Comparative study on piezoresistive properties of CFRP tendons prepared by two different methods

Accepted Manuscript Comparative study on piezoresistive properties of CFRP tendons prepared by two different methods Jie Yin, Rong-gui Liu, Jun-jie Huang, Ge Liang, Dan Liu, Gui-hua Xie PII:

S1359-8368(16)31731-0

DOI:

10.1016/j.compositesb.2017.07.064

Reference:

JCOMB 5200

To appear in:

Composites Part B

Received Date: 25 August 2016 Revised Date:

31 May 2017

Accepted Date: 29 July 2017

Please cite this article as: Yin J, Liu R-g, Huang J-j, Liang G, Liu D, Xie G-h, Comparative study on piezoresistive properties of CFRP tendons prepared by two different methods, Composites Part B (2017), doi: 10.1016/j.compositesb.2017.07.064. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Comparative study on piezoresistive properties of CFRP tendons prepared by two different methods

a

RI PT

Jie Yina, b, *, Rong-gui Liua, Jun-jie Huanga, Ge Lianga, Dan Liua, Gui-hua Xiea ,

Department of Civil Engineering, Faculty of Civil Engineering and Mechanics, Jiangsu

University, Zhenjiang 212013, China

Department of Civil and Environmental Engineering, University of Wisconsin–Madison,

SC

b

Madison, WI 53706, USA

M AN U

* Corresponding Author, e-mail [email protected]; [email protected] Abstract

AC C

EP

TE D

Carbon fiber reinforced polymer (CFRP) material is an exceptionally and light fiber-reinforced polymer that can be used as a smart sensor due to its remarkable piezoresistivity. This study presents a comparative study on piezoresistive characteristics of CFRP tendons manufactured by two different methods, i.e., the vacuum process method (VPM) and the brush process method (BPM). The tensile and resistance tests were conducted simultaneously on two groups of CFRP tendon specimens respectively by VPM and BPM to investigate their differences in initial resistances, piezoresistive performance and sensitivity. Test results showed that the initial resistance of the CFRP tendons prepared by the VPM is larger than those prepared by the BPM. The resistances of the CFRP tendon specimens by two methods increase uniformly with increasing strain. The resistance increases more quickly at the same strain increment for specimens prepared by BPM compared with those for VPM. Three stages can be observed according to the variation of fractional change in resistance (dR/R) with tensile strain. The scope of dR/R varies from 0% to 55% for BPM specimens, which is much wider than that for VPM specimens, which fall within 8.5%. The result is that BPM specimens are more sensitive than that of VPM. The values of sensitivity ‘K’ obtained for BPM specimens varying from 22 to 28 are much bigger than that for VPM ranging from 2.7 to 3.5. For BPM specimens, the pre-tension process can tighten up the fibers to reduce crimp and significantly reduce the amounts of bubbles during the brush process. However, the specimens prepared by VPM without undergoing the pre-tension process showed a relative looser state and carbon fibers might crimp.

Keywords CFRP tendons; Piezoresistivity; resistance; sensitivity.

1

ACCEPTED MANUSCRIPT

1 Introduction

Carbon fiber reinforced polymer (CFRP) materials have been extensively used in recent years as substitutes for the conventional pre-stressed steel due to their notably advantages, such as high strength-to-weight ratio, light weight, considerable flexibility, excellent fatigue performance, corrosion resistance and other notable properties (Crouch et al., 2013; Cai et al., 2015; Cai et al.

RI PT

2016; Kim et al. 2016; Barbosa et al. 2016; Liu et al. 2017; Xie et al. 2017). In addition to these superior mechanical properties, CFRP materials also possess good self-awareness behaviors as a self-monitoring materials resulting from their remarkable piezoresistivity (Bakis et al., 2001; Chung et al., 2004; Haj-Ali et al. 2014; Todoroki et al. 2014; Ma et al., 2015; Shen et al., 2015; Zuo et al

SC

2015; Kwon et al. 2016; Liu et al. 2016; Yang et al. 2016), which can be used as smart sensing materials in the application of structural health monitoring (SHM) systems for large-scale structures

M AN U

and infrastructures, such as long-span bridges, large dams, high-rise buildings, etc. Self-monitoring capability comes from the piezoresistive effect in carbon fibers. Therefore, the real-time monitoring and non-destructive evaluation (NDE) in-service of the structure health status can be obtained based on the evaluation of the piezoresistive properties of CFRP materials (Yuen, et al., 2007; Kwon et al.; Arai et al., 2011; Wang and Chung, 2013). Extensive researches have been

TE D

undertaken to examine the piezoresistive effects of CFRP materials, such as CFRP tendons (Kwon et al.,2015; Bakis et al.,2001; Suzuki et al., 2015; Yang and Wu, 2006), CFRP layers(Chung et al., 2004; Todoroki and Yoshida, 2004; Wang et al., 2006; Sevkat et al., 2008; Todoroki et al., 2010; Vavouliotis

EP

et al., 2011;Wang et al.2017), and carbon powder /epoxy resin matrix composites (Yuen et al., 2007; Dong et al., 2015; Guadagno et al., 2015; Ma et al., 2015; Shen et al., 2015).

AC C

Till now, most studies on the piezoresistive properties of CFRP materials have been limited in the scientific premise. Although CFRP tendons used as force-bearing materials have been industrialized for some time, the CFRP tendon as smart sensing materials has no standard manufactured process yet. The processing methods do affect the quality of the CFRP tendon specimen and its piezoresistivity. This study presents a comparative study on piezoresistive characteristics of CFRP tendons manufactured by two different methods. The tensile and resistance tests were carried out to obtain characteristics of the piezoresistive properties of CFRP tendons and some recommendations are provided to improve the manufacturing method and get better effects in practical application.

2

ACCEPTED MANUSCRIPT

2 Materials and methods 2.1 Materials

PAN-based monofilament carbon fiber bundle (T300B-3K), which was produced by the HENGSEHN Co., Ltd., in Zhenjiang city of China. Table 1 lists the main properties of monofilament carbon fiber bundle. Type HZ9901 epoxy resin matrix with Constituent A as resin, Constituent B as curing agent

RI PT

was produced by Nanjing HAOZHUO Materials Technology Co., Ltd. in China. Mold release agent is produced by Mirror Glaze. Besides, silver conductive adhesives (DAD-40) manufactured by Shanghai Synthetic Resin Research Institute in China as well as copper foils and copper wires were also used to prepare the CFRP tendon.

Tensile

Carbon

Name

Elongation (%) 3480

Content (%)

M AN U

Strength (MPa) T300B-3K

SC

Table 1 Properties of monofilament carbon fiber bundle

96

1.5

Resistivity ( Ω⋅ cm ) 1.5×10-3

2.1.1 CFRP tendon prepared by Vacuum Process Method (VPM)

The main procedures using the VPM to prepare the CFRP tendon specimens are listed as follows:

TE D

(1) Brush the mold release agent evenly onto the inside wall of the steel mold. Lay a tow of carbon fiber monofilament (3000 filaments) with 60 cm in length one by one into the steel mold (length=50 cm, section size=10 mm×10 mm and thickness=1 mm), as shown in Fig. 1 (a). Check the appearance

EP

of the tow of filament and make sure they are smooth and tidy before we lay them to the mold. (2) When the steel mold is full of carbon fiber monofilaments, press a steel bar (length=50 cm,

AC C

breadth=8 mm and thickness=2 mm) as a cap on the carbon fibers to make them dense. Wrap up the whole steel mold with vacuum membrane to make a firm seal, as shown in Fig. 1 (b). (3) Cap the steel mold filled with rubber plugs at both ends before cutting the excess fibers, as shown in Fig. 1(c). Prepare the mixed epoxy liquid with constituent A as resin and constituent B as curing agent and put the liquid into the funnel. Turn on the vacuum pump and let mixed epoxy liquid flow through the carbon fibers into the jar under the negative vacuum pressure. The jar can prevent vacuum pump from being polluted by the epoxy liquid. (4) After a time, the carbon fibers will be infiltrated by the mixed epoxy liquid. Turn off the vacuum pump and take out the steel mold. Cure the specimen until it becomes solid, and release it out of 3

the mold, as shown in the Fig. 1ACCEPTED (d).

MANUSCRIPT

(5) Cut off the surplus of the both ends of the specimen to 40 cm in length and keep the section clean and smooth. Cut the copper foil into the same size as the section with 8 mm by 7 mm. Weld the copper wire onto the copper foil. Brush the silver conductive adhesive on the copper foil and stick it to the section of specimen as an electrode (see Fig. 1 (e)).

RI PT

(6) Prepare a protective layer by brushing the epoxy resin onto the electrode. The CFRP tendon specimen prepared by VPM will be done after curing for 3 days (see Fig. 1 (f)).

Fig.2 shows the photographs of three replicate specimens prepared by VPM and the length of each

AC C

EP

TE D

M AN U

SC

specimen is 40 cm and sectional area is 8 mm by 7 mm.

4

ACCEPTED MANUSCRIPT

Wrap

(b)

RI PT

(a) Funnel

SC

Rubber plug

TE D

(e)

M AN U

(c)

(d)

(f)

AC C

EP

Fig.1. Steps of VPM to prepare CFRP tendon specimen

Fig. 2. Photographs of CFRP tendon prepared by VPM

5

MANUSCRIPT 2.1.2 CFRP tendon prepared byACCEPTED Brush Process Method (BPM) The main procedures of the BPM to prepare CFRP tendons are listed as below: (1) Select a large glass plate for placing the carbon fibers and keep the surface of the plate smooth and clean. Use adhesive tape to fix one side and apply a pre-tension force on the other side to ensure the carbon fiber bundles are tightened and adhered to the glass plate. The carbon fiber

RI PT

bundles are arranged approximately parallel, as shown in Fig.3 (a). (2) Prepare the epoxy matrix liquid with constituent A as resin and constituent B as curing agent. Softly brush the liquid onto the carbon fiber bundles on the plate. Keep the process of brushing evenly and prevent forming bubbles (see Fig. 3 (b)).

SC

(3) Remove the plate surface carefully followed by putting it into a curing chamber for cure/solidification after brushing, as shown in the Fig. 3 (c). Keep the carbon fiber undisturbed and

M AN U

prevent damaging or breaking of it.

(4) Cut off the surplus solid epoxy around the carbon fiber bundle, as shown in the Fig. 3 (d). Prevent cutting off the carbon fibers to make them continuous and guarantee their conductivity. (5) Brush the resin matrix liquid again on the trimmed carbon fiber bundles (see Fig.3 (e)). (6) Place the prepared carbon fiber bundle one by one into a mold (length=50 cm, section size=10

TE D

mm×10 mm and thickness=1 mm) which is pre-coated with the model release agent, as shown in Fig.3 (f). After filling up the mold, use a steel bar (length=50 cm, breadth=8 mm and thickness=2 mm) to compress the mold, and thereafter put the mold into the curing box. The first brush process

reinforcement.

EP

is to solidify the carbon fiber and this second brush step is to make the carbon fiber into

AC C

(7) Take out the CFRP tendon from mold after maintaining process. Cut off the surplus of the both ends of the specimen to 40 cm in length and keep the section clean and smooth. Cut the copper foil into the same size as the section with 8 mm by 7 mm. Weld the copper wire onto the copper foil. Brush the silver conductive adhesive on the copper foil and stick it to the section of specimen as an electrode. Trim and clean two sides of tendon. Make sure the cross section is flush and clean (see Fig. 3 (g)). (8) Prepare a protective layer by brushing the epoxy resin onto the electrode. The CFRP tendon specimen prepared by BPM will be done after curing for 3 days (see Fig. 3 (h)). Fig.4 shows the photographs of three replicate specimens prepared by BPM and the size of each specimen is the same as VPM specimen with 40 cm in length and 8 mm by 7 mm in sectional area. 6

ACCEPTED MANUSCRIPT

(b)

M AN U

SC

RI PT

(a)

(d)

(f)

AC C

EP

(e)

TE D

(c)

(g)

(h)

Fig.3. Steps of BPM to prepare CFRP tendon specimen

Fig. 4. Photographs of CFRP tendon prepared by BPM 7

ACCEPTED MANUSCRIPT

2.2 Testing Methods

For comparison, two groups of CFRP tendon specimens were prepared with the proposed two methods, i.e. VPM and BPM, respectively. For each group, three replicate specimens numbered as 1#, 2#, and 3# for the tensile and resistance tests with a displacement-controlled tensile testing system (Electronic universal testing machine) and a digital avometer.

RI PT

The tensile and resistance tests were conducted at the same time. The tensile test was carried out on the electronic universal testing machine and the resistance test was performed by the digital avometer. The electronic universal testing machine can exert tensile load on the CFRP tendon specimens. The strain is calculated from the displacement measured by the extensometer under

SC

different tensile loads while the digital avometer recorded the change of the resistance at the same time.

M AN U

Before the tensile and resistance tests, we need to measure the initial resistance for each CFRP tendon specimens first. Since silver conductive adhesives used to prepare the electrode at both ends of CFRP need time to curing, we measure the initial resistance of the tendon specimens after 3 days. For checking the resistance stability for CFRP tendon as a smart material, we can use voltage-current (V-C) curves where the X-axis and Y-axis represent the current and the voltage

TE D

respectively. For a material with stable resistance, the V-C curve should be a straight line and the slope of the curve refers to the resistance, which means the resistance of the material will not increase or decrease with the change of voltage or current. Therefore, the CFRP tendon can be used

EP

as a smart material to sensor the mechanical filed change. In this study, we use stable currents as 2mA, 4mA, 8mA and 10mA, and measure the corresponding voltage under each current to check

AC C

the resistance stability.

3. Results

3.1 Initial Resistance

Fig. 5 shows the comparison of the initial resistances for each CFRP specimen prepared by VPM and BPM. The statistic mean values and standard deviations for each method are also shown in Fig. 5. We see that the initial resistance of the CFRP tendons prepared by the VPM is larger than those prepared by the BPM. It is estimated that the resin volume fraction for VPM is greater than that for BPM since the carbon fibers laid tow-by-tow for BPM undergoing a pre-tension process that can tighten up the fibers. For VPM, the specimen without undergoing pre-tension process shows a relative looser state which will result in higher resin volume of VPM than that of BPM, i.e. for the 8

ACCEPTED MANUSCRIPT same sectional area of the tendon, the carbon fibers of BPM outnumber that for VPM specimen.

20 16

VPM BPM

14.8

14.7 12.4

12 8 5.1

4.8

3.8

4

4.6

RI PT

Initial Resistance R0 (Ω)

17.2

2.4

0.68

0

1#

2#

3#

Mean Value

Standard Deviation

SC

Fig.5. Initial resistances of specimens prepared by VPM and BPM

M AN U

Defects

VPM

BPM

TE D

Fig.6. Exterior Comparison of CFRP

As shown in Fig. 5, the standard deviation for VPM is 2.4, which is greater than 0.68 for BPM indicating that the BPM has a better performance with relatively stable results than VPM.

EP

Accordingly, the initial resistance for VPM specimens varying from 12.4 to 17.2 shows a wider range compared to BPM specimen ranging from 3.8 to 5.1. Fig. 6 shows some obvious defects on the

AC C

surface of the CFRP tendon prepared by the VPM while the tendon manufactured by the BPM has a smoother and clean surface. These defects may be due to the gas bubbles retained in the mold before vacuum process and thus make the results of VPM more fluctuant. For VPM specimen, as shown in Fig. 7, the epoxy liquid flows and infiltrates the carbon fiber under the action of negative vacuum pressure. The epoxy liquid prefers to flow along conducted channels rather than open new channels. This will result in some defects in the specimen since some carbon fiber cannot be infiltrated during the vacuum process. Because the location and the size of defects are random, the initial resistance of the specimen prepared by the VPM is not stable in Fig. 5.

9

ACCEPTED MANUSCRIPT

Fig.7. Schematic of Defect Analysis for VPM specimen

RI PT

3.2 Voltage versus Current Fig. 8 shows the voltage-current (V-C) characteristics for the two types of CFRP tendons prepared by VPM and BPM. Although the initial resistances of the tendons using different methods are different, their V-C curves are almost linear. This result indicates that the CFRP tendons prepared in this study

for a smart material to the mechanical field changes. 60 VPM

50

120 90 60

Voltage (mV)

Voltage (mV)

150

M AN U

180

SC

has the potential to be used as sensors because their linear V-C relationship is a favorable condition

1# 2# 3#

30

BPM

40 30 20

1#

10

2# 3#

0

0

4

6

8

10

TE D

2

2

Current (mA)

4

6

8

10

Currnet (mA)

EP

Fig.8. V-C Characteristic Curve

3.3 Resistance versus Strain

AC C

Fig. 9 shows the variation of resistance with strain for different CFRP tendon specimens prepared by VPM (see Figs. 9(a) to 9(c)) and BPM (see Figs. 9(d) to 9(f)). Generally, the resistances of the CFRP tendon specimens increase uniformly with the increasing strain. Comparing the results, we can find out that the resistance increases more quickly at the same strain increment for BPM compared to VPM. For example, the resistance increases by 31.4% from 5.1Ω to 6.7Ω with the increase in strain from 0 to 2.4% in Fig.9 (d) with BPM, while the resistance increases by 5.2% from 17.2Ω to 18.1Ω at the same strain increment of 2.4 in Fig.9(a) with BPM. The same comparative results also can be found in the rest figures of Fig. 11 indicating that the CFRP tendon specimen manufactured by BPM is more sensitive to strain than that by VMP. 10

ACCEPTED MANUSCRIPT It should be mentioned that in Figs. 9 (b) and 9(c), the resistance for CFRP tendon specimens prepared by VPM exhibit a small decrease at the initial stage of the test with the strain increasing from 0 to 0.1%. The possible reason for this negative piezoresistive performance observation is that in the initial stage the electrodes to sample resistance are changing due to bedding in. Meanwhile, several fluctuations on resistance can be observed in Fig.9 for both VPM and BMP specimens with

15.6

12.8

VPM

17.4 17.2 17

2#

ε (%)

12.2 0 0.5 1 1.5 2 2.5 3 3.5

R (Ω)

7.5 7

6

6.5

5.5

6

BPM

5.5 5 4.5

5

(e)

5

0.5

1

1.5

2

2.5

1

2

3

4

(f)

4.4

BPM

2#

ε (%)

4.5

0

0

4.7

TE D

ε (%)

1#

ε (%)

3#

14.4

0 0.5 1 1.5 2 2.5 3 3.5 8

(d)

14.7

M AN U

7

VPM

15

12.4

1# ε (%)

15.3

VPM

12.6

(c)

SC

17.6

6.5

15.9

R (Ω)

17.8

16.2

(b)

R (Ω)

13

R(Ω)

(a)

R (Ω)

18

13.2

R (Ω)

18.2

RI PT

increasing strain, which is possibly caused by the measurement error of the instrumentation.

0 0.5 1 1.5 2 2.5 3 3.5

BPM

4.1 3.8

ε (%)

3#

3.5 0

0.5

1

1.5

2

2.5

3

EP

Fig.9. Variation of Resistance with Strain for different CFRP tendon specimens

3.4 Fractional change in resistance versus strain

AC C

Fig. 10 shows the variation of fractional change in resistance (dR/R) with tensile strain for CFRP tendon specimens prepared by two different methods. Three stages designated as Stage I, Stage II and Stage III, can be observed to be common for CFRP tendon specimens prepared by VPM and BPM.

At Stage I, the dR/R increases quickly as the strain increases from 0% to 1.0% for VPM specimens and from 0% to 0.8% for BPM specimens. In this stage, different curves show good linearity relations between dR/R and strain. The dR/R increases quickly with the increasing strain, indicating that the CFRP tendon shows good piezoresistive performance at this stage. The same linear increasing dR/R at lower strain range (from 0% to 0.3%) can be found in the literature (Wang, 11

2012) as shown in Fig. 11(b) ACCEPTED for single carbonMANUSCRIPT fiber tow(CFT) under tension, where test fiber T300B-3000 was manufactured by the Toray company in Japan. One single CFT with 50cm in length was infiltrated by epoxy resin before tension. The resistance of CFT increased from 75.7Ω to 76.2Ω under tension as shown in Fig. 11(a). Since the carbon fiber tows are all aligned, the resistance between fibers would contribute negligibly to the bulk resistance. The main contributor is the

RI PT

piezoresistivity of carbon fiber. Several sources (Bakis et al. 2001) may cause the increase in resistance for CFRP tendons and CFT: (1) geometry change caused by elastic deformation of fibers; (2) resistivity change caused by electrical changes within fibers; (3) resistivity change due to strain-dependent inter-fiber contact in a composite. Besides, the volume fraction of carbon fiber

SC

obtained for BPM specimen (23.8%) is greater than that for VPM specimen (15.9%), which will result in the different piezoresistive behavior between BPM and VPM specimens.

M AN U

At Stage II, the dR/R increases relatively slowly compared with Stage I as the strain increases from 1% to 2.6% for VPM specimens and from 0.8% to 2.4% for BPM specimens. Since the piezoresistivity is purely a function of resistance change as the carbon fiber as it is extended, the slower increase in resistance in this stage should be a characteristic of fibers. Besides, there are some fluctuations for three specimens prepared by VPM comparing with the BPM specimens

TE D

indicating that BPM specimen has a better performance than VPM.

At Stage III, the dR/R increases faster than Stage II as the strain is bigger than 2.6% for VPM specimens and 2.4% for BPM specimens. With the stain increases, the corresponding increasing

EP

tensile stress will enable carbon fibers breakage or fracture (Bakis et al. 2001; Chung, 2002), which make the conductive pathways decrease faster than that in stage II and therefore a fast-growing of

AC C

dR/R.

Comparing the CFRP tendon specimens prepared by VPM and the BPM, the difference is not

significant for the strain at each intersection point between stages I and II where stain is 0.8% for BPM and 1% for VPM, and between stages II and III where stain is 2.4% for BPM and 2.6% for VPM. The average strain that separate stages I and II is about 0.9% as well as stages II and III is about 2.5%. However, the change spectrum of dR/R for CFRP tendon specimens prepared by VPM is different from that by BPM. The scope of dR/R varies from 0% to 55% for BPM specimen which is much wider than that for VPM specimen falls within 8.5%. It can be found that the result of BPM is more sensitive than that of VPM. The variation of dR/R with strain by BPM is comparable with the testing result from the literature (Wang et al., 2007). 12

ACCEPTED MANUSCRIPT 9

60 (a)

8

(b)

VPM

Stage I

Stage III

Stage II

5 4

dR/R (%)

40 Stage I

Stage II

Stage III

30

3 20

2

1# 2# 3#

1 0

1# 2# 3#

10 0

-1 1

2

3

ε (%)

0

4

1

2

3

ε (%)

4

SC

0

RI PT

dR/R (%)

6

BPM

50

7

50

70

CFT VPM BPM

(a)

60 50

30 20 10 0 1

2

3

BPM VPM CFT

40

(b)

30

8

20

4 0

10

4

0

0.3

0.6

0 0

1

2

3

4 ε (%)

EP

0

TE D

40

dR/R (%)

R (Ω)

80

M AN U

Fig.10. dR/R versus ε for CFPR tendons prepared by (a) VPM and (b) BPM

ε (%)

Fig. 11. Comparison between the properties of CFRP tendons in this study and carbon fiber tow

AC C

(CFT) in the literature with (a) R versus ε and (b) dR/R versus ε

3.5 Sensitivity

The sensitivity of piezoresistive performance for a given material refers to the dR/R per unit strain. It is an important measure index for piezoresistive materials. The expression of the sensitivity K can be written as,

K=

dR / R

ε

where dR/R is fractional change in resistance and ε is the strain. 13

(3)

MANUSCRIPT When CFRP tendon is usedACCEPTED as a smart sensor, it works at a low stress state that corresponds to stage I in Fig. 10. The linear fitting method can be used to obtain the sensitivity, as illustrated in Fig. 11, where linear fitting equations were obtained for each specimen and R2 is the coefficient of determination.

(a) VPM

y3# = 3.498x - 0.302 R² = 0.959

2

y1# = 2.912x + 0.196 R² = 0.990

1

1# 2# 3#

0 -1 0

0.2

25

(b)

0.4

0.6

0.8

1

ε ( %)

BPM

y1# = 27.719x + 1.782 R² = 0.984

20

y2# = 27.60x + 2.16 R² = 0.969

TE D

dR/R (%)

M AN U

y2# = 2.705x - 0.380 R² = 0.985

SC

dR/R (%)

3

RI PT

4

15

1# 2# 3#

10

EP

5

y3# = 22.048x + 3.142 R² = 0.955

0

0.2

0.4

0.6

0.8

ε ( %)

AC C

0

Fig.12 Linear fittings to obtain sensitivity K for CFPR tendons prepared by (a) VPM and (b) BPM We can see from Fig.12 that all of the specimens have good linearity no matter it is prepared

by VPM or BPM. The coefficients of determination (R2) are consistently more than 0.95. However, the values of sensitivity K obtained from two preparing methods are different. For specimens prepared by VPM, the sensitivity K ranges from 2.7 to 3.5, while specimens prepared by BPM vary from 22 to 28. The K values obtained for BPM specimens are much bigger than that for VPM specimens. This might be due to the different performance subjected to lateral effect for specimens 14

prepared by VPM and BPM, asACCEPTED shown in Fig. 13.MANUSCRIPT Because of the pre-tension process for specimen prepared by BPM, carbon fibers in the specimen would be tighten up to reduce fibers crimp. In contrast, specimen prepared by VPM without undergoing the pre-tightening process will show a relative looser state and many carbon fibers might crimp together. When the VPM specimen is tensioned under axial tensile force (FA), the inside carbon fibers will be straightened first before

RI PT

they were tensioned.

SC

Force FL

M AN U

FA

FA

εL

FL

Force

TE D

Fig.13. Schematics of lateral effect

According to the theory of elastic mechanics, the lateral sides of CFRP tendons will be compressed when they are under the axial tension. For specimens prepared by VPM, we can find some gas bubbles (defects) that cannot be eliminated (see Fig.13). Therefore, those gas bubbles will be

EP

compressed during the tensile test due to the lateral effect. The lateral force will be transmitted onto the air bubbles via the carbon fibers which are surrounding the bubbles. As shown in Fig. 13,

AC C

for a given bubble, the force caused by lateral effect on the bubble can be simplified as an arch structure, where the force will be undertaken by the carbon fiber at the both ends of the bubble. The lateral force and the counter-force at both ends developed the external forces that acted on the arch. The internal force of the arch will result in compressive stress, which will partly release the tension of the whole carbon fiber. The total released tension is considerable because of large amounts of bubbles in the CFRP tendon specimen prepared by VPM, which make the dR/R change more slowly and consequently a lower sensitivity. In contrast, for the CFRP tendon specimen prepared by BPM, the pre-tension process will tighten up the fibers to reduce crimp and significantly reduce the amounts of bubbles during brush process, which is recommended in the 15

ACCEPTED MANUSCRIPT

practice application.

4. Conclusions This study presents a comparative study on piezoresistive properties of CFRP tendons prepared by two different methods, i.e., the vacuum process method (VPM) and the brush process method

RI PT

(BPM). The initial resistances, the piezoresistive performance and the sensitivity of CFRP tendon prepared by the two methods were measured and compared in the tensile tests. Main conclusions are drawn as below,

(1) The initial resistance of the CFRP tendons prepared by the VPM is larger than those prepared

SC

by the BPM. The resistances of the CFRP tendon specimens by two methods increase uniformly with the increasing strain. The resistance increases quickly at the same strain

M AN U

increment for specimens prepared by BPM than that for VPM.

(2) Three stages can be observed based on the relationship between dR/R and tensile strain for CFRP tendon specimens prepared by two methods. The difference between two methods is not significant for the strain at each intersection point between stages I and II, and between stages II and III. But the change spectrum of dR/R for CFRP tendon specimens prepared by

TE D

VPM is different from that by BPM. The scope of dR/R varies from 0% to 55% for BPM specimen which is much wider than that for VPM specimen falls within 8.5%. The result of BPM is more sensitive than that of VPM.

EP

(3) The sensitivity K obtained for specimens prepared by BPM specimens varies from 22 to 28 are much bigger than that ranges from 2.7 to 3.5 for VPM specimens. Lateral effect is responsible

AC C

for the difference between VPM and BPM specimen. The pre-tension process is recommended because it can tighten up the fibers to reduce crimp and significantly reduce the amounts of bubbles during brush process.

Acknowledgments This study was financially sponsored by the National Nature Science Foundation of China (No. 51478209, No. 51508235 and No. 41402251), and the Jiangsu Province Science Foundation for Youths (No. BK20140553). All of these supports are gratefully acknowledged.

16

ACCEPTED MANUSCRIPT References Arai M., Matsushita K., Hirota S. Criterion for inter laminar strength of CFRP laminates toughened with carbon nano fiber interlayer. Composites Part A, 2011, 42:703-711. Bakis C. E., Nanni A., Terosky J. A., et al. Self-monitoring, Pseudo-ductile, Hybrid FRP Reinforcement

RI PT

Rods For Concrete Applications. Compos Sci Technol 2001, 61: 815-823. Barbosa A P C, Fulco A P P, Guerra E S S, et al. Accelerated aging effects on carbon fiber/epoxy composites. Composites Part B, 2016, 110:298-306.

Cai D. S., Yin, J. Liu R.G., Experimental and analytical investigation into the stress performance of

SC

composite anchors for CFRP tendons. Compos Part B Eng, 2015, 79:530-534.

Cai, D. S., Xu Z. H., Yin, J. Liu, R.G., Liang, G. A numerical investigation on the performance of

M AN U

composite anchors for CFRP tendons. Construction and Building Materials, 2016 (112), 845-855. Dong JK, Zuo JW, Jin YC et al. Interfacial evaluation of carbon fiber/epoxy composites using electrical resistance measurements at room and a cryogenic temperature. Compos: Part A, 2015, 72:160-166.

Chung D D L. Piezoresistive Cement-Based Materials for Strain Sensing. Journal of Intelligent Material

TE D

Systems & Structures, 2002, 13(9):599-609.

Chung D. D. L., Wang S., Kowalik D. P. Self-sensing attained in carbon fiber polymer-matrix structural composites by using the interlaminar interface as a sensor. Smart Mat and Struct, 2004, 13(3):570-592.

EP

Crouch R. D., Clay S. B., Oskay C. Experimental and computational investigation of progressive damage accumulation in CFRP composites. Compos Part B Eng, 2013, 48:59-67.

AC C

Guadagno L., Vietri U., Raimondo M., et al. Correlation between electrical resistivity and manufacturing. Composites Part B 2015, 80:7-14. Haj-Ali, R., Zemer, H., El-Hajjar, R. and Aboudi, J. Piezoresistive fiber-reinforced composites: a coupled nonlinear micromechanical–microelectrical modeling approach. International Journal of Solids and Structures, 2014, 51(2), pp.491-503. Kim K W, Kim D K, Kim B S, et al. Cure behaviors and mechanical properties of carbon fiber-reinforced nylon6/epoxy blended matrix composites. Composites Part B Engineering, 2016, 112:15-21.

Kwon DJ, Wang ZJ, Choi JY, Shin PS, DeVries KL, Park JM. Interfacial evaluation of carbon fiber/epoxy composites using electrical resistance measurements at room and a cryogenic temperature. 17

ACCEPTED Composites Part A. 2015, 72:160-166.

MANUSCRIPT

Kwon, D.J., Wang, Z.J., Choi, J.Y., Shin, P.S., DeVries, K.L. and Park, J.M., 2016. Detection of damage in cylindrical parts of carbon fiber/epoxy composites using electrical resistance (ER) measurements. Composites Part B: Engineering. Liu, R., Xu, Z., Yin, J., Huang, J., Liu, D., Xie, G. A coupled mechanical and electrical model concerning piezoresistive effect of CFRP materials. Composites Part B: Engineering. 2016(96), 125-135

RI PT

Liu W, Wang Y, Wang P, et al. A biomimetic approach to improve the dispersibility, interfacial interactions and toughening effects of carbon nanofibers in epoxy composites. Composites Part B Engineering, 2017, 113:197-205.

SC

Ma C. G., Liu H. Y., Du X. S., et al. Fracture resistance, thermal and electrical properties of epoxy composites containing aligned carbon nanotubes by low magnetic field. Compos Sci Technol, 2015,

M AN U

114:126-135.

Sevkat E., Li J., Liaw B., et al. A statistical model of electrical resistance of carbon fiber reinforced composites under tensile loading. Compo Sci Technol, 2008; 68(10): 2214-2219. Shen J. T., Buschhorn ST, Hosson JTM, Schulte K, Fiedler B. Pressure and temperature induced electrical resistance change in nano-carbon/epoxy composites. Compos Sci Technol, 2015,

TE D

115:1-8.

Suzuki Y., Todoroki A, Matsuzaki R. Impact-damage visualization in CFRP by resistive heating: Development of a new detection method for indentations caused by impact loads. Composites Part A, 2015, 43: 53-64.

EP

Todoroki, A., Kurokawa, H., Mizutani, Y., Matsuzaki, R. and Yasuoka, T., 2014. Self-sensing time domain reflectometry method for damage monitoring of a CFRP plate using a narrow-strip

AC C

transmission line. Composites Part B: Engineering, 58, pp.59-65. Todoroki A., Suzuki K, Mizutani Y, et al. Electrical resistance change of CFRP under a compression load. J Solid Mechan and Mater Eng, 2010, 4(7): 864-874. Todoroki A., Yoshida J. Electrical resistance change of unidirectional CFRP due to applied load. JSME Inter J Series A, 2004, 47(3): 357-364. Vavouliotis A., Paipetis A., Kostopoulos V. On the fatigue life prediction of CFRP laminates using the electrical resistance change method. Compos Sci Technol, 2011, 71(5): 630-642. Xie G., Yin J., Liu R., Chen B., and Cai D. Experimental and numerical investigation on the static and dynamic behaviors of cable-stayed bridges with CFRP cables. Composites Part B: Engineering, 18

ACCEPTED MANUSCRIPT

2017(111):235-242.

Wang C, Liu G, An Q, et al. Occurrence and formation mechanism of surface cavity defects during orthogonal milling of CFRP laminates. Composites Part B Engineering, 2017, 109:10-22. Wang H T. Study on the Self-Monitoring FRP-Concrete Structures Based on Carbon Fiber Sensing[D]. Southeast University, 2012. (in Chinese).

Wang, B., Ou, J.P., Zhang, X.Y. and HE, Z., 2007. Experimental research on sensing properties of CFRP

RI PT

bar and concrete beams reinforced with CFRP bars. Journal of Harbin Institute of Technology, 39(2), pp.220-224. (in Chinese).

Wang D. J., Chung D. D. L. Through thickness piezoresistivity in a carbon fiber polymer-matrix structural composite for electrical resistance-based through-thickness strain sensing. Carbon,

SC

2013, (60)129-138.

Wang S., Wang D., Chung D. D. L., et al. Method of sensing impact damage in carbon fiber

M AN U

polymer-matrix composite by electrical resistance measurement. J Mater Sci, 2006, 41(8):2281-2289.

Yang, C.Q., Wang, X.L., Jiao, Y.J., Ding, Y.L., Zhang, Y.F. and Wu, Z.S., 2016. Linear strain sensing performance of continuous high strength carbon fibre reinforced polymer composites. Composites Part B: Engineering, 102, pp.86-93.

TE D

Yang C. Q., Wu Z. S. Self-structural Health Monitoring Function of RC Structures with HCFRP Sensors. J Intelligent Mater Systems and Structures, 2006, 17(10): 895-906. Yuen, S. M., Ma C. C. M., Wu H. H., et al. Preparation and thermal, electrical, and morphological

EP

properties of multiwalled carbon nanotube and epoxy composites. J Appl Polym Sci, 2007, 103(2):1272-1278.

AC C

Zuo, J., Chen, S., Luo, C. and Chen, D. Preparation of electroless copper coated glass fiber and piezoresistive properties of copper coated glass fiber reinforced plastics. Applied Surface Science, 2015, 349, pp.319-326.

19