Instantaneous Baseline Research Articles

Appraisal of linear baseline-free techniques for guided wave based structural health monitoring

This paper offers a comprehensive critical appraisal and experimental comparison of leading linear baseline-free techniques applied in guided wave-based structural health monitoring (GWSHM). The paper extensively examines the most popular linear baseline-free techniques, namely Time Reversal (TR), Virtual Time Reversal (VTR), Instantaneous Baseline (IB), and reciprocity-based methods. Detailed discussions on the principles, strengths, and limitations of each technique provide a thorough understanding of their capabilities and challenges. Critical factors affecting performance that influence the performance of baseline-free techniques in damage detection and localization is the main focus of the paper. These factors encompass varying environmental conditions such as temperature fluctuations, geometric and structural complexities, and diverse damage scenarios.The research reported conducts experimental comparisons among VTR, IB, and reciprocity-based techniques as related to the challenging case of composite materials, considering single and dual Barely Visible Damage (BVID) scenarios, temperature variations, boundary reflections, and structural complexities like stiffeners. The results demonstrate that the investigated baseline-free techniques are capable of identifying and localizing damages, albeit with differing capabilities.

Improving Probability-Based Diagnostic Imaging Algorithm by Modeling Weight Distribution Coefficient for Instantaneous Baseline Damage Identification

This study used a combination of the instantaneous baseline method and a probability-based diagnostic imaging (PDI) algorithm for damage identification in a stiffener-reinforced aluminium plate. Selecting an appropriate weight distribution function plays a significant role in the PDI algorithm by expressing the probability of damage existence around a sensing path. Weight distribution functions are highly dependent on the weight distribution coefficient (β). This parameter (β) normally is estimated by trial and error, which is incorrect. In addition, determining this parameter is quite costly and time-demanding. To address these issues thoroughly, a new approach is proposed for modelling the weight distribution coefficient based on the physics of wave propagation and the actuator–sensor distance. An appropriate value for β is estimated for each sensing path between actuators and sensors in a specific structure based on the proposed model, making the results more accurate and reasonable. The proposed model was validated by identifying damages at three different locations of a structure for five scenarios (three single-damage and two multiple-damage scenarios). Moreover, the results obtained from proposed model using linear and Gaussian weight distributions were compared with the results extracted from an existing approach. The acceptable localization results clearly indicate the effectiveness of proposed approach.

Self-referenced robust guided wave based defect detection: Application to woven composite parts of complex shape

The use of a reference state, i.e. a baseline, for data analysis in guided wave based structural health monitoring is limiting the range of applicability of such approaches. This has been largely demonstrated in the literature in the presence of temperature changes but is also true in the presence of other varying environmental and operational conditions, as well as aging effects. This paper presents a novel self-referenced methodology, which is built to be intrinsically robust to the influence of external effects, allegedly including the ones not identified during system design. The method relies on the concept of instantaneous baseline, i.e. the comparison of multiple measurements acquired on structures with a high degree of similarity from a guided wave perspective. The subtlety of the approach is the detection of flaws of small influence on the measurements in the presence of extrinsic and intrinsic variabilities of comparatively larger influence over several similar samples. The method also includes a dimension reduction through the extraction of quantities of interest from the acquired signals, potentially enabling the analysis of several samples with limited data volume. The proposed method is successfully validated on 12 aluminum plates, each with a through-hole crack from 1 to 30mm in length in a laboratory environment with limited external parameters. Next, the approach is tested on 2 woven composite samples of complex shape with up to 7 similar paths from a guided wave perspective. The detection of an added mass is successful over the whole temperature range under consideration (−30°C to 30 °C) for all studied interrogating frequencies (50, 100, and 150 kHz). The proposed methodology performs significantly better than a state of the art imaging algorithm with temperature compensation applied to the same data.

Application Study on Fiber Optic Monitoring and Identification of CRTS-II-Slab Ballastless Track Debonding on Viaduct

China Railway Track System (CRTS)-II-slab ballastless track is a new type of track structure, and its interlayer connection state is considerably important for the operation safety and ride comfort of high-speed trains. However, the location and multiple influencing factors of interlayer debonding lead to difficulties in monitoring and identification. Here, the research on the design and application of a monitoring scheme that facilitates interlayer debonding detection of ballastless track and an effective indicator for debonding identification and assessment is proposed. The results show that on-site monitoring can effectively capture the vibration signals caused by train vibration and interlayer debonding. The features of the data acquired in the situations with and without interlayer debonding are compared after instantaneous baseline validation. Some significant features capable of obviously differentiating a debonding state from the normal state are identified. Furthermore, a new indicator, combining multiple debonding-sensitive features by similarity-based weights normalizing the initial difference between mutual instantaneous baselines, is developed to support rational and comprehensive assessment quantitatively. The contribution of this study includes the development and application of an interlay-debonding monitoring scheme, the establishment of an effective-feature pool, and the proposal of the similarity-based indicator, thereby laying a good foundation for debonding identification of ballastless track.

An instantaneous-baseline multi-indicial nonlinear ultrasonic resonance spectral correlation technique for fatigue crack detection and quantification

This article presents an instantaneous-baseline multi-indicial nonlinear ultrasonic resonance spectral correlation technique for fatigue crack detection and quantification. A reduced-order nonlinear oscillator model is tailored to illuminate the contact acoustic nonlinearity (CAN) and the nonlinear resonance phenomena. The analytical formulation considers the rough surface condition of the fatigue cracks, with a crack open–close transitional range for the effective modeling of the variable-stiffness nonlinear mechanism. Multiple damage indices (DIs) associated with the degree of nonlinearity of the interrogated structures are then proposed by correlating the ultrasonic resonance spectra. Three perspectives of the nonlinear resonance phenomena are investigated to detect and monitor the fatigue crack growth: (1) time-history dependence, which evolves different resonance states depending on the loading history; (2) amplitude dependence, which renders significantly different nonlinear responses under various levels of excitation amplitudes; (3) breakage of superposition, which effectively distinguishes nonlinear resonant responses from the linear counterparts. These DIs are established using instantaneous baselines, facilitating the fatigue damage monitoring without the prior knowledge of a pristine structure. Fatigue tests on a thin aluminum plate with a rivet hole are conducted to induce fatigue cracks in the specimen. The experimental results demonstrate that the proposed technique shows remarkable sensitivity to the nucleation and growth of the fatigue cracks. This paper differs from the existing literature on nonlinear resonance-based techniques in that it focuses on the resonance phenomenon aroused by the contact acoustic nonlinearity from localized fatigue cracks, rather than the diffused material nonlinearity. The novelty of the paper resides in the establishment of an instantaneous baseline technique utilizing the nonlinear resonance features without the need of referring to a pristine baseline situation. The paper finishes with discussion, concluding remarks, and suggestions for future work.

Recurrent time-to-event models with ordinal outcomes.

A model to accommodate time-to-event ordinal outcomes was proposed by Berridge and Whitehead. Very few studies have adopted this approach, despite its appeal in incorporating several ordered categories of event outcome. More recently, there has been increased interest in utilizing recurrent events to analyze practical endpoints in the study of disease history and to help quantify the changing pattern of disease over time. For example, in studies of heart failure, the analysis of a single fatal event no longer provides sufficient clinical information to manage the disease. Similarly, the grade/frequency/severity of adverse events may be more important than simply prolonged survival in studies of toxic therapies in oncology. We propose an extension of the ordinal time-to-event model to allow for multiple/recurrent events in the case of marginal models (where all subjects are at risk for each recurrence, irrespective of whether they have experienced previous recurrences) and conditional models (subjects are at risk of a recurrence only if they have experienced a previous recurrence). These models rely on marginal and conditional estimates of the instantaneous baseline hazard and provide estimates of the probabilities of an event of each severity for each recurrence over time. We outline how confidence intervals for these probabilities can be constructed and illustrate how to fit these models and provide examples of the methods, together with an interpretation of the results.

General range model for multi‐channel SAR/GMTI with curvilinear flight trajectory

The platform of multi-channel synthetic aperture radar/ground moving target indication (MC-SAR/GMTI) system may encounter the problem of curvilinear trajectory due to the existence of acceleration in several applications, e.g. manoeuvring responding, orbital motion, and long-time observing. The curvilinear trajectory changes the attitude of the array, which leads to instantaneous baseline changing during echo reception. Thus, the traditional constant baseline range model is no longer valid since it will introduce severe phase error. In this Letter, a general range model that it applies to both stationary and moving targets for MC-SAR/GMTI is proposed. Simulation results of the point spread functions demonstrate that the proposed model can achieve good performance.

An improved instantaneous baseline method for pipe structures based on distance compensation using guided waves

Among the baseline-free damage detection techniques based on guided waves (GWs), instantaneous baseline measurement is easily applied in practical situations. However, the number of transducers increases when large structures are inspected, and the operation becomes quite complicated because the excitation signal has to be applied to the actuators one by one. In this paper, an improved instantaneous baseline technique based on distance compensation is proposed. This technique has two advantages: (1) a smaller number of transducers is required; and (2) the operation of the excitation and receiving of GW signals is very simple. A distance compensation algorithm is developed to predict the signals in intact pipe. The efficiency of the wave prediction in intact pipe is validated through simulation. A baseline-free damage detection method for the pipe structure is proposed in this paper. The feasibility of the presented method is demonstrated by the simulation of two damage cases. Moreover, experiments are performed to verify the wave propagation and the feasibility of the proposed method.

A novel approach for epipolar resampling of cross-track linear pushbroom imagery using orbital parameters model

This paper presents a novel approach to epipolar resampling of cross-track linear pushbroom imagery using orbital parameters model (OPM). The backbone of the proposed method relies on modification of attitude parameters of linear array stereo imagery in such a way to parallelize the approximate conjugate epipolar lines (ACELs) with the instantaneous base line (IBL) of the conjugate image points (CIPs). Afterward, a complementary rotation is applied in order to parallelize all the ACELs throughout the stereo imagery. The new estimated attitude parameters are evaluated based on the direction of the IBL and the ACELs. Due to the spatial and temporal variability of the IBL (respectively changes in column and row numbers of the CIPs) and nonparallel nature of the epipolar lines in the stereo linear images, some polynomials in the both column and row numbers of the CIPs are used to model new attitude parameters. As the instantaneous position of sensors remains fix, the digital elevation model (DEM) of the area of interest is not required in the resampling process. According to the experimental results obtained from two pairs of SPOT and RapidEye stereo imagery with a high elevation relief, the average absolute values of remained vertical parallaxes of CIPs in the normalized images were obtained 0.19 and 0.28 pixels respectively, which confirm the high accuracy and applicability of the proposed method.

A semi-baseline damage identification approach for complex structures using energy ratio correction technique

Summary Because damage identification results based on the Lamb waves propagation approach can be influenced by varying environmental and operational conditions, development of a robust monitoring system with no need of the prior measured data of the structure has gained much attention recently. The instantaneous baseline damage detection technique is one of the promising methods that overcome the mentioned obstacles. For this, the material properties, electromechanical characteristics, and the geometric features must be identical. Also, sensor distances in similar paths and the geometric dimensions of the transducers must be the same. So, implementing the instantaneous baseline damage identification in complex structures is rather complicated due to the inherent non-homogeneities involved. To ease the complexity, this article proposes a semi-instantaneous baseline damage identification approach by modifying an existing instantaneous baseline method. This is used to detect and characterize fatigue cracks that initiate around the rivet holes of lap joint structures. In this method, an active sensing network is mounted on the lap joint, and a robust and effective feature called energy ratio change is extracted from the collected time-domain signals using the wavelet transform. The introduced “identicality coefficient” for all the sensing paths in pristine condition of the structure is obtained and used to remove any inequalities that may occur to the signals of each path. The obtained results show that the proposed method can detect fatigue cracks around the lap joint rivet holes and estimate the crack size. An experiment as well as numerical simulations is performed to verify the method.

Integrated GPS + BDS instantaneous medium baseline RTK positioning: Signal analysis, methodology and performance assessment

BDS system has become regionally operational and it is expected that it will reach its full operational status in the next few years. Therefore application of fully operational GPS system together with dynamically developing BDS is of interest for scientific, navigation and land surveying communities. In the first part the study demonstrates an analysis of measurement noise for both aforementioned systems. In the second part the mathematical and stochastic models of combined multi-GNSS medium range RTK positioning were presented. Furthermore, the performance assessment of single-epoch multi-baseline positioning was analyzed. The analysis of dispersion of undifferenced pseudorange observations caused by noise and multipath effect has confirmed slightly higher precision of code observations on the first in respect to second frequency for both systems. Moreover, the more detailed analysis suggests that in the case of strong ionospheric disturbances the internal smoothing of code measurements can be a source of their strong discrepancies. Due to the medium length of baselines (∼40km) the investigation of phase measurement noise was executed using triple differenced data. Generally it has shown the similar level of discrepancies for both systems. Only for geostationary BDS satellites one can observe measurable increase of noise. The quality assessment of instantaneous multi-baseline RTK (Real Time Kinematic) positioning based on single GPS or BDS systems, as well as on combined multi-GNSS observations, was evaluated simulating different observing conditions with terrain obstacles. The performance assessment of combined GPS and BDS precise relative positioning has proved the advantage of multi-GNSS positioning especially in harsh observing conditions. The usage of observational data from both systems allowed instantaneous positioning even with high elevation mask.

Epipolar Resampling of Cross-Track Pushbroom Satellite Imagery Using the Rigorous Sensor Model.

Epipolar resampling aims to eliminate the vertical parallax of stereo images. Due to the dynamic nature of the exterior orientation parameters of linear pushbroom satellite imagery and the complexity of reconstructing the epipolar geometry using rigorous sensor models, so far, no epipolar resampling approach has been proposed based on these models. In this paper for the first time it is shown that the orientation of the instantaneous baseline (IB) of conjugate image points (CIPs) in the linear pushbroom satellite imagery can be modeled with high precision in terms of the rows- and the columns-number of CIPs. Taking advantage of this feature, a novel approach is then presented for epipolar resampling of cross-track linear pushbroom satellite imagery. The proposed method is based on the rigorous sensor model. As the instantaneous position of sensors remains fixed, the digital elevation model of the area of interest is not required in the resampling process. Experimental results obtained from two pairs of SPOT and one pair of RapidEye stereo imagery with different terrain conditions shows that the proposed epipolar resampling approach benefits from a superior accuracy, as the remained vertical parallaxes of all CIPs in the normalized images are close to zero.

Reference-Free Damage Identification in Plate-Like Structures Using Lamb-Wave Propagation with Embedded Piezoelectric Sensors

AbstractStructural health monitoring (SHM) has been adopted in the aircraft and civil industries over the last two decades. The ultrasonic guided wave propagation technique is a promising damage detection approach in airplane thin wall structures because of their long distance traveling ability and low attenuation. In fact, by efficiently designing the pattern of sensors one can effectively pick up the changes that happen in propagated signals due to presence of any kind of structural damage. This detection approach could be effectively assisted by numerical simulation techniques like finite-element modeling. To obtain a damage detection algorithm based on instantaneous baseline data, the finite-element technique is employed for simulation of wave propagation in thin wall structures. Also, the influence of artificial crack (cut-damage) on propagated Lamb waves is investigated. An instantaneous baseline damage detection based on cross-correlation (CC) analysis is carried out to detect the damage, and the r...

Detection and localization of fatigue crack with nonlinear instantaneous baseline

This article presents a fatigue crack characterization technique using a nonlinear instantaneous baseline not only to identify but also to locate fatigue damage in an aluminum plate. In conventional ultrasonic structural health monitoring approaches, damage is identified by comparing the newly measured signal with the baseline signal prerecorded when the structure is in its pristine state. The need to compare the current signals with prerecorded signals makes these methods unattractive, since the measured signals are also affected by the changing environment and load conditions of the structure. To overcome this deficiency, the technique proposed here replaces the prerecorded baseline with an instantaneous baseline which can be obtained when the structure is inspected. The measurement of instantaneous baseline utilizes the nonlinear properties of fatigue cracks when subjected to different amplitudes of excitations. Then damage is characterized in terms of a damage index value which is defined as the spectral changes in each path. The short-time Fourier transform is adopted to obtain the spectrogram. The experimental tests are presented to validate the effectiveness of this damage detection technique. A fatigue crack on an aluminum plate was located accurately with a set of surface-bounded piezoceramic (lead zirconate titanate) transducers by adopting the instantaneous baseline.

Closed Crack Detection with Nonlinear Instantaneous Baseline

Closed cracks, which stay in contact unless the excitation exceeds a certain threshold, exist as great menace to structures. Since nonlinear response is more sensitive to micro damage than conventional linear approaches, analyzing the nonlinear part of the collected response of structures to an input ultrasonic excitation is more promising in damage detection. In this paper, in order to image the location of a closed crack, an instantaneous baseline measurement is adopted and the nonlinear response is extracted by using scaling subtraction method. A three-dimensional finite element model of a plate with a closed crack is developed and the behavior of the closed crack is simulated with nonlinear springs at the crack interfaces. A network of actuators and sensors which constitutes of two arrays of surface-bonded piezoelectric transducers is built. The instantaneous baselines of each path are collected when the model is excited with low amplitude excitation. To diagnose the closed crack, a higher amplitude excitation over the threshold is applied to the model and the response signals of each path are recorded. The result shows that the differences caused by the crack can be observed from the scaling subtraction of these two recorded responses and the location of closed crack can be accurately imaged.

A NOVEL ESTIMATION APPROACH OF DYNAMIC AND COUPLING BASELINE FOR DISTRIBUTED SATELLITE SAR

In distributed satellite synthetic aperture radar (DS- SAR), along-track and cross-track baselines couple with each other and change dynamically due to formation ∞ying, which makes it di-cult to estimate interferometric baseline accurately. To solve the problem, a novel high-precision baseline estimation approach based on interferometric phase is proposed. By modeling accurate relationship between coupling baselines and two-dimensional (azimuth and range) inteferometric fringe frequency under the ellipsoid earth model, the along-track and cross-track baseline can be estimated separately by interferometric phase decoupling. By selecting several segments from interferometric phase during the whole data-take time and estimating instantaneous baseline of each segment, the dynamic baseline can be obtained via a linear flltering. Besides, to improve the baseline estimation accuracy, Semi-Newton iterative method is applied to acquire high-precision fringe frequency estimation, which can make the baseline estimation achieve centimeter level precision. The simulation validates the approach.

Lamb Waves Signal Analysis in Structural Health Monitoring Systems

Many of the wave propagation based structural health monitoring techniques rely on some knowledge of the structure in a healthy state in order to identify damage. Baseline measurements are recorded when a structure is pristine and are stored for comparison to future data. A concern with the use of baseline subtraction methods is the ability to discern structural changes from the effects of varying environmental and operational conditions when analyzing the vibration response of a system. The use of a standard baseline subtraction technique may falsely indicate damage when environmental or operational variations are present between baseline measurements and new measurements. A procedure was outlined for the method, including excitation and recording of Lamb waves, and the use of damage detection algorithms. In this paper, several tests are performed and the results are used to help develop the damage detection algorithms previously described, and to evaluate the performance of the instantaneous baseline SHM technique. Analytical testing is first performed by feeding known input signals into each damage detection algorithm and analyzing the output data. The results of the analytical testing are used to help develop the damage detection algorithms.

Instantaneous baseline structural damage detection using a miniaturized piezoelectric guided waves system

In recent years, new Structural Health Monitoring (SHM) methodologies with a concept of “instantaneous baseline damage detection” are being developed by many researchers since it has been found that the most of SHM technologies are too vulnerable to environmental and/or operational variations. In this context, this paper presents online instantaneous baseline structural damage detection using a low cost and low power, in-situ piezoelectric guided waves-SHM system. Firstly, four small, low cost and light weight smart Piezoelectric Ceramic (PZT) patches are surface-mounted and assumed to have the same bonding conditions to detect structural defects on an aluminum plate. Then, a miniaturized low power guided waves-SHM system with a Digital Signal Processing (DSP) module is employed for signal generation/excitation, signal sensing, and data processing. The instantaneous baseline damage detection based on Wavelet Transform (WT) and Cross Correlation (CC) analysis is carried out on the DSP module. Finally, effects of Lamb waves due to artificial ‘cut-damage’ at different locations are investigated using both “pitch-catch” and “pulse-echo” wave propagation schemes. Conclusively, this study shows a good potential for online and in-situ crack monitoring on panel structures such as an aircraft wing.

Reference-Free Damage Detection Using Instantaneous Baseline Measurements

A novel method of guided wave-based structural health monitoring is developed in which no direct baseline data are required to identify structural damage. Conventional wave propagation structural health monitoring techniques involve the comparison of structural response data to a prerecorded baseline or reference measurement taken while the structure is in pristine condition. The need to compare new data to a prerecorded baseline can present several complications, including data management issues and difficulty in accommodating the effects of varying environmental and operational conditions on the data. To address the complications associated with baseline comparison, this new method accomplishes reference-free damage detection by acquiring what is referred to as an instantaneous baseline measurement for analysis. The instantaneous baseline technique is validated through both analytical and experimental testing. Analytical tests show that the instantaneous baseline method is able to correctly identify simulated damage. It is found experimentally that nonpermanent damage in the form of removable putty as well as permanent damage in the form of corrosion and cuts are all identifiable in thin aluminum plate test structures without direct comparison to baseline data when implementing the instantaneous baseline method.

SURVIVAL ANALYSIS OF FLOWER AND FRUIT ABORTION IN SWEET PEPPER

In order to obtain a crop growth model that can simulate inter- and intra-plant variation in fruit set, fruit abortion times in sweet pepper were analysed by means of survival analysis. Survival analysis is a statistical technique dealing with the timing of events. The Cox proportional hazards model estimates the instantaneous baseline probability of abortion per time (hazard rate), given that the fruit has not aborted yet. It also estimates the effect of explanatory factors (covariates) on the abortion rate. Two important factors known to influence abortion in sweet pepper are the supply and demand for assimilates (source and sink strength, respectively). A plant density experiment was analysed, as density influences plant source and indirectly also sink strength. Flowering as well as abortion or harvest dates were recorded for all individual flowers and fruits. LAI was measured to calculate the source strength with a photosynthesis-based simulation model. Empirical curves showed that survival of flowers and fruits (per plant) was higher at 1.6 plants/m2 than at 3.1 or 4.6 plants/m2. Plant density, source strength and source/sink ratio all correlated with the treatment plant density, but source/sink ratio explained most of the variation in the data in a Cox proportional hazards model. Averaging the source/sink ratio over seven days gave the best fit. Adding the position within the node (main or side branch) improved the fit; flowers from the side branch had a higher probability of abortion per unit time than flowers from the main branch. The susceptibility for fruit abortion differed among plants. The baseline hazard rate indicated that between 0 and 13 days after anthesis, flowers were most susceptible for abortion.