MULTI-DISCIPLINARY APPLICATIONS OF PIEZO-SENSORS: STRUCTURAL HEALTH MONITORING, BIO-MECHANICS AND ENERGY HARVESTING S. Bhalla1, R. Suresh, S. Moharana, T. Visalakshi, N. Kaur and S. Naskar
ABSTRACT Piezoelectric materials, especially the ceramic version lead zirconate titanate (PZT), are the among the most widely used smart materials today. This paper presents the state-of-the art in the application of PZT patches in various disciplines of science and technology. The most striking application of the PZT patches in the field of non-destructive evaluation (NDE) is in the form of the electro-mechanical impedance (EMI) technique, and its several variants, the notable being the metal wire based approach. The paper presents the recently developed modelling approaches involving the EMI technique taking into account the PZT-bondstructure interaction. It also covers very recent applications of the PZT patches for foot pressure monitoring (bio-medical application), rebar corrosion assessment (civil engineering application) and energy harvesting (civil/ mechanical/ aerospace application). These striking applications undoubtedly establish the true potential of PZT patches in multi-disciplinary fields.
Keywords: Structural health monitoring (SHM), Lead zirconate titanate (PZT), Electro mechanical impedance (EMI) technique, Bio-mechanics, Energy harvesting.
1
Associate Professor (Corresponding Author), Department of Civil Engineering, Indian Institute of Technology (IIT) Delhi, Hauz Khas, New Delhi ‐ 110 016, (India).Email:
[email protected],Phone: (91)‐11‐2659‐1040, Fax : (91)‐11‐2658‐1117
INTRODUCTION The phenomenon of piezoelectricity occurs in certain classes of noncentro-symmetric crystals, such as quartz, in which electric dipoles (and hence surface charges) are generated due to mechanical deformations. The same crystals also exhibit the converse effect; that is, they undergo mechanical deformations when subjected to electric fields. The constitutive relations for piezoelectric materials for 1D interaction, such as for a piezoelectric plate shown in Fig. 1, are (Shanker et al., 2011) T D3 = ε 33 E3 + d 31T1
S1 =
T1 YE
+ d 31 E3
(1)
(2)
where S1 is the strain in direction ‘1’, D3 the electric displacement over the PZT patch, d31 the piezoelectric strain coefficient and T1 the axial stress in direction ‘1’. Y E = Y E (1 + ηj ) is the complex Young’s modulus of elasticity of the PZT patch at constant electric field and T T ε 33 = ε 33 (1 − δj ) the complex electric permittivity (in direction ‘3’) at constant stress, with
j = − 1 . In these expressions, η and δ respectively denote the mechanical loss factor and the dielectric loss factor of the PZT material. Equation(1) is used in sensing applications and Equation (2) in actuation applications of the piezo-electric materials.
3
E3
2 1
w T1
h
l
Fig. 1. A piezoelectric plate under the action of stress and electric field (1D interaction).
The EMI technique makes use of both the direct and the converse effects simultaneously. The governing equation of the EMI technique is (Bhalla and Soh, 2004) 2 E 2d 312 Y E l 2 ⎡ T 2d 31 Y + Y = G + Bj = 4ωj ⎢ε 33 − h ⎢ (1 −ν ) (1 − ν ) ⎣
⎛ Z a ,eff ⎜ ⎜Z ⎝ s ,eff + Z a ,eff
⎞ ⎤ ⎟T ⎥ ⎟ ⎥ ⎠ ⎦
(3)
where Za,eff is the effecive mechanical impedance of the PZT patch and Zs,eff that of the host structure.
T
is the complex tangent ratio, ideally equal to {tan(κl)/κl}, with
κ = ω ρ (1 − ν 2 ) / Y E being the wave number. Any damage to the host structure will alter Zs,eff.
Assuming that other parameters in the equation are not influence, the damage will get detected easily as change in the admittance signature. The forthcoming sections describe applications of PZT patches in multiple engineering and technological domains.
BIO-MEDICAL APPLICATIONS Bhalla and Suresh (2013) successfully demonstrated the potential of the EMI technique (which is conventionally employed for SHM of structural systems) for monitoring condition of bones, covering detection of fracture as well as the healing process that follows. In addition to detecting cracks and fracture, the conductance signatures of the PZT patches could suitably detect changes occurring in bone density, both increase as well as decrease. Suresh et al. (2015) demonstrated the proof-of-concept experimentation to measure foot pressure distribution using PZT patches and fibre-Bragg grating (FBG) sensors (Fig. 2). The PZT sensors carried out the measurement using d33 coupling, acting as sensors. A pressure resolution up to 0.89 kPa was achieved with the PZT sensors. The FBG sensors employed a special arch type configuration for higher sensitivity. The pressure values measured by the two sensors were comparable in nature. The pressures at two locations (forefoot/heel) were found to increase with the walking speed. The PZT patches, which can measure the pressure with a high sampling rate (typically few kHz per second), can provide near real time pressure
measurement and are most suitable for fast walking speeds, typically higher than 3 kmph. The FBG sensors, on the other hand, are suitable for both static as well as low frequency dynamic measurements typically less than 3 Kmph. Acting in synergy, both the PZT and the FBG sensors enabled measuring the foot pressure in wide speed range, starting from static case to high speed walking.
FBG sensor
PZT patch
(a)
(b)
Fig. 2 Foot pressure measurement using a combination of PZT and FBG sensors. (a) Shoes instrumented with both sensors (b) Test under progress
MODELLING PZT-BOND-STRUCTURE INTERACTION Bhalla and Moharana (2013) developed a refined model to take into account the interaction between a PZT patch and the host structure through the medium of an adhesive bond layer. This model duly considered the elastodynamic aspects of problem incorporating both shear lag as well as inertia terms in the framework of lumped impedance. Later, Moharana and Bhalla (2014) further improved the model by considering the variation in continuum rather
than a lumped impedance approach. The results of the continuum approach are so far the best reported analytical results as far as closeness to the experimental results is concerned (Fig. 3).
1.00E-01
hs/hp = 0.417 6.00E-02
hs/hp = 0.417
8.00E-02
Susceptance (S/m)
Susceptance (S/m)
8.00E-02
4.00E-02
2.00E-02
hs/hp = 0.834
6.00E-02
4.00E-02
hs/hp = 0.834
2.00E-02
0.00E+00 0
50
100
150
200
250
0.00E+00 0
Frequency (kHz)
(a)
50
100
150
200
250
Frequency (kHz)
(b)
Figure 3 Comparison of susceptance of experimental result with continuum shear lag model (a) Normalized analytical susceptance (continuum model) for hs/hp=0.417 and hs/hp=0.834 (b) Normalized experimental susceptance for hs/hp=0.417 and hs/hp=0.834
REBAR CORROSION MONITORING Talakokula et al. (2014) presented the first ever comprehensive monitoring of rebar corrosion using the EMI technique (Fig. 4). Through accelerated corrosion testes on RC specimens, it was demonstrated that PZT patches bonded to rebars could provide information regarding the extent of corrosion (initiating stage, propagation stage or splitting stage) in terms of piezo identified stiffness. The approach has several advantages in comparison with conventional corrosion detection techniques.
Steel bar (Anode) Copper rod (Cathode) --
PZT patch Brine solution Brine Brine sBBrimnnfion solution
+ + +
Constant power supply device
Concrete
(a)
(b)
Fig. 4 Rebar corrosion detection using EMI technique (a) Experimental set-up (b) Condition of a specimen after 120 days of accelerated corrosion exposure
ENERGY HARVESTING APPLICATIONS Energy harvesting is another fascinating multi-disciplinary application of PZT patches. Conventionally, built up configurations, such as stack arrangement or secondary structures are employed for energy harvesting purposes. These are not only cumbersome, but also expensive and interfere with functional service of the structure. Kaur and Bhalla (2014a, b) evaluated the possibility of employing normal thin PZT patches for this purpose and developed a coupled electro-mechanical model for this purpose. The model was first extended to a typical real-life city flyover, modelled through FEM, whereby it was estimated that two day operation of the PZT patch for energy extraction is sufficient enough to enable one time operation of AD5933 for acquisition of structural health monitoring (SHM) related data in surface-bonded configuration. The coupled electro-mechanical model for surfacebonded and embedded patches were extended to eight real-life bridges across the world. It has been estimated that Low power consuming circuits like typical A/D convertor, such as TMP 112 (Texas Instruments, 2014) would warrant energy harvesting for 1 second and 2.5 minutes when powered by a surface bonded PZT patch on steel bridge and an embedded CVS
in a RC bridge, respectively. Hence, using the PZT patch both for SHM and energy harvesting in real-life structures is certainly feasible. The same patch, which scavenges energy during the idle period, could be used for SHM as and when warranted (Fig. 5). With the ongoing developments in electronics, as lesser power consuming circuits are emerging, it is believed that the energy scavenging time will drastically come down.
Fig. 5 Concept of dual use of PZT patch for energy harvesting as well as SHM (Kaur and Bhalla, 2014a)
NEW VARIANTS OF EMI TECHNIQUE Naskar and Bhalla (2014) successfully demonstrated the feasibility of indirect excitation of structure using PZT patches bonded to metal wires (Fig. 6). This enables monitoring of structures under inaccessible and hazardous conditions. In addition, the proposed approach warrants minimum number of sensors as compared to conventional array configuration.
Fig. 5 Metal wire based variant of EMI Technique (Naskar and Bhalla, 2014)
CONCLUSIONS This paper has presented the state-of-the-art in the multi-disciplinary applications of PZT patches covering wide ranging fields such as SHM, bio-mechanics and energy harvesting. These highlight the high potential of piezo materials in modern scientific and technical applications.
REFERENCES 1. BHALLA, S. AND MOHARANA, S. (2013) A Refined Shear Lag Model for Adhesively Bonded Piezo-Impedance Transducers. Journal of Intelligent Material Systems and Structures, 24(1), pp 33-48. 2. BHALLA, S. AND SOH C. K. (2004) Structural Health Monitoring by Piezo-Impedance Transducers. Part I Modeling. Journal of Aerospace Engineering, ASCE, 17(4), pp 154165. 3. BHALLA, S. AND SURESH, R. (2013) Condition Monitoring of Bones using PiezoTransducers. Meccanica 48(9), pp 2233-2244. 4. KAUR, N. AND BHALLA, S. (2014a) Combined Energy Harvesting and Structural Health Monitoring Potential of Embedded Piezo-Concrete Vibration Sensors. Journal of Energy
Engineering,
ASCE,
accepted
on
08
July
2014,
under
press.
DOI: 10.1061/(ASCE)EY.1943-7897.0000224 5. KAUR, N. AND BHALLA, S. (2014b) Feasibility of Energy Harvesting from Thin Piezo Patches via Axial Strain (d31) Actuation Mode. Journal of Civil Structural Health Monitoring 4(1), pp 1-15.
6. MOHARANA, S. AND BHALLA, S. (2014) A Continuum Based Modelling Approach for Adhesively Bonded Piezo-Transducers for EMI Technique. International Journal of Solids and Structures 51(6), pp 1299-1310. 7. NASKAR, S. AND BHALLA, S. (2014) Experimental Investigations of Metal Wire Based EMI Technique for Steel Structures. Proceedings of 7th ISSS Conference on Smart Materials Structures and Systems (ISSS 2014), 08-11 July, Bangalore, paper no 1569922649.
8. SHANKER, R., BHALLA, S. AND GUPTA, A. (2011) Dual Use of PZT Patches as Sensors in Global Dynamic and Local EMI Techniques for Structural Health Monitoring. Journal of Intelligent Material Systems and Structures, 22(16), pp 1841-1856. 9. SURESH, R., BHALLA, S., SINGH, C., KAUR, N., J. HAO AND ANAND, S. (2015) Combined Application of FBG and PZT Sensors for Plantar Pressure Monitoring at Low and High Speed Walking. Technology and Health Care 23(1), pp 47-61. 10. TALAKOKULA, V., BHALLA, S., AND GUPTA, A. (2014) Corrosion Assessment of RC Structures Based on Equivalent Structural Parameters Using EMI Technique. Journal of Intelligent Material Systems and Structures 25(4), pp 484-500.
