The performance of three Electromechanical Impedance damage indicators on structural element with bolted joints

Tomasz WANDOWSKI1, Szymon OPOKA1, Pawel MALINOWSKI1, Wieslaw OSTACHOWICZ1,2

1 Department of Mechanics of Intelligent Structures, Institute of Fluid-Flow Machinery, Polish Academy of Sciences, Gdansk, Poland;

Phone: +48 58 6995 260, Fax: +48 58 3416 144; e-mail: tomaszw@imp.gda.pl, 2 Faculty of Automotive and Construction Machinery, Warsaw University of Technology, Warsaw, Poland;

AbstractIn the paper the results of application of Electromechanical Impedance (EMI) method to metal structure with bolted and riveted joints is presented. The principle of electromechanical impedance method is based on the measurement of electric parameters of piezoelectric transducer mounted on the host metal structure. Transducer is supplied by low voltage source (usually in range 1 V – 5 V). Due to electromechanical coupling between the piezoelectric transducer and the host structure mechanical resonances (peaks) are visible in the electric impedance spectrum of transducer. The appearance of anomalies in signals (shift of resonance peaks in frequency and change in their magnitude) bear witness of change in the structural condition of the test piece. In this research CeramTec SONOX P5 transducer was used which behaved as actuator and detector simultaneously. The structural part of real airplane wing was investigated. This structure is a part of trailing edge of the wing with joints connecting wing skin to the stiffeners and is prone to increased level of vibrations caused by flow separation and vortices. Measurements were conducted using HIOKI Impedance Analyzer IM3570. The EMI method with correlation distance or chessboard distance seem to be valuable local method for damage detection even in changing temperature conditions for geometrically more complicated structures.

Keywords: bolted joint, riveted joint, electromechanical impedance, local damage detection method, wing

1. Introduction

Electromechanical impedance (EMI) method is based on measurements of electrical impedance of piezoelectric transducer bonded to the structure. Due to electromechanical coupling of piezoelectric transducer mechanical resonances of the structure can be seen as peaks in the electrical impedance characteristics of piezoelectric transducer. Very often damage existing in the structure is source of stiffness change and can be noticed as peak shifts in measured electrical impedance characteristics. In electromechanical impedance method actuation and sensing are done by the same piezoelectric transducer placed in predefined location. Very small value of loading is imposed on the specimen (low voltages on the transducer) and hence linear behavior of the structure is assured. The beginning of electromechanical impedance method is connected with paper published by Liang, Rogers and Sun [1]. Paper shows the essence of the method and the model of piezoelectric transducer bonded to 2 degree-of-freedom structure. Electromechanical impedance has been utilized for assessment of beams, [2], plates [3],[4], railroad tracks [5], pipes with bolted joints [2], bonded joints [3],[6], concrete structures [7] and riveted panels [8],[9].

In last few years, the usage of integrated electric circuits in EMI, e. g. AD5933 – impedance analyzer, has been in the centre of interest of many scientists. This device is very popular in the field of Structural Health Monitoring based on electromechanical impedance method [10-14].

Different signal processing methods have been also developed for electromechanical method. Generally two types of damage indexes are commonly utilized in literature [3]: first is based on root mean square deviation (RMSD) and second is based on signal cross-correlation (CC). In [8] different variation of classical RMSD index were tested. In [2] neural network-based pattern analysis tool to identify damage-sensitive frequency ranges

autonomously and to provide information about damage type and severity was proposed. In [4] time domain approach combined with multilevel wavelet decomposition was utilized in order to process electromechanical impedance signals. Results show that it is more sensitive to damage than methods based on impedance measurements in the frequency domain. Interesting approach which allow to improve the effectiveness of electromechanical impedance method for composite elements with large surface was proposed in [15]. The idea is to use additional metallic part between composite part and piezoelectric transducer in order to modify impedance characteristic. Currently combined method joining electromechanical impedance method with guided wave propagation method is utilized [9], [18]. Such a combination of methods is more effective.

The critical problem related to EMI method is the similar influence of temperature and damage cases on electromechanical impedance measurements. Results of experimental study of this influence were presented for example in [16]. Authors showed that the temperature effects were also strongly frequency-dependent. Algorithm for temperature compensation in EMI method was proposed in [17].

The RMSD and CD are well known indexes which are used as potential damage indicators. With these two the new one Chessboard Distance (CB) is checked against their potential applicability in damage detection problems. It seems that this damage indicator was not used previously in the literature in this context.

2. Application of EMI method to curved element from wing structure

2.1 Investigated structures and damage indicators

The investigated structure was the section of plane’s wing cut off from its trailing edge. The structure was composed of thin and curved upper shell which was originally fixed to the wing ribs and spars by rivets, see Fig. 1a, b. For the purpose of the experiment one line of rivets was replaced by bolts in order to simulate normal and degenerate behavior of rivets in repetitive manner. The bolts were located on the left side of the element, see Fig. 1c.

a) b) c)

d) e)

Fig. 1. Investigated structures: a), b) c) different views of the section of plane wing, d) cantilever aluminium beam with lap joint fixed by bolts, e) CFRP beam in cantilever configuration. Black dot in c) indicates the location of PZT transducer which is on the rear side of the shell, see b).

The considered damage scenarios were:a) damage d1: release of the nut for the bolt in location 1, see Fig. 1c;b) damage d2: release of the nuts for the bolts in location 1, 2;c) damage d3: release of the nuts for the bolts in location 1, 2, 3;

The loosened bolts were remained in their locations simulating improperly working rivets. The piezoelectric transducer (Sonox P5) that one located closer to the left edge of the section in Fig. 1b was used to generate results shown in the paper. The remaining structures were simple cantilever aluminium beam with lap joint fixed by bolts and CFRP dog bone composite, see Fig. 1d, e. The results from these structures were only used to show differences in impedance characteristics between simple and geometrically more complicated structural elements. The frequency step was equal to 1 Hz.The considered damage indicators were:

Root Mean Square Deviation (RMSD)

RMSD (u ,d )=∑i=1

n √ (d i−ui)2

ui2

Chessboard Distance(CB)

CB (u ,d )=Max(|(u−d)|)

Correlation Distance (CD)

CD (u ,d )=1−(u−u )∘(d−d)

√u∘u¿ √d ∘d¿

where uand d denote vectors of the measurement variable for respectively undamaged and damaged structure with components uiand d i, respectively. The subscript i is directly related to frequency which is independent variable in this measurement method. Each component of u is equal to mean value of vector u in order to perform component wise subtraction. The operator ∘ denotes scalar product of two vectors and the operator ¿ denotes complex conjugate of the vector. The variables that can be derived from impedance measurement were: impedance modulus, admittance modulus, phase angle, resistance, reactance, susceptance, conductance, loss coefficient and Q factor. The behaviour of considered damage indicators was dependent on chosen variable. The best damage detection performance of RMSD was obtained using Q factor. For CB and CD the best results were obtained from phase angle and resistance, respectively. The further experiments were performed for the section of the wing placed in temperature chamber for two different temperature levels 30°C and 40°C in order to check the robustness of the damage indicators against temperature variations. Each measurement was repeated three times for the structure in the same mechanical and temperature conditions. The mean curve was calculated from single approved measurements. The temperature variation in the chamber was about 1°C for stable temperature level. It should be mentioned that even such a small temperature variation causes changes in characteristics of real part of electromechanical impedance. That influence manifests as a vertical shift of the characteristics for the lower frequency range (up to few kilohertz). Larger changes of temperature causes frequency shift of peaks and amplitude reduction in characteristic for lower frequencies with growing temperature what corresponds to results from literature e.g.: [16], [17].

2.2 Complicated impedance curves for more geometrically complex structures

The basic assumption of impedance measurements of PZT transducer bonded to the investigated structure is the existence of electromechanical coupling between the transducer and the structure. In particular, the peaks from impedance curves are directly related to vibration modes of simple structures. This dependence is essential to infer correctly about the state of the structure.The comparison of impedance measurements with the vibration modes obtained from laser scanning vibrometry for clamped beam shows that admittance is directly related to frequency response function of the investigated structure [19]. For simple structures like that one in Fig.1d resonance peaks are directly indicated in this curve Fig. 2a. For each curve the determination of frequency and damping ratio is straightforward. The situation is more complicated for geometrically complex structures such as the investigated element of the wing. The peaks on the corresponding curve (see Fig. 2c) except several ones are more wide suggesting overlapping of different vibration modes (mode mixing) and suppressing the use of simplest procedures for calculating damping ratios. Even frequency determination for artificial simple peak composed of two vibration modes can lead to incorrect interpretation (recognition of simple value whilst there are two frequency peak values).

Fig. 2 Comparison of real part of electromechanical impedance for piezoelectric transducer placed on: (a) cantilever beam with bolted joint (transducer close to joint), (b) CFRP beam in cantilever configuration, (c) section of aeroplane wing (trailing edge).

The interpretation problems regarding determination of the two modal parameters as well as their temperature dependence caused that other damage indicators should be proposed.

2.3 Performance of damage indicators for loosened bolts in the section of aeroplane wing.

In Fig. 3 the sensitivity of RMSD to increased level of damage is shown for low (1-10 kHz), high (10-20 kHz) and entire (1-20 kHz) frequency intervals calculate from Q factor variable. Damage was imposed by subsequent loosening of three bolts connecting outer shell and internal rib in the section of aeroplane wing (trailing edge). For stable temperature level RMSD behaves well regardless the considered frequency interval used for calculation of this damage indicator.

a) b)

Fig. 3. Performance of RMSD as damage indicator for two independent measurements done in temperatures 30°C (left column) and 40°C (right column) for increasing damage – measurements on the section of the wing.

The RMSD case gets worse if one considers indication of damage based on two measurements performed at different temperatures, compare Fig. 4. The reference measurement was performed at 30°C, further the damage was introduced into the structure and the structure with damage state at 40°C was measured once again. The first two cases of damage scenario d1 and d2 were obscured by temperature differences. Only the third case d3 had bigger influence on RMSD than temperature variations. The damage indicator more resistant to temperature differences need to be proposed.

Fig. 4. Performance of RMSD as damage indicator between the reference (undamaged) state which was measured at temperature 30°C and the final state measured in 40°C with damage – measurements on the section of the wing panel.

a) b)

Fig. 5 Performance of CB as damage indicator for two measurements done in temperatures 30°C (left column) and 40°C (right column) for increasing damage – measurements on the section of the wing.

The best performance of CB was obtained from phase angle. The CB as damage indicator in constant temperature does not perform better than RMSD especially for intervals containing significant changes in overall trend of the curves. This is evident for 1-10 kHz interval in Fig.5b. The better performance of this damage indicator is obtained in 10-20 kHz frequency range. The better performance of CB over RMSD occurs for the damage introduced between the measurements in temperatures 30°C and 40°C, see Fig. 6. It seems that this damage

indicator is consistent regardless frequency interval and has the best performance of three damage indicators considered here.

Fig. 6 Performance of CB as damage indicator between the reference (undamaged) state which was measured at temperature 30°C and the final state measured in 40°C with damage – measurements on the section of the wing.

a) b)

Fig. 7. Performance of CD as damage indicator for two independent measurements done in temperatures 30°C (left column) and 40°C (right column) for increasing damage – measurements on the section of the wing panel. For undamaged structure the same mean signal was used resulting in CD=0 for that case.

The best performance of CD was obtained from resistance values. In Fig. 7 the sensitivity of CD to increased level of damage is shown for low (1-10 kHz), high (10-20 kHz) and entire (1-

20 kHz) frequency intervals. For stable temperature levels CD behaves well regardless the considered frequency interval used for calculation of this damage indicator.

Fig. 8. Performance of CD as damage indicator between the reference (undamaged) state which was measured at temperature 30°C and the final state measured in 40°C with damage – measurements on the section of the wing panel.

The CD performs well as damage indicator also for measurements performed at different temperatures, compare Fig. 8. The reference measurement was performed at 30°C, further the damage was introduced into the structure and the structure with damage state at 40°C was measured once again. All cases of damage scenario differ from undamaged case. However the robustness of damage indicator depends on the frequency interval considered. The CD damage indicator is less effective in the flat part of the resistance curve (Fig. 2c). This damage indicator seems to be more resistant to temperature differences than the RMSD considered in this paper. The CD seems to be consistent damage indicator as it increases with subsequent damage levels.

3. Conclusions

The considered structural element was the section of aeroplane wing cut off from its trailing edge. The trailing edge was chosen because this part of the wing experiences instabilities caused by detachment of the flow from the wing surface and the appearance of eddies and vortices. These unstable phenomena are responsible for increased local vibrations on the trailing edge and , in turn, could be the cause of fatigue damage in rivets.Generally the PZT transducer must be very close to possible localizations of damage. The distance not longer than few centimetres to the location of damage is the necessary condition for this method to be able to record changes in transducer’s impedance signatures. For such condition the experiments for damage detection using three indicators were performed in different temperatures. The basis assumption used during drawing conclusions was that the subsequent increase of severity of damage should be visible in damage indicator. Such damage indicator is then consistent measure. The performance of damage indicators was strongly dependent on chosen variable vector derived from impedance vector. For the measurements done in constant temperature the damage detection performance of all indicators was good. However, the probability of executing the measurements in the same temperature conditions for real structure is very low. The robustness of damage indicators to

temperature effects is therefore important problem that needs to be addressed. The RMSD did not performed well considering the indication of damage stages under changing temperature. On the other hand CB (and to some extent also CD) properly detected increased level of damage even in different temperature conditions regardless frequency interval considered.

Acknowledgements

This research was partially supported by the project titled: Non-invasive Methods for Assessment of Physicochemical and Mechanical Degradation (PBS1/B6/8/2012) granted by National Centre for Research and Development in Poland. Tomasz Wandowski also would like to acknowledge the support provided by research project IUVENTUS Plus No. IP2011 058971 founded by Ministry of Science and Higher Education from the budget for science in years 2012-2013.

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