantly different among the surveys, but the distribution pattern was different and suggested seasonal migration. The cargo ship based passive acoustic survey was effective in detecting phonating animals and can potentially monitor the distribution and population trend over time. Compared to surveys that used dedicated research ships, the present method is more cost effective.     

利用环境噪声互相关实现散射体无源成像

最近研究表明利用环境噪声的互相关可以恢复两点之间的时域格林函数(声脉冲响应),这一原理在文献中被称为格林函数恢复。基于此原理,通过对多个传声器所接收的环境噪声进行互相关处理,获取与散射体相关联的散射波的到达时延信息,结合最小二乘反演算法和改进克希霍夫移位算法,分别获得道路交通噪声场中石柱以及海浪噪声场中塑料桶的空间位置,且其估计结果与实际测量相一致。实验结果表明将环境噪声作为探测信号进行散射体无源成像是可行的。这为设计室内无源声监测系统以及通过海洋环境噪声实现对水中静默目标成像提供了新思路和有益参考。     

Quantifying the complexity of excised larynx vibrations from high-speed imaging using spatiotemporal and nonlinear dynamic analyses

In this paper, we investigate the biomechanical applications of spatiotemporal analysis and nonlinear dynamic analysis to quantitatively describe regular and irregular vibrations of twelve excised larynges from high-speed image recordings. Regular vibrations show simple spatial symmetry, temporal periodicity, and discrete frequency spectra, while irregular vibrations show complex spatiotemporal plots, aperiodic time series, and broadband spectra. Furthermore, the global entropy and correlation length from spatiotemporal analysis and the correlation dimension from nonlinear dynamic analysis reveal a statistical difference between regular and irregular vibrations. In comparison with regular vibrations, the global entropy and correlation dimension of irregular vibrations are statistically higher, while the correlation length is significantly lower. These findings show that spatiotemporal analysis and nonlinear dynamic analysis are capable of describing the complex dynamics of vocal fold vibrations from high-speed imaging and may potentially be helpful for understanding disordered behaviors in biomedical laryngeal systems.