Structural Health Monitoring by Piezo-Impedance Transducers. I: Modeling

The electromechanical impedance (EMI) technique, which employs piezoelectric–ceramic lead–zirconate–titarate (PZT) patches as impedance transducers, has emerged as a powerful nondestructive evaluation technique during the last few years. This series of two papers present a new simplified methodology to diagnose structural damages by means of surface bonded piezo-impedance transducers. The first part introduces a new PZT–structure electroelastic interaction model based on the concept of “effective impedance.” The proposed formulations can be conveniently employed to extract the mechanical impedance of any “unknown” structural system from the admittance signatures of a surface bonded PZT patch. This is an improvement over the existing models, whose complexity prohibits direct application in similar practical scenarios. This also eliminates the requirement of any a priori information concerning the phenomenological nature of the structure. The proposed model is experimentally verified by means of test on a smart system comprising an aluminum block with a PZT patch instrumented on it. Part II of this paper outlines a new methodology to evaluate structural damages using the extracted impedance spectra. The proposed approach is found to be suitable for diagnosing damages in structures ranging from miniature precision machine and aerospace components to large civil structures.     

Damping and Viscoelastic Dynamic Response of RC Flexural Members Strengthened with Adhesively Bonded Composite Materials

A theoretical approach for the dynamic viscoelastic response of reinforced concrete (RC) beams and one-way slabs strengthened with adhesively bonded composite materials is developed. The analytical model is based on variational principles, dynamic equilibrium, and compatibility of deformations between the structural components (RC beam/slab, adhesive, composite material). The model accounts for the deformability of the adhesive layer and for its high order stress and displacement fields. The equations of motion and the boundary, continuity, and initial conditions are derived via the extended Hamilton’s principle. The Kelvin-Voigt approach is adopted for the consideration of the viscoelastic response of the adhesive material and the internal damping in the composite material and the RC member. The Rayleigh damping model is used for the external viscous damping of the RC member. The dynamic governing equations are solved using the Newmark time integration and a multiple shooting algorithm is used for the solution in space. A numerical example is presented to examine the capabilities of the model, to highlight the unique phenomena associated with the viscoelastic response of the adhesive material, and to demonstrate its influence on the local and global behavior. The results obtained using the analytical model show that the viscoelastic response of the adhesive material may significantly modify the critical shear and peeling stresses at the interfaces of the adhesive layer.     

Structural Health Monitoring by Piezo–Impedance Transducers. II: Applications

This paper, the second in a two-part series, presents a new methodology for structural identification and nondestructive evaluation by piezo–impedance transducers. The theoretical development and experimental validation of the underlying lead–zirconium–titanate (PZT)–structure interaction model was presented in the first part. In our newly proposed method, the damage in evaluated on the basis of the equivalent system parameters “identified” by the surface-bonded piezo–impedance transducer. As proof of concept, the proposed method is applied to perform structural identification and damage diagnosis on a representative lab-sized aerospace structural component. It is then extended to identify and monitor a prototype reinforced concrete bridge during a destructive load test. The proposed method was found to be able to successfully identify as well as evaluate damages in both the structures.     

Characterization of Wireless Smart Sensor Performance

A critical aspect of using wireless sensors for structural health monitoring is communication performance. Unlike wired systems, data transfer is less reliable between wireless sensor nodes owing to data loss. While reliable communication protocols are typically used to reduce data loss, this increase in communication can drain already limited power resources. This paper provides an experimental investigation of the wireless communication characteristics of the Imote2 smart sensor platform; the presentation is tailored toward the end user, e.g., application engineers and researchers. Following a qualitative discussion of wireless communication and packet delivery, a quantitative characterization of wireless communication capabilities of the Imote2 platform, including an assessment of onboard and external antenna performance, is provided. Herein, the external antenna was found to significantly outperform the onboard antenna in both transmission and reception reliability. However, the built environment, including building materials and other wireless networks, can significantly reduce reception rate and thus increase packet loss. Finally, implications of these results for a full-scale implementation are presented.     

Three-Dimensional Electromechanical Impedance Model. I: Formulation of Directional Sum Impedance

Piezoceramic transducers (PZTs) are extensively used in vibration and noise control, and damage detection of various engineering structures. In the last decade, its application has been extended to include their interactions with the host structure in electromechanical impedance models. The interaction between the host structure and PZT is governed by both the extensional and longitudinal vibrations of the transducer. However, the interaction models developed in the last decade consider only the one-dimension or two-dimension extensional actuations, ignoring the longitudinal actuations. This study examines the three-dimensional (3D) interaction of a transducer with the host structure, considering both the extensional and the longitudinal actuations of the transducer. It does not impose any restriction on the shape, size, and electrical properties of the PZT and thus contains additional features over the existing PZT-structure interaction models. This paper is Part I of a two-part paper, which presents a new “directional sum” numerical–analytical admittance formulation with experimental verification. Part II of this paper will elaborate on the damage analysis and characterization of PZT properties for the new 3D model.     

Development and Evaluation of an Adhesively Bonded Panel-to-Panel Joint for a FRP Bridge Deck System

A fiber-reinforced polymer (FRP) composite cellular deck system was used to rehabilitate a historical cast iron thru-truss structure (Hawthorne St. Bridge in Covington, Va.). The most important characteristic of this application is reduction in self-weight, which raises the live load-carrying capacity of the bridge by replacing the existing concrete deck with a FRP deck. This bridge is designed to HL-93 load and has a 22.86?m clear span with a roadway width of 6.71?m. The panel-to-panel connections were accomplished using full width, adhesively (structural urethane adhesive) bonded tongue and groove splices with scarfed edges. To ensure proper construction, serviceability, and strength of the splice, a full-scale two-bay section of the bridge with three adhesively bonded panel-to-panel connections was constructed and tested in the Structures Laboratory at Virginia Tech. Test results showed that no crack initiated in the joints under service load and no significant change in stiffness or strength of the joint occurred after 3,000,000 cycles of fatigue loading. The proposed adhesive bonding technique was installed in the bridge in August 2006.     

Sensor Network for Structural Health Monitoring of a Highway Bridge

A bridge monitoring TestBed is developed as a research environment for sensor networks and related decision-support technologies. A continuous monitoring system, capable of handling a large number of sensor data channels and three video signals, is deployed on a four-span, 90-m long, reinforced concrete highway bridge. Of interest is the integration of the image and sensor data acquisition into a single computer, thereby providing accurate time synchronization between the response and corresponding traffic loads. Currently, video and acceleration records corresponding to traffic induced vibration are being recorded. All systems operate online via a high-speed wireless Internet network, allowing real-time data transmission. Elements of the above health monitoring framework are presented herein. Integration of these elements into an automated functional system is emphasized. The recorded data are currently being employed for structural system identification via a model-free technique. Effort is also underway to correlate the moving traffic loads with the recorded accelerations. Finally, the TestBed is available as a resource for verification of new sensor technologies, data acquisition/transmission algorithms, data mining strategies, and for decision-support applications.     

Three-Dimensional Electromechanical Impedance Model. II: Damage Analysis and PZT Characterization

Piezoceramic transducers (PZTs) are extensively used in the nondestructive evaluation of damages in various engineering structures. This paper, the second of a two-part paper, focuses on the application of a PZT using three-dimensional (3D) directional sum impedance (DSI). The semianalytical 3D admittance has been formulated and experimentally validated in the first paper. This part deals with the application of the 3D DSI model in damage analysis where by damages were numerically simulated for various types of specimens and the DSI admittance signatures were predicted and compared. The deviations of the signature from that of the undamaged state provide an indicator for the health of the structure. This technique is nondestructive in nature, and the damages were quantified using root-mean-square deviation in signatures with respect to the undamaged state signature. In Part I, the properties of the PZT and their influences on admittance signatures were briefly presented. In this part, a thorough investigation was made and the importance of all the PZT properties in damage analysis was presented.     

Simplified Procedure for Estimating Earthquake-Induced Deviatoric Slope Displacements

A simplified semiempirical predictive relationship for estimating permanent displacements due to earthquake-induced deviatoric deformations is presented. It utilizes a nonlinear fully coupled stick-slip sliding block model to capture the dynamic performance of an earth dam, natural slope, compacted earth fill, or municipal solid-waste landfill. The primary source of uncertainty in assessing the likely performance of an earth/waste system during an earthquake is the input ground motion. Hence, a comprehensive database containing 688 recorded ground motions is used to compute seismic displacements. A seismic displacement model is developed that captures the primary influence of the system’s yield coefficient (ky), its initial fundamental period (Ts), and the ground motion’s spectral acceleration at a degraded period equal to 1.5Ts. The model separates the probability of “zero” displacement (i.e., ? 1?cm) occurring from the distribution of “nonzero” displacement, so that very low values of calculated displacement do not bias the results. The use of the seismic displacement model is validated through reexamination of 16 case histories of earth dam and solid-waste landfill performance. The proposed model can be implemented rigorously within a fully probabilistic framework or used deterministically to evaluate seismic displacement potential.     

Computer Modeling for the Complex Response Analysis of Nonstandard Structural Dynamics Problems

Over the past several decades, two intriguing classes of problems, having a wide range of applications in engineering, have been of interest to many researchers: (1) coupled dynamics of a distributed parameter system traversed by one or more moving oscillators; and (2) transient dynamic analysis of axially moving media (and associated phenomena of parametric resonances). Bridge vehicle interaction falls into the first class of problems, and the analysis of flexible appendages deployed from a satellite or a spacecraft is typical of the second class. Mathematically, these two problems are dual to each other, and they often are highly nonlinear in nature and typically involve large overall motion in space with complex effects of convective inertia terms in their governing equations of dynamic equilibrium. The “nonstandard” analytical nature of these problems stems from the fact that we are dealing with one or more of the following peculiarities: (1) variable problem domain; (2) varying spatial distribution of forces over the time duration of the analysis; and/or (3) changing location and type of constraints. Many researchers are trying to formulate the response of these problems, each having a different approach, but applicable only to certain specific details. Moreover, few researchers have concluded that these problems are beyond the scope of the present commercial finite-element (FE) software packages. However, we believe that if the nature and details of these problems are studied properly and carefully, it is immediately possible to simulate these problems in available commercial FE programs. An added advantage would also be the avoidance of many unrealistic simplifying assumptions that are often introduced to reduce the mathematical complexity; e.g., neglecting possible separation (after periods of prior contacts) in beam-moving vehicle problems, assuming linear behavior of suspension systems, and restriction to beam configuration only, among many others. For demonstration, we use ABAQUS in a large number of test cases to be presented. The results are compared with those presented in literatures.     

Generic Impedance-Based Model for Structure-Piezoceramic Interacting System

This paper describes a generic impedance-based model for predicting the electromechanical (EM) impedance of one-dimensional (1D) and two-dimensional (2D) structure-piezoceramic (PZT) interacting systems. The vibration of a PZT patch is first analyzed. The effect of the host structure is then represented by its force impedance, which is obtained by a semianalytical method. Finally, experiments are carried out on beams and plates to simulate 1D and 2D problems, respectively. It is found that the predicted EM impedance of the structure-PZT interacting system coincides very well with the experimentally measured data. The results show that the predicted peaks match the natural frequencies of the host structure. The very small shift of the peaks from the natural frequencies caused by the interaction between the host structure and the PZT patch indicates that the small size PZT patch can be permanently bonded to the structure for on-line health monitoring without changing the mechanical properties of the structure too much. It is concluded that the proposed impedance-based model for general structure-PZT interacting systems can be potentially used in structural health monitoring.     

Viscoplastic Response of Structures for Intense Local Heating

A thermoviscoplastic finite element method employing the Bodner‐Partom constitutive model is used to investigate the response of simplified thermal‐structural models to intense local heating. The computational method formulates the problem in rate form and advances the solution in time by numerical integration. The thermoviscoplastic response of simplified structures with prescribed temperatures is investigated. With rapid rises of temperature, the nickel alloy structures display initially higher yield stresses due to strain rate effects. As temperatures approach elevated values, yield stress and stiffness degrade rapidly and pronounced plastic deformation occurs.     

Experiments and Cohesive Zone Model Parameters for T650/AFR-PE-4/FM680-1 at High Temperatures

This paper reports an experimental program to establish a cohesive zone model for the T650/AFR-PE-4 (laminate) and FM680-1 (adhesive) system. The cohesive zone model is based on a four parameter characterization: in each mode, a range of values for the critical energy release rate and cohesive strength are computed from a set of experimental results. Values of each parameter are determined over the temperature range of 20–350°C. Owing to experimental limitations, two methods for determining the Mode I critical energy release rate are reported from the double cantilever beam test: the area method and the inverse method. The Mode I strength is determined from a button peel stress test. The values of the Mode II parameters are determined by using a mapping procedure that accounts for multiparameter dependence in models of the end notch flexure and single lap joint tests.