C A R B O N 5 0 ( 2 0 1 2 ) 3 8 1 6 – 3 8 2 5

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Localized functionally modified glass fibers with carbonnanotube networks for crack sensing in composites usingtime domain reflectometry

Gaurav Pandey a,b, Mitchell Wolters d, Erik T. Thostenson a,b,*, Dirk Heider a,c

a Center for Composite Materials, University of Delaware, Newark, DE 19716, United Statesb Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, United Statesc Department of Electrical and Computer Engineering, University of Delaware, Newark, DE 19716, United Statesd Department of Composite Materials Engineering, Winona State University, Winona, MN 55987, United States

A R T I C L E I N F O

Article history:

Received 12 August 2011

Accepted 3 April 2012

Available online 13 April 2012

0008-6223/$ - see front matter � 2012 Elsevihttp://dx.doi.org/10.1016/j.carbon.2012.04.008

* Corresponding author at: Department of MeE-mail address: thosten@udel.edu (E.T. Th

A B S T R A C T

An electric time domain reflectometry (TDR) based sensing approach with an external par-

allel plate transmission line has been developed to evaluate high-frequency electromag-

netic changes in composites due to applied load and internal damage. A model system

of cross-ply glass fiber/vinyl ester composites with and without the selective integration

of localized carbon nanotube (CNT) networks was studied where microcracking and delam-

ination are introduced during tensile loading. A sizing technique has been used for local-

ized functional modification using CNTs. The TDR sensing approach has been correlated

with strain and acoustic emission (AE) measurements as well as micrographs of edge rep-

licas capturing the damage state. Both the nanotube modified and baseline composites

have similar mechanical properties and damage progression which is reflected in similar

stress–strain plots, AE measurements and edge replica studies. However, the CNT intro-

duced composites have enhanced strain and damage dependent TDR response. Hence,

through localized functional modification of the composite electromagnetic properties

using CNTs and the electromagnetic–mechanical property coupling of CNTs, it is possible

to (1) increase TDR sensitivity to strain and (2) sense development of micro-scale cracks.

This approach offers potential for use in existing composite structures or permanently

integrated during the manufacturing process and is in situ and non-invasive.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Composite materials exhibit multiple damage mechanisms at

different scales such as micro and transverse cracking, fiber–

matrix debonding, delamination and fiber breakage. These

various damage mechanisms interact and affect the ultimate

performance of the material at the structural level [1]. Failure,

durability, and damage tolerance are a critical part of any

high-performance structural design process but current fail-

ure theories suffer from large variations between theoretical

er Ltd. All rights reserved

chanical Engineering, Unostenson).

predictions and experimental data [2]. As a consequence

there is a critical need to develop robust and versatile struc-

tural health monitoring (SHM) techniques to evaluate the per-

formance of composites in long-term structural applications.

An ideal SHM system would be capable of measuring the var-

ious micro and macro damage modes that occur over the life-

span of the composite structure.

Recent advances in detecting damage have been made

using X-ray radiography [3], fiber Bragg grating [4,5], acoustic

emission (AE) [6,7], ultrasonic [3,8], eddy current [9,10] and

.

iversity of Delaware, Newark, DE 19716, United States.

C A R B O N 5 0 ( 2 0 1 2 ) 3 8 1 6 – 3 8 2 5 3817

piezoelectric sensing methods [11]. X-ray radiography is suit-

able only for laboratory-scale damage detection due to large

equipment required to perform radiography. Embedded fiber

optic Bragg grating sensing approaches suffer from of being

invasive and difficult to integrate during the composite manu-

facturing process. Acoustic emission sensors are firmly estab-

lished in laboratory environments but are prone to high

interference in practical implementations due to vibration

and noise.

Advances in the understanding of nanocomposite struc-

ture/property relations have enabled scientists and engineers

to produce functionally tailored matrix and fibers which have

coupled electromagnetic and mechanical properties. In partic-

ular, CNTs can form electrically conductive networks at small

concentrations in a polymer matrix and displays piezoresistive

behavior [12–14]. Due to their vast difference in scale CNTs can

form networks around the structural fiber reinforcement and

be utilized to sense damage based on breaking-up of the nano-

tube network. CNTs impart inherent damage sensing capabil-

ities to probe the bulk of the material [15–17]. Typically, this

measurement method relies on relating the bulk changes of

the direct current conductivity and low frequency dielectric

properties of the composite dopedwith CNTs to provide insight

into the overall material behavior and damage accumulation.

Electric time domain reflectometry (TDR) is a high-fre-

quency electrical measurement technique that can detect

spatial changes in dielectric properties. A key advantage of

the TDR technique is in the potential for distributed measure-

ment as the TDR signal provides time-domain feedback as a

function of the local dielectric material property and trans-

mission line geometry. The TDR approach has been utilized

for fault detection in printed circuit boards and cables [18]

and also for geological measurements to monitor soil mois-

ture content and faults in rock [19]. In addition, TDR ap-

proaches have been used for flow monitoring in liquid

composite molding [20], distributed strain sensing [21] and

as a test method to measure the crack length in the double

cantilevered beam fracture toughness tests [22]. Recently,

TDR has been used to study defects in concrete structures

[23,24] but the scale of faults in these structures is macro-

scopic as opposed to microscopic scale damage which initi-

ates in composites. While both TDR and DC based

monitoring (piezoresistivity) are electrical characterization

techniques, piezoresistivity based measurements require me-

tal electrodes to be bonded directly to the composite speci-

men using a conductive polymer. The degradation/

environmental effects (for example humidity) can damage

the composite-electrode joint and can hence lead to a false

signal. In TDR measurements, the metal TDR connectors

need to be connected to metal transmission line conductors

and not the actual specimen. Hence, TDR is less invasive as

compared to piezoresistivity measurements. Further, damage

localization using DC based techniques requires the integra-

tion of multiple arrays of electrodes (which will result in more

composite-metal electrode contacts) and multiplexing of sig-

nals whereas TDR can probe at the nanosecond time scale

along a single sensing line, without any potentially damaging

contact with the composite specimen.

For fiber composites the scale of damage ranges from the

scale of microns (microcracks, fiber/matrix debonding) to

centimeters or larger (ply delamination). The length scale res-

olution of the TDR measurement is directly proportional to

wavelength of the electromagnetic signal, which in turn is in-

versely proportional to the frequency. With recent advances

in microwave engineering and the advent of picoseconds rise

time and Gigahertz frequency pulses, TDR has been used to

probe length scales providing high enough resolution to mea-

sure many of the damage modes observed in composites.

Through nanoscale hybridization and localized tailoring of

the dielectric properties it is likely that the scale can be ex-

tended to detect microcracks. The TDR approach also over-

comes some challenges for SHM since the sensors are non-

invasive; they can be surface mounted yet detect sub-surface

damage in situ. Furthermore, the technique is not affected by

vibration and other acoustic events.

The main objectives of present study are: (1) to establish a

TDR based non-invasive technique for damage sensing in

CNT introduced fiber composite materials; (2) to investigate

the role of CNT networks in high frequency electromagnetic

response of composite materials; and (3) to demonstrate the

utility of selective functional modification using CNTs as de-

scribed in [26,29] for non-invasive damage sensing. The basic

scientific question addressed by this study is to observe the

effect of CNT networks in fiber composites on TDR response.

The present study fulfills these objectives and addresses the

scientific question by TDR measurements in conjunction with

strain measurement, acoustic emission and optical

microscopy.

The current study evaluates the sensitivity of the high fre-

quency dielectric property changes for sensing of micro-scale

damage CNT–polymer composites using the TDR technique.

Model experiments have been conducted to measure the

TDR signal changes for various damage modes with focus

on early detection of microcracking. The TDR data has been

correlated with AE and strain measurements. An enhanced

electromagnetic response due to localized functional modifi-

cation of the electrical properties in regions prone to damage

has been observed with an external TDR sensor at low input

voltage (250 mV). A uniform electromagnetic field has been

achieved using a novel parallel plate sensor design. Previous

TDR related researches have focused mostly on parallel wire

sensing. Parallel plate sensing enables enhanced sensitivity

due to due uniform penetration of electromagnetic fields.

The present work provides the foundation for an embedded

or surface-mounted sensor capable to measure distributed

micro and macro scale damage mechanisms in advanced

composite materials doped with CNTs. Together, the TDR sys-

tem combined with localized functional modification of com-

posite dielectric properties with CNT networks provides the

potential for future scale-up to a viable structural health

monitoring system in large composite structures.

2. Experimental

2.1. Electric time domain reflectometry: configuration anddata reduction

TDR refers to a technique of sending a high-frequency electri-

cal pulse through a transmission line and subsequently

3818 C A R B O N 5 0 ( 2 0 1 2 ) 3 8 1 6 – 3 8 2 5

detecting the reflected signal. Fig. 1(a) shows the equivalent

electric circuit diagram of a typical TDR setup and Fig. 1(b)

shows the test configuration for the current investigation. In

Fig. 1(b), the pulse propagates along the transmission line

and the electromagnetic field interacts with the dielectric

material surrounding the transmission line. Because the time

domain signal response is a function of the sensor configura-

tion and surrounding dielectric properties, the test, in theory,

allows reconstruction of the material electrical properties

along the entire length of the transmission line [20,23]. Since

cracks in the material result in discontinuity in dielectric

Fig. 1 – (a) Equivalent electrical circuit diagram of TDR setup;

and (b) schematic of experimental setup.

properties this reconstruction could be subsequently ana-

lyzed to find the location and extent of damage. The electrical

properties of the specimen and transmission line are defined

by the characteristic impedance, Z. The characteristic Imped-

ance and propagation speed, v, of a lossless transmission line

is given by:

Z ¼ffiffiffiffiLC

rð1Þ

v ¼ 1ffiffiffiffiffiffiLCp ð2Þ

where, L is the distributed inductance and C is the distributed

capacitance of the specimen under investigation. The imped-

ance of the parallel plate transmission line system can then

be expressed in terms of the material dielectric constant (e,

magnetic permeability (l), specimen geometry (d and trans-

mission line width (w) as [25]:

Z ¼ffiffiffiffiffilep

C¼

ffiffiffile

rdw

ð3Þ

It should be noted that the impedance given by Eq. (3) is

not a material property of the specimen but a property of

the specimen and sensor (transmission line). For example,

for the specimen with same dielectric constant and magnetic

permeability, the impedance will depend not only on the

dielectric constant and the magnetic permeability of the spec-

imen but also on the distance between the transmission line

conductors (d) and the width of transmission line conductors

(w). Also, due to multiple reflections, the measured imped-

ance will also depend on the contact impedance between

the connector and the specimen and the coaxial cables [11].

The effect of CNT on actual impedance has been measured

using a ‘TDR test cell’. This data is included as Supplementary

data. Fig. S1 shows the test setup and Fig. S2 of the Supple-

mentary information shows the TDR response of the neat

and CNT-reinforced specimens. CNTs have been found to

lower the impedance of the vinyl ester resin. Similar results

have been found for other resins as well and ongoing research

is aimed at quantifying the influence of CNTs on electrical

properties. For monitoring purposes, effective impedance

change given by Eq. (5) can be used assuming that it is only

the specimen which is changing during the experiment. Also,

from Eq. (3), it can be observed that the impedance of the

specimen depends on magnetic and dielectric material prop-

erties. Since the specimen used in this study were not mag-

netic, the electromagnetic response presented here can be

assumed to be primarily dielectric.

In this study, a parallel plate transmission line made from

copper foil strips is used. The copper strips are the conductors

for the transmission line and the specimen forms the dielec-

tric medium. There is no direct electrical connection between

the copper strips and the specimen but the copper strips are

in contact with the specimen surface. Electrical contact is not

important to the impedance measurement since the speci-

men acts like a dielectric material between the copper strips

(Fig. 2). Adhesive tape was used to fix the transmission line

conductors in place on the specimen. The copper strips are

connected via a coaxial cable to the TDR signal generator

and detector (HL-8200, Hyperlabs Inc., OR, USA). The TDR

module has a high frequency step generator and an oscillo-

Fig. 2 – Parallel plate transmission line using CNT sized sample (similar copper sheet on other side of the specimen).

Adhesive tapes were used to fix the transmission line conductors in place, fully covering the copper sheet.

Fig. 3 – Relationship between voltage response and load

impedance.

C A R B O N 5 0 ( 2 0 1 2 ) 3 8 1 6 – 3 8 2 5 3819

scope together with a very accurate timing device. The step

generator of the TDR module has a rise time of 200 ps and

an amplitude of 250 mV. A step pulse generated by the step

generator is made up of several different frequency compo-

nents. Based on the rise time specified by the manufacturer,

the frequencies are in the 5 GHz range. The high frequency

electrical signal travels along the specimen transmission line

and an electromagnetic field develops and penetrates

throughout the specimen thickness.

As the specimen elongates under tension (without any

damage accumulation), the thickness and hence the effective

dielectric constant changes due to Poisson contraction so the

characteristic impedance changes with applied strain. In addi-

tion, the dielectric constant also depends on strain as well [31].

At the end of an elastic loading cycle, the impedance should

return to its original value for an undamaged specimen. Sim-

ilarly, with crack accumulation and delamination, the imped-

ance is expected to increase due to a decrease of the effective

dielectric constant due to cracks and air inclusions.

The relationship between voltage of the TDR line (El,m) at

each measurement point (m), incident voltage (Ei), specimen

impedance (Zl) and the effective characteristic impedance of

the cable (Zcable) connecting the TDR module to the load

impedance is given by:

El;m ¼ Ei2ZL;m

Zcable þ ZL;m

� �ð4Þ

and the effective impedance change can be calculated by sub-

tracting the initial average impedance of the untested sample

from the current test.

DZEff ¼P

ZL;m

m� Zuntested ð5Þ

In the experiments, the incident voltage was set at 250 mV

and standard Bayonet Neill–Concelman (BNC) and SubMinia-

ture version A (SMA) cables with 50 X impedances were used

for attaching the TDR module to the specimen. TDR traces for

several known resistances were obtained to verify that Eq. (4)

is valid for the test setup (Fig. 3). The results indicate that

impedance can be extracted accurately using Eq. (4).

2.2. Composites manufacturing

The composite laminates were manufactured by laying-up

five unidirectional E-glass fiber plies (COM-VEW130-50,

Jamestown Distributers) with a cross-ply stacking sequence

of [02/90/02]. The ply lay-up was sealed in a vacuum bag with

a resin distribution layer to enhance matrix infiltration and

subsequently infused with an epoxy vinyl-ester resin (Dera-

3820 C A R B O N 5 0 ( 2 0 1 2 ) 3 8 1 6 – 3 8 2 5

kane� 501A-40, Ashland Inc.). Two percent MEKP (Chemtura

Inc.) and 0.03% Cobalt naphthenate (6% Cobalt, Mahagony

Co.) were used as initiator and promoter, respectively. The

laminates were cured at room temperature for 24 h followed

by post cure at 120 �C for 2 h. Identical processing conditions

were used for manufacturing both the CNT-sized and baseline

composites. The center cross-ply configuration is used to ini-

tiate microcracking in the 90� glass layer under tensile loading

[15–17]. It is widely established that micro-scale cracks initi-

ate in the 90� layers at relatively low strain as compared to

the ultimate strain. As cracking is initiated stress is redistrib-

uted through the interface with the 0� plies. The cracks con-

tinue to develop within the 90� layers as the material is

strained until the ply is completely saturated with cracks.

After the cross ply laminate is saturated with transverse

cracks in the 90�, the transverse cracks coalesce to form inter-

ply delaminations [32]. Ultimate failure of the laminate fol-

lows immediately after these interply delaminations.

Previous research has established improved sensitivity to

microcracking with DC conductivity measurements when

percolating networks of CNTs were added to the composite

[15–17,26]. Changes in the dielectric properties of the speci-

men due to strain or damage will result in a change in the re-

flected TDR signal. In order to evaluate the influence of

nanoscale modification on the sensitivity of the impedance

change two different sets of laminates were produced with

and without CNT modification. Since damage accumulation

occurs primarily in the 90� layer of the composite, this layer

was selectively modified with CNTs in order to assess the sen-

sitivity of the dielectric properties on the sensing response.

The 90� ply was conductively modified by creating CNT net-

works at the surfaces of the fibers through the application

of a fiber sizing prior to composite manufacturing. The sizing

process, described in Ref. [26], involves the infiltration of a

commercially available liquid sizing agent containing highly

dispersed nanotubes (SizicylTM CNT sizing agent from Nanocyl

s.a.) followed by drying. The solid content in the sizing agent

is reported in the manufacturer’s data1 as 6.2% but this solid

content is mostly the polymeric film-former. The mass

change of the glass fibers after the sizing application is less

than 0.5% which is partially due to the deposition of CNTs

[26]. The as-sized layer was electrically conductive after dry-

ing and indicates that electrical percolation (formation of

CNT networks [33]) had been achieved in the layer. Earlier re-

search work [26] provides DC electrical characterization and

microscopy data for CNT sized glass fiber composites. Based

on the microscopic images in [26] the sizing layer has a thick-

ness of less than 500 nm. In addition to being electrically con-

ductive, CNT sized glass fiber layer has the capability to detect

early onset of damage as reported in [29].

2.3. Mechanical/electrical characterization

Tensile specimens were produced based on ASTM Standard

D3039 [27] by bonding fiber glass end tabs onto the ends of

the laminates and machining the laminate using a diamond

slot grinding wheel to a final width of 2.54 cm (1 in.). After

1 SIZICYLTM XC R2G product data: http://www.nanocyl.com/Products

machining the test specimens the copper foils were applied

to the surface of the specimen as described in Section 2.1

(Fig. 2). The copper foil (0.05 mm thickness) was de-greased

using isopropyl alcohol and stretched over the length of the

specimen on both sides to form a parallel plate transmission

line. The parallel transmission line conductors (copper foils)

were soldered to a SMA connector. The specimen strain was

measured by bonding a surface-mounted resistive strain gage

in a region outside the copper strips (Micro-Measurements,

Inc.).

A custom data acquisition system was developed to track

the TDR signals and compute the difference between the ori-

ginal baseline TDR trace and all subsequent traces. Load,

strain and the TDR signals were further integrated to assess

the real-time response. The TDR module acquired an inci-

dent/reflected signal approximately every 1 s. Standard strain

gauges were mounted on the specimen surface and continu-

ously monitored using a National Instruments DAQ system.

In addition, conventional damage detection and assess-

ment techniques were utilized for validation of the observed

TDR sensing response. AE measurements were used to non-

invasively monitor the progression of damage due to micro-

cracking. An AE sensor (35–100 kHz, Physical Acoustics,

Princeton Junction, NJ) was placed on the surface of the spec-

imen and measures the number of elastic wave pulses gener-

ated during each crack. Microscopic edge replication was

utilized to track the state of damage in the specimen at differ-

ent stages of loading. Tensile testing under a progressively

increasing cyclic loading profile with step size of 1000 N was

applied using a displacement controlled load frame (Instron

5567) at 1.26 mm/min. At each loading step the machine

was stopped for approximately 2 min and the load was subse-

quently released with the same rate. During the hold under

tension, microscopic edge replicas were acquired by applying

an acetate replica tape to the specimen (Ted Pella, Inc.). Opti-

cal micrographs of edge replicas of the laminate were taken at

various loading steps and reveal the progressive micro crack

damage accumulation.

3. Results and discussion

Stepwise cyclic loading in steps of 2000 N was applied to both

specimens (Fig. 4(a)). Fig. 4(b) summarizes the stress–strain

behavior. Fig. 4(c) shows the normalized stiffness (E/E0) with

respect to number of loading cycles undergone, where E0 is

the stiffness of the specimen during the first loading cycle.

The modulus does not decrease significantly due to the for-

mation of transverse cracks in the polymer matrix since the

elastic stiffness is dominated by the stiff 0� plies. An almost

identical stress–strain response is observed for all loading cy-

cles, indicating that the CNTs did not change the overall elas-

tic properties as compared to the baseline. In order to verify

that the transverse CNT layer does not significantly change

the tensile strength of the composite laminate, uniaxial ten-

sile tests were performed on three specimens each of baseline

and CNT modified laminates, shown in Fig. 4(d). The baseline

laminates have an average tensile strength of 383 MPa (stan-

-Solutions/Products/SIZICYL.

Fig. 4 – (a) Applied loading to both the specimen, (b) stress–strain behavior for both samples, (c) normalized stiffness for both

specimen, and (d) results of quasi static mechanical tests till failure for both specimen.

Fig. 5 – TDR traces of 0-0-90-0-0 laminate with 90� CNT

introduced layer.

C A R B O N 5 0 ( 2 0 1 2 ) 3 8 1 6 – 3 8 2 5 3821

dard deviation 15 MPa) and the nanotube-modified laminates

have an average strength of 368 MPa (standard deviation

22 MPa). Based on manufacturer specified information,1 CNT

sizing agent marginally improves the flexural properties of

the glass fiber composites. The slope of the stress–strain

curve decreases slightly after the 4th loading cycle (4000 N

or approximately 150 MPa) indicating a reduced stiffness after

the loading has been applied. The stiffness reduction is due to

damage accumulating in the sample, in particular, micro-

cracking in the 90� layer.

Fig. 5 shows the TDR trace obtained for a CNT introduced

sample at 0 s (unloaded sample), 2000 s (7th hold stage),

4000 s (12th peak) and at 5900 s (just before failure). The

time-domain information can be divided into three different

regions. The TDR trace from 149 to 150 ns is the location of

the coaxial SMA cable connecting the TDR module to the

specimen. The TDR cables have an impedance of 50 X corre-

sponding to an output voltage of approximately 250 mV

(Fig. 3). Data from approximately 150 to 151.5 ns corresponds

to the specimen while any data afterwards corresponds to

‘‘open circuit reflection’’ [28]. Ideally, there should be a sharp

step transition between the specimen and discontinuity at

the open circuit reflection. In the present experiments the

transition is not sharp but is continuous. The reasons for

3822 C A R B O N 5 0 ( 2 0 1 2 ) 3 8 1 6 – 3 8 2 5

the gradual transition at the open circuit arise from are multi-

ple reflections and equivalent time sampling by the TDR mod-

ule. The frequency of the pulse is also not a single frequency

but rather a bandwidth of frequencies in the GHz range.

Hence, as it is not possible to explicitly delimit the specimen

region, a fixed time interval, from 150 to 153 ns has been used

as the gradual open circuit reflection contains electromag-

netic property information about the specimen state as the

TDR reflection at any point depends on all previous reflec-

tions as well [11]. This helps in maintaining consistency from

specimen to specimen.

The TDR timescale can be accurately converted to spatial

scale using the velocity of electromagnetic waves propagating

inside the sample as:

Dx ¼ 1

2ffiffiffiffiffiffiLCp Dt ð6Þ

In order to apply Eq. (6), we must have an accurate induc-

tance and capacitance profile of the sample at each time step.

Since this study is more concerned with gross damage state of

the specimen and not with tracking damage location, the

effective material change based on Eq. (5) was calculated.

Nevertheless, the approach could be used to calculate distrib-

uted damage parameters (spatial damage along the sensor)

with the appropriate signal processing. This approach will

be evaluated in future work.

The effective impedance change between 150 and 153 ns

was calculated (Fig. 6). At the peak of 9th loading cycle prior

to total failure of the specimen, an impedance change of

approximately 5.5 X in the CNT specimen is seen versus 2 X

in the specimen without CNTs indicating significantly higher

sensitivity in the CNT introduced specimen. Although the

same qualitative trend was observed from testing multiple

specimens, there was a quantitative specimen to specimen

variation due to different contact impedances (mainly due

to variation in soldering between the transmission line con-

Fig. 6 – Effective impedance change for CNT sized and

baseline specimen under progressively increasing cyclic

loading.

ductors and the SMA connectors). The standard deviation of

noise for the measurement of the TDR system is approxi-

mately 0.028 and 0.04 X for CNT introduced and baseline

specimens, respectively, based on data from four specimens

each tested under the same loading conditions. The scatter

data can be found in Figs. S5 and S6 in the Supplementary

information section. Two observations can be made from

the Fig. 6. First, the TDR data seems to follow the loading cy-

cle. Increasing the specimen stresses or strain increases the

effective impedance of the specimens, while reducing the

tensile load reduces the impedance. Secondly, a hysteresis

behavior is seen where the TDR response does not reach ori-

ginal impedance values when the specimens are unloaded.

This is particularly true for the CNT modified specimen and

for higher peak loads.

Fig. 7 illustrates this behavior in more detail. The maxi-

mum impedance change is plotted for each loading cycle dur-

ing the peak (Fig. 7(a)) load hold stage and unloaded/

unstrained stage (Fig. 7(b)). At a higher peak load, higher

impedance is observed for both the baseline and the CNT

introduced specimen. However, the CNT introduced specimen

shows higher impedance sensitivity due to the fact that CNTs

have coupled electrical and mechanical properties [12–15].

The peak data suggest a linear relationship between stres-

ses/strain and the TDR signal until the 3rd to 4th load cycle,

with an increasing TDR response afterwards.

The unstrained baseline and CNT modified specimen

show a different TDR response (Fig. 7(b)). In Fig. 7(b), the per-

manent impedance change is plotted against the peak strain

undergone by the specimen in the previous cycle. The imped-

ance of the specimen without CNTs falls back to its original

value as the specimen is unloaded until the 6th to 7th cycle

while a significant impedance change is observed in the

CNT introduced specimen starting with the 4th cycle The hys-

teresis behavior can be attributed to permanent breakage of

CNT interconnects within the sizing layer [29]. After the sev-

enth loading cycle, the micro cracks coalesce to form delam-

inations between the plies. At this stage of higher damage

(delamination), even the baseline composite shows a perma-

nent impedance change. Increasing the load further does lead

to permanent damage in both specimens and significant

impedance residuals.

The test specimens were designed to introduce micro-

cracking in the center ply even at intermediate loading condi-

tions. In the previous section, several observations of the TDR

and strain sensor data suggested damage accumulation after

the 3rd loading cycle for the CNT sized specimen or 4th load-

ing for baseline specimen. AE tests and micrographic tests

were conducted to validate the damage mode. AE measure-

ment is an established method to monitor local damage in

materials and has been successfully employed to validate

CNT network based damage sensing techniques for cross

ply laminates [30]. Basic principle of acoustic emission is

monitoring elastic waves which are generated by crack open-

ing. A sensor placed on the surface of specimen generates a

given number of AE counts corresponding to each elastic

wave pulse it encounters. Recursively summing up these AE

counts results in the accumulated AE counts. A sudden in-

crease in accumulate AE counts corresponds to damage initi-

ation. Fig. 8(a) and (b) shows the accumulated AE counts

Fig. 7 – Impedance change for each consecutive loading step (a) at peak load plotted with peak strain and (b) during unloaded

stage plotted with previous peak strain.

Fig. 8 – Acoustic emission studies of: (a) CNT sized specimen and (b) baseline specimen.

C A R B O N 5 0 ( 2 0 1 2 ) 3 8 1 6 – 3 8 2 5 3823

versus time for baseline and CNT modified specimens,

respectively. It can be seen that after the 3rd loading cycle

for the CNT sized specimen or 4th loading for baseline spec-

imen, the acoustic hits significantly increase. This suggests

development of damage due to transverse cracking. As a con-

sequence the permanent change observed in the impedance

of unloaded impedance of CNT introduced specimen during

3rd–4th loading step onwards is microcracking. The micro-

cracking results in transverse cracking which shows a large

number of acoustic hits for higher loading cycles.

An important observation is that even though the acoustic

emission plots are similar for both the baseline and the CNT

introduced specimen, the unloaded impedance changes ob-

served are markedly different. While the CNT introduced

specimen enable a much earlier indication of damage, the

baseline specimen show a significant impedance changes

only at the delamination stage. The final composite fracture

then occurs due to the fiber-dominated fracture of the 0� plies

at the last cycle. Here it is observed that there is no change in

the initiation strain for the formation of transverse cracks

and the ultimate fracture of the composite specimen. Thus,

the addition of nanotubes to the composite is non-invasive

and does not alter the composite properties.

Visual inspection of the specimens using micro graphs of

edge replicas further validate the occurrence of micro cracks

in the specimens. Fig. 9 shows micrographs of edge replica of

both the specimens at 2nd, 4th and 7th loading cycle. The

micrographs indicate similar progressive damage accumula-

tion in both the CNT introduced and baseline specimen. It

can be seen that during the 2nd loading step, microscopic

damage is initiated. At the 4th loading step, the specimen

has a significant accumulation of transverse cracks. The for-

mation of these transverse cracks results in higher acoustic

hits observed around 3rd–4th loading cycle. The formation

of microscopic cracks as well as transverse cracks results in

permanent impedance change of CNT introduced specimen.

Finally, at near failure loading cycles (7th loading step and be-

yond), these transverse cracks start coalescing to form delam-

ination’s which results in a permanent impedance change of

both CNT introduced and baseline specimen. Hence, while in

a baseline composite, delaminations can be detected only

when they have already occurred, the CNT networks can be

Fig. 9 – Micrographs of edge replicas of baseline (a) and CNT loaded (b) laminate at various loading stages: (left) 2nd loading

step (4000 N), (center) 4th loading step (8000 N), (right) 7th loading step (14,000 N).

3824 C A R B O N 5 0 ( 2 0 1 2 ) 3 8 1 6 – 3 8 2 5

used for an early warning about delaminations as they are fol-

lowed by transverse cracking and transverse cracking shows

up in permanent impedance change of CNT introduced spec-

imen. An earlier work has also used TDR for detecting delam-

inations in baseline composites during double cantilevered

beam (DCB) testing using TDR [22].

All sensor and image data suggest that the accumulation

of microcracking starts around the 3rd or 4th cycle, leading

to delaminations and stiffness reduction and ultimately total

failure of the specimens. Nevertheless, the TDR system pro-

vides this feedback non-destructively with the integrated sen-

sor at a significantly higher sensitivity compared to the

stiffness reduction measured by the attached strain gauge

system. In addition, at zero load level the TDR system pro-

vides a change in signal which can be related to the perma-

nent damage of the specimen. For the specimen with the

CNT layer, small-scale damage is detected significantly earlier

with the TDR system compared to the baseline. The TDR sen-

sor is also sensitive to strain and thus provides multi-func-

tional feedback of the state of the specimen.

4. Conclusions

The multifunctional properties of CNTs can be utilized to tai-

lor the dielectric properties of fiber-reinforced composites. In

this work we have established that localized functional mod-

ification of the composite structure with networks of CNTs re-

sults in enhanced sensitivity to crack detection. As damage

accumulates in the CNT-graded region the network of CNTs

breaks-up and changes the dielectric response. Through

tracking changes in impedance a TDR system can be used

to evaluate the effective dielectric changes in composites

specimen during applied loading and damage accumulation.

This research relates the TDR signal to strain when an exter-

nal load is applied, accumulation of microcracks in the spec-

imen, delamination and ultimate failure. The TDR sensor

correlates well with the strain of the sample and was able

to measure the accumulated damage when microcracking

and delamination occurred at various loading stages. An

important feature of the TDR sensor is that the effect of the

damage can be measured without external loading and thus

provides a successful sensor system at any loading condition

of the sample. In addition, the distributed sensing (along the

transmission line) capability of the TDR method eliminates

the need for multiple sensors to monitor gross structural

changes in large structures.

Acknowledgment

The authors would like to acknowledge the Department of the

Navy, Office of Naval Research Grant No. N00014-06-1-1000 for

funding this work.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at http://dx.doi.org/10.1016/j.carbon.

2012.04.008.

C A R B O N 5 0 ( 2 0 1 2 ) 3 8 1 6 – 3 8 2 5 3825

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