Structural Health Monitoring current issue

Sensitivity Enhancement of Long-gage FBG Sensors for Macro-strain Measurements

Suzhen Li, , Zhishen Wu, — Tue, 13 Oct 2009 02:29:50 PDT

In our recent study, a structural health monitoring strategy based on distributed fiber optic sensing techniques has been proposed to utilize the strain responses throughout the full or some partial areas of structures to detect the arbitrary and unforeseen damage. However, to perform this strategy more effectively, there is an increasing demand for improving the ability of such sensors to measure small structural responses, especially for the cases such as damage identification based on ambient vibration tests, fatigue crack monitoring of steel structures and crack detection of reinforced concrete structures. This work puts forward a novel packaging design for long-gage fiber Bragg grating (FBG) sensors to enhance the measurement sensitivity of strain responses. The basic idea is to utilize two materials of different stiffness to package the in-tube optical fiber and impose the deformation within the gage length largely on the short-gage sensing part of FBG. Proof-tests indicate that the measurement sensitivity of the innovative FBG sensor can be artificially enhanced by adjusting the two recoating materials and their respective lengths. A series of tests are carried out to verify the ability of the improved sensors to detect small crack and measure slight vibration.

Three-point Frequency Tracking Method

Pai, P. F. — Tue, 13 Oct 2009 02:29:50 PDT

This paper presents and evaluates in detail a harmonics tracking method (HTM) for tracking the instantaneous frequency and amplitude of a vibration signal by processing only three most recent data points. Teager—Kaiser algorithm (TKA) is a popular four-point method for online frequency tracking, but its accuracy is easily destroyed by measurement noise due to the use of finite difference. Moreover, because a signal is assumed to be a pure harmonic in TKA, any moving average in the signal can destroy the accuracy of TKA. On the other hand, HTM uses a constant and a pair of harmonics to fit three recent data points and estimate the instantaneous frequency and amplitude, and it dramatically reduces the influence of any moving average. Moreover, noise filtering is an implicit capability of HTM even if only three points are processed, and this capability increases with the number of processed points. However, HTM depends on TKA to provide the first frequency estimation in order to start online tracking. To compare HTM and TKA and evaluate the accuracy of HTM, Hilbert— Huang transform (HHT) is used to extract accurate time-varying frequency and amplitude by processing the whole data set without assuming the signal to be harmonic. Frequency and amplitude tracking of different amplitude- and/or frequency-modulated signals and nonlinear nonstationary dynamic signals is studied. Results show that HTM is more accurate, robust, and versatile than TKA for online frequency tracking. Moreover, the frequencies and amplitudes tracked by HTM have about the same accuracy as those extracted by HHT, but without the edge effect that HHT suffers from.

Assessment of Vibration-based Damage Identification Methods Using Displacement and Distributed Strain Measurements

Philips Adewuyi, A., Wu, Z., Kammrujaman Serker, N.H.M. — Tue, 13 Oct 2009 02:29:51 PDT

Accurate measurement and monitoring of vibration characteristics is critical for proper detection of the location and severity of damage. However, experimentally measured modal properties are inevitably corrupted by measurement noise and errors, which could render most vibration-based structural damage identification algorithms unreliable for civil structural health monitoring (SHM). This article, through computer simulation and experimental investigation of a simply supported beam, comparatively evaluates the performance of these techniques for practical civil SHM by using displacement modes from accelerometers and long-gage distributed strain measurements. Most of all the techniques proved unreliable for damage identification using noisy measurements from accelerometers, while successful with distributed strain measurements. The findings reveal that long-gage distributed strain measurements are much more efficient choice over the traditional measurement techniques for reliable civil SHM. It may therefore be concluded that the performance of some algorithms might be improved for application to civil infrastructure by using distributed strain fiber optic sensing measurement techniques.

Time Domain Reflectometry Automatic Bridge Scour Measurement System: Principles and Potentials

Yu, X., Yu, X. — Tue, 13 Oct 2009 02:29:51 PDT

Bridge scour is a major factor contributing to the failures of highway bridges constructed over waterways. During flood events, sediments can be washed away and bridge substructures (piers and abutments) are left inadequately supported. Field monitoring of bridge sour process is necessary to study the scour mechanism, to develop scour-resistant design of bridge piers and abutments, to implement effective scour countermeasures, and to deploy safety warning systems. The current instruments are not completely satisfactory in providing real-time monitoring data during critical flood events. This study introduces the development of an automatic scour monitoring system using time domain reflectometry (TDR) principle. It presents the principles and system design of the TDR sediment scour monitoring system. An analyses algorithm for scour signals has been developed. It was found to be robust and can be implemented to automate scour signal interpretation. Simulated experiments were conducted to validate the performance of the scour monitoring system prototype. The results showed that this TDR technology can accurately measure the scour depth. Besides, the properties of the sediments, such as the porosity and density can be estimated with reasonable accuracy. At the end of the paper, a few important issues associated with field deployments of TDR scour monitoring system are discussed.

Onset of Resin Micro-Cracks in Unidirectional Glass Fiber Laminates with Integrated SHM Sensors: Experimental Results

Ghezzo, F., Huang, Y., Nemat-Nasser, S. — Tue, 13 Oct 2009 02:29:51 PDT

This article presents the results of experiments conducted in order to identify and locate the failure initiation in glass fiber/epoxy laminates with integrated structural health monitoring sensors (SHM) and electronics. Recent advances in health monitoring technologies have resulted in the development of micro-dimensional devices that can be embedded into composite laminates. Notwithstanding their small size, such inclusions may affect the response of the composite. Damage induced by the peak values of stress concentration around the embedded inclusion is, in fact, one of the main concerns in smart structures technology. To address this specific issue, unidirectional S2 glass fiber/epoxy laminated composites are fabricated with embedded small implants that mimic potential sensors and microprocessors. Quasi-static tensile tests are then performed on those samples while monitoring them by the acoustic emission (AE) technique. Additionally, the microstructure of the material with and without implants is explored. The AE results show that early low-medium amplitude events are detected at the implant location and the micrographic inspections reveal that micro-cracks initiate at the device-composite matrix interface and grow around the implant causing the debond of the external component from the surrounding resin system.

Onset of Resin Micro-cracks in Unidirectional Glass Fiber Laminates with Integrated SHM Sensors: Numerical Analysis

Huang, Y., Ghezzo, F., Nemat-Nasser, S. — Tue, 13 Oct 2009 02:29:51 PDT

The embedment of micro-sensors and micro-devices into composite laminates for structural health monitoring systems leads to stress/strain concentrations due to geometrical and material discontinuities around such embedded inclusions, with high potential to initiate premature failures. This article presents the efforts to estimate the effects of these stress/strain concentrations induced by the integration of rectangular-shape sensors within unidirectional fiber-glass composites. The micro-crack initiation sites and the failure load are predicted using finite-element simulations. Good agreement has been found between the numerical results and the experimental findings presented in an accompanying paper.

Development of a Real-time Remote Structural Monitoring Scheme for Civil Infrastructural Systems

Tue, 13 Oct 2009 02:29:51 PDT

Long-term structural monitoring has become an important requisite to ensure structural and operational safety for critical civil infrastructures. Long-term structural monitoring by acquiring data continuously or at small intervals may be difficult to achieve by employing conventional on-site monitoring methods. In order to overcome these difficulties remote structural monitoring (RSM), an advanced structural monitoring technique, can be used to acquire continuous data from the remotely instrumented structure. RSM methodology requires an interdisciplinary approach integrating areas such as structural engineering, sensor technology, communication technology, statistical mechanics, information technology for online data transmission (over larger distances) and damage detection/ health assessment of the structure. RSM has many advantages like, continuous monitoring, early alarm of any incipient damage, and data acquisition even in adverse climatic/environmental conditions. This article describes the development of an RSM scheme, developed at SERC, Chennai. The article outlines the laboratory and field investigations carried out to validate the developed scheme. Brief information about the observations and modifications made during these trials are also presented. A part of the work carried out for synthesizing online data using Auto Regressive Moving Average model is also presented.

Correlating Low-energy Impact Damage with Changes in Modal Parameters: A Preliminary Study on Composite Beams

Shahdin, A., Morlier, J., Gourinat, Y. — Tue, 13 Oct 2009 02:29:51 PDT

This article is an experimental study of the effects of multi-site damage on the vibration response of a composite beam damaged by low-energy impact. The variation of the modal parameters with different levels of impact energy and density of impact is studied. Specimens are impacted symmetrically in order to induce a global rate of damage. A damage detection tool Damage Index is introduced in order to verify the estimation of damping ratios. Design of Experiments is used to establish the sensitivity of both energy of impact and density of damage. The DOE analysis results (using natural frequency only) indicate that impact energy for 2nd, 3rd, and 4th bending modes is the most significant factor contributing to the changes in the modal parameters for this kind of symmetrical dynamic test.

Structural Health Monitoring of a Metallic Sandwich Panel by the Method of Virtual Forces

White, J. R., Adams, D. E., Jata, K. — Tue, 13 Oct 2009 02:29:51 PDT

A structural health monitoring system for assessing thermal damage in a metallic sandwich panel was developed in this article. The method of virtual forces was utilized to detect, locate, and quantify damage. The method assumes that a damaged structural response is a summation of the undamaged response and the response from an effective forcing function, which represents the effects of the material damage. The effective forcing function, or virtual force, is an estimate of the forces that the damage mechanism exerts on the undamaged structure. Virtual forces were shown to detect changes in the mass, stiffness, and damping matrices in a lumped parameter dynamic model. A finite element model of a mechanically attached sandwich metallic panel was used to detect a stiffness loss at different locations between two sensor locations. These results were then compared with experimental results involving thermal debond between the face sheet and core of an Al—Al honeycomb panel. Virtual forces indicated there was damage associated with magnitude and frequency shifts in the frequency response function measurements made on the damaged panel. Damage was located relative to the sensor/actuator measurement locations. Damage that was not adjacent to measurement locations was less accurately located and quantified. Variability in producing localized thermal debond using an acetylene torch was also apparent in the virtual force estimates. The virtual force estimates also indicated that damage was spread over several measurement locations because highly localized thermal debond was difficult to produce.

Experimental Investigation on Statistical Moment-based Structural Damage Detection Method

Xu, Y.L., Zhang, J., Li, J.C., Xia, Y. — Tue, 13 Oct 2009 02:29:51 PDT

Although vibration-based structural damage detection methods have demonstrated various degrees of success, the damage detection of civil structures still remains as a challenging task. The main obstacles include the insensitivity to local damage and the high sensitivity to measurement noise. A new structural damage detection method based on the statistical moments of dynamic responses of a structure has been recently proposed by the authors, and the numerical study manifested that the proposed method is sensitive to local structural damage but insensitive to measurement noise. The experimental investigation on this method is presented in this article. Three shear building models with and without damage were built and subjected to ground motions generated by a shaking table. The displacement and acceleration responses of each building model at each floor were recorded. The recorded ground motion and building responses as well as identified structural damping ratios were then used to identify damage locations and severities using the statistical moment-based damage detection method. The identified damage locations and severities were compared with the theoretical values. The comparison is found satisfactory, and the method proposed is effective and feasible.

Erratum

Tue, 13 Oct 2009 02:29:51 PDT

Erratum

Tue, 13 Oct 2009 02:29:51 PDT

Erratum

Tue, 13 Oct 2009 02:29:51 PDT

Erratum

Tue, 13 Oct 2009 02:29:51 PDT

Erratum

Tue, 13 Oct 2009 02:29:51 PDT

Erratum

Tue, 13 Oct 2009 02:29:51 PDT

Erratum

Tue, 13 Oct 2009 02:29:51 PDT

Erratum

Tue, 13 Oct 2009 02:29:51 PDT

Erratum

Tue, 13 Oct 2009 02:29:51 PDT

Erratum

Tue, 13 Oct 2009 02:29:51 PDT

Erratum

Tue, 13 Oct 2009 02:29:51 PDT