robust flexible capacitive surface sensor for structural
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Civil, Construction and Environmental Engineering Publications
Civil, Construction and Environmental Engineering
7-2013
Robust Flexible Capacitive Surface Sensor for Structural Health Robust Flexible Capacitive Surface Sensor for Structural Health
Monitoring Applications Monitoring Applications
Simon Laflamme Iowa State University, laflamme@iastate.edu
Matthais Kollosche Universität Potsdam
Jerome J. Connor Universität Potsdam
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Robust Flexible Capacitive Surface Sensor for Structural Health Monitoring Robust Flexible Capacitive Surface Sensor for Structural Health Monitoring Applications Applications
Abstract Abstract Early detection of possible defects in civil infrastructure is vital to ensuring timely maintenance and extending structure life expectancy. The authors recently proposed a novel method for structural health monitoring based on soft capacitors. The sensor consisted of an off-the-shelf flexible capacitor that could be easily deployed over large surfaces, the main advantages being cost-effectiveness, easy installation, and allowing simple signal processing. In this paper, a capacitive sensor with tailored mechanical and electrical properties is presented, resulting in greatly improved robustness while retaining measurement sensitivity. The sensor is fabricated from a thermoplastic elastomer mixed with titanium dioxide and sandwiched between conductive composite electrodes. Experimental verifications conducted on wood and concrete specimens demonstrate the improved robustness, as well as the ability of the sensing method to diagnose and locate strain.
Keywords Keywords CNDE, Strain gauge, Structural health monitoring, Strain monitoring, Capacitive sensor, Dielectric polymer, Stretchable sensor, Flexible membrane, Sensing skin
Disciplines Disciplines Structural Engineering
Comments Comments This is an author's final manuscript of an article from Journal of Engineering Mechanics 139 (2013): 879–885, doi:10.1061/(ASCE)EM.1943-7889.0000530. Posted with permission.
Authors Authors Simon Laflamme, Matthais Kollosche, Jerome J. Connor, and Guggi Kofod
This article is available at Iowa State University Digital Repository: https://lib.dr.iastate.edu/ccee_pubs/17
Robust Flexible Capacitive Surface Sensor for Structural Health1
Monitoring Applications2
Simon Laflamme 1, A.M. ASCE
Matthias Kollosche 2
Jerome J. Connor 3, F. ASCE
Guggi Kofod 4
3
ABSTRACT4
Early detection of possible defects in civil infrastructure is vital to ensure timely mainte-5
nance and extend structure life expectancy. A novel method for structural health monitoring6
based on soft capacitors was recently proposed by the authors. The sensor consisted of an7
off-the-shelf flexible capacitor that could be easily deployed over large surfaces, with the8
main advantages of being cost-effective, easy to install, and allowing simple signal process-9
ing. In this paper, a capacitive sensor with tailored mechanical and electrical properties is10
presented, resulting in a greatly improved robustness while retaining measurement sensitiv-11
ity. The sensor is fabricated from a thermoplastic elastomer mixed with titanium dioxide,12
then covered with conductive composite electrodes. Experimental verifications conducted on13
wood and concrete specimens demonstrate the improved robustness, as well as the ability of14
the sensing method to diagnose and locate strain.15
Keywords: structural health monitoring, strain monitoring, capacitive sensor, dielectric16
polymer, stretchable sensor, flexible membrane, sensing skin17
1Department of Civil, Construction, and Environmental Engineering, Iowa State University, Ames, IA,50011
2Applied Condensed-Matter Physics, Institute of Physics and Astronomy, University of Potsdam, D-14476Potsdam, Germany
3Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge,MA, 02139 USA
4Applied Condensed-Matter Physics, Institute of Physics and Astronomy, University of Potsdam, D-14476Potsdam, Germany
1
INTRODUCTION18
Structural health monitoring (SHM), defined as methods for damage diagnosis and local-19
ization, is expected to play a predominant role in management of large-scale infrastructure.20
Their long lifespan, often combined with insufficient maintenance and increased utilization,21
enhances the risks associated with their failure (Karbhari 2009; Harms et al. 2010). In the22
U.S., the number of bridges classified as in need of repairs, due either to deficiency or obso-23
lescence, is excessive, and estimated costs of repairs is beyond 2 trillions USD (Mason et al.24
2009; Simon et al. 2010). Timely maintenance and inspection programs are essential in25
improving health and ensuring safety of civil infrastructure (Brownjohn 2007), consequently26
enhancing their sustainability.27
The complexity of damage diagnosis and localization in large-scale infrastructure arises28
from their inherent size, which impacts monitoring solutions both technically and economi-29
cally. Damage localization techniques, such as non-destructive evaluation using pulse-echo,30
dynamic response, or acoustic emission, are too expensive to deploy over the entire moni-31
tored infrastructure for complete monitoring. In fact, known complete SHM systems rapidly32
become economically unfeasible as the scale is increased.33
The authors recently proposed a novel monitoring technique for civil structures (Laflamme34
et al. 2012a), the sensing skin, consisting of soft elastomeric capacitors arranged in a matrix35
form which could be deployed over an entire structure. This low-cost approach is capable36
of discretely monitoring changes in strain over large areas without compromising structural37
integrity, a combination of properties hardly found in other SHM technologies. It is anal-38
ogous to biological skin, where local changes can be monitored over an entire surface, and39
therefore represents one possible solution to the complexity of applying damage localization40
methods globally.41
The skin-type property of bio-inspired sensing methods arises from the use of flexi-42
ble films, for which elastomeric substrates are common (Lumelsky et al. 2001). For in-43
stance, Harrey et al. (Harrey et al. 2002) researched a humidity sensor using polymide and44
2
polyethersulphone (PES) substrates. Zhang et al. (Zhang et al. 2003) and Hurlebaus & Gaul45
(Hurlebaus and Gaul 2004) developed sensor skins using polyvinylidene (PVDF) substrates.46
Carlson et al. (Carlson et al. 2006) and Tata et al. (Tata et al. 2009) opted for a poly-47
imide substrate. Lipomi et al. (Lipomi et al. 2011) developed a stretchable capacitor made48
from conducting spray-deposited films of single-walled carbon nanotubes on polydimethyl-49
siloxane (PDMS). Carbon nanotubes have also been used as resistance-based strain sensors50
(Gao et al. 2010), and for damage localization by measuring changes in electrical impedance51
tomographical conductivity (Loh et al. 2009). Here, carbon nanotubes (carbon black) are52
added to the SEBS substrate to form the sensor electrodes.53
Other bio-inspired solutions have been proposed using stiff ceramic piezoelectric (PZT)54
sensors. Often used in civil engineering for dynamic strain measurements (Zhang et al.55
2003; Lin et al. 2009; Giurgiutiu 2009), PZT patches can be arranged in an array to form56
piezoelectric wafer active sensors (PWAS), which allows damage localization (Zhang 2005;57
Yu and Giurgiutiu 2009; Zagrai et al. 2010; Yu et al. 2011). Fiber optic-based health58
monitoring techniques are also commonly used in the field. Their concept is analogous to a59
nervous system, where fiber optic sensors can be deployed linearly for damage localization.60
Large-scale deployments of fiber optic systems have been studied in Ref. (Chen et al. 2005;61
Zou et al. 2006; Lin et al. 2007).62
Previously, the sensing skin was constructed using a commercially available silicone63
elastomer-type material (Danfoss PolyPowerTM) with pre-defined smart metallic electrodes.64
Results showed that the sensing solution was promising at monitoring large-scale surfaces,65
capable of both detecting and localizing cracks on concrete specimens (Laflamme et al. 2010;66
Laflamme et al. 2012a). Despite these promising results, practical issues arose due to the67
concurrence of relatively low stiffness and thickness (80 µm) of the sensing material. In the68
experiments presented by the authors in Ref. (Laflamme et al. 2012a), practical challenges69
arose in the electrical contact on the electrodes, and small surface features on the tested70
concrete specimens sometime led to tears in the sensing patches. Also, a few months after71
3
the experiments were conducted and the results published, the authors noticed degradation72
of the silver electrodes due to environmental condition. These issues inspired the authors’73
efforts to develop a robust capacitive-based sensor using the same monitoring principle, with74
the objective to improve the applicability, robustness, and life cycle of the sensing technology75
for broad implementation onto civil infrastructure.76
As it will be demonstrated in this paper, the replacement of materials results in a greatly77
enhanced robustness, a fundamental property for SHM solutions applied to civil infras-78
tructure. Here, both the dielectric elastomer and the sensing electrodes are replaced with79
nanocomposites based on a thermoplastic elastomer and the sensor itself has a higher thick-80
ness. It follows that this paper also demonstrates the generality of the sensing principle, and81
the practical opportunities for improving the technology.82
The paper is organized as follows. The next section describes the robust capacitive-based83
sensor. The sensing principle is discussed, the sensor fabrication process is briefly described,84
and the proposed sensor is verified. The subsequent section verifies the measurement method85
in a laboratory setup. Here, the sensor robustness is demonstrated, along with its capacity86
to diagnose and locate damages on wood and concrete specimens. The last section concludes87
the paper.88
ROBUST CAPACITIVE-BASED SENSOR89
Sensing Principle90
The sensing skin consists of several individual capacitive sensors, termed sensing patches,91
which can have individually shaped areas. A single sensing patch is constructed using a soft92
incompressible polymer membrane covered by highly compliant and stretchable electrodes93
to form a stretchable capacitor.94
The capacitance C of such a sensing patch depends on the surface area A, the thickness95
d, the vacuum permittivity ε0, and the permittivity of the polymer εM:96
C = ε0εMA/d (1)
4
The change in the geometry of the capacitor due to deformation is directly related to its97
capacitance, allowing measurement of high precision strain responses.98
Fig. 1 illustrates a sensor patch deployed over a monitored surface. In the figure, a crack99
causes a change ∆A in the sensor area, which can be measured as a change in capacitance100
(Laflamme et al. 2010; Kollosche et al. 2011). A differential measurement mode can also be101
employed in which two sensors are compared directly (Laflamme et al. 2010).102
Sensor Fabrication103
The dielectric material for the capacitive sensor is a nanocomposite with soft dielectric104
elastomer and ceramic insulating nanoparticles. The elastomer matrix is poly-styrene-co-105
ethylene-co-butylene-co-styrene, (SEBS - Dryflex 500120, ρ = 930 kg/m3), provided by VTC106
Elastoteknik AB (Sweden). The incompressible thermoplastic SEBS is a soluble tri-block107
copolymer, which forms a reversible physically crosslinked elastomer network (Laurer et al.108
1996). The initial stiffness of the SEBS is Y = 312 kPa (Kollosche et al. 2009) and the109
permittivity of the pure material is εM = 2.2 (McCarthy et al. 2009). In this study the110
handling and robustness of the capacitive sensors is improved by increasing the sensor film111
thickness in the range of 200 µm to 400 µm. The increase in thickness leads to a reduction in112
basic capacitance (Eq. (1)), which can be adjusted by increasing the permittivity εM through113
addition of nanoparticles. The authors have demonstrated a significant sensing enhancement,114
approximately 46 times, aimed towards sensing skin for robots; this was achieved by grafting115
a small amount of highly polarizable polyaniline (PANI) on the matrix elastomer backbone116
to form a molecular composite (Stoyanov et al. 2010; Kollosche et al. 2011).117
Here, an elastic nanocomposite of SEBS and ceramic nanoparticles of rutile titanium118
dioxide TiO2 (R 320 D, ρ = 4200 kg/m3, Sachtleben Chemie GmbH, Germany) is proposed119
to form a soft capacitive-based sensor for direct application to health monitoring of civil120
infrastructure. The nanoparticles have an average diameter of 300 nm, a static dielectric121
constant of ε = 128, and are coated with a silicon oil. This coating facilitates the dispersion122
of the nanoparticles in the SEBS matrix, which improves the mechanical and dielectric prop-123
5
erties of the composite (Stoyanov et al. 2011). The fabrication of the dielectric membrane124
is as follows:125
1. Two SEBS/toluene solutions are prepared by dissolving the polymer at concentrations126
of 200 g/L and 300 g/L.127
2. A mass of TiO2 particles is weighed to obtain a concentration of 15%vol of the pre-128
pared elastomer solutions. The TiO2 is added to the dissolved elastomer and stored129
in an ultra sonic bath for 1 hr to homogenize the mixture. The nanoparticles are130
dispersed in the polymer solution using an ultra-sonic tip (high intensity ultrasonic131
processor Vibracell 75041, Sonics & Materials Inc.,USA). The dispersion of the TiO2132
particles within the polymer matrix is investigated for selected samples using scanning133
electron microscopy (SEM) (Zeiss Ultra 55+, cryo-fracture with 5 nm sputtered Au,134
2.0 kV). Fig. 2 shows the dispersion of the nanoparticles within the polymer matrix.135
3. The polymer concentrations are drop-casted from solution on glass slides and covered136
to control the evaporation rate. The samples are left to dry on a heat plate at 50C137
for 24 hr and then stored in an oven at 50C for 4 more days to allow evaporation of138
the remaining solvent.139
4. All membranes are visual inspected and the thicknesses are measured using a mi-140
crometer screw at randomly selected locations. Membranes with inhomogeneities141
or varying thicknesses are discarded. The selected polymer concentrations result in142
thicknesses of approximately 200 µm or 400 µm.143
5. The membrane are peeled of from the glass slides and equipped with plastic masks on144
both sides to define an area of dimension 70 × 70 mm for application of the electrodes145
(Fig. 3(a)).146
Subsequent to the fabrication of the elastomeric membrane, stretchable electrodes are147
applied on both surfaces of the doped polymer. These elastic electrodes are made of carbon148
black (CB) (Printex XE 2-B) particles dispersed in SEBS solution. The fabrication of the149
6
electrodes is as follows:150
1. A batch of SEBS/toluene solution is prepared by dissolving the polymer at a concen-151
tration of 400 g/L, followed by the incorporation of the CB for a final concentration152
of 15%vol.153
2. The CB solution is stored in an ultra sonic bath for 2 hrs to disperse the CB. The154
solution is sonicated with a ultra-sonic tip for 10 min and stored a second time in the155
ultra sonic bath for 2 hrs to homogenize.156
3. A mask is applied to the membrane to define an area of approximately 70 × 70 mm.157
(Fig. 3(a)). The conductive elastomer solution is brushed onto the membrane, and let158
drying for 20 min . The process is repeated onto the other side. Because the solvents159
used to fabricate the membrane and the electrodes are identical, the CB solution is160
coalesced with the dielectric membrane, forming a uniform conductive layer.161
The completed process gives a highly stretchable capacitor of known dimensions, shown162
in Fig. 3(b). The prepared sample is attached to plastic frames equipped with aluminum163
strips to form electrical connections. The resistance of the electrodes is verified using a164
circuit analyzer. Typical values range between 2-10 kΩ. Finally, the relative permittivity of165
elastic nanocomposite is backtracked from the capacitive measurement performed with an166
LCR meter (HP 4284A, sampling rate of 100 s−1) and the dimension of the capacitor. The167
permittivity of the materials ranges between εM = 6.5−7, in agreement with results obtained168
for well dispersed particles at identical volume concentration (Stoyanov et al. 2011).169
Experimental verification of a sensing patch170
The strain sensing properties of a sensing patch were determined in a tensile tester171
(Zwick Roell Z005) combined with a capacitance meter (LCR meter, HP 4284A, sampling172
rate of 100 s−1). These experiments were performed using a direct capacitance measurement173
mode, where the capacitance is measured directly for a freestanding sample, in opposition174
to a differential measurement mode, where the differential capacitance between two sensors175
7
is measured directly. A sample was clamped into the tensile tester on the frame pieces,176
and metallic leads were connected to the capacitance meter using shielded cable to record177
the change in capacitance under variation in strain. The sample was stored in an oven to178
avoid variation in ambient conditions, and pre-strained to approximately 5% of its initial179
length while the electrodes retained their conductivity. The capacitive measurements were180
performed far below of the cut off frequency to avoid any influence of changes in resistance181
or capacitance on the sensing electronics (RC behavior). Remark: local strain is transduced182
by a variation in capacitance of the dielectric membrane, not by a variation in resistance of183
the electrodes.184
An experiment was also conducted in which parts of the sensing materials have been185
removed using a regular hole punch (see Fig. 4(b)). The sample was placed on a stack of186
paper, then punched through with the hole punch. The diameter of the hole was 14 mm.187
Afterwards, the sample was characterized as described. Remark that the sample capaci-188
tance was 692.7 pF before the hole was punched, and 667.8 pF after. The capacitance of189
the removed section was 23.8 pF, for a total of 691.6 pF, which is almost identical to the190
initial value. This is an indication that the film can survive this disruptive procedure while191
conserving its sensing properties almost intact.192
The sensor capacitance with respect to strain is shown in Fig 5 for a time history of193
increasing strain targets (unfiltered data). Measurements are presented for a freestanding194
sensor, and for the same sensor after the 14 mm hole was punched through it. Both curves195
clearly show a capacitive response to the variation in strain. Already at about 0.00001%196
(0.1 ppm) strain, jumps in the capacitive signal can be discerned, and above 0.2 ppm the197
signal raises well above the noise. Here, 0.1 ppm corresponds to approximately 0.750 µm.198
The hole punched through the sample does not affect the sensing properties, the signal199
appears to be as responsive as before the intrusive procedure. This experiment demonstrates200
the high level of the sensor robustness with respect to controlled mechanical tempering.201
An obvious feature of both capacitive curves is the baseline drift, which is as large as the202
8
signal. The authors suggest that these drifts could be caused by the impact of relaxation203
behavior due to pre-stretching of the free-standing sensor. A successful use of these sensors204
must therefore take these drifts into account. In what follows, the differential capacitance205
measurement mode is used, with the benefits to cancel out measurement drifts and envi-206
ronmental impacts (e.g. changes in temperature and humidity), similar to bridge circuits in207
resistance-based strain gauges (Hannah and Reed 1992).208
LABORATORY VERIFICATION OF THE SENSING METHOD209
The sensing method is tested on wood and concrete specimens using the differential210
capacitance measurement mode, where the change in capacitance between two adjacent211
patches is measured directly. This way, strain (e.g. cracks) can be localized among several212
patches.213
The objective of the tests discussed in this section is to demonstrate that the novel flexible214
membrane is robust, and capable of detecting and localizing damages. First, a robustness215
test is conducted by damaging the sensor. Then, experiments employing one differential216
patch set on wood specimens are presented. Finally, four patches are installed on a concrete217
beam to demonstrate two independent patch sets, enabling a sensing solution capable of218
localizing cracks.219
Experimental Setup220
The newly developed sensing patches are shown in Fig. 6 glued to a concrete beam. Each221
individual patch was connected at two corners using electrical connectors, which were glued222
using the previously described electrode solution. Each single patch covers an area of 80 ×223
80 mm (3.15 × 3.15 in) with an active sensing area of 70 × 70 mm (2.75 × 2.75 in). The224
wood and concrete specimens were sanded, then a primer was brushed on the surface and225
allowed to dry. The sensor patches were glued to this area and smoothed by hand (which226
may cause a slight prestretch). The glue was a two-component epoxy. Electrical connectivity227
could easily be established using wires and clips.228
9
The wood specimens were mounted on an Instron servo-hydraulic testing machine, while229
experiments on concrete beams were performed using a Baldwin universal compression/tension230
machine with an Admit computer control servo hydraulic system. Each differential sensor231
pair was attached to an ACAM PSA21-CAP signal comparison amplifier, which was con-232
nected directly to the measurement computer. The sampling rate was set to 10 s−1. Remarks:233
the ACAM PSA21-CAP outputs the differential capacitance between two capacitors. Thus,234
it is not possible to obtain plots of individual sensors. Also, while the ACAM PSA21-CAP235
is an inexpensive data acquisition system, the use of this equipment involves wiring of the236
sensors, which decreases applicability of the sensing method. However, development of im-237
proved electronics will result in fewer wires, and ultimately allows wireless data acquisition238
of several patches using a single data acquisition system (Laflamme et al. 2012b).239
Robustness Test240
Here, the sensor robustness with respect to physical damages (e.g. caused during the241
installation or vandalism) is verified experimentally. During the experiment, five long cuts242
were made into one patch (a short cut followed by four cuts running over approximately243
50% of the patch length, see Fig. 7(a)). The change in differential capacitance after each cut244
stabilized immediately (Fig. 7(b)), demonstrating that the sensor was still operational.245
Strain and Crack Detection on a Wood Specimens246
A set of experiments have been conducted on wood specimens of dimension 185 × 185 ×247
38 mm (7.25 × 7.25 × 1.5 in). Each test consisted of inducing a flexural crack in the specimen248
using a three-point load setup, under constant displacement at a rate of 0.500 mm/min until249
failure. For these tests, the reference sensing patch of identical geometry and comparable250
capacitance was not glued to the beam, but placed nearby as shown in Fig. 8(d). The251
measurement setup is shown in Fig. 8(b), showing the sensing patch glued to the bottom-252
center of the wood specimen. Tests have been conducted on a total of five specimens.253
Fig. 8(a) shows a typical result (sensor data are filtered). During the experiment, the sen-254
sor was capable of detecting the crack before it caused failure of the specimen, as illustrated255
10
by the two jumps in the differential capacitance measurements. These two jumps indicate256
the initial deformation of the sensor resulting from the formation of the crack and secondary257
impulsive expansion of the crack at failure. The decrease in the sensor capacitance after fail-258
ure corresponds well with a recovery of the sensor after the specimen was unloaded. After259
unloading, the measured differential signal change reported by the PSA21-CAP was 28.3%.260
Remark that the direction of the capacitance signal, in this case positive with increasing261
strain, depends on the pole connections with respect to the reference sensor. Fig. 8(c) shows262
the cracked specimen. This crack detection test has been successfully repeated on the four263
additional specimens.264
The sensor capability to measure a quasi-static strain history can be determined by265
computing the correlation capacitance-load, as the load-curvature relationship can be ap-266
proximated as linear in the case of the 3-point load setup for small deformations. Table ??267
lists the regression study for the load-capacitance relationship before the formation of the268
first crack. Specimens 1-2 used thin sensing patches while specimens 3-5 used thick sensing269
patches. All samples depicted a good fit in the linear regression (R2), showing that the270
sensor accurately captured the change in curvature of the beam. Thin samples exhibited271
a larger variance, which can be explained by the higher sensitivity associated with smaller272
thicknesses d.273
Strain and Crack Detection on a Concrete Beam274
A second set of experiments have been conducted on a medium-scale concrete beam of275
dimension 1370 × 90 × 150 mm (54 × 3.5 × 6 in). The objective was to test the capacity of276
the nanocomposite sensor to both detect and localize cracks. Four sensing patches have been277
installed in a matrix arrangement to form a sensing skin, using a vertical pattern. The sensing278
patches were organized in pairs in order to measure changes in differential capacitance, one279
pair at the extremities (sensor 1), and one pair in the middle (sensor 2). Fig. 6 illustrates280
the monitored surface of the bottom flange of the beam, along with the location of the281
comparative sensing patches. The sensor patches were applied off-center to avoid regions282
11
of constant differential curvature. The localization of two comparative patches with respect283
to each other in the differential capacitance measurement mode does not affect the sensing284
accuracy. For enhanced precision on strain localization, the differential capacitance can285
be measured between all sensors. Such additional precision was out of the scope of this286
experimental campaign.287
Flexural cracks were induced using a four-point load setup, with a constant load rate of288
6000 N/min, until failure of the beam. Remark: the failure mode was in shear, thus not289
reflected in the measurements. This is a consequence of the underlying pedagogic purpose of290
the beam failure experiment, which was performed in the context of an undergraduate class.291
Fig. 9 (a) plots the differential capacitance measurements for both sets of sensors against292
time, and Fig. 9 (b) shows the cracked specimen at the end of the test, where the sensing293
patches have been removed while the beam was loaded to verify the final locations of the294
cracks. A first substantial jump in the measurements is observed with sensor 1, at the295
beginning of the experiment (less than 100 sec). That jump indicates the formation of a296
crack (crack 1) at the extremity of the sensor patches. The crack was visually observable at297
that time. A second jump in the measurement curve can also be seen, which coincides with298
the formation of two additional cracks (cracks 2a and 2b) which could have been formed299
simultaneously, along with a symmetrical crack (crack 2c) monitored by the second set of300
sensors. The small jumps in sensor 1 measurements and the slope in sensor 2 measurements301
probably result from a combination of the slow expansion of the cracks and the increasing302
beam curvature. At the end of the sensor 2 curve (after 900 sec), there is a significant jump303
in the data, which coincided with the formation of a third crack (crack 3) that was visually304
verifiable.305
Additional features in Fig. 9(a) data include a jump in sensor 1 measurements after 1000306
sec, a fluctuation in sensor 2 measurements after 900 sec (at crack 3), and a higher level in307
the change in capacitance compared with results obtained in Fig. 8 (experiments on wood308
specimens). The upward jump in the sensor 1 is a result of an inversion of the electrode poles309
12
for ’sensor 1 left’. This inversion allowed an additional level of damage localization with two310
comparative patches, provided that the direction of strain was known to be increasing and311
constant for both patches. Thus, the crack formed under the patch ’sensor 1 left’ (crack 4)312
was measured as a positive change in capacitance. The fluctuation in the measurements at313
crack 3 could be explained by the patch oscillating between two states of strain due to a local314
plastic deformation of the material. This fluctuation tends to converge to the higher strain315
state. Finally, the higher level of differential capacitance compared with the experiments316
on wood specimens are partly attributed to the use of thicker patches (approximately 10%317
thicker).318
The good correspondence between visually observable results and the sensor pair curves319
demonstrated that the sensing skin was capable of both detecting and localizing cracks in320
the concrete beam.321
CONCLUSION322
In this paper, the concept of the sensing skin was substantially improved by developing a323
capacitive sensing patch with tailored mechanical and electrical properties, de facto demon-324
strating the generality of the sensing method. The sensing patch consists of a thermoplastic325
elastomer doped with titanium dioxide and sandwiched by stretchable CB composite elec-326
trodes. The proposed sensing method comprises several practical advantages, making the327
technology directly applicable for health monitoring of civil infrastructure:328
• Inexpensive material: SEBS-based materials are inexpensive and easy to process.329
• Can cover large areas: the flexible nature of the membrane allows full-scale imple-330
mentation, a powerful advantage compared against conventional mechanical strain331
gauges.332
• Low voltage (1-2.5 volts) requirement: capacitance-based measurements only require333
a fraction of the electrical energy compared to resistance-based strain sensors.334
• Easy to install: the sensor can be applied easily on irregular surfaces using off-the-shelf335
13
epoxy and necessitates minor surface preparation.336
• Customizable surface monitoring: shapes and sizes of the electrodes sandwiching the337
polymer can be user-defined.338
• Robust with respect to physical damages: damaged sensing patches can still be used339
for measuring strain.340
• Damage localization: individual sensor pairs can localize damage, and the polarity of341
a signal change may give further precision within a sensor pair.342
Tests on wood have shown that the new sensor is robust towards mechanical tampering343
or accidents, and is capable of detecting cracks. Test on a medium-size concrete specimen344
demonstrated the promising capability of the sensing skin to both detect and locate cracks,345
an advantage over most existing SHM technologies. Potential impacts of the sensing skin346
include:347
1. Enhanced life-span of civil infrastructure via timely maintenance and visual inspection348
programs, improving sustainability of civil infrastructure.349
2. Cost savings arising from timely repairs and reduced number of visual inspections.350
3. Improved safety by diagnosing and localizing problems before they endanger struc-351
tural integrity.352
ACKNOWLEDGMENTS353
The authors thank John T. Germaine for his priceless help and guidance during the354
testing process, as well as Andreas Pucher and Stephen W. Rudolph for their valuable355
support with the measurement setups. The authors also acknowledge support from the356
WING initiative of the German Ministry of Education and Research through its NanoFutur357
program (Grant No. NMP/03X5511).358
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18
List of Tables445
19
List of Figures446
1 Sketch of a capacitive unit, showing the sandwich structure of compliant elec-447
trodes surrounding an electrically insulating elastomer layer, deployed over a448
monitored surface using a bounding agent. Layer thicknesses are not scaled. 21449
2 SEM picture of a cryo-fractured surface of the polymer nanocomposite material 22450
3 (a) Schematic of the plastic mask used to define the electrode area, (b) Soft451
stretchable capacitor with conductive electrodes . . . . . . . . . . . . . . . . 23452
4 (a) The capacitive sensor with an electrode area of 70 mm × 70 mm and453
thickness of 386 µm (b) The sensor after a hole of diameter 14 mm was454
punched through it. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24455
5 Sensitivity of a sensing patch with respect to strain (regular sensor above,456
sensor with hole punched below). The bottom graph shows the strain history. 25457
6 (a) Four sensing patches deployed on the bottom flange of the concrete beam,458
forming a sensing skin. (b) Locations of sensing patches on the concrete beam459
used for the experiment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26460
7 (a) Sensor after five cuts. (b) Relative capacitance against time. . . . . . . . 27461
8 (a) Relative capacitance and applied load against time - specimen 4. (b)462
Typical three-point load setup for the wood specimens. (c) Cracked specimen463
(bottom flange view). (d) Locations of sensing patches. . . . . . . . . . . . . 28464
9 (a) Differential capacitance measurements for both set of sensors against time.465
(b) Cracked specimen (bottom flange view) . . . . . . . . . . . . . . . . . . . 29466
20
polymer layer
bonding agent
monitored structure
stretchable electrodes
crack
A+ AD
FIG. 1. Sketch of a capacitive unit, showing the sandwich structure of compliant elec-trodes surrounding an electrically insulating elastomer layer, deployed over a monitoredsurface using a bounding agent. Layer thicknesses are not scaled.
21
1 µm Mag = 10 KX
FIG. 2. SEM picture of a cryo-fractured surface of the polymer nanocomposite material
22
(b)
glass substrate
nanocomposite
80mm
80mm
~70mm
(a)
mask
FIG. 3. (a) Schematic of the plastic mask used to define the electrode area, (b) Softstretchable capacitor with conductive electrodes
23
(a)
C =692.7 pF0
(b)
C=667.8 pF
C =23.8 pFcut
14 mm
FIG. 4. (a) The capacitive sensor with an electrode area of 70 mm × 70 mm andthickness of 386 µm (b) The sensor after a hole of diameter 14 mm was punchedthrough it.
24
Cap
acit
ance
[pF
]
SEBS 50012015 vol% TiO2
0
Str
ain
[%]
0 1000 2000Time [s]
3000 4000 5000
0.00004
0.00008
0.00012
SEBS 50012015 vol% TiO2
761.4
761.6
762.0
761.8
733.8
734.0
734.2
734.4
„WITH HOLE“
FIG. 5. Sensitivity of a sensing patch with respect to strain (regular sensor above,sensor with hole punched below). The bottom graph shows the strain history.
25
sensor 1right
sensor 1left
sensor 2left
sensor 2right
beam centerline
(a)
sensor 1left
sensor 2left
sensor 2right
4 patches @ 80mm = 320 mm
sensor 1right
90 mm
beamcenterline
pointload
pointload
(b)
(a)
FIG. 6. (a) Four sensing patches deployed on the bottom flange of the concrete beam,forming a sensing skin. (b) Locations of sensing patches on the concrete beam usedfor the experiment.
26
0
10
20
30
40
50
60di
ffer
enti
al c
apac
itan
ce [
ppm
]
-100 10 20 30 40 50 60 70
Time [s]
1. Cut2.
3.
5.
4.
4.
5.2.
1. Cut.
3.
(a) (b)
FIG. 7. (a) Sensor after five cuts. (b) Relative capacitance against time.
27
3.5
2.5
3.0
2.0
1.5
1.0
0.5
0
Loa
d [k
N]
Time [s]0 100 200 300 400 500
diff
eren
tial
cap
acit
ance
[pp
m] Unloading
Crack-expansion
Crack
Load
Sensor
(a)
0
10
20
30
40
50
60
70
80
beam centerline &load point
185mm
185m
mSensor
Ref.Sensor
(d)
crack(c)(b)
FIG. 8. (a) Relative capacitance and applied load against time - specimen 4. (b)Typical three-point load setup for the wood specimens. (c) Cracked specimen (bottomflange view). (d) Locations of sensing patches.
28
Time [s]
Crack 40 200 400 600 800 1000 1200
-50
-100
-150
-200
-250
-300
-350
-400
0
rela
tive
cha
nge
in c
apac
itan
ce [
ppm
]
Crack 2c
Crack 3
Crack 2a &2b
Crack 1
Sensor 1Sensor 2
sensor 1right
sensor 1left sensor 2
left sensor 2right
beam centerline
crack 4crack 3
crack 2ccrack 1
crack 2a&2b
(b)(a)
FIG. 9. (a) Differential capacitance measurements for both set of sensors against time.(b) Cracked specimen (bottom flange view)
29
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