Impedance-based Damage Detection Research Articles

Experimental Study on Impedance-Based Damage Detection Considering Temperature Effect

The impedance-based damage detection technique has the potential for health monitoring of different types of structures. However, it is necessary to consider the temperature effect on the impedance signal in applying this technique to actual structures. In this study, an effective impedance-based damage detection method that compensates for the temperature effect was developed. Experimental tests on a steel frame structure connected with high tensile bolts were performed. Moreover, the temperature effect on the impedance damage index was compensated for detecting damage caused by bolt looseness; that is, the relationship between the impedance damage index and the temperature was established through long-term measurements. Based on this relationship, damage detection was performed by compensating for the temperature effect. Because the damage index after the bolt loosening reflects the effects of temperature and damage, it is difficult to evaluate the damage by monitoring only the damage index. However, after compensating for the temperature effect, it was observed that the damage could be estimated precisely. The damage was effectively monitored after measuring the impedance signal and temperature over a specific period for the initial healthy structural state, analyzing the correlation between the impedance damage index and temperature, and setting an appropriate warning criterion based on the correlation.

Method for removing temperature effect in impedance-based structural health monitoring systems using polynomial regression

Structural health monitoring systems are employed to evaluate the state of structures to detect damage, bringing economical and safety benefits. The electromechanical impedance technique is a promising damage detection tool since it evaluates structural integrity by only measuring the electrical impedance of piezoelectric transducers bonded to structures. However, in real-world applications, impedance-based damage detection systems exhibit strong temperature dependence; therefore, variations associated with temperature changes may be confused as damage. In this article, the temperature effect on the electrical impedance of piezoelectric ceramics attached to structures is analyzed. Besides, a new methodology to compensate for the temperature effect in the electromechanical impedance technique is proposed. The method is very general since it can be applied to nonlinear (polynomial) temperature and/or frequency dependences observed on the horizontal and vertical shifts of the impedance signatures. A computer algorithm that performs the compensation was developed, which can be easily incorporated into real-time damage detection systems. This compensation technique is applied successfully to two aluminum beams and one steel pipe, minimizing the effect of temperature variations on damage detection structural health monitoring systems in the temperature range from −40°C to 80°C and the frequency range from 10 to 90 kHz.

Removal of temperature effect in impedance-based damage detection using the cointegration method

Electromechanical impedance technique has been widely used in the area of structural health monitoring. However, both damage and variation in environmental temperature can cause the changes in electromechanical impedance signature, which may cause false damage diagnosis. The temperature effect on the electromechanical impedance-based method has been one of its main drawbacks in practical application. This article proposes a new approach based on cointegration to eliminate temperature interference in the electromechanical impedance technique. The augmented Dickey–Fuller test is used to analyze the stationary characteristics of the time series and determine the degree of non-stationarity. The Johansen test is used to obtain the cointegrating residuals instead of the direct electromechanical impedance responses for damage detection. The proposed method is verified on the undamaged and damaged steel plates with the consideration of environmental temperature variations. The damage detection was based on the electromechanical impedance technique in which the peak frequency is chosen as a cointegrated variable. The experimental results show that the cointegration method can remove the temperature effect on the electromechanical impedance responses, and the cointegrating residuals are effective indices to indicate the occurrence of damage.

Sensing Region Characteristics of Smart Piezoelectric Interface for Damage Monitoring in Plate-Like Structures

For impedance-based damage detection practices, the sensing range of piezoelectric devices is an important parameter that should be determined before real implementations. This study presents numerical and experimental analyses for characterizing the sensing region of a smart PZT (lead–zirconate–titanate) interface for damage monitoring in plate-like structures. First, a finite element (FE) model of the PZT interface mounted on a plate structure is established. The impedance responses of the PZT interface are numerically simulated under different damage locations inflicted in the plate domain. The impedance features are extracted from the impedance signatures to analyze the sensing distance and the damage detectability of the PZT interface. Next, the splice plate of a bolted connection is selected as a practical plate-like structure for the experimental examination of the PZT interface’s sensing region on a limited plate domain. The damage sensitivity behavior of the PZT interface is analyzed with respect to the damage location on the splice plate. An FE analysis of the corresponding PZT interface-splice plate system is also conducted to support the experimental results.

Lamb waves and electro-mechanical impedance based damage detection using a mobile PZT transducer set

Lamb waves and electro-mechanical impedance (EMI) based methods are increasingly used in damage detection owing to their high sensitivity to small structural defects. Lamb wave based methods are effective in detecting damages in a large area and electro-impedance based methods are suitable for characterizing the identified damage. Based on these two methods, a novel combined damage detection method is presented in this research. To achieve this, first, a mobile transducer set is developed, which can be used for both the Lamb waves and EMI based methods. Then, a baseline-free damage detection strategy that combines the Lamb waves and EMI methods is presented. Finally, a laboratory-sized test piece is used to validate the effectiveness of the proposed approach. The results achieved with the application of the presented combined method for characterizing an L-shape crack in an aluminum plate show better location accuracy and detection efficiency than those obtained by applying only one method.

Compound Damages Detection for Rotors by the Coupled Piezoelectric Impedance

The rotor is a key component of rotating machinery, thus it is of great importance to quantitatively detect its damage. Since the sensitivity matrix in the impedance-based rotor damage identification model with a single piezoelectric transducer has lower rank and the available impedances are not sufficient enough, a highly underdetermined inverse problem arises, which may seriously affect the identification of the damaged location and severity. It follows that a coupled impedance-based composite rotor damages detection method with double piezoelectric transducers is proposed to improve the accuracy of damage identification. In this method, the dynamics characteristics of the electromechanically coupled system can be changed by adding a piezoelectric transducer and a corresponding circuit. Thus the coupled impedance is different from the rotor impedance based on a single piezoelectric transducer. Moreover, owing to the different locations of the two piezoelectric transducers, two sets of sensitivity matrix which are linearly independent with each other are derived, and then more available admittances can be obtained by exciting the two piezoelectric transducers successively. Obviously, the rank of the sensitivity matrix can be increased and the ill-posed inverse problem of damage identification equations can be well improved, which leads to an accurate damage detection. Simulated and experimental results show that the proposed coupled impedance-based composite rotor damages identification method with double piezoelectric transducers can more accurately identify the locations and severities of the compound damages and be more suitable for the detection of composite tiny damages, compared to the impedance-based damage detection method with a single piezoelectric transducer.

Impedance-based damage detection under noise and vibration effects

The electro-mechanical impedance technique has been extensively studied in recent decades as a non-destructive method for detecting structural damage in structural health monitoring applications using low-cost piezoelectric transducers. Although many studies have reported the effectiveness of this detection method, numerous practical problems, such as the effects of noise and vibration, need to be addressed to enable this method’s effective use in real applications. Therefore, this article presents an experimental analysis of noise and vibration effects on structural damage detection in impedance-based structural health monitoring systems. The experiments were performed on an aluminum bar using two piezoelectric diaphragms, where one diaphragm was used to measure the electrical impedance signatures and the other diaphragm was used as an actuator to generate noise and controlled vibration. The effects of noise and vibration on impedance signatures were evaluated by computing the coherence function and basic damage indices. The results indicate that vibration and noise significantly affect the threshold of the lowest detectable damage, which can be compensated by increasing the excitation signal of the piezoelectric transducer.

Piezoelectric impedance based damage detection in truss bridges based on time frequency ARMA model

Electromechanical impedance (EMI) based structural health monitoring is performed by measuring the variation in the impedance due to the structural local damage. The impedance signals are acquired from the piezoelectric patches that are bonded on the structural surface. The impedance variation, which is directly related to the mechanical properties of the structure, indicates the presence of local structural damage. Two traditional EMI-based damage detection methods are based on calculating the difference between the measured impedance signals in the frequency domain from the baseline and the current structures. In this paper, a new structural damage detection approach by analyzing the time domain impedance responses is proposed. The measured time domain responses from the piezoelectric transducers will be used for analysis. With the use of the Time Frequency Autoregressive Moving Average (TFARMA) model, a damage index based on Singular Value Decomposition (SVD) is defined to identify the existence of the structural local damage. Experimental studies on a space steel truss bridge model in the laboratory are conducted to verify the proposed approach. Four piezoelectric transducers are attached at different locations and excited by a sweep-frequency signal. The impedance responses at different locations are analyzed with TFARMA model to investigate the effectiveness and performance of the proposed approach. The results demonstrate that the proposed approach is very sensitive and robust in detecting the bolt damage in the gusset plates of steel truss bridges.

Direct compensation of lift-off oscillation effect in magnetic impedance–based damage detection

Owing to its magneto-mechanical coupling characteristics, a magnetic transducer such as the electro-magnetic acoustic transducer can excite a metallic structure by means of the Lorenz force, and its electrical impedance is actually directly related to the mechanical impedance of the structure. Therefore, the change in electrical impedance measured can be used as an indicator of damage occurrence. As such, this type of magnetic transducer can be used in impedance-based structural damage detection, which features a non-contact advantage. Nevertheless, one key issue is that the coupling between the magnetic transducer and the structure monitored is strongly influenced by the lift-off distance (i.e. the distance from the transducer to the structure) which oscillates as the structure is oftentimes subject to oscillation/movement due to environment disturbance. In this research, we propose a new data analysis approach of transformed impedance that is immune to the lift-off distance oscillation during measurements. This data analysis approach takes advantage of the lift-off distance information embedded in the impedance measurement and is capable of removing the lift-off oscillation effect without explicitly measuring it. By doing so, the damage signature can be highlighted directly. Numerical simulations and experimental validations are carried out to demonstrate the effectiveness.

Enhanced modeling of magnetic impedance sensing system for damage detection

There has been recent interest in utilizing the magneto-mechanical coupling characteristics of a magnetic transducer to perform impedance-based damage detection of electrically conductive structures. This approach is non-contact in nature and has potential advantages in many applications. One important parameter in this approach is the lift-off distance, i.e. the distance from the transducer to the structure monitored, the change of which changes the magneto-mechanical coupling. In the past, the magneto-mechanical coupling is extracted completely or partially from experiment in an ad hoc manner. A predictive capability of magneto-mechanical coupling under given lift-off distance would play a significant role in damage detection practice and in sensor design/optimization. In this research, we formulate detailed first-principles-based modeling of a magnetic impedance transducer. In particular, the complete electrical effect of the structure is explicitly taken into consideration. Comprehensive analyses and experiments are carried out, which validate the underlying hypothesis as well as the accuracy of the new model proposed for impedance response prediction.

Damage Detection in Riveted Aircraft Elements Based on the Electromechanical Impedance Measurements

This work is devoted to the use of the electromechanical impedance method for the damage detection in a riveted aircraft element. In the first part of the paper a theoretical background of the impedance-based damage detection technique is made. Next, the description of the utilised experimental set-up is described. Then, an application of the method used to detect damage in a wing sheathing of a turboprop training aircraft is presented. A damage scheme incorporating multiple notches through the selected rivets is considered. Finally, the suitability of the described method to distinguish close and far field damages is discussed.

Experimental Investigation on Improving Electromechanical Impedance Based Damage Detection by Temperature Compensation

The sensitivity of the electromechanical impedance to structural damage under varying temperature is investigated in this paper. An approach based on maximizing cross-correlation coefficients is used to compensate temperature effects. The experiments are carried out on an air plane conform carbon fiber reinforced plastic (CFRP) panel (500mm x 500mm x 5mm) instrumented with 26 piezoelectric transducers of two different sizes. In a first step, the panel is stepwise subjected to temperatures between-50 °C and 100 °C. The influence of varying temperatures on the measured impedances and the capability of the temperature compensation approach are analyzed. Next, the sensitivity to a 200 J impact damage is analyzed and it is set in relation to the influence of a temperature change. It becomes apparent the impact of the transducer size and location on the quality of the damage detection. The results further indicate a significant influence of temperature on the measured spectra. However, applying the temperature compensation algorithm can reduce the temperature effect at the same time increasing the transducer sensitivity within its measuring area. The paper concludes with a discussion about the trade-off between the sensing area, where damage should be detected, and the temperature range, in which damage within this area can reliably be detected.

Damage detection in beam-type structures via PZT's dual piezoelectric responses

In this paper, practical methods to utilize PZT\'s dual piezoelectric effects (i.e., dynamic strain and electro-mechanical (E/M) impedance responses) for damage detection in beam-type structures are presented. In order to achieve the objective, the following approaches are implemented. Firstly, PZT material\'s dual piezoelectric characteristics on dynamic strain and E/M impedance are investigated. Secondly, global vibration-based and local impedance-based methods to detect the occurrence and the location of damage are presented. Finally, the vibration-based and impedance-based damage detection methods using the dual piezoelectric responses are evaluated from experiments on a lab-scaled beam for several damage scenarios. Damage detection results from using PZT sensor are compared with those obtained from using accelerometer and electric strain gauge.

Spectral element modeling based structure piezoelectric impedance computation and damage identification

This paper presents a numerical simulation study on electromechanical impedance technique for structural damage identification. The basic principle of impedance based damage detection is structural impedance will vary with the occurrence and development of structural damage, which can be measured from electromechanical admittance curves acquired from PZT patches. Therefore, structure damage can be identified from the electromechanical admittance measurements. In this study, a model based method that can identify both location and severity of structural damage through the minimization of the deviations between structural impedance curves and numerically computed response is developed. The numerical model is set up using the spectral element method, which is promised to be of high numerical efficiency and computational accuracy in the high frequency range. An optimization procedure is then formulated to estimate the property change of structural elements from the electric admittance measurement of PZT patches. A case study on a pin-pin bar is conducted to investigate the feasibility of the proposed method. The results show that the presented method can accurately identify bar damage location and severity even when the measurements are polluted by 5% noise.

Impedance based damage detection under varying temperature and loading conditions

The impedance based damage detection technique utilizing piezoelectric materials has become a promising and attractive tool for structural health monitoring due to its high sensitivity to small local damage. However, impedance signals are also sensitive to time-varying environmental and operational conditions, and these ambient variations can often cause false-alarms. In this study, a data normalization technique using Kernel principal component analysis (KPCA) is developed to improve damage detectability under varying temperature and external loading conditions and to minimize false-alarms due to these variations. The proposed technique is used to detect bolt loosening within a metal fitting lug, which connects a composite aircraft wing to a fuselage. Model and full-scale tests are performed under realistic temperature and loading variations to validate the proposed technique. The uniqueness of this paper lies in that (1) a data normalization technique tailored for impedance based damage detection has been developed (2) multiple environmental parameters, such as temperature and static/dynamic loading are considered simultaneously for data normalization and (3) the effectiveness of the proposed technique is examined using data collected from a full-scale composite wing specimen with a complex geometry.

Reference-free impedance-based crack detection in plates

Impedance-based damage detection techniques gained popularity among structural health monitoring (SHM) and nondestructive testing (NDT) communities due to their sensitivity to local damage and applicability to complex structures. In general, conventional impedance-based techniques identify damage by comparing “current” impedance signals with “baseline” ones obtained from the pristine condition of a structure. However, in-situ structures are often subject to changing temperature and loading conditions that can adversely affect measured impedance signals and cause false-alarms. In this paper, a “reference-free” impedance method, which does not require direct comparison of the current impedance signals with the previously obtained baseline impedance signals, is developed for crack detection in a plate-like structure. The proposed technique utilizes a single pair of PZTs collocated on the opposite surfaces of a structure to extract mode conversion produced by crack formation. Then, a reference-free damage classifier is developed and performed on the extracted mode conversion for instantaneous damage diagnosis. Numerical simulations and experimental tests have been conducted explicitly considering varying temperature and loading conditions to demonstrate the robustness of the proposed damage detection technique under varying operational and environmental conditions.

CFRP 판 보강 RC보의 균열 및 박리 손상 모니터링

CFRP 보강공법은 구조물에 내하력을 증가시키기 위해 사용되고 있는데 실제 교량에 적합하고 연구 활용성이 좋다. 하지만, CFRP로 보강된 콘크리트 보에서 휨파괴와 전단파괴 뿐만 아니라 부착파괴 또한 추가적으로 발생하게 된다. 이러한 CFRP 부착파괴는 취성파괴를 유발하게 된다. 따라서 이러한 CFRP로 보강된 콘크리트보 박리에 대한 모니터링은 매우 중요한 의미를 갖는다. 본 논문에서는 국부적인 손상 파악에 유리한 PZT센서를 이용한 임피던스 기반 손상검색 방법을 사용하여 CFRP로 보강된 콘크리트보에서 박리 모니터링에 대한 적용가능성을 검증해 보았다. A CFRP (Carbon Fiber-Reinforced Plastic) strengthening method being widely used to increase the load-carrying capacity of structures is very suitable for existing bridge structures. However, not only flexure and shear failures but also debonding failure might be additionally occured in reinforced concrete(RC) beams strengthened with the CFRP plates. The CFRP debonding failure would cause a brittle fracture of the beam. Therefore, health monitoring for the CFRP bonding condition is strongly required. In this study, a feasibility of the impedance-based damage detection method using PZT sensors was investigated through a series of experimental studies for realtime structural health monitoring(SHM) for the CFRP laminated concrete structures.

Structural damage detection using a magnetic impedance approach with circuitryintegration

Magnetic transducers have recently been explored for use in impedance-based damagedetection as a part of online structural health monitoring. The basic idea is toexcite the structure being monitored through a magneto-mechanical effect due tothe sensor, and infer the structural status by examining the sensor’s impedancewhich is coupled to the impedance of the structure. Since sensors can be hungabove the surface of the structure instead of being in direct contact, the magneticimpedance approach is promising, especially for structures with complicated surfacegeometries and boundaries. They may also be deployed in a movable manner,monitoring a large area with a small number of sensor units. Nevertheless, there arecurrently practical issues concerning implementation. If a small number of turnsof wire are wound in the electrical coil of the sensor, the magneto-mechanicalcoupling between the transducer and the host structure is generally weak, yieldinglow damage detection capability. On the other hand, if we simply increase thenumber of wire turns in the magnetic sensor, its parasitic resistance and inherentinductance are both simultaneously increased, which not only introduces excessiveelectrical damping that deteriorates the damage detection performance but may alsoresult in unstable circuit dynamics. In this research, a new magnetic impedancesensing scheme based on circuit integration is proposed, i.e. a tunable capacitorand a negative resistance element are integrated to the sensor unit. Since theseadditional circuit elements can neutralize the high inductive load and reduce thelarge parasitic resistance of the transducer, the signal-to-noise ratio in the sensormeasurement can be significantly increased. Moreover, circuit integration canamplify the damage-induced change in magnetic admittance, leading to muchenhanced damage detection sensitivity. Correlated numerical and experimentalstudies are carried out to validate this new magnetic impedance sensing system.

Design of Impedance Spectroscopy Based Structural Health Monitoring Sensors for Fatigue Damage Detection in Aluminum Beams

Piezoelectric sensors have been previously used for continuous structural health monitoring using impedance spectroscopy. However, three important issues still remain to be investigated for efficient design of such sensors, namely (i) the effect of sensor geometry on sensitivity of impedance based damage detection, (ii) the selection of frequency band for continuous health monitoring, and (iii) identification of damage quantification metric. In this study, we address these issues and report the results on aluminum beam subjected to mechanical fatigue. A commercially available Materials Testing Machine was used in order to perform a load controlled fatigue test (250–1350 lbs @ 20 cycles/sec) on a notched aluminum beam. Sensor geometry was optimized with the help of damage index metric based health monitoring. Next, optimized geometry was selected to further study the selection of frequency band with a new approach based on number of peaks in the impedance signature under identical testing conditions. The results of this study provide a benchmark for designing the sensors for structural health monitoring.

Reference-free crack detection using transfer impedances

A new concept of reference-free damage detection methodology is developed using transfer impedances to detect crack damage in a plate-like structure without using previously collected baseline data. Conventional impedance-based damage detection techniques have been shown to be vulnerable to other types of changes such as temperature variation that may not be relevant to defects of interest. One of the potential disadvantages of the conventional techniques is frequent false-alarms due to these undesirable variations that may occur in field applications. In order to reduce these false-alarms, this paper proposes a new methodology that utilizes transfer impedances obtained between two pairs of collocated PZT patches instead of the electromechanical impedance obtained at one PZT patch. The proposed technique utilizes Lamb mode conversion effects caused by the presence of crack damage in plate structures. Furthermore, an instantaneous damage classification is carried out by comparing mode conversion energy among several combinations of measured signals without any user-specified threshold or relying on the baseline data. The feasibility of the proposed reference-free methodology using transfer impedances is investigated via a series of experiments conducted on an aluminum plate.