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