One of the most promising NDT technologies is the guided ultrasonic wave technology. The most useful guided ultrasonic wave is Lamb waves. Damage detection and (or) nondestructive evaluation of structures using Lamb waves may be completed by the comparison between the analytic and the experimental dispersion diagram of Lamb waves. In order to construct the experimental dispersion diagram, the estimation of group velocities of each Lamb modes is necessary. Time of arrival information is needed to calculate the group velocity of each Lamb mode, and frequency information is needed to tell the Lamb modes from receiving signals because at least two modes are present at same frequency. Current research for detection of the time of arrival and the frequency is either in time domain or in frequency domain. However, these methods were not designed to give simultaneous information of the time and the frequency. Furthermore the scattering of waves and the background noise often mask the signal, leading difficulties in its estimation of the time of arrival and the frequency of the reflected and transmitted of dispersive Lamb waves. In this study, the authors introduce a detection method to estimate the time of arrival and the frequency simultaneously via time-frequency representation and to resolve the noise problem based on statistical signal detection theory. Numerical experiments were conducted to verify and validate the capability of the proposed method. The results of experiments demonstrate the utility of the proposed method.
Piezoelectric materials have been widely used in ultrasonic nondestructive testing (NDT). PZT ceramics can be used to receive and generate surface acoustic waves. It is a common application to attach PZT transducers to the surface of structures for detecting cracks in nondestructive testing. However, not until recently have piezoelectric polymers attracted more and more attention to be the material for interdigitated (IDT) surface and guided-wave transducers. In this paper, an interdigitated gold-on-polyvinyldine fluoride (PVDF) transducer for actuating and sensing Lamb waves has been introduced. A specific etching technology is employed for making the surface electrodes into a certain finger pattern, the spacings of which yield different single mode responses of Lamb waves. Experiments have been performed on steel plates. Results from PVDF IDT sensors have been compared with those from PZT transducers for verification.
In this paper Jiles-Atherton model, a phenomenological model, is proposed to model physical properties of structural steel and magnetite(corrosion product). The Jiles-Atherton model parameters based on mean field approximation were optimized to simulate the curves obtained from magnetic measurements using conventional quasi-static method. Results from hot rolled steel, a low carbon steel, were simulated using Jiles model to understand and correlate the measured and simulated curves. Hysteresis curves for magnetite, one of the most prevalent corrosion product and the only ferromagnetic component, are obtained to simulate the effect of corrosion products on the magnetic measurements of corroded structural steel. Since corrosion is initially a surface phenomenon, high frequency measurements were suggested from the simulations obtained to reduce the skin depth estimates and increase the accuracy of corrosion measurement.
The Kishwaukee bridge (circa 1979) is a five-span pre-cast post-tensioned segmental concrete box girder bridge. The structure has been under increasingly stringent inspection since extensive cracking adjacent to the piers in the webs was observed. There is limited observational evidence that continued propagation of the cracks has been occurring, and static load testing of the structure in 2000 provided direct indications of locally excessive stresses in the shear reinforcement. Since that time, the authors have developed
instrumentation which continuously monitors the bridge. Our previous thermal analyses and modeling based upon the data collected
demonstrates that thermal equilibrium is rare for this structure, a situation that undoubtably applies to most bridges of this type. Partly due to this constant disequilibrium, basing alarm criteria
on real time measurements is untenable. In this paper we develop a two-step monitoring strategy which can be applied to both global and local deformation data. The strategy is based on preliminary modeling to identify the thermomechanical influences, followed by a second step using a bootstrap comparator.
Polyvinylidene fluoride (PVDF) is a piezoelectric polymer material. One of its most attractive applications is being used as a sensor for structure monitoring. A suitable circuit interface plays an important role in sensor design. PVDF sensor can be used in a large variety of situations according to different design of circuit. The approach to a special circuit interface, which enables PVDF sensor to be utilized as a wireless “dynamic strain gage”, is presented in this paper. The wireless PVDF sensor was then tested and all the results have been compared with strain gage output for strain and displacement measurements.
Magnetic measurements were performed on steel cables subjected to a magnetic field and the response measured without contact using Faraday's law, to estimate the effect of temperature and corrosion on magnetic properties of structural steel. Magnetic measurements were compared with electrochemical measurements to correlate corrosion quantitatively in terms of mass loss. The results obtained from the present work are helpful in bounding the achievable sensitivity for conventional magnetoelastic corrosion sensing and for suggesting the need for alternate techniques.
A sizeable number of efforts have sought to instrument bridges
for the purpose of structural monitoring and assessment. The outcomes
of these efforts, as gaged by advances in the understanding of the
definition of structural damage and their role in sensor selection as well as in the design of cost and data-effective monitoring systems, has itself been difficult to assess. The authors' experience
with the design, construction, and operation of a monitoring system for the south-bound Kishwaukee Bridge has provided several lessons that bear upon these concerns. In this paper we describe certain aspects of the design of our Unix-based monitoring system. The system, patterned after similar systems developed for kick detection and well-control in the oil industry, has performed well in providing a continuous, low-cost monitoring platform for bridge engineers with immediately relevant information.
Despite the increasing popularity of cable-stayed bridges there is no convenient and accurate means available to measure the forces in the cable stays. The measurement of the forces is important for monitoring excessive wind or traffic loadings, to gage the redistribution forces which may occur after seismic events, and for detecting corrosion via loss of the cross-section. Although magnetoelastic stress sensors have been extensively tested on many types of prestressing cables, and have demonstrated accuracies of < 1%, to-date they have been based upon a solenoid geometry, which is not practical for cable force measurements in existing bridges having hundreds of cables. In order to address this problem a magnetoelastic sensor for the direct measurement of stress in steel cables is currently under development. The sensor differs from previous magnetoelastic sensors in that the cable is magnetized by a removable C- shaped circuit, rather than by a solenoid. We report preliminary results on measurement of the initial permeability curve indicating adequate sensitivity to stress with this geometry, but further work is necessary to understand the influence of the more complicated field geometry on data reduction and calibration procedures.
Polyvinylidene fluoride (PVDF) is a semicrystalline polymer exhibiting piezoelectric and pyroelectric properties which has been used for sensing infrared energy and the measurement of high frequency stress and strain, such as shocks. Despite the low material cost, durability, high sensitivity, and fast response, PVDF-based sensors have not been successfully used for infrastructure monitoring, where these benefits would seem appropriate. One reason is that the low frequency response of the sensor and integrated charge amplifier (the hybrid characteristic of the sensor) has not been adequately measured and modeled. In this paper we report on the development of a PVDF displacement sensor designed for infrastructure monitoring.
The magnetic characteristics of ferromagnetic steels, such as hysteresis loops, total permeability and differential permeability, are dependent on mechanical stress and temperature. This dependency can be the basis for a sensitive and non invasive sensing method for measuring the active stress in steel tendons and cables. This paper describes the current status of a stress sensor which can reliably monitor stress in tendons and cables. A transient magnetic field generated by a solenoid is utilized to bring the material to technical saturation. The induced voltage, which is affected by the presence of the ferromagnetic material, is measured and related to material characteristics. In order to obtain a stable and linear calibration curve between permeability and monitoring stress for a given material, an optimal applied field H0 must be determined. Initially, this field is estimated from the measured relationship between permeability and input voltage. The optimal working point is determined by searching for a linear relationship between permeability and applied stress at different temperatures. Temperatures from -20 C to 40 C were used. Experimental results for two common tendons, at two sizes (0.5' and 0.6'), were investigated using this technique. The results were demonstrated to match the accuracy and repeatability of a reference load cell for loading up to 70% of the yield stress. Additional tests were conducted on multi-strand cable assemblies. The experimental results indicate that the calibration procedure can be extended without significant error to accommodate such assemblies.
The damage index method is an intuitively attractive method for detecting localized stiffness perturbations through their influence on mode shapes. In spite of its attractiveness, and a large literature on the formulation of the damage index method for different types of structures and its application to specific test problems, the practical consequences which can result from its limitations, as well as its overall effectiveness, have not been demonstrated or discussed. This is an important problem since, at minimum, the qualitative characteristics and performance of a proposed diagnostic algorithm should be understood. In this paper, the authors review the damage index formulation and examine the traditional assumption step by step. Then this method has been used for numerous cases at different locations and degrees of stiffness perturbation for a large pre-stressed segmental concrete bridge. The finite element models have been used as test structures. The object is to evaluate the feasibility of the damage index method.