PZT Patches Research Articles

Monitoring of Pre-Load on Rock Bolt Using Piezoceramic-Transducer Enabled Time Reversal Method.

Rock bolts ensure structural stability for tunnels and many other underground structures. The pre-load on a rock bolt plays an important role in the structural reinforcement and it is vital to monitor the pre-load status of rock bolts. In this paper, a rock bolt pre-load monitoring method based on the piezoceramic enabled time reversal method is proposed. A lead zirconate titanate (PZT) patch transducer, which works as an actuator to generate stress waves, is bonded onto the anchor plate of the rock bolt. A smart washer, which is fabricated by sandwiching a PZT patch between two metal rings, is installed between the hex nut and the anchor plate along the rock bolt. The smart washer functions as a sensor to detect the stress wave. With the increase of the pre-load values on the rock bolt, the effective contact surface area between the smart washer and the anchor plate, benefiting the stress wave propagation crossing the contact surface. With the help of time reversal technique, experimental results reveal that the magnitude of focused signal clearly increases with the increase of the pre-load on a rock bolt before the saturation which happens beyond a relatively high value of the pre-load. The proposed method provides an innovative and real time means to monitor the pre-load level of a rock bolt. By employing this method, the pre-load degradation process on a rock bolt can be clearly monitored. Please note that, currently, the proposed method applies to only new rock bolts, on which it is possible to install the PZT smart washer.

Finite Element Analysis of Grouting Compactness Monitoring in a Post-Tensioning Tendon Duct Using Piezoceramic Transducers.

With the development of the post-tensioning technique, prestressed concrete structures have been widely used in civil engineering. To ensure the long-term effectiveness of the prestressed tendon, the grouting quality of the tendon duct is one of the important factors. However, it is still a challenge to monitor the grouting quality of post-tensioning tendon ducts, due to the invisibility of the grouting. The authors’ previous work proposed a real-time method that employed a stress wave-based active sensing approach with piezoceramic transducers to monitor the grouting compactness of a Post-Tensioning Tendon Duct (PTTD). To further understand the piezoceramic induced stress wave propagation in the PTTD with different grouting levels, this paper develops a two-dimensional finite element model for monitoring the grouting compactness of the tendon duct with a piezoceramic transducer. A smart aggregate (SA) developed to utilize one Lead Zirconate Titanate (PZT) transducer with marble protection is installed in the center location of the tendon duct as an actuator. Two PZT patches are bonded on the bottom and top surface of the tendon duct as the sensors. The analysis results show that the finite element analysis results are in good agreement with the experimental results, which demonstrates that the finite element analysis is feasible and reliable. For the top half of the specimen, not much stress wave could be detected before the full grouting level, except for negligible signals that may propagate through the walls of the tendon duct. When the tendon duct grouting is at 100%, the stress wave propagates to the top of the specimen, and the displacements are symmetric in both left-right and top-bottom directions before the stress waves reach the boundary. The proposed two-dimensional finite element model has the potential to be implemented to simulate the stress wave propagation principle for monitoring grouting compaction of the post-tensioning tendon duct.

A novel embeddable spherical smart aggregate for structural health monitoring: part I. Fabrication and electrical characterization

Recently developed piezoceramic-based transducers, known as smart aggregates (SAs), have shown their applicability and versatility in various applications of structural health monitoring (SHM). The lead zirconate titanate (PZT) patches embedded inside SAs have different modes that are more suitable for generating or receiving different types of stress waves (e.g. P and S waves, each of which has a unique role in SHM). However, due to the geometry of the 2D PZT patch, the embedded SA can only generate or receive the stress wave in a single direction and thus greatly limits its applications. This paper is the first of a series of two companion papers that introduces the authors’ latest work in developing a novel, embeddable spherical smart aggregate (SSA) for the health monitoring of concrete structures. In addition to the 1D guided wave produced by SA, the SSA embedded in concrete structures can generate or receive omni-directional stress waves that can significantly improve the detection aperture and provide additional functionalities in SHM. In the first paper (Part I), the detailed fabrication procedures with the help of 3D printing technology and electrical characterization of the proposed SSA is presented. The natural frequencies of the SSA were experimentally obtained and further compared with the numerical results. In addition, the influence of the components’ thickness (spherical piezoceramic shell and epoxy) and outer radius (spherical piezoceramic shell and protection concrete) on the natural frequencies of the SSA were analytically studied. The results will help elucidate the key parameters that determine the natural frequencies of the SSA. The natural frequencies of the SSA can thus be designed for suitability in the damage detection of concrete structures. In the second paper (Part II), further numerical and experimental verifications on the performance of the proposed SSA in concrete structures will be discussed.

Embedded PZT Sensor for Monitoring Mechanical Impedance of Hydrating Cementitious Materials

An embedded PZT (Lead Zirconate Titanate)-based sensor is developed for real-time, continuous, in-situ monitoring of hydrating cementitious materials after casting. The development of a multi-layer protection for a PZT patch, which provides a physical barrier with the surrounding medium while ensuring the sensitivity of measurement is described. Electrical impedance measurements from the sensor embedded inside mortar mixtures of different compositions are shown to sensitively provide an indication of changes in the state and the mechanical impedance of the material during periods associated with setting and early strength gain. An analytical procedure is developed for extracting the mechanical impedance of the surrounding cementitious material from the electromechanical measurements of the embedded PZT sensor. Changes in the mechanical impedance of mortars through periods of setting and early strength gain obtained from the embedded PZT sensor are validated using pin penetration, isothermal calorimetry and vibration-based measurements. Kinetics of hydration reaction obtained from isothermal calorimetry and increase in the penetration resistance during the setting behavior in the material, are accurately reflected in the increase in the mechanical impedance of the surrounding mortar obtained from the embedded PZT sensor. The continued increase in the mechanical impedance of the mortar after setting, up to 28 days, correlates well with the increase in elastic modulus of material obtained from vibration-based measurements. The durability of the sensor protection scheme is verified by evaluating the performance of sensors recovered from inside the mortar after long-term embedment. The embedded PZT sensor offers the potential for monitoring the local property development in a cementitious material from within the bulk of the structure and for use in quality assessment.

Tunable sub-wavelength acoustic energy harvesting with a metamaterial plate

We report theoretically on sub-wavelength acoustic energy harvesting (AEH) using a thin acoustic metamaterial (AM) made of spring-mass resonators attached to the surface of a homogeneous elastic thin plate. Considering an incident acoustic wave hitting the AM plate, tunable and highly efficient AEH is achieved by introducing a sub-wavelength defect inside the AM structure to confine the elastic energy into a spot which is then electromechanically converted into electrical power using a ceramic PZT patch. Several types of sub-wavelength cavities capable of confining acoustic energy at the sonic regime are extensively investigated for the optimization of AEH. Three analytical approaches—band structure, sound transmission loss and electrical-to-mechanical energy conversion—are proposed to fully describe the system interaction with the acoustic wave and quantify the AEH performance. The computed results show that an average power of 18 μW can be harvested using a specific cavity design of only size from an incident acoustic wave with a sound pressure level of 100 dB at 520 Hz. Such a system can open up a way through the design of effective tunable sub-wavelength acoustic energy harvesters based on AM applied to scavenge energy from sound.

Grouting monitoring of post-tensioning tendon duct using PZT enabled time-reversal method

The post-tensioning method, which involves the post-tensioning tendon duct (PTTD), is often used to reinforce concrete beams. An important procedure of the post-tensioning method is to fill the tendon duct with grout to protect the prestressed reinforcement from moisture and air. The grouting quality has a great impact on the corrosion prevention for the prestressed reinforcement and the bearing capacity of the concrete structures. In this paper, a method using Lead Zirconate Titanate (PZT) enabled time-reversal method to monitor the grouting quality is proposed. PZT is a commonly used piezoceramic material with strong piezoelectric effect. In the proposed method, one PZT ring installed on the prestressed reinforcement is used as an actuator to generate stress waves and the other distributive PZT patches bonded on the outer surface of the tendon duct are applied as sensors to detect the propagated stress waves. The grout in tendon duct forms conduits between PZT ring actuator and PZT patch sensors. To quantitatively represent the grout’s presence, the time-reversal method is used. The received signals from the sensors are reversed by the time-reversal operation and re-emitted. The grouting quality is evaluated by analyzing the peak value changes of the time-reversal focused signal. Experiments with 0%, 50%, 75%, and 100% grouting levels are conducted by the proposed method. The experimental results show that the grouting quality in the tendon duct can be estimated by monitoring the peak value changes of the time-reversal focused signal.

Detection of Debonding Between Fiber Reinforced Polymer Bar and Concrete Structure Using Piezoceramic Transducers and Wavelet Packet Analysis

Fiber reinforced polymer (FRP), a composite material with high corrosion resistance and high strength-to-weight ratio, has been increasingly used in reinforced concrete structures. The effectiveness of the structures depends on the bonding behavior between FRP composites and concrete structures. Therefore, detection of the debonding between the FRP materials and the hosting concrete structure is of great importance to ensure the structural safety. This paper proposes a stress wave-based active sensing approach to monitor the debonding process of FRP bar with the hosting concrete structure. One shear-type lead zirconate titanate (PZT) patch bonded on the outer surface of the FRP bar was used as an actuator to generate stress wave. Two smart aggregates (SAs), which were fabricated by sandwiching a shear type PZT patch between two protection marble pieces, were embedded in the hosting concrete structure to detect the wave response. The occurrence of debonding between the FRP bar and the hosting concrete structure attenuates the wave propagation. An FRP bar reinforced concrete specimen was designed and fabricated in laboratory. A pullout test was conducted to simulate different degrees of debonding damage. The attenuation of the stress wave due to debonding was clearly observed from the signal received by SAs in both time and frequency domain. Furthermore, a damage index based on wavelet packet analysis was developed to evaluate the debonding status. Experimental results demonstrate that the proposed method has potentials to detect different degrees of debonding damage of FRP bar reinforced concrete composite structures.

Experiments on coupled axial–flexural wave propagation in a sagged rod with structural discontinuity using piezoelectric transducers

A damage evaluation methodology in a sagged prismatic rod through application of wave propagation mechanics is presented, and an experimental investigation an experimental investigation is illustrated for damage detection using lead zirconate titanate (PZT) piezoelectric transducers. Discontinuity in the form of variation of the cross-sectional area (impedance mismatch) is induced in the longitudinal direction of the experimental specimen by butt welding two steel rods, as described in the previous work. The proposed methodology outlined for a sagged prismatic rod can be adopted for damage detection in a transmission line conductor made of wire rope. The corresponding wave reflections from the damage and boundaries are highlighted. Experiments are conducted on a predominantly single frequency based amplitude modulated narrow-banded, burst wave propagation. The novelty of the work is the experimental demonstration of narrow-banded single frequency wave generation with PZT patches with facility for a variable frequency excitation and controlled by a function generator. This is different from the conventional resonating piezo-crystal at ultrasonic frequency range, where the excitation is strong but the frequency is not variable. Also, the methodology is proposed for bifurcating the initial portion of the nondispersive wave propagation from a mixed-up response. For locating the bifurcation zone for axial–flexural wave interaction, a moving correlation coefficient method is employed. Thus, the use of PZT transducers for online monitoring of damage identification in sagged rod is promising for the future work.

Free vibration analysis of smart composite beam

The integration of composite beam with piezoelectric materials has been under a considerable attention to obtain active lightweight material. Smart materials will control the system in terms of reducing the vibrations amplitude and frequency so as to improve the efficiency of the system. Piezoelectric materials are often used to design smart structures in industrial, medical, military and scientific areas. The aim of the present study is to study the free vibration behaviour of a smart composite beam structure. Smart beam consists of composite beam modelled in cantilever configuration with PZT patches. The modal analysis of the smart cantilever beam was performed using the ANSYS software to reveal the fundamental modal frequencies and modal shapes. It can be inferred that the Modal analysis performed is usefull for applications in structural health monitoring of the smart composite structures. The changes in the mode shapes are used as an indicator to ascertain the damage to the smart composte beam structure.

Studies on mechanical and electrical characteristics of PZT-5H patch

Lead Zirconate Titanate(Pb (Zrx Ti1-x)o3) is one of the perovskite can be used for generating electricity by applying mechanical pressure and also can be used of developing deflection by applying electrical field. Many researchers are employing this material for both the applications because of their excellent electromechanical response by using ambient vibrations. Mechanical properties such as compressive strength and hardness of Lead Zirconate Titanate coated brass patch called as PZT patch were studied. The morphology of the PZT powder was studied in scanning electron microscopy (SEM).Electrical properties such as impedance was studied by using electrochemical impedance spectroscopy. The geometry of PZT -5H patch is 35 mm diameter of Brass patch and 28mm diameter is coated with Lead Zirconate Titanate(PZT-5H). The compressive strength of PZT-5H patch was 4267.516 N/sq.mm and the BHN 78 for PZT material was recorded for designing the energy harvester (EH) by using PZT-5H patches and also recommended that the EH can be operated at both sinusoidal and random excitations.

Damage Monitoring of Dry and Saturated Rocks Using Piezo Transducers

Abstract This paper explores the possibility of using the electro-mechanical impedance (EMI) technique for structural health monitoring (SHM) of underground rock structures. The underground structures are complex in design and require continuous inspection for their maintenance. However, some of the underground areas are inaccessible for the inspection team or might warrant a special clearing arrangement, leaving the structure out of service. The complexity in their design, their construction, and the conditions encountered during service life necessitate the deployment of a dedicated SHM system. Sometimes the stress ratio in rock because of overburden stress and tectonic activities may reach a high value exceeding threshold, which may lead to rock bursts and subsequent failure of the structure. The SHM technique becomes important for underground infrastructure to enable timely detection of the damage caused by any anticipated or unanticipated loads. In the present study, rock specimens have been tested in a laboratory environment under saturated, as well as dry, conditions, under pre-loaded stress conditions by applying continuous quasi-static loading, so as to simulate the near-real-life underground conditions. The EMI technique is used for damage monitoring. The vibrations generated by the actuation of lead zirconate titanate (PZT) patch are sensed by the same PZT patch and recorded in the form of conductance and susceptance signatures. The signatures have been quantified using root-mean-square-deviation (RMSD) method. The test results clearly establish the potential of the EMI technique in detecting and quantifying load-induced damage in dry, as well as saturated, conditions in the underground structures, opening avenues for applications in real-life situations.

Performance analysis of simplified Fuzzy ARTMAP and Probabilistic Neural Networks for identifying structural damage growth

Structural Health Monitoring (SHM) has been advancing worldwide as evidenced by recent papers and practical applications. SHM systems increase safety and reduce maintenance costs in a variety of applications contributing significantly to prevent structural failures. Studies focusing on the structural damage identification have been proposed in literature by using methods based on Probabilistic Neural Networks (PNN). Although Fuzzy based methods are recurrent in SHM approaches, the Simplified Fuzzy ARTMAP Network (SFAN) has not been explored for analyzing structural damage. Furthermore, there is no evidence of comparative studies of these two methods when applied to the problem of progressing structural damage. Thus, this paper presents a comparative analysis of these two methods in the context of identifying structural damage growth. The comparison is made based on the factors of success rate and training/testing times. Additionally, this approach has carried out the performance analysis of suitable parameters setup for SFAN: choice parameter, training rate and vigilance parameter. As a practical case study, both methods were applied to a unidirectional composite plate containing four PZT (Lead Zirconate Titanate) patches where the damage growth scenarios were also simulated by loosening bolts for three different levels. In addition, the repaired structural condition was also considered by retightening bolts. The results have shown that both methods are suitable for the problem of damage growth, particularly to supporting decisions about the structural damage assessment. In short, the comparative analysis has shown that the SFAN method is better suited to the problem of damage growth especially in regard to training and testing times. Thus, this comparative study contributes to helping researchers and industrialist in choosing more effective approaches for structural damage growth.

Load Monitoring of the Pin-Connected Structure Using Time Reversal Technique and Piezoceramic Transducers—A Feasibility Study

An effective approach using time reversal technique and lead zirconate titanate (PZT) transducer was described in this paper to monitor the loading status of pin-connected structures, which are widely used in the construction industry. A tension-controllable structure with a pin connection was fabricated and used for experimentation. Two PZT patch sensors were mounted on the pin and on the connected structural surface. In the experiment, the time reversal process was performed by varying the tension on the pin joint, and the experimental results demonstrate that the peak value of the focused signal, detected by the PZT sensor, increases with the load on the pin connection. In addition, the entire experimental process was repeated seven times to verify the repeatability and consistency of the proposed method. Subsequently, the reliability of the experimental results was verified in an environment with high disturbances. All the test results show that the proposed method using PZT sensors along with the time reversal technique has potential to be employed in practical applications, especially monitoring the load of a cable-deck connection in a cable-strut bridge in an environment with severe disturbances.

Experimental Study on Damage Detection in Timber Specimens Based on an Electromechanical Impedance Technique and RMSD-Based Mahalanobis Distance.

In the electromechanical impedance (EMI) method, the PZT patch performs the functions of both sensor and exciter. Due to the high frequency actuation and non-model based characteristics, the EMI method can be utilized to detect incipient structural damage. In recent years EMI techniques have been widely applied to monitor the health status of concrete and steel materials, however, studies on application to timber are limited. This paper will explore the feasibility of using the EMI technique for damage detection in timber specimens. In addition, the conventional damage index, namely root mean square deviation (RMSD) is employed to evaluate the level of damage. On that basis, a new damage index, Mahalanobis distance based on RMSD, is proposed to evaluate the damage severity of timber specimens. Experimental studies are implemented to detect notch and hole damage in the timber specimens. Experimental results verify the availability and robustness of the proposed damage index and its superiority over the RMSD indexes.

Bolt Looseness Detection Based on Piezoelectric Impedance Frequency Shift

In this paper, a piezoelectric impedance frequency shift method is developed to estimate the bolt preload for the detection of bolt looseness in engineering structures. An experimental device that allows the precision control of the axial preload force on a bolt is designed and fabricated. A universal testing machine is used to preload accurately on the bolt in the experiments. Under different bolt preload conditions, the impedance analyzer measures the admittance (inverse of the impedance) signal of the PZT (Lead ZirconateTitanate) patches which are bonded on the bolt head. Firstly, a wide frequency band is swept to find a sensitive frequency band of the piezoelectric admittance with the imaginary part. Then in the sensitive frequency band, a specified peak frequency of the admittance signature is chosen to investigate the frequency shift with different bolt preloads. The relationship between the specified frequency shift and the bolt preload is established. The experimental results show that the specified peak frequency decreases as the bolt preload increases for both M16 and M12 bolts, and the frequency shift has a linear relationship with the preload on the bolt. The frequencies of the real and imaginary parts of the admittance signature have the same results. Therefore, the bolt preload can be determined by measuring the specified frequency shift and this method has a good application prospect.

Identification of structural parameters including crack using one dimensional PZT patch model

This article presents a new concept of using the one-dimensional piezo-electric patch on beam model for structural identification (SI). A hybrid element constituted of one-dimensional beam element and a PZT sensor is used with reduced material properties. Accuracy of this element is first verified against a corresponding 3D finite element model. Then SI is carried out as an inverse problem whereby parameters are identified by minimizing the deviation between the predicted and measured voltage response of the patch, when subjected to impulse excitation. A non-classical optimization algorithm Particle Swarm Optimization is used to minimize this objective function. Identified parameters involve stiffness, damping as well as the depth and location of crack in a beam. The validity of the proposed approach is proved by numerical studies on a beam, nine member frame and crack depth and location identification using various patch lengths. The signals are polluted with 5% Gaussian noise to simulate experimental noise. The results show there is a significant improvement in identification accuracy compared to other methods. The proposed method is also successfully verified experimentally.

Monitoring of Grouting Compactness in a Post-Tensioning Tendon Duct Using Piezoceramic Transducers.

A post-tensioning tendon duct filled with grout can effectively prevent corrosion of the reinforcement, maintain bonding behavior between the reinforcement and concrete, and enhance the load bearing capacity of concrete structures. In practice, grouting of the post-tensioning tendon ducts always causes quality problems, which may reduce structural integrity and service life, and even cause accidents. However, monitoring of the grouting compactness is still a challenge due to the invisibility of the grout in the duct during the grouting process. This paper presents a stress wave-based active sensing approach using piezoceramic transducers to monitor the grouting compactness in real time. A segment of a commercial tendon duct was used as research object in this study. One lead zirconate titanate (PZT) piezoceramic transducer with marble protection, called a smart aggregate (SA), was bonded on the tendon and installed in the tendon duct. Two PZT patch sensors were mounted on the top outside surface of the duct, and one PZT patch sensor was bonded on the bottom outside surface of the tendon duct. In the active sensing approach, the SA was used as an actuator to generate a stress wave and the PZT sensors were utilized to detect the wave response. Cement or grout in the duct functions as a wave conduit, which can propagate the stress wave. If the cement or grout is not fully filled in the tendon duct, the top PZT sensors cannot receive much stress wave energy. The experimental procedures simulated four stages during the grout pouring process, which includes empty status, half grouting, 90% grouting, and full grouting of the duct. Experimental results show that the bottom PZT sensor can detect the signal when the grout level increases towards 50%, when a conduit between the SA and PZT sensor is formed. The top PZT sensors cannot receive any signal until the grout process is completely finished. The wavelet packet-based energy analysis was adopted in this research to compute the total signal energy received by PZT sensors. Experimental results show that the energy levels of the PZT sensors can reflect the degree of grouting compactness in the duct. The proposed method has the potential to be implemented to monitor the tendon duct grouting compactness of the reinforced concrete structures with post tensioning.

Electromechanical properties of smart aggregate: theoretical modeling and experimental validation

Smart aggregate (SA), as a piezoceramic-based multi-functional device, is formed by sandwiching two lead zirconate titanate (PZT) patches with copper shielding between a pair of solid-machined cylindrical marble blocks with epoxy. Previous researches have successfully demonstrated the capability and reliability of versatile SAs to monitor the structural health of concrete structures. However, the previous works concentrated mainly on the applications of SAs in structural health monitoring; no reasonable theoretical model of SAs was proposed. In this paper, electromechanical properties of SAs were investigated using a proposed theoretical model. Based on one dimensional linear theory of piezo-elasticity, the dynamic solutions of a SA subjected to an external harmonic voltage were solved. Further, the electric impedance of the SA was computed, and the resonance and anti-resonance frequencies were calculated based on derived equations. Numerical analysis was conducted to discuss the effects of the thickness of epoxy layer and the dimension of PZT patch on the fundamental resonance and anti-resonance frequencies as well as the corresponding electromechanical coupling factor. The dynamic solutions based on the proposed theoretical model were further experimentally verified with two SA samples. The fundamental resonance and anti-resonance frequencies of SAs show good agreements in both theoretical and experimental results. The presented analysis and results contribute to the overall understanding of SA properties and help to optimize the working frequencies of SAs in structural health monitoring of civil structures.

Sensing of damage and substrate stress in concrete using electro-mechanical impedance measurements of bonded PZT patches

The influence of stress and induced damage in concrete on the electro-mechanical (EM) impedance response of bonded PZT patches is evaluated for applied compressive loading. Full field displacements obtained from digital image correlation are used to evaluate the level of stress-induced damage in concrete. Stress in the substrate produces an imposed strain on the PZT. A change in the imposed strain produces a rightward frequency shift and an increase in the amplitude of the resonant peak in the EM conductance spectrum of the PZT. An increase in the substrate compliance produces a decrease in the resonant frequency and an increase in the amplitude of the resonant peak. Changes in the resonant peak in the conductance spectrum induced by increasing substrate stress are of a significant magnitude when compared with the changes induced by damage. In the early stages of damage associated with distributed microcracking, the counteracting influences of increasing level of damage and increasing stress on the resonant peak result in no shift in frequency for measurements under applied load. There is however an increase in the amplitude of the resonance peak. When the applied stress is removed, there is a net decrease in frequency resulting from damage in the form of distributed microcracks. Measures of changes in the resonant peak based on root mean square deviation (RMSD), do not show any observable change when measurements are performed under applied loading. There is a consistent increase in RMSD values and frequency shift with increasing damage when the applied stress is removed. The centroidal measure of the normalized frequency spectrum reflects changes in substrate stress. At higher applied stress levels, there is a nonlinear increase in damage, leading to localization and cracking. The influence of damage is dominant in this region and significant changes are obtained in the RMSD values in both loaded and unloaded conditions.

Simplified fuzzy ARTMAP network-based method for assessment of structural damage applied to composite structures

This paper addresses a novel method for structural damage assessment based on the electromechanical impedance (EMI) technique applied to composite structures. This method is based on a high-frequency excitation range in order to overcome the difficulties caused when the low vibration modes are excited. A structure made up of unidirectional carbon fiber pre-impregnated with resin-epoxy is excited using a chirp signal which varies the frequency in a wide range through the PZT (Lead Zirconate Titanate) patches. From the structural response signal, the Euclidean Distance (ED) is computed from each structural condition considering the pristine condition as a reference (baseline). The ED is used as input to the Simplified Fuzzy ARTMAP Network (SFAN) which is responsible for identifying various different structural conditions. It is shown that the robustness provided by SFAN allows the evaluation of the progress of structural damage even under critical conditions. The structural damage was simulated by loosening bolts in a structure. Furthermore, the repaired condition was also considered by retightening bolts. The paper discusses the advantages and drawbacks of the approach in light of the experimental results.