Composite Structures 261 (2021) 113280
Contents lists available at ScienceDirect
Composite Structures
journal homepage: www.elsevier.com/locate/compstruct
High-sensitive flexural sensors for health monitoring of composite materials
using embedded carbon nanotube (CNT) buckypaper
Meng Wang a, Nan Li a,b, Gong-Dong Wang a,b,⇑, Shao Wei Lu a,⇑, Qi Di Zhao a, Xi Liang Liu a
a
b
School of Aerospace Engineering, Shenyang Aerospace University, Shenyang 110136, China
Key Laboratory of Fundamental Science for National Defense of Aeronautical Digital Manufacturing Process, China
A R T I C L E
I N F O
Keywords:
Flexural sensors
Health monitoring
CNT buckypaper sensor
CNT conductive networks
Piezoresistive property
A B S T R A C T
The health monitoring for the service status of the composite materials is particularly crucial, calling for highly
sensitive piezoresistive strain sensors. This paper outlined the working mechanisms of CNT buckypaper sensor
during loading and releasing processes by analyzing the electrical conduction mechanism of the adjacent CNT
and developing the macroscopical and mesoscopic models of CNT electrical networks inside of the bukypaper.
The CNT‐based nanocomposite specimens underwent monotonic and cyclic flexural loading in low strain range
to examine the dynamic stability and durability of the in‐situ nano sensor. The different support span configurations were also employed and operated to obtain the recommended application scenarios. The long‐term
response behavior of the nano sensor was also concluded as three typical phases. The desirable agreement
was demonstrated by comparing the responses of ΔR/R0 signals and phenomena of the breakage and propagation of micro cracks and the rearrangement and reconnection of CNT conductive networks within the Scanning
electron microscope (SEM) images of the buckypaper. The proposed flexural nano sensor is suitable for potential health monitoring application of composite materials.
1. Introduction
Static and Dynamic health monitoring (HM) for structural applications such as wearable devices, virtual reality and product structure
monitoring, have always been attracting attention from aerospace,
automobile and sports industries. In order to evaluate and further
improve infrastructures performance, the advanced and smart devices
which can provide operational state details and predict service life of
advanced structures call for the development. Conventionally, HM is
performed by evaluating the micro deformation (strain) of the key
objects, depending on capacitive, piezoelectric, photoelectric or
piezoresistive sensing principles. However, the sensors currently in
service offer limited sensitivity and stability and especially affect structural integrity. The development of flexible, lightweight and structurally friendly sensors can therefore accelerate and motivate the
application of HM technology in emerging areas, especially in aero
polymer composites.
Given the progress being made, the polymer composites have
received extensive applications on the aircraft. The anisotropic nature
of the composite laminates induced by the fragile fibers and the ductile
resins as well as the low interlaminar strength, contributes to the fac-
ing challenges generally including delamination and micro cracks during the service stage, especially when suffering from mechanical and
thermal stress. For example, the wing spar is a beam which takes the
main load along the wing, from wing tip to fuselage [1], as well as
it is constantly exposed to threatening environmental effects such as
temperature changes, impact by birds or hailstones, lightning strikes,
etc [2]. Health monitoring, especially in‐flight strain monitoring, of
these wing spar beam is very critical as the lack of information about
the structural health of the composites during the operation phase
results in wrong judgment on the life cycle of composites, and even
emerging failures. Thus, the needs to monitor and optimize the lifespan of composites have become of great importance.
Traditional strain sensors for in situ health monitoring are fabricated by employing various metals and semiconductors [3], and
embedded into the composite parts or attached on the surface. Yang
Yang et al. [4] designed and fabricated a dielectric sensor by flexible
circuit board technology with off‐the‐shelf electronic components
(polyimide and copper) for in situ and real‐time production monitoring of composites. The optimal design was determined of a metallization ratio equals to 0.5, which gave the highest sensitivity per unit
area. Nickel based alloys are also commonly embedded into the com-
⇑ Corresponding authors at: School of Aerospace Engineering, Shenyang Aerospace University, Shenyang, Liaoning, China.
E-mail addresses: daidaidegaga@gmail.com (N. Li), cadcam119@126.com (G.-D. Wang).
https://doi.org/10.1016/j.compstruct.2020.113280
Received 24 March 2020; Revised 28 September 2020; Accepted 1 November 2020
Available online 7 November 2020
0263-8223/© 2020 Elsevier Ltd. All rights reserved.
M. Wang et al.
Composite Structures 261 (2021) 113280
piezoresistive properties for the nanocomposites was also explored:
the constant destruction and reconstruction of electrically conductive
paths connected by some CNF caused the piezoresistive responses of
the nanocomposites.
CNT is a novel one‐dimensional material with outstanding mechanical properties and high conductivity [23]. The small dimensions make
macroscopic applications difficult [31], it therefore has always been
introduced into an insulating matrix or coated in certain area as functional fillers. The strain sensors were fabricated with printed carbon
nanotube layers by Jihua Gou et al. [24], offering strain gauge factors
measured in a range of 0.61–6.42. The dynamic loading test results
revealed that the printed carbon‐nanotube strain sensors exhibited
excellent durability and stability at cyclic strain. Fu‐Zhen Xuan et al.
[25] found the resistance of Multi‐walled carbon nanotube (MWCNT)
sensor showed a linear dependence on test length, while it reached a
percolation threshold after 5‐cycle’s immersion. Lee et al. [26] also
demonstrated that SWCNT film gauges had a linear relationship
between resistance changes and externally applied strain. In summary,
the superior current carrying capacity and sensitivity for subtle strain
variation of the continuous entangled CNT networks make the fabricated CNT sensor a promising candidate for wearable smart electronics
and structural health monitoring applications.
Although many above efforts have been made including the damage monitoring and strain inspection using strain gauges, fiber optic
sensors and nano particles, the facing problems remain crucial: 1)
the sensing scale of the strain gauges is limited where damages can
be only detected in designated directions and locations. 2) the embedded optic sensors with diameter larger than fibers acting as the precrack can compromise the inherent strength of the laminates, and
what counts is that optical fiber sensors are often insensitive to cracks
propagating parallel to the optical fiber orientation. Additionally, the
expensive and complicated collocation equipment is needed for structural health monitoring. 3) the agglomeration tendency induced by
high specific surface area and non‐uniform dispersion of the fillers
within the resin requires further analysis to eliminate the induced
noise signal. These problems mentioned above can distinctly affect
the mechanical robustness and the electrical responses of the CNT
buckypaper sensor [20].
CNT buckypaper possesses the superior properties of one dimensional CNT, the multi‐tunneling effect simultaneously enhanced by
the CNT entanglements, evidencing the ideally implantable sensor.
The distribution and magnitude of the CNT could be effectively controlled in the buckypaper, the agglomeration tendency induced by
high specific surface area of CNT hence could be reduced [27]. In addition, much attention has been paid that CNT buckypaper could not
only provide sensitive self‐sensing function when monitoring the subtle strain [28,29], but also serve as an ideal interlaminar reinforcement
offering mechanical robustness and structural contribution [27]. There
is still a gap in the development of high‐sensitive embedded CNT buckypaper sensor for health monitoring of composite materials. Especially, the needs for health condition and service life of airfoil tip
are crucial and necessary when subjecting to continually flexural
stress. Therefore, the real‐time health monitoring of flexural property
of composite components using embedded CNT buckypaper sensor
would be the burning question, which could receive more practical
interests. In the present study, a promising self‐sensing nanocomposite
embedded with carbon nanotube buckypaper sensor will be investigated to realize the health monitoring function. The three‐point bending test with different support span lengths will be employed to detect
the dynamic stability and dynamic durability of the electrical resistance signal response of the nano sensor. The long‐term response
behavior of the nano sensor will be also concluded by analyzing the
experimental data. The developed macroscopical and mesoscopic
models of CNT electrical networks inside of the buckypaper and the
changing of ΔR/R0 signal and the morphological SEM images will
posites to monitor the strain and damage induced in the composites. R.
Balaji and M. Sasikumar [5] inferred that there were two phases such
as deformation phase and failure phase based on the piezoresistivity
response of the nickel alloy when subjected to uni‐axial tension and
flexural loading, which can be used to predict the induced strain and
damage state of the composites. In order to avoid the a low resistivity
introduced by metal electrodes, Sungwoo Chun et al. [6] proposed an
all‐graphene strain sensor that can detect various types of strain
induced via stretching, bending, and torsion. This sensor was successfully demonstrated the bi‐directional response and the infinitesimal
strain as low as 0.1% with a relative resistance change (ΔR/R0) of
~0.005 via experiments. Alternatively, the Fiber Bragg Grating (FBG)
sensors are also commonly employed to predict the strain distributions
[7] and monitor the propagation of delamination crack [8]. Although
the conventional health monitoring sensors can be utilized in form of
small and thin patches, leading to minor structural modifications combined with a relative high electromechanical coupling, the integrity of
the host composite structure is compromised by the presence of the
embedded heterogeneous device [9,10].
The commonly ill‐suited configuration of traditional metallic sensors requests for miniaturized sensors, inspiring the exploration of
nano‐sensors. Graphene is a typical 2D carbon nanometer material
which is composed of a single layer of sp2‐bonded carbon atoms
[1,11]. Due to the unique atomic arrangement, the electric tunnel
can be easily formed when suffering the changing of external environment, like strain, sound or light. The appearance of graphene nanoplatelets and other modified films allow them existing in a more stable
form, improving their conductivity and motivating the application in
sensor field. Recently, graphene nanoplatelets have been considered
as alternative materials for piezoresistive pressure sensors [12]. Jun‐
Wei Zha et al. [13] synthesized the amino‐functionalized graphene
nanoplates composite sensor to monitor the deformation and damage
in the structural composites and explore the underlying sensing mechanism by the quasi‐static and cyclic loading tests as well. Damage
detection and location of multiscale composite materials based on
functionalized graphene nanoplatelets (f‐GNPs) coated fabrics were
demonstrated by analyzing electro‐mechanical response of sensors
during flexural and tensile tests [14]. Although these nano sensors
exhibited the potential in the health monitoring by exhibiting prefect
tunable‐sensitivity, flexibility and reliability, some phenomena represented that the sensitivity of the microstructural sensor was related
with the properties of graphene electrodes and the layout and the
geometry of Polydimethylsiloxane (PDMS) pyramids [15]. It was also
found the f‐GNPs coated fabrics creating a 2D electrical network presented orientation‐related feature, showing more sensitive to damage
oriented at 90° than 0° [14]. Briefly, these kind of flexible pressure sensors are sensitive to hand bending and facial muscle movements [16],
so that possessing the potential to be used for highly sensitive wireless
structural monitoring [17,18]. However, the random stacking of graphene layers during the self‐assembly process and severe structural
defects induced during exfoliation and reduction processes can be
two main issues which limited the application of GNP [12].
The usage of other nanoscale particles (e.g. nanotubes, nanofibers)
offers an alternative approach to implement continuous or periodic
self‐sensing for deformation or cracks detection of composite components by forming a 3‐dimensional electrically‐conductive network at
low volume contents in situ [19] without compromising the structural
integrity of the components, and that leading to multifunctional components integrating structural and sensing capabilities [20]. Gil‐Yong
Lee et al. [21] printed silver nanoparticle strain sensors onto composites for the entire repair process monitoring, delivering the non‐
destructive evaluation purposes during the service periods and showing good feasibility as a strain measurement of composites. Yanlei
Wang et al. [22] observed the Cellulose nanofiber (CNF)/epoxy composites exhibited a stable and repeatable piezoresistive property under
different types of compression cyclic loadings. The mechanism of
2
Composite Structures 261 (2021) 113280
M. Wang et al.
jointly demonstrate the working mechanisms of CNT buckypaper sensor, deducing a high‐sensitive flexural sensor.
The contact resistance where an equivalent circuit of N parallel
resistors with the value of MRt is arranged, leads the whole composite
resistance (see Fig. 2).
The tunneling‐type contact resistance can be further expressed by
the formula Eq. (3), derived by Simmons [33].
h2 d
4πd pffiffiffiffiffiffiffiffiffi
pffiffiffiffiffiffiffiffiffi exp
2mλ
ð3Þ
Rt ¼
h
Ae2 2mλ
2. Theoretical analysis
2.1. Electrical conduction mechanism
The cylindrical CNT with one‐dimensional quantum characteristics
have high aspect ratio possessing the properties of radial nano‐scale
and axial micron‐grade, and the high specific surface area of CNT especially makes the typical clustering and tangling between adjacent
MWCNT and forms multi‐directional conductive networks. CNT buckypaper acts as the multi‐scale material which integrates the outstanding performance of the CNT, containing the randomly oriented
networks of CNT, contributes to be the outstanding sensitive sensor
for structural health monitoring.
According to Ruschau [30], the electrical conduction mechanism of
the randomly dispersed CNT‐based composite can be attributed to two
main sources:
where h is the Planck's constant, d is the distance between adjacent
CNT, A is the cross‐sectional area of tunnel conjunction (here the average cross‐sectional area of CNT is approximately adopted), e is the single electron charge, m is the mass of the electron, and λ is the height of
energy barrier for CNT‐based composite system.
Ultimately, the overall resistance of the CNT‐based composite can
be dominated by tunneling resistance and calculated by the following
improved Eq. (4) [31]:
MRt M
h2 d
4πd pffiffiffiffiffiffiffiffiffi
pffiffiffiffiffiffiffiffiffi exp
2mλ
ð4Þ
¼
Rc ≅
N Ae2 2mλ
h
N
By analyzing the Eq. (4), the composite resistance is especially
determined by the tunneling critical distance d apart from other constant parameters. Thus, the changes in the tunneling gaps play crucial
roles in the piezoresistive response [34] when undergoing micro
strain, also explaining the tunneling effect as a critical factor to dictate
the piezoresistive response [35,36] and why CNT buckypaper can be
used to monitor the structural health condition based on the electrical
signal changes.
1) The intrinsic electrical resistance of the nanotubes, i.e., Rp;
2) The contact resistance among two adjacent nanotubes, i.e., Rt.
When there are N conduction paths, M particles forming one conduction path, herein the resistance of CNT‐based composite (i.e., Rc)
can be described by the following relation Eq. (1), improved on the
basis of Yasuda and Nagata [30].
Rc ¼
MRp þ ðM 1ÞRt
N
ð1Þ
2.2. Working mechanisms of CNT buckypaper sensor
where the lead resistance and electrodes resistance have been
neglected. The contact resistance (Rt) is mainly stimulated via the electrical tunneling effect among the neighboring carbon nanotubes at spatial domains [31], playing the dominant role in the tunneling resistance
of piezoresistively CNT‐based composite. Considering the randomly distributed CNT, the possible electrical tunneling effect predominantly
includes two types of conductive mechanisms: 1) in‐plane conductive
network formed by closely situated CNT; 2) out‐plane conductive network formed by closely overlapped CNT [32], as shown in the schematic Fig. 1, a and b respectively.
Since the gap area between CNT contact points is filled, more or
less by matrix, the contact resistance or tunneling resistance (Rt) is several orders of magnitude higher than the carbon nanotube inherent
resistance, Rp therefore has been neglected. Eq. (1) can be modified
into Eq. (2):
Rc ¼
MRp þ ðM 1ÞRt ðM 1ÞRt MRt
≅
≅
N
N
N
To better understand the working mechanisms of strain sensors
made from CNT, the evolution process of the distributed CNT conductive networks in the interface between CNT buckypaper and composite
laminate, during stretching and releasing behaviors (e.g., bending
test), schematically described in the Fig. 3. The conductive networks
are initially formed by the adjacent CNT under static state, the zoomed
details of CNT arrangement are depicted into 3 typical conductive
paths (see the Fig. 3b, d, f). No matter the overlapping configuration
of the CNT or the tunneling effect stimulated by closely situated
CNT, the layout of the conductive networks are identified along with
the original CNT buckypaper, offering the stable electrical signal
response. Subsequently, when the flexural stress is imposed, the inflection will be generated accordingly. The schematic of the changes of
CNT conductive networks and damage evolution during flexural loading are depicted in Fig. 3c and 3d. In this case, the homeostatic distribution of CNT has been destructed, followed by the changes of inter‐
nanotube distance and the matrix region. Following the classical material mechanics theory, at the loading process, the concave part of the
ð2Þ
Fig. 1. Schematics of in-plane a and out-plane b conductive networks.
3
M. Wang et al.
Composite Structures 261 (2021) 113280
Fig. 2. Schematics of the whole composite resistance.
complete the dispersion process for 40 min at the centrifugal force
of 8000g.
Upon the completion of the dispersion process, the solution was
ejected onto a porous filtration membrane with 0.45 μm diameter
micropore and then filtered by a spray‐vacuum filtration setup. A large
amount of water was used to remove any residual surfactant. The filtration membrane was dried at 80 °C for 3 h and the carbon nanotubes
buckypaper was peeled off the membrane. The process parameters for
the fabrication of MWCNT are presented in Table 1 whereas the flow
diagram is schematically illustrated in Fig. 4.
curved laminate (above the neutral plane) will mainly suffer the compressive stress (σc) with the decrease of inter‐nanotube distance (see
the path 1 of the Fig. 3d), whereas the convex part of the curved laminate (below the neutral plane) will mainly suffer the tensile stress (σt)
with the increase of inter‐nanotube distance (see the path 3 of the
Fig. 3d). The breakup of the original network contact points and tunneling effect will simultaneously promote the forming of new adjacent
conductive networks (see the path 2 of the Fig. 3d) at both concave
and convex parts. It is worth noting that the compression of the CNT
will predominate over elongation in the concave part while the elongation is predominant in convex part, the electrical resistance change
will present a negative correlation with the distance changes of adjacent CNT. At the releasing process, the position, orientation and connection of the CNT will be rearranged along with the deformation
recovery of the laminate, contributing to the updated conductive networks (see the path 2 and 3 of the Fig. 3f).
If the strain is high enough to result in permanent rupture in the
inspection regions, i.e., micro cracks or CNT movement in the matrix,
the elongation of inter‐nanotube distance cannot be reduced and the
breakup of the original networks cannot be reconnected, as shown in
the path 2 and 3 of the Fig. 3d (‘Break’ marks). The abrupt loss of numbers of conductive networks communication will certainly give rise to
the dramatic variations in electrical resistance signal, providing the
possible damage indications for structural health monitoring.
By summing up the aforesaid phenomenon, the changes of CNT
conductive networks cover: a) the positions rearrangement and the
distance changing of CNT particles within the mixture interface; b)
the reconstruction of CNT conductive networks (disconnection and
reconnection).
3.2. Fabrication of glass fiber reinforced plastic (GFRP) embedded with
CNT buckypaper sensor
3. Experiment setup
The purchased unidirectional glass/epoxy prepreg (6501/G1500)
with 33% resin volume fraction (supplied by Guangwei Composites
Co., Ltd, China), was used as the carrier to help verify the multi‐
sensing properties of CNT buckypaper sensor when GFRP laminates
experiencing bending test. The GFRP laminates to tested were fabricated by hand lay‐up process where a simple stacking sequence of
[0°] 8s with a total of 16 layers was adopted. The CNT buckypaper with
measuring rectangular shape of 30 mm × 10 mm was directly
attached on the top and the bottom of the GFRP laminate where the
silver electrodes with two thin copper wires were printed on the both
sides of buckypaper strips to minimize the contact resistance. After
cured 2 h by employing the autoclave method at 120 °C and 1 MPa curing pressure, followed by 3 h natural cooling, the GFRP specimens
were tailored into required configurations according to the ASTM standard D790‐17 (but not limited to) for flexural tests (three point bending mode). The schematic diagram illustrating the position of CNT
buckypaper sensor on top and bottom surface is drawn in Fig. 5.
3.1. Fabrication of CNT buckypaper
3.3. Characterization of CNT buckypaper sensor
The commercially available Multi‐Walled Carbon Nanotube
(MWCNT), with over 98% purity (brand TN1M1), 5–12 nm diameter
and 30–50 nm length were purchased from Chengdu Organic Chemicals Co., Ltd (Chengdu, China). The different volumes 500 mg,
1000 mg and 1500 mg of MWCNT were respectively functionalized
by adequately mixing with the aqueous solution (Triton X‐100 surfactant, Tianjin Shakespeare Company). The deionized water (100 ml)
was added to the mixture while constantly stirring for 2.5 h. Sonication of the mixture followed where the Misonix Sonicator Q700 (Sonicator Co., Ltd., USA) was operated in pulse mode (2 s on, 2 s off) at a
power of 100 W for 30 min. The centrifugal machine was utilized to
3.3.1. Observation
In order to preliminarily investigate the morphological characterization of CNT buckypaper and distribution characteristics of CNT
within the interface of the fiber and resin, which determine the electrical percolation network and the formation of conductive junctions due
to the tunneling effect between neighbor tubes [2], the scanning electron microscopy (SEM, Philips 505 at 10 kV) was employed to detect
the hybrid composite laminates, as shown in Fig. 6. The homogeneous
distribution of the CNT within the cross‐sectional of the buckypaper
and the interlaced conductive networks (Fig. 6a) guarantee the reliable
monitoring sensitivity. The acceptable infiltration and diffusion of
4
Composite Structures 261 (2021) 113280
M. Wang et al.
Fig. 3. Macroscopical and mesoscopic models of CNT electrical networks inside of the bukypaper before loading, after loading and after releasing.
Table 1
Parameters for preparation of MWCNT.
Parameters
MWCNT concentration (mg/L)
MWCNT/surfactant ratio
Sonication time/power (min)/(w)
Centrifugal time/force (min)/(×103g)
Selected values
500, 1000, 1500
1:8
30/100
40/8
3.3.2. Measurement
Apart from the high sensitivity under various cyclic tensile and
compressive loading, the stability and durability of the nanocomposite
are also crucial aspects to investigate [38]. In order to investigate the
long term stability and durability of the CNT buckypaper sensor
beforehand to be useful, the averaged electrical resistance of sensors
epoxy resin into buckypaper due to the porous network (Fig. 6b) indicates the perfect interfacial combination, potentially contributing to
the increase of flexural strength. Furthermore, the exposed and interlinked CNT displayed in Fig. 6 also indicated the sensor signals and
inner stress could reliably be conducted through the porous network
and multichannel conductive paths [37].
5
M. Wang et al.
Composite Structures 261 (2021) 113280
Fig. 4. Flow diagram for the CNT buckypaper preparation process.
Fig. 5. The layout of CNT sensors on the GFRP laminate.
Fig. 6. SEM images of buckypaper cross-sectional and buckypaper embedded in composite.
observed, meaning the stability of in‐situ potential and the continuity
of the electrical signal exchange of the CNT buckypaper sensors.
was measured (plotted in Fig. 7) without external load over 10 h at
room temperature where the Data Acquisition Unit (Fluke 2638A)
was employed in the frequency range of 20 Hz to 1 MHz using an
AC voltage of 220 V. It is evident that the specimen shows stable outputs of the resistance by observing the stable fluctuation of the electrical resistance change ratio ΔR/R0 (where R0 is the steady‐state
electrical resistance without any strain applied and ΔR = R − R0 is
the real‐time change of the electrical resistance). Along with the long
term detection at steady state, there was no obvious signal drifting
3.4. Mechanical tests
To further understand the mechanical and piezoresistive behavior
of the CNT buckypaper sensor, the mechanical tests were performed
in agreement with ASTM standards D790‐17 (three point bending test)
under different levels of applied strain and mechanical cycles, utilizing
6
Composite Structures 261 (2021) 113280
M. Wang et al.
metal on the resistance response of buckypaper sensor. Each kind of
flexural test was repeated at least five times to make sure the reliability
of the experimental data and then obtain average values of the flexural
strength of composite laminates and the electrical signal changes at
different user‐defined loading levels.
4. Result and discussion
The electrical responses of the CNT buckypaper sensors with different volumes 500 mg, 1000 mg and 1500 mg of MWCNT were respectively tested, however, the sensors with 500 mg content expressed
much higher sensitivity, stability, and durability of resistance changes.
Hence, the electromechanical experiments were conducted in over 5
samples with the same percentage of CNT (500 mg), for different
bending/releasing cycles, deflections and deformation velocities at
room temperature. The relationships of stress, strain and electrical
responses of two CNT buckypaper sensors planted on top and bottom
surfaces of the laminate were studied and their operating principles
were also analyzed thereinafter in detail, for the purpose of comparison of the difference of the piezoresistive response between the two
CNT buckypaper sensors at the same mechanical behavior.
Fig. 7. The long term detection of the electrical resistance change ratio ΔR/R0
at steady state.
the universal testing machine (INSTRON 3365) set with a crosshead
speed of 2 mm/min for loading and 2 mm/min for unloading, respectively. The applied force was recorded by the machine load cell and
converted to the mechanical stress (σ), while the crosshead displacement was normalized by the gage length of the test specimen and
defined as the flexural strain (ε), which was detected by means of a
conventional strain gauge. Theoretically speaking, the mechanical
stress and strain can be also expressed as:
σ¼
3PL
2bh2
4.1. Piezoresistive characterization
The Fig. 9 expresses the typical experimental results observed from
the specimens with support span (80 mm), concerning the electrical
resistance responses corresponding to the flexural stress when subjected to three‐point bending test up to failure. Generally, the flexural
stress (i.e., σ) and the resistance change ratio of the CNT buckypaper
sensors (i.e., ΔR/R0 upper and lower) follow nonlinearly the mechanical deformation. However, it is worth noting that the resistance
change ratio of two sensors shows the characteristics of piecewise
function, in terms of the increase of the flexural stress and strain. Initially, the flexural stress of the composite laminate increases linearly
with the machine cross‐head motion, while the strain that falls in
the elastic regime of the material. The composite laminate starts to
yield up to failure ultimate strength when the incremental stress surpasses the elastic limit strength of the composite laminate, the strain
that afterwards falls in the plastic regime showing the nonlinearly
increase. Similarly, the normalized change of electrical resistance
ΔR/R0 of sensor at upper surface suffering the compressive stress
shows an initial linear dependence (a slope of 1) upon the flexural
strain, followed with nonlinearly decrease, due to the constantly
decreasing of inter‐nanotube distance during the bending process
and the observed saltation of this curve most likely means the irre-
ð5Þ
where P is the load (N), L is the support span length (mm), b is the specimen width (mm) and h is the specimen thickness (mm).
ɛ¼
6δh
L2
ð6Þ
where h, and L are the same as in Eq. (5), δ is the deflection of the crosshead (mm).
The experimental set up and the required geometric specification of
specimens are shown in Fig. 8. The generated electrical signals when
the crosshead loaded at the midpoint of the composite laminates
was amplified by the external resistance measuring apparatus (Fluke
2638A) which helped transmit the signal to the PC end by signal
amplification and signal acquisition. Importantly, the Teflon thin
membranes were cover the contact surface between the buckypaper
sensor and the metallic crosshead in case of the interference of the
Fig. 8. The experimental setup.
7
M. Wang et al.
Composite Structures 261 (2021) 113280
the relative direction of the sensor, namely gauge factor K (slope of a
line), see Eq. (7).
K¼
ΔR=R0
ɛ
ð7Þ
For the bending best of the composite material beam, the permanent and irreversible tensile fractures on the convex part of the curved
laminate dominate the main position among all the damages and will
occur primarily, therefore the mechanical behavior of the composite
laminate and the relatively electrical response of the CNT buckypaper
sensor, especially on the convex part, will be the research emphasis of
many authors’ work [2,20,38,39]. Based on the definition of the gauge
factor, it can be viewed that the CNT buckypaper sensor at lower surface expresses typical gauge factor values of 46, 176.7, 11.72, which
are higher than the piezoresistive sensitivity of the commercial metallic gauge factor K ≈ 2.0. Especially, when the resistance variation ΔR/
R0 located in the elastic regime (CD phase), the sensitivity of CNT
buckypaper sensor exceeds most gauge factor values of scholar’s
researches (around 0.1 [20], 3.45 [2], 8 [40], respectively) where
equipped with the similar CNT sensors under flexural behavior.
While in comparison, the CNT buckypaper sensor at upper surface
shows the lower sensitivity with the gauge factor values of 1.0 when
the resistance variation ΔR/R0 located in the elastic regime, which is
comparable with the gauge factor values of other similar sensor supported by research (Giovanni Spinelli’s 0.1 and 1.28 [2,20], C. Camerlingo’s 0.67 [41]). Apparently, the gauge factor of the CNT buckypaper
sensor did not reach the K value of the commercial metallic gauge factor, the possible reason may be attributed to the relatively small compression deformation on the concave part of the curved laminate.
However the CNT buckypaper sensor at upper surface can still express
precise and sensitive signal variation (shown on the following figures
and contents) according to the mechanical behavior of the laminates,
indicating the potential application acting as the micro strain sensor.
Most importantly, the carbon nanotube buckypaper can co‐cure with
the prepreg and improve the interlaminar strength if it is inserted
between layers, which have been demonstrated in our previous
researches [27,29]. Therefore, the carbon nanotube buckypaper can
give a superior adhesive property than other physical strain gauges
which may compromise the laminates strength to some extent.
In order to detect the dynamic stability of the piezoresistive
response, the CNT‐based nanocomposites were subjected to over 10
bending/releasing cycles at different levels of strain by means of
Instron Testing System. The applied strains were limited in the elastic
regime of the material (i.e., up to 1 mm, 2 mm, 3 mm of the displacement amplitudes and separately around 10%, 20% and 30% of the
fracture stress, see Table 2) at every loading process and returned to
zero at each unloading process thereafter. In this case, the electrical
resistance ratio of CNT‐based nanocomposites also followed the similar trend with the strain growth curve, as shown in Fig. 10. The
Fig. 10a expresses the cyclic variations of stress and strain vs. time
when subjected to flexural loading cycles. Clearly seen that the flexural
stress increases and decreases monotonically respect to the applied
strain within the elastic regime of the material. The Fig. 10b presents
the relative electrical resistance ratio ΔR/R0 vs. time curves for the
CNT buckypaper sensor at different given deflections (δ = 1 mm,
2 mm, 3 mm). The electrical responses at these given deflections fol-
Fig. 9. Mechanical response (i.e., σ) and normalized changes of electrical
resistance of sensors (i.e., ΔR/R0) on the lower and upper surface.
versible rupture of percolating networks or the permanent deformation of the material at the loading area. However, the convex part of
the curved laminate will primarily occur the permanent and irreversible fracture instead of the concave part of the curved laminate.
At lower surface, the normalized change of electrical resistance ΔR/
R0 of sensor suffering the tensile stress increases according to a nonlinear curve characterization with a style of multi‐stage function. As
marked in the pink dashed lines, there are several evident linear
increase. Initially, the AB phase (up to 0.070% strain) expresses linear
increase with a slope of 46 referring to the flexural strain at the early
linear stage of elastic strain. Subsequently, with the strain extension,
the breakage of the conductive networks inside of the sensor became
serious, inducing a temporary nonlinear increase (BC phase). Until
around the 0.256% strain to 0.64% strain (CD phase), the evident
abrupt change occurs with linear slope of 176.7, followed by another
nonlinearly increase (DE phase), thereafter the breakage phenomenon
of the conductive networks became gentle. While in the nonlinearly
plastic stage of the flexural strain, a relatively gradually increase of
electrical resistance ΔR/R0 is displayed accompanied by once ladder‐
shaped increase, showing the linear slope of 11.72. The ladder‐
shaped increase occurred with the cross‐head displacement revealed
the constant and dramatic broken styles of the percolating networks
and even some irreversible failures occurred on the convex part of
the curved laminate. Note that there is a remarkable difference (at
least one order of magnitude) on the normalized change of electrical
resistance ΔR/R0 between upper and lower sensors, indicating the
elongation of inter‐nanotube distance of CNT buckypaper sensor (contact resistance Rt) is predominant at the convex part of the curved laminate when subjected to the flexural stress.
This electro‐mechanical response of the composite laminate also
indicates that it should keep the level of strain under 0.03 for subsequent experimental characterization so that to avoid failure of the
laminates.
Generally, the sensitivity of produced sensor was quantified by the
resistance variation ΔR/R0 of the sensor versus the micro strain ε upon
Table 2
The average flexural stress at different loading stages for specimens with three support spans.
Stress at different stage
Final fracture
1 mm Deflections/its ratio in fracture stress
2 mm Deflections/its ratio in fracture stress
3 mm Deflections/its ratio in fracture stress
Specimens with 60 support span
1000 MPa
92 MPa/9.2%
173 MPa/17.3%
254 MPa/25.4%
8
Specimens with 80 support span
700 MPa
75 MPa/10.7%
130 MPa/18.57%
180 MPa/25.7%
Specimens with 100 support span
500 MPa
64.3 MPa/12.86%
96 MPa/19.2%
134.5 MPa/26.9%
Composite Structures 261 (2021) 113280
M. Wang et al.
Fig. 10. The mechanical and electrical responses of the CNT-based nanocomposites with the support spans (80 mm).
when the support spans of 80 mm and 100 mm were separately
adopted for flexural tests, meanwhile generated highest resistance
changes ratio ΔR/R0 at every given deflections (shown in Fig. 11b),
which clearly evidenced higher sensitivity of the CNT buckypaper sensor. Comparatively, the needed loads (i.e. 134.5 MPa at 3 mm deflection shown in Fig. 12a) to maintain the same deflections when the long
support span (100 mm) was adopted were lowest than that generated
on the samples with the support spans of 60 mm and 80 mm. The
piezoelectric sensitivity (shown in Fig. 12b) from the samples subjected to the long support span (100 mm) was relatively lowest than
that generated on the samples with the support spans of 60 mm and
80 mm. Significantly, the accumulated micro‐damages eventually
induced the abrupt fluctuation of the stress curve after the 11th cyclic
loading where the whole curve decreased to a certain extent (blue dotted circle in Fig. 12a), while the buckypaper sensor have already given
the indication of partial fractures of the CNT‐based nanocomposites
where the obviously electrical signal drift were observed at the beginning of the 12th cyclic loading (marked in Fig. 12b) ahead of the aforesaid changing viewed from stress curve. This slight difference indicates
that the CNT buckypaper sensor can sense the subtle strain or damage
inside of the composite materials and then further offer the prediction
of the mechanical behavior of the composite components. It is interesting to note, the irreversibly internal damage occurred on the samples
when subjected to the support spans of 100 mm was earlier compared
with other samples with the support spans of 60 mm and 80 mm (no
obvious resistance drift), mainly due to the maximum stress generated
at every deflection taking a larger proportion (i.e. 26.9%, 19.2% and
12.86% respectively marked in Fig. 12b) in the final fracture stress,
although the flexural stress generated on the samples with support
spans of 100 mm at every deflection is inferior to other samples with
the support spans of 60 mm and 80 mm. This slight difference can contribute to explore more reliable and desirable sensor configuration,
low linearly along with the mechanical deformation during both bending and releasing phases, showing stably piezoresistive response till
after the twelfth loading cycles. Furthermore, the electrical responses
at deflections (δ = 1 mm, 2 mm) which undergo lower than 20% of
the fracture stress, can return to initial value after each single cycle
thus pointing that the applied loads at this scope induce reversible
variations inside of the nanotube network configuration [2]. Nevertheless, when the accumulated micro strains or cracks inside of the material were over than the threshold value of intrinsic strength of the
CNT‐based nanocomposites, the critical damages would occur, resulting in the inreversible breaks in the electrical network and further the
shifting of the ΔR/R0 curve.
Considering the practical service conditions of sensors for composite components health monitoring, e.g. wing spar, more information
about the electrical responses of the CNT buckypaper sensor at various
mechanical behaviors need to be obtained, the piezoresistive properties of the sensors were therefore examined in regard to different support span lengths. Due to the difference of the support span length, the
measured loads induced by the uniform deflections (δ = 1 mm, 2 mm,
3 mm) were also different accordingly.
Furthermore, Table 2 intuitively outlines the average flexural stress
at different loading stages for specimens with three support spans
(60 mm, 80 mm and 100 mm). The average flexural stress observed
from the specimens with 60 mm support span length shows the maximum value no matter the deflections the specimens changed and that
stress shows a decline trend along with the increasing of support span
lengths. While, the ratio of flexural stress at different loading deflection in the final fracture stress displays an increasing trend along with
the increasing of support span lengths.
Specifically, a short distance configuration of support span
(60 mm) led to highest load (i.e. 254 MPa at 3 mm deflection shown
in Fig. 11a) to maintain the same deflections comparing with the loads
Fig. 11. The mechanical and electrical responses of the CNT-based nanocomposites with the support spans (60 mm).
9
M. Wang et al.
Composite Structures 261 (2021) 113280
Fig. 12. The mechanical and electrical responses of the CNT-based nanocomposites with the support spans (100 mm).
II) Stable propagation process of irreversible damage: at BC phase,
the microscopic damages started to accumulate within the
material, the progressively increase of the inter‐nanotube distance therefore led to the break of the mentioned equilibrium,
contributing to the irreversible breakage of the conductive network of the composite interphase and the increasingly electrical
resistance ratio. It is noteworthy that the increase of the electrical resistance ratio showed linear relationship, indicating the
stable damage propagation.
III) Accumulative process of creep damages: after the weak percolation network has been broken permanently and the rearrangement of the new conductive networks have completed, the
health monitoring of the CNT buckypaper sensor fell into a relatively stable phase (CD) where the drift of the electrical resistance ratio was quite imperceptible, which can be roughly
considered as the undamaged response. This balance state of
the disconnection and reconnection of the conductive networks
[42] at loading cycle would be disturbed when the creep damages induce suddenly evidenced change in the electrical resistance ratio, such as BC phase. At this CD phase, the CNT
buckypaper sensor still possesses reliable and reversible signal
indication to predict the mechanical behavior of the composite.
However, the electrical resistance is apparently going to keep
health monitoring modes and service conditions using the multifunctional nano‐sensor.
To perform the dynamic durability investigation, the electrical
resistance behavior of composite under cyclic loading was monitored
for the given deformation of 1 mm with the crosshead speed of
2 mm/min at room temperature, where the samples with the support
spans of 80 mm was employed. Specifically, the partial curve of the
electrical resistance ratio ΔR/R0 for bending/releasing cycle from 40
to 132 cycles was plotted in Fig. 13, dividing into three typical phases.
Generally, the repeatable values of the ΔR/R0 are measured and the
strong correlation between the deformation of the specimen and the
variation of the electrical resistance is evidently observed. There is
no sharp change occurred mainly ascribed to the real‐time stress which
was only around 20% of the fracture stress, where the applied strain
was limited in the elastic regime of the material.
I) Initial undamaged response: initially, the electrical resistance
ratio ΔR/R0 at AB phase (up to 94th cycle) presented repeatable
response with no obvious drift observed due to the competition
between the increase and decrease of the tunneling effects in
bending/releasing processes were almost in equilibrium.
Fig. 13. The electrical resistance behavior of CNT-based nanocomposites under cyclic loading (around 100 bending/releasing cycles).
10
Composite Structures 261 (2021) 113280
M. Wang et al.
growing along with duration of monitoring process until up to
the infinity, afterwards the accumulated damages will result
in irreversible fracture of the composite laminates and conductive networks.
It can be inferred that the changing trend of the electrical resistance
ratio within the whole monitoring process is related with the time‐
dependent characterization of the gradual damages accumulation
inside of the composite material during the whole lifecycle, verifying
a high‐sensitive flexural sensor for health monitoring, self‐diagnosis
and damage prediction of composite materials.
4.2. Morphologies analysis of the CNT-based nanocomposites
As evident from SEM images (see Fig. 6 in Section 3.3), the morphological micrographs describe the details of the random distribution
and possible tunneling paths between the entangled networks and the
acceptable infiltration and diffusion of epoxy resin into the buckypaper due to the porous networks, introducing a superior combination
of CNT‐resin‐fiber interphase. The strong interaction among CNT,
resin and fiber and the interlaminar enhancement effect of CNT buckypaper have been deeply discussed in previous work [27,29]. SEM
fracture surfaces were examined after different types of cyclic loading–unloading tests, the typical details further explained the health
monitoring principle of the CNT buckypaper sensor. As mentioned in
the Fig. 3 (Section 2.2), the weak conductive paths or combination
of CNT‐resin‐fiber interphase will be interrupted upon bending, like
the ‘Break’ marks in Fig. 3d. During the releasing process (Fig. 3f),
the divided phases tend to merge each other together again, the original conductive paths will face to reconnect over again or the new tunneling effect will emerge to form the conductive network and transmit
the electrical response of the monitoring information. Similarly, as
reflected on the physical picture (see Fig. 14), the red marked crack
in Fig. 14a was observed which contributes to the changing of the electrical resistance ratio ΔR/R0. Upon bending process (see Fig. 14a), the
micro crack will accumulate in the weak connection parts of CNT‐
resin‐fiber interphase and increasingly expand till to next stable phase
or the formation of new crack. Upon releasing process (see Fig. 14b),
the enlarged cracks will recover and the conductive paths will reconnect. The exposed CNT played especially important role in synergistically bridging the conductive gap between the neighboring CNT at the
releasing process, contributing to the reason of the stability and
repeatability of the CNT buckypaper sensor. It can be considerably
equal to the self‐healing process.
Furthermore, Fig. 15 revealed the micro view of the fracture surface to gain a further insight into the operating principle of the CNT‐
based nanocomposite. Corresponding to the ladder‐shaped curve in
Fig. 9, when microcrackings initiated from the material (as shown in
Fig. 15), the electrical resistance increases from a current level to a
higher level, it will probably result in the first step and following steps
Fig. 15. The physical pictures of micro cracks under bending and releasing
processes.
of that ladder‐shaped curve (the Red curve in Fig. 9), which can be act
as the warning instructions for operators. The final state of the electrical resistance ratio ΔR/R0 suddenly jumping to the infinity will be
associated with the broken (see Fig. 16a and 16b) of CNT‐based
nanocomposite. Interestingly, the Fig. 16a obtained from the specimens with 60 mm support span, expresses that the pioneered failure
tends to appear in the fiber and interface, due to the superior averaged
cyclic loading. While the Fig. 16b captured that micro cracks (obtained
from the specimens with 100 mm support span) are prone to extend
primitively on buckypaper and interface, likely due to the frangibility
of the CNT buckypaper. The variation of the electrical resistance ratio
and the accumulated cracks in fiber‐buckypaper interface of the CNT‐
based nanocomposite perfectly demonstrated the damage evolution of
material, identifying the flexural sensor as a serviceable indicator with
the health monitoring feature for structural detects in composites.
5. Conclusion
This research aimed to understand the reaction of the high sensitive
CNT buckypaper sensor on health monitoring of composite materials
under flexural behavior. The working mechanisms of CNT buckypaper
sensor during loading and releasing processes were deeply elaborated
by analyzing the electrical conduction mechanism of the adjacent CNT
and developing the macroscopical and mesoscopic models of CNT electrical networks inside of the bukypaper. The CNT‐based nanocomposite specimens were investigated under monotonic and cyclic flexural
loading, and the electrical resistance change ratios were collected
Fig. 14. The physical pictures of micro cracks under bending and releasing processes.
11
M. Wang et al.
Composite Structures 261 (2021) 113280
Fig. 16. The physical pictures of micro cracks at final state of the laminate (up to fractured state).
nautical Digital Manufacturing Process and Liaoning Key Laboratory of
Advanced Polymer Matrix Composites Manufacturing Technology for
supporting the materials, equipment and other research activities.
and compared to examine the dynamic stability and durability of the
nano sensor.
By setting different support span configurations to simulate various
application scenarios of the CNT‐based nanocomposite, the slight differences among nano sensors responses were noted to help understanding the desirable sensor configuration, health monitoring
modes and service conditions. The failure rule of the CNT‐based
nanocomposite in long‐term service was drawn through long‐term cyclic bending/releasing experiments: when the subtle creep damages
accumulated to the breakage threshold of weak percolation networks
(failure of CNT conductive networks or fiber‐resin‐CNT interface),
the ΔR/R0 would increase sharply till to a stable phase where the
remained CNT conductive networks proceeded with stable signal output, and then the nanocomposite would repeat the accumulative process of creep damages and the stable propagation process of
irreversible damage until to the final fracture limit.
The SEM images demonstrated the desirable agreement between
the changing of ΔR/R0 signal and phenomena of the breakage and
propagation of the micro cracks and the rearrangement and reconnection of the CNT conductive networks within the buckypaper.
However, in order to achieve industrial applications, the mathematical relationship function between the ΔR/R0 changing and the
strain propagation of the CNT‐based nanocomposite under long‐term
service are necessary to explore thoroughly, and the need of more relevant mechanical tests is essential to conclude the regular sensing
characteristics of the CNT buckypaper sensor. It still requires advanced
technologies to optimize the combination degree between the CNT
buckypaper and the composite laminates. A better support substrate
calls for further inquiry to permit the stretchability and flexibility of
the buckypaper sensor to a greater extent. The above gaps will be covered in future work to pursue all‐purpose CNT buckypaper sensor for
related fields.
Funding
This work was supported by.
Foundation of Liaoning Province Education Administration No.
L201734.
Key Program of Natural Science Foundation of Liaoning Province of
China No. 20170520019.
Data Availability Statement
All data included in this study are available upon request by contact
with the corresponding author.
References
[1] Li H, Zhu L, Sun G, Dong M, Qiao J. Deflection monitoring of thin-walled wing spar
subjected to bending load using multi-element FBG sensors. Optik
2018;164:691–700. https://doi.org/10.1016/j.ijleo.2018.03.067.
[2] Vertuccio L, Guadagno L, Spinelli G, Lamberti P, Tucci V, Russo S. Piezoresistive
properties of resin reinforced with carbon nanotubes for health-monitoring of
aircraft primary structures. Compos B Eng 2016;107:192–202. https://doi.org/
10.1016/j.compositesb:2016.09.061.
[3] Liu Y, Zhang D, Wang K, Liu Y, Shang Yu. A novel strain sensor based on graphene
composite films with layered structure. Compos A Appl Sci Manuf
2016;80:95–103. https://doi.org/10.1016/j.compositesa:2015.10.010.
[4] Yang Y, Chiesura G, Vervust T, Bossuyt F, Luyckx G, Degrieck J, et al. Design and
fabrication of a flexible dielectric sensor system for in situ and real-time
production monitoring of glass fibre reinforced composites. Sens Actuators, A
2016;243:103–10.
[5] Balaji R, Sasikumar M. Development of strain and damage monitoring system for
polymer
composites
with
embedded
nickel
alloys.
Measurement
2017;111:307–15. https://doi.org/10.1016/j.measurement.2017.07.036.
[6] Chun S, Choi Y, Park W. All-graphene strain sensor on soft substrate. Carbon
2017;116:753–9. https://doi.org/10.1016/j.carbon.2017.02.058.
[7] Rito RL, Crocombe AD, Ogin SL. Health monitoring of composite patch repairs
using CFBG sensors: experimental study and numerical modelling. Compos A Appl
Sci
Manuf
2017;100:255–68.
https://doi.org/10.1016/
j.compositesa:2017.05.012.
[8] Kakei A, Epaarachchi JA, Islam M, Leng J. Evaluation of delamination crack tip in
woven fibre glass reinforced polymer composite using FBG sensor spectra and
thermo-elastic response. Measurement 2018;122:178–85. https://doi.org/
10.1016/j.measurement.2018.03.023.
[9] Lampani L, Sarasini F, Tirillò J, Gaudenzi P. Analysis of damage in composite
laminates with embedded piezoelectric patches subjected to bending action.
Compos
Struct
2018;202:935–42.
https://doi.org/10.1016/
j.compstruct.2018.04.073.
[10] Butler S, Gurvich M, Ghoshal A, Welsh G, Attridge P, Winston H, Urban M, Bordick
N. Effect of embedded sensors on interlaminar damage in composite structures. J
Intell Mater Syst Struct 2011;22(16):1857–68. https://doi.org/10.1177/
1045389X11414225.
[11] Chiacchiarelli LM, Rallini M, Monti M, Puglia D, Kenny JM, Torre L. The role of
irreversible and reversible phenomena in the piezoresistive behavior of graphene
epoxy nanocomposites applied to structural health monitoring. Compos Sci
Technol 2013;80:73–9. https://doi.org/10.1016/j.compscitech.2013.03.009.
[12] Yang J, Ye Y, Li X, Lü X, Chen R. Flexible, conductive, and highly pressuresensitive graphene-polyimide foam for pressure sensor application. Compos Sci
Technol 2018;164:187–94. https://doi.org/10.1016/j.compscitech.2018.05.044.
CRediT authorship contribution statement
Meng Wang: Writing ‐ original draft, Validation, Investigation.
Nan Li: Conceptualization, Methodology, Software, Data curation,
Writing ‐ original draft, Writing ‐ review & editing. Gong‐Dong Wang:
Conceptualization, Methodology, Supervision. Qi Di Zhao: Validation.
Xi Liang Liu: Validation.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to thank the Shenyang Aerospace University,
Key Laboratory of Fundamental Science for National Defense of Aero12
Composite Structures 261 (2021) 113280
M. Wang et al.
[28] Rein MD, Breuer O, Wagner HD. Sensors and sensitivity: carbon nanotube
buckypaper films as strain sensing devices. Compos Sci Technol 2011;71
(3):373–81.
[29] Dong Wang G, Li N, Kirwa Melly S, Peng T, chi Li Y, Di Zhao Qi, De Ji S.
Monitoring the drilling process of GFRP laminates with carbon nanotube
buckypaper sensor. Compos Struct 2019;208:114–26. https://doi.org/10.1016/
j.compstruct.2018.10.016.
[30] Ruschau GR, Yoshikawa S, Newnham RE. Resistivities of conductive composites. J
Appl Phys 1992;72(3):953–9. https://doi.org/10.1063/1.352350.
[31] Panozzo F, Zappalorto M, Quaresimin M. Analytical model for the prediction of the
piezoresistive behavior of CNT modified polymers. Compos B Eng
2017;109:53–63. https://doi.org/10.1016/j.compositesb:2016.10.034.
[32] Takeda T, Shindo Y, Kuronuma Yu, Narita F. Modeling and characterization of the
electrical conductivity of carbon nanotube-based polymer composites. Polymer
2011;52(17):3852–6. https://doi.org/10.1016/j.polymer.2011.06.046.
[33] Simmons JG. Generalized formula for the electric tunnel effect between similar
electrodes separated by a thin insulating film. J Appl Phys 1963;34:1793–803.
https://doi.org/10.1063/1.1702682.
[34] Wang X, Meng S, Tebyetekerwa M, Li Y, Jürgen P, Jürgen BS. Highly sensitive and
stretchable piezoresistive strain sensor based on conductive poly(styrenebutadiene-styrene)/few layer graphene composite fiber. Compos A
2018;105:291–9.
[35] Luo S, Obitayo W, Liu T. SWCNT-thin-film-enabled fiber sensors for lifelong
structural health monitoring of polymeric composites - from manufacturing to
utilization to failure. Carbon 2014;76:321–9. https://doi.org/10.1016/
j.carbon.2014.04.083.
[36] Hu H, Fukunaga H, Atobe S, Liu Y, Li J. Piezoresistive strain sensors made from
carbon nanotubes based polymer nanocomposites. Sensors 2011;11
(11):10691–723.
[37] Dürkop T, Getty SA, Cobas E, Fuhrer MS. Extraordinary mobility in
semiconducting carbon nanotubes. Nano Lett 2004;4(1):35–9. https://doi.org/
10.1021/nl034841q.
[38] Sanli A, Benchirouf A, Müller C, Kanoun O. Piezoresistive performance
characterization of strain sensitive multi-walled carbon nanotube-epoxy
nanocomposites. Sens Actuators, A 2017;254:61–8.
[39] Costa P, Ferreira A, Sencadas V, Viana JC, Lanceros-Méndez S. Electro-mechanical
properties of triblock copolymer styrene–butadiene–styrene/carbon nanotube
composites for large deformation sensor applications. Sens Actuators, A
2013;201:458–67.
[40] Ferreira A, Rocha JG, Ansón-Casaos A, Martínez MT, Vaz F, Lanceros-Méndez S.
Electromechanical performance of poly(vinylidene fluoride)/carbon nanotube
composites for strain sensor applications. Sens Actuators, A 2012;178:10–6.
[41] Bonavolontà C, Camerlingo C, Carotenuto G, De Nicola S, Longo A, Meola C, et al.
Characterization of piezoresistive properties of graphene-supported polymer
coating for strain sensor applications. Sens Actuators, A 2016;252:26–34.
[42] Zheng Y, Li Y, Dai K, Wang Y, Zheng G, Liu C, Shen C. A highly stretchable and
stable strain sensor based on hybrid carbon nanofillers/polydimethylsiloxane
conductive composites for large human motions monitoring. Compos Sci Technol
2018;156:276–86. https://doi.org/10.1016/j.compscitech.2018.01.019.
[13] Zha J-W, Zhang Bo, Li RKY, Dang Z-M. High-performance strain sensors based on
functionalized graphene nanoplates for damage monitoring. Compos Sci Technol
2016;123:32–8. https://doi.org/10.1016/j.compscitech.2015.11.028.
[14] Moriche R, Jiménez-Suárez A, Sánchez M, Prolongo SG, Ureña A. High sensitive
damage sensors based on the use of functionalized graphene nanoplatelets coated
fabrics as reinforcement in multiscale composite materials. Compos B Eng
2018;149:31–7. https://doi.org/10.1016/j.compositesb:2018.05.013.
[15] Luo S, Yang J, Song X, Zhou Xi, Yu L, Sun T, Yu C, Huang D, Du C, Wei D. TunableSensitivity flexible pressure sensor based on graphene transparent electrode. SolidState Electron 2018;145:29–33. https://doi.org/10.1016/j.sse.2018.04.003.
[16] Niu D, Jiang W, Ye G, Wang K, Yin L, Shi Y, Chen B, Luo F, Liu H. Grapheneelastomer nanocomposites based flexible piezoresistive sensors for strain and
pressure detection. Mater Res Bull 2018;102:92–9. https://doi.org/10.1016/
j.materresbull.2018.02.005.
[17] Lampani L, Gaudenzi P. Innovative composite material component with embedded
self-powered wireless sensor device for structural monitoring. Compos Struct
2018;202:136–41. https://doi.org/10.1016/j.compstruct.2018.01.011.
[18] Kou HR, Zhang L, Tan QL, Liu GY, Lv W, Lu FX, et al. Wireless flexible pressure
sensor based on micro-patterned Graphene/PDMS composite. Sens Actuators, A
2018;277:150–6.
[19] Kravchenko OG, Pedrazzoli D, Bonab VS, Manas-Zloczower I. Conductive
interlaminar interfaces for structural health monitoring in composite laminates
under fatigue loading. Mater Des 2018;160:1217–25. https://doi.org/10.1016/
j.matdes.2018.10.045.
[20] Spinelli G, Lamberti P, Tucci V, Vertuccio L, Guadagno L. Experimental and
theoretical study on piezoresistive properties of a structural resin reinforced with
carbon nanotubes for strain sensing and damage monitoring. Compos B Eng
2018;145:90–9. https://doi.org/10.1016/j.compositesb:2018.03.025.
[21] Lee G-Y, Kim M-S, Yoon H-S, Yang J, Ihn J-B, Ahn S-H. Direct printing of strain
sensors via nanoparticle printer for the applications to composite structural health
monitoring. Procedia CIRP 2017;66:238–42. https://doi.org/10.1016/j.
procir.2017.03.279.
[22] Wang Y, Wang Y, Wan B, Han B, Cai G, Li Z. Properties and mechanisms of selfsensing carbon nanofibers/epoxy composites for structural health monitoring.
Compos
Struct
2018;200:669–78.
https://doi.org/10.1016/
j.compstruct.2018.05.151.
[23] Wang Q, Arash B. A review on applications of carbon nanotubes and graphenes as
nano-resonator sensors. Comput Mater Sci 2014;82:350–60. https://doi.org/
10.1016/j.commatsci.2013.10.010.
[24] Wang X, Sparkman J, Gou J. Strain sensing of printed carbon nanotube sensors on
polyurethane substrate with spray deposition modeling. Compos Commun
2017;3:1–6. https://doi.org/10.1016/j.coco.2016.10.003.
[25] Yang B, Xuan F-Z, Lei H, Wang Z, Xiang Y, Yang K, Tang X, Liang W.
Simultaneously enhancing the IFSS and monitoring the interfacial stress state of
GF/epoxy composites via building in the MWCNT interface sensor. Compos A Appl
Sci Manuf 2018;112:161–7. https://doi.org/10.1016/j.compositesa:2018.06.006.
[26] Lee D, Hong HP, Lee CJ, Park CW, Min NK. Microfabrication and characterization
of spray-coated single-wall carbon nanotube film strain gauges. Nanotechnology
2011;22(45):455301. https://doi.org/10.1088/0957-4484/22/45/455301.
[27] Li N, Wang GD, Melly SK, Peng T, Li YC, Zhao QD, et al. Interlaminar properties of
GFRP laminates toughened by CNTs buckypaper interlayer. Compos Struct
2019;208:13–22.
13