INTRODUCTION:

The Electro Mechanical impedance (EMI) technique, which employs piezoelectric Ceramics(PZT) patches as impedance transducers, has emerged as a powerful NDE Technique in last few years. Inspite of substantial improvements in material strength and increased accuracy in analysis in Design and Analysis with the aid of digital computers, gradual detoriation of structures during prolonged usage cannot be completely ruled out. This has been proven by many recent accidents such as sudden Bridge Collapses and in-flight break downs of Aircrafts and shuttles, causing immense loss in lives and properties. Hence, the idea of equipping structures with sensors and actuators in an attempt to impart “smartness” has great potential in cost-effective predictive maintenance of structures, particularly for high performance components not easily accessible for manual inspections.

Structural safety is an evolutionary accomplishment, and Structural Health Monitoring (SHM) is the key to its achievement. It is an area of growing interest and worthy of new and innovative approaches. SHM assesses the state of structural health and, through appropriate data processing and interpretation, may predict the remaining life of the structure. Many aerospace and civil infrastructure systems are at or beyond their design life; however, it is envisioned that they will remain in service for an extended period. SHM is one of the enabling technologies that will make this possible. It addresses the problem of aging structures, which is a major concern of the engineering community. Finally, significant developments have been made regarding deterioration mechanisms and environmental loads on structures. These developments open the way for a wide range of applications related to efficient operation and maintenance of structures. The overall aims for structural monitoring systems include one or several of the following main objectives;

 To ensure safe structures.

 To obtain rational and economic maintenance planning.

 To attain safe and economic operation.

 To identify causes for unacceptable responses

SHM Principles

The SHM process is defined in terms of a four step statistical pattern recognition paradigm as described in Phil. Trans. R. Soc. A article (Farrar et al.).

This process involves the following steps:

1. Operational evaluation

2. Data acquisition, normalization and cleansing,

3. Feature selection and information condensation.

4. Statistical model development for feature discrimination

Operational evaluation answers about life safety and/or economic justification for performing SHM, definition of damage for a system, operational and environmental conditions under which the system is monitored and limitations on acquiring data in the operational environment.

The statistical models are also used to minimize false indications of damage.

False indications of damage fall into two categories:

(1) false-positive damage indication (indication of damage when none is present) and

(2) false-negative damage indication (no indication of damage when damage is present).

Errors of the first type are undesirable, as they will cause unnecessary downtime and loss of revenue as well as loss of confidence in the monitoring system. More importantly, there are clear safety issues if misclassifications of the second type occur.

ELECTROMECHANICAL IMPEDANCE (EMI) METHOD

The surface bonded piezoelectric patches, because of their inherent direct and converse mechatronic coupling, can be effectively utilized as mechatronic impedance transducers (MITs) for SHM

The MIT-based technique has evolved during the last 8 years and is commonly called the electro-mechanical impedance (EMI) technique In principle, this technique is similar to the conventional ultrasonic wave propagation techniques but it uses low-cost transducers, which can be permanently bonded to the structure and can be interrogated without removal of any finishes, or rendering the structure temporarily unusable. No complex data processing or any expensive hardware is necessary in this technique. The data is directly acquired in the frequency domain as opposed to the time domain as in the case for conventional ultrasonic techniques.

DESCRIPTION OF EMI TECHNIQUE

The transducers used in the EMI technique are made up of piezoelectric materials, such as Lead Zirconate Titanate (PZT), often referred to as piezoceramic patches or PZT patches. These materials generate surface charges in response to applied mechanical stresses. Conversely, they undergo mechanical deformations in response to applied electric fields. This unique capability enables the material to be used both as a sensor and as an actuator, and eventually as an MIT. In the EMI technique, a PZT patch is bonded to the surface of the monitored structure by means of high-strength epoxy adhesive and electrically excited by an impedance analyser. In this configuration, the patch behaves as a thin bar undergoing axial vibration and mechanically interacting with the host structure, as shown in Fig. 1(a). The PZT patchhost structure system can be equivalently represented by a mechanical impedance Z connected to an axially vibrating thin bar, as shown in Fig. 1(b). In this figure, the PZT patch has half-length la; width wa and thickness ha; and it expands/contracts dynamically in direction „1‟ due to an alternating electric field in direction, 3. It can be assumed to be infinitesimally small and possessing negligible mass and stiffness as compared to the host structure. Hence, the two end points of the patch can be assumed to encounter equal mechanical impedance Z (from the host structure). Under these conditions, the patch invariably has zero displacement at the mid-point, irrespective of its location on the host structure.

Figure 1(a). A PZT patch bonded to structure. (b) Interaction model of one half of the PZT patch and the host structure.

Liang et al. solved the governing onedimensional wave equation for the generic system comprising one half of the patch and the structur (Fig. 6.1(b)), using the impedance approach. Using Liang‟s generic derivation, the following expression can be written for the complex electro-mechanical admittance _Y (inverse of electrical impedance), of the coupled system shown in Fig. 6.1(a)

Where d31 is the piezoelectric strain coefficient,

Y11E the complex Young‟s modulus of the PZT patch at constant electric field,

T33 the complex electric permittivity of the PZT material at constant stress,

Z the mechanical impedance of the structural system, v the angular frequency

Za the mechanical impedance of the PZT patch, k the wave number..

The mechatronic coupling represented by Eq. (6.1) is utilized in damage detection in the EMI technique (and hence the name mechatronic impedance transducer or MIT for the PZT transducers). The mechanical impedance Z in this equation is a function of the structural parameters, i.e. the stiffness, the damping and the mass. Any damage to the structure will cause these parameters to change, and hence changes the drive point mechanical impedance Z: Consequently, as can be seen from Eq. (6.1), the electro-mechanical admittance, Y; will undergo change, and this serves as an indicator of the state of health of the structure. Measuring Z directly may not be feasible practically, but Y can be easily measured by using an electrical impedance analyzer. The measured admittance is a complex quantity consisting of real and imaginary parts, the conductance (G) and the susceptance (B); respectively, and its unit is Siemens (ohm-1). The imaginary part has very weak interaction with the structure. The real part, on the other hand, actively interacts with the structure and is, therefore, preferred in SHM applications. A plot of G over a sufficiently wide band of frequency serves as a diagnostic signature of the structure and is called the conductance signature.

Fig. 2(a) shows an aluminum beam, 200 X 25 X 2 mm3 in size, instrumented with two circular PZT patches (marked „1‟ an d „2‟), 10 mm in diameter, at a distance of 60 mm from each end. Fig. 2(b) shows the conductance signature acquired from the PZT Patch in the frequency range 120–140 kHz. The sharp peaks in the signature correspond to the modes of vibration of the structural system. Thus, the conductance sig-nature „identifies‟ the local structural system in the vicinity of the patch. Also shown in Fig. 2(b) is the effect of an incipient damage, a hole of 5 mm diameter, at the mid point of the beam, on the conductance signature. It is clearly evident from Fig. 2(b) that the induced incipient damage, which amounts to a negligible loss of mass and stiffness of the structure, has significant effect on the conductance signature. Hence, any change in the „identified‟ local system due to any damage manifests as a change in the conductance signature of the patch, thereby giving an indication of the damage.

Figure 2 Illustrating the application of EMI technique for NDE. (a) An aluminum beam instrumented with two PZT patches (marked „1‟ and „2‟). (b) Effect of damage on conductance signature of PZT Patch 1.

SIGNAL PROCESSING TECHNIQUES AND DAMAGE QUANTIFICATION

As is apparent in Fig.6.2(b), the prominent effects of damage on the conductance signature are the appearance of new peaks in the signature, and lateral and vertical shifts of the peaks. These changes are the main indicators of damage in the vicinity of the PZT patch. Many pattern recognition techniques have been reported to quantify changes occurring in the frequency response functions of structures (similar to conductance signatures in the EMI technique) due to damage; such as the waveform chain code (WCC) technique, the signature assurance criteria (SAC), the equivalent level of degradation system and the adaptive template matching (ATM). Many similar statistical techniques have been proposed by researchers working on the EMI technique; such as the root mean square deviation (RMSD), relative deviation, and the difference of transfer function between damaged and undamaged conditions.

The RMSD algorithm was found to be the most robust and the most convenient. Hence, RMSD, which was used in the previous case studies, is also adopted in the present investigation.

The RMSD deviation in signatures is defined as

Where

G1j is the post-damage conductance at the jth measurement point and

G0j is the corresponding pre-damage value.

APPLICATION OF EMI TECHNIQUE FOR DIAGNOSIS OF SEISMIC/BLAST INDUCED DAMAGES

The test structure was a two-storeyed portal frame, made of RC. The details of the scaled structural model are shown in Fig. 6.3. The model represented a prototype frame with storey height 2.9 m and span length 3.3 m, at a scale of 1:10. Also shown in the figure are the cross-sectional sizes of the various structural members, i.e. the beams and the columns. The test model was also characterized by a floor slab protruding 100 mm from the faces of the two floor beams. The proportion of the various members was determined in accordance with the basic model similitude rules for scaling , not only for the member sizes, but also for the size of reinforcement bars as well as the aggregate used in concrete. An enlarged base plate was provided for the anchorage of the column reinforcing bars and to allow for proper attachment of the test model to the shaker. The model structure was constructed using microconcrete and model reinforcement. The concrete had a characteristic strength of 30 MPa, and the reinforcement (3 mm deformed bars) had yield strength of 200 GPa. The shaker was an electromagnetic shaking table, rated to a maximum acceleration of 120 g and a maximum frequency of 3000 Hz.

In order to monitor the state of the test frame during the loading process, it was instrumented with two PZT patches, shown in Fig. 6.3. as Patches #1 and #2. The patches were bonded to the structure using the RS 850-940 epoxy adhesive . The optimal locations of the sensors were decided, based on the theory of structural engineering, to be the locations of peak bending moment and shear. Patch #1 was instrumented on the first floor beam, very close to the beam-column joint, a location very critical from the point of view of shear cracks. Patch #2 was instrumented at the bottom face of the second floor beam, near the mid point, a location critical from the point of view of flexural cracks. Both the patches were 10 mm square and 0.2 mm thick (Fig. 3), conforming to grade PIC 151 (PI Ceramic). Flexural and shear cracks are the most common types of damage in RC structures and it was intended to evaluate the PZT patches in diagnosing these damage types. The test structure was also instrumented with accelerometers at the base and roof levels (to measure the accelerations), with LVDTs at the base and roof levels (to measure the displacements), and with strain gauges (to record the concrete strain). This part of the instrumentation was done by the research group of Professor H. Hao and Professor Y. Lu, which was interested in monitoring the condition of the structure by means of conventional low frequency vibration-based techniques.

Fig 3 : EQ load setup

APPLICATION OF SEISMIC LOADING

The test loads were applied in the form of vertical base motions of varying frequencies and amplitudes, as the buildings are normally subjected to such base motions when excited by underground explosions or earthquakes. The test was performed in eight phases divided according to the range of the base motion frequencies and the velocity and acceleration amplitudes. The applied base motions are graphically depicted in Fig.6.4. The intermediate states between consecutive vertical loading phases are referred to as States 1, 2, 3,…,8 in the figure. Based on stress analysis, the concrete was expected to remain uncracked up to State 3 and cracks were expected to appear from State 4 onwards. After each excitation, the patches were scanned to acquire their conductance signatures. The frequency range 100–150 kHz was found to contain several peaks, and was selected as the appropriate frequency band for this particular test (as per the empirical guideline of Sun et al. The bonded PZT actuator/sensor patches were connected to the HP 4192A impedance analyzer for the acquisition of their conductance signatures. The impedance analyzer was controlled by a personal computer via a GPIB interface. At any stage of the test, the acquired signature was compared to the baseline signature and the damage was quantified using the RMSD index defined by Eq. (6.2).

FLEXURAL DAMAGE CHARACTERIZATION BY PZT PATCH #2

In Fig. 6.5 shows the signatures of Patch #2 at various intermediate stages during the loading process. The peaks are not as prominent as those of the signature acquired from the aluminum beam shown in Fig. 2(b). This is due to lower stiffness of concrete as compared to aluminum, and to the presence of higher damping in concrete.

From States 1 to 3, only minor deviations were observed in the signature as compared to the baseline signature, suggesting that the localized sensing zone of the patch was still very much intact.

From Phase 4 onwards, the velocity and acceleration amplitudes were increased so as to cause flexural cracking in the concrete, especially around the midpoint of the second floor beam. At State 4, a very prominent vertical shift was observed in the signature, suggesting damage in the surroundings of the patch. However, there was no visible sign of external damage at this stage. This trend in the signature further continued from States 4 to 6, and very prominent and sequential vertical shifts were observed. The area around the patch was continuously monitored for the occurrence of cracks, and observable cracks were first detected at State 6. Thus, the patch signature indicated damage much earlier than it was visible to the naked eye. The sudden jump in the signature at State 4 was probably due to the occurrence of micro-cracks at the very onset of damage (strain level at roughly 300–400 micro strain near the PZT Patch #2). This incipient damage finally took the form of visible flexural cracks at State 6. The conductance signature at this state showed a prominent downward shift. A similar shift was again observed at State 8.

SHEAR DAMAGE CHARACTERIZATION BY PZT PATCH #1

From the graph in Fig. 4 shows the signatures acquired from Patch #1, which was instrumented to monitor the occurrence of shear cracks at the critical beam-column joint region. It was observed that from the Baseline State to State 6, the signatures underwent only minor changes. At State 7, a prominent shift was observed in the signature, as can be seen from Fig. 5. At State 8, a sudden and more prominent vertical shift of the signature was observed. Close examination of the region surrounding the patch showed the presence of a hairline shear crack near the beam-column joint. The patch signature, however, provided an indication of this deterioration much earlier, at State 7, which manifested as a prominent signature pattern change (Fig. 5).

Figure 4. Conductive signatures of patch #2 at various stages of test.

Figure 5. Conductive signatures of patch #1 at various stages of test.

FUTURE WORKS

The following works are to be done in the 2nd stage of my project,

1. Liang et al Electro Mechanical Impedance relation will be reviewed. And to predict the Mechanical Impedance of the structure by considering critical Forces and Velocities at major points in a structure, thereby reducing the complex analysis.

2. Coupling of Piezoelectricity and Electro-mechanical components by considering the Stiffness and Flexibility of the Structure.

3. Study of existing PZT-Structure Interaction models.

4. Work on Advantages and Limitations of Existing modelling Approaches.

5. Define Electromechanical Impedance and to derive Electro Mechanical admittance Formulations based on Effective Impedance.

6. Determination of Effective Drive Point(EDP) Impedance by Finite Element Method.

7. Modelling of Structural Damping.

8. If Time permits, Experimental Verification will be done and will be compared with Theoretical values.

REFERENCES:

1. S.O Reza Moheimani and Andrew J.Fleming, Text Book of Piezoelectric Transducers for Vibration Control and Damping, Springer Publications 2006.

2. Sudhakar A Kulkarni and Kamal M Bajoria, Large deformation analysis of piezolaminated smart structures using higher-order shear deformation theory, Smart Materials and Structures, 2007 1506–1516.

3. Suresh Bhalla, Chee Kiong Soh, High frequency piezoelectric signatures for diagnosis

1. of seismic/blast induced structural damages, NDT&E International 37 (2004) 23–33

4. Jürgen Nuffer, Thilo Bein Fraunhofer, Application of Piezoelectric Materials in Transportation Industry, Institute for Structural Durability and System Reliability, LBF Bartningstrasse 47 64281 Darmstadt Germany 2006.

5. A.V. Srinivasan and D. Michael Mc Farland , Text Book of Smart Structures-Analysis and Desing; Cambridge University Press 2000.

6. ANSI/IEEE Std 176-1987 IEEE Standard on Piezoelectricity, The Institute of Electrical and Electronic Engineers 1987.

7. Scott W.D., Farrar R.F., Prime M.B. and Shevitz D.W. (1996) – Damage identification and health monitoring of structural and mechanical systems for changes in their vibration characteristics: a literary review. Technical Report, LA-13070-MS, UC 900, Los Alamos National Laboratory, USA.

8. S. W. Doebling, L. D. Peterson, K. F. Alvin- Experimental determination of Local structural stiffness by Disassembly of measured Flexibility matrices, Journal of Vibration and Acoustics Copyright © 1998 by ASME October 1998, Vol. 120 / 949.

9. Charles R. Farrar and Keith Worden - An introduction to structural health monitoring, Phil. Trans. R. Soc. A (2007) 365, 303–315.

10. James Brownjohn, United Kingdom, Swee-Chuan Tjin, Guan-Hong Tan, Boon-Leong Tan, Singapore and Sushanta Chakraboorty, United Kingdom - A Structural Health Monitoring Paradigm for Civil Infrastructure.

11. Charles R. Farrar and Scott W. Doebling - An overview of modal-based damage identification methods, Engineering Analysis Group, Los Alamos National Laboratory, Los Alamos, NM.

Received on 01.11.2015 Accepted on 02.12.2015

Int. J. Tech. 5(2): July-Dec., 2015; Page 291-296

DOI: 10.5958/2231-3915.2015.00038.3