上海交通大学：《Measurement Systems：Application and Design》课程教学资源（扩展知识）Strain rate-dependent tensile properties and dynamic electromechanical response of carbon nanotube fibers

CARB0N50(20I2)3876-388I Available at www.sciencedirect.com Carbon SciVerse ScienceDirect ELSEVIER journal homepage:www.elsevier.com/locate/carbon Strain rate-dependent tensile properties and dynamic electromechanical response of carbon nanotube fibers Amanda S.Wu,Xu Nie,Matthew C.Hudspeth,Weinong W.Chen Tsu-Wei Chou a.,David S.Lashmore Mark W.Schauer,Erick Tolle Jeff Rioux Center for Composite Materials and Department of Mechanical Engineering,University of Delaware,201 Comp.Sci.Mnfg.Lab,Newark, DE 19716,USA bSchool of Aeronautics and Astronautics,Purdue University,West Lafayette,IN 47907-2045,USA Nanocomp Technologies,Inc.,162 Pembroke Road,Concord,NH 03301,USA ARTICLE INFO ABSTRACT Article history: This investigation into the rate-dependent tensile behavior of carbon nanotube(CNT)fibers Received 4 January 2012 provides insight into the role of strain rate and specimen gage length on tensile strength. Accepted 6 April 2012 Chemical vapor produced CNT continuous fibers made of single and dual wall CNTs are Available online 13 April 2012 evaluated and the potential for fiber improvement by post-process stretching to improve alignment is explored.Post-processed CNT fibers exhibit significantly higher strengths (3-5 GPa)and moduli(80-200 GPa)than untreated fibers.During dynamic tension evalua- tion,real-time electrical measurements provide correlations between high rate deforma- tion/damage mechanical behavior and electrical resistance of the fiber specimens. Furthermore,this first look into the dynamic tensile behavior of CNT fibers demonstrates their potential to serve as sensors in high rate applications. 2012 Elsevier Ltd.All rights reserved. Introduction arising from chemical treatment and stretching in conjunc- tion with added twisting and spinning compared with ace- Carbon nanotube(CNT)fiber performance has improved sig- tone drawn/spun CNT fibers.CNT fibers or yarns are post- nificantly since 2000 [1],now reaching strengths(9 GPa)and processed in two different ways:(1)acetone condensed moduli (350 GPa)comparable or greater than those reported and post-spun to a 15 pitch angle,(2)stretched up to 50% for carbon fibers [2,3].Fiber performance is influenced by in a proprietary chemical followed by spinning to a 15 pitch changes in CNT properties and processing method (wet- angle. spinning [1,4-6],forest spinning [7-12],aerogel spinning The high strain rate characterization of CNT fibers is [2,13-17]).However,the post-processing drawing and spin- spurred by these recent improvements and the potential of ning phase is also significant and mesoscale fiber morphol- CNT fibers to drive high performance applications.In this ogy (twist angle [18],linear density)can affect nanoscale study,Kolsky tension bar methodology for single fiber evalu- behavior (e.g..CNT collapse [19))as well [20].In this study, ation is implemented to study the high rate electromechani- we compare the tensile properties of CNT-based fibers cal response of CNT fibers [21-23].Through this study,we produced by CVD and discuss the improvements in perfor- gain insight into gage length and strain rate effects on CNT mance achieved by post-processing.Specifically,we demon- fiber strength,as well as a clear demonstration of their poten- strate a signifcant improvement in mechanical properties tial for sensing in high rate applications. Corresponding author:Fax:+1302 8312904. E-mail address:chou@udel.edu (T.-W.Chou). 0008-6223/$-see front matter 2012 Elsevier Ltd.All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.04.031

Strain rate-dependent tensile properties and dynamic electromechanical response of carbon nanotube fibers Amanda S. Wu a , Xu Nie b , Matthew C. Hudspeth b , Weinong W. Chen b , Tsu-Wei Chou a, * , David S. Lashmore c , Mark W. Schauer c , Erick Tolle c , Jeff Rioux c a Center for Composite Materials and Department of Mechanical Engineering, University of Delaware, 201 Comp. Sci. Mnfg. Lab, Newark, DE 19716, USA b School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN 47907-2045, USA c Nanocomp Technologies, Inc., 162 Pembroke Road, Concord, NH 03301, USA ARTICLE INFO Article history: Received 4 January 2012 Accepted 6 April 2012 Available online 13 April 2012 ABSTRACT This investigation into the rate-dependent tensile behavior of carbon nanotube (CNT) fibers provides insight into the role of strain rate and specimen gage length on tensile strength. Chemical vapor produced CNT continuous fibers made of single and dual wall CNTs are evaluated and the potential for fiber improvement by post-process stretching to improve alignment is explored. Post-processed CNT fibers exhibit significantly higher strengths (3–5 GPa) and moduli (80–200 GPa) than untreated fibers. During dynamic tension evalua￾tion, real-time electrical measurements provide correlations between high rate deforma￾tion/damage mechanical behavior and electrical resistance of the fiber specimens. Furthermore, this first look into the dynamic tensile behavior of CNT fibers demonstrates their potential to serve as sensors in high rate applications. 2012 Elsevier Ltd. All rights reserved. 1. Introduction Carbon nanotube (CNT) fiber performance has improved sig￾nificantly since 2000 [1], now reaching strengths (9 GPa) and moduli (350 GPa) comparable or greater than those reported for carbon fibers [2,3]. Fiber performance is influenced by changes in CNT properties and processing method (wet￾spinning [1,4–6], forest spinning [7–12], aerogel spinning [2,13–17]). However, the post-processing drawing and spin￾ning phase is also significant and mesoscale fiber morphol￾ogy (twist angle [18], linear density) can affect nanoscale behavior (e.g., CNT collapse [19]) as well [20]. In this study, we compare the tensile properties of CNT-based fibers produced by CVD and discuss the improvements in perfor￾mance achieved by post-processing. Specifically, we demon￾strate a significant improvement in mechanical properties arising from chemical treatment and stretching in conjunc￾tion with added twisting and spinning compared with ace￾tone drawn/spun CNT fibers. CNT fibers or yarns are post￾processed in two different ways: (1) acetone condensed and post-spun to a 15 pitch angle, (2) stretched up to 50% in a proprietary chemical followed by spinning to a 15 pitch angle. The high strain rate characterization of CNT fibers is spurred by these recent improvements and the potential of CNT fibers to drive high performance applications. In this study, Kolsky tension bar methodology for single fiber evalu￾ation is implemented to study the high rate electromechani￾cal response of CNT fibers [21–23]. Through this study, we gain insight into gage length and strain rate effects on CNT fiber strength, as well as a clear demonstration of their poten￾tial for sensing in high rate applications. 0008-6223/$ - see front matter 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.04.031 * Corresponding author: Fax: +1 302 8312904. E-mail address: chou@udel.edu (T.-W. Chou). CARBON 50 (2012) 3876 – 3881 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon

CARB0N50(2012)3876-388I 3877 Load cell Photo diode Optical lens Strain gage Fiber sample Optical lens Incident bar Striker Laser generator Fig.1-Kolsky tension apparatus for single fiber evaluation [21]. 10 Experimental 2.1. CNT fibers Untreated CNT fibers (UF)and chemically treated and 8 stretched CNT fibers(CSF)were provided by Nanocomp Tech- nologies,Inc.Single-,dual-and multi-walled CNTs comprise the untreated fibers,which have a linear density (tex)of 1.4 gkm and average fiber diameter of 57.3+5.9 um [17]. The chemically treated and stretched fibers underwent addi- tional stretching and twisting,resulting in a higher linear density of 3.81g km-1 with a comparable average fiber diam- eter of55.2±2.5um. 三 2.2. Static testing Quasi-static tensile experiments were performed at an exten- sion rate of 1.0 mm min-1 using a displacement controlled 500 load frame (Instron 5848 Micro Tester)equipped with micro- 400 pneumatic grips(Instron 54851B).5 and 500 N load cells were used to test the UF and CSF specimens,respectively. 300 边 200 2.3. Dynamic tension evaluation 100 Dynamic tension evaluation of the fiber specimens was per- formed using the experimental apparatus depicted in Fig.1, 0.0 0.1 02 0.30.40.50.60.70.8 courtesy of Prof.Weinong Chen at Purdue University.During t (ms) testing,the air-propelled striker tube impacts the flanged end of the incident bar(6.35 mm diameter,1.8 m length).The Fig.2-Typical conditions during a dynamic tension fiber specimen is attached using set screws to the opposite experiment. end of the incident bar and to a 22.24 N(5 lbf)quartz-piezo- electric load cell(Kistler 9712B5).A laser emitter-detector pair provides displacement measurements.Experiments are de- Strength is calculated based on average fiber linear den- signed to provide a constant strain rate after an initial ramping sity,assuming a CNT density of 2.0 gcm-3[7,14].This value up period [22.Typical strain (e),force (F)and strain rate ( may provide an overestimate of the fiber strength and histories during the dynamic tension experiment are provided modulus,since these fibers are comprised of single-,dual- in Fig.2.Strain rate is determined using curve-fitting and dif- and multi-walled CNTs.However,it is important to note that ferentiation of the displacement curve.An in-house circuit the actual fiber density is lower than the density of the indi- was used to measure electrical resistance along the length of vidual CNTs.The denser,CSF specimens possess superior the fiber specimens during each dynamic tension experiment. mechanical properties under dynamic tension loadings (Fig.3)compared with the untreated CNT fibers (UF).In fact, 3. Results and discussion the strength of the CSFs is comparable to that of several high performance fibers,also measured using the Kolsky tension Ultimate tensile strength(urs),elastic modulus(E),applied bar method(Fig.4).Moreover,CSF specimens exhibit signifi- strain rate ()gage length (1)and strain to failure (ep)for each cant resistance to cutting as compared to the more frangible experimental set is provided in Table 1. UF specimens.This behavior is attributed to an improvement

2. Experimental 2.1. CNT fibers Untreated CNT fibers (UF) and chemically treated and stretched CNT fibers (CSF) were provided by Nanocomp Tech￾nologies, Inc. Single-, dual- and multi-walled CNTs comprise the untreated fibers, which have a linear density (tex) of 1.4 g km1 and average fiber diameter of 57.3 ± 5.9 lm [17]. The chemically treated and stretched fibers underwent addi￾tional stretching and twisting, resulting in a higher linear density of 3.81 g km1 with a comparable average fiber diam￾eter of 55.2 ± 2.5 lm. 2.2. Static testing Quasi-static tensile experiments were performed at an exten￾sion rate of 1.0 mm min1 using a displacement controlled load frame (Instron 5848 Micro Tester) equipped with micro￾pneumatic grips (Instron 54851B). 5 and 500 N load cells were used to test the UF and CSF specimens, respectively. 2.3. Dynamic tension evaluation Dynamic tension evaluation of the fiber specimens was per￾formed using the experimental apparatus depicted in Fig. 1, courtesy of Prof. Weinong Chen at Purdue University. During testing, the air-propelled striker tube impacts the flanged end of the incident bar (6.35 mm diameter, 1.8 m length). The fiber specimen is attached using set screws to the opposite end of the incident bar and to a 22.24 N (5 lbf) quartz–piezo￾electric load cell (Kistler 9712B5). A laser emitter-detector pair provides displacement measurements. Experiments are de￾signed to provide a constant strain rate after an initial ramping up period [22]. Typical strain (e), force (F) and strain rate (e_) histories during the dynamic tension experiment are provided in Fig. 2. Strain rate is determined using curve-fitting and dif￾ferentiation of the displacement curve. An in-house circuit was used to measure electrical resistance along the length of the fiber specimens during each dynamic tension experiment. 3. Results and discussion Ultimate tensile strength (rUTS), elastic modulus (E), applied strain rate (e_), gage length (l) and strain to failure (ef) for each experimental set is provided in Table 1. Strength is calculated based on average fiber linear den￾sity, assuming a CNT density of 2.0 g cm3 [7,14]. This value may provide an overestimate of the fiber strength and modulus, since these fibers are comprised of single-, dual￾and multi-walled CNTs. However, it is important to note that the actual fiber density is lower than the density of the indi￾vidual CNTs. The denser, CSF specimens possess superior mechanical properties under dynamic tension loadings (Fig. 3) compared with the untreated CNT fibers (UF). In fact, the strength of the CSFs is comparable to that of several high performance fibers, also measured using the Kolsky tension bar method (Fig. 4). Moreover, CSF specimens exhibit signifi- cant resistance to cutting as compared to the more frangible UF specimens. This behavior is attributed to an improvement Strain gage Incident bar Striker Optical lens Optical lens Laser generator Photo diode Load cell Fiber sample Fig. 1 – Kolsky tension apparatus for single fiber evaluation [21]. 0 2 4 6 8 10 0 2 4 6 8 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 100 200 300 400 500 ε (%) F (N) ε (s-1 ) t (ms) . Fig. 2 – Typical conditions during a dynamic tension experiment. CARBON 50 (2012) 3876 – 3881 3877

3878 CARB0N50(2012)3876-388I Table 1-Experimental parameters and mechanical properties of CNT fibers. Specimen type Quantity (s-) 1(mm) 可(%) aurs (N/tex) aurs(GPa) E(GPa) UF[17] 2.1(10-3 10±2 0.4±0.1 0.70±0.2 9±3 UF 8 3.5(10-) 9±1 0.49±0.03 0.97±0.08 32±4 UF 12 5.2(10-) 9±2 0.49±0.04 0.99±0.08 50±10 UF 22 450±60 3 5±2 0.6±0.1 1.2±0.2 80±20 UF 11 1300±300 4±1 0.7±0.1 1.5±0.3 70±30 UF 13 790±30 3±1 0.9±0.1 1.8±0.2 90±203 CSF 2.1(10- Pull-out Pull-out 120±10 CSF 3 5.2(10- 5.59 2.22 4.45 90±20 CSF 17 490±50 2.3±0.4 1.7±0.2 3.5±0.5 140±30 CSF 10 760±50 6±1 2.0±0.3 3.9±0.5 110±203 Elastic modulus is measured during the strain rate ramping up period Q-S 5 450s1 1300s1 ● ◆ 色 790s1 1.0 ● Q-S ◆ 490s1 3 760s1 0.8 △ 且 ● 4 g80 00 8 0.2 8 0 50 100 150 200 E(GPa) 0.0 3 5 6 7 8 Fig.3-Quasi-static and dynamic mechanical performance 1 (mm) comparison of untreated(UF,hollow points)and chemically treated and stretched(CSF,filled points)CNT fibers. Fig.5-Effect of gage length on untreated CNT fiber ultimate tensile strength under quasi-static loading.The 8 mm gage length UF specimen behavior has been measured previously 17八. ☐quasi-static Z7☑700-1500s (eds) 3.1. Gage length effect Due to a greater availability of the untreated fiber,these spec- imens were evaluated at gage lengths of =3.18 mm(1/8 in). 12=4.76 mm (3/16 in)and I3=7.94 mm (5/16 in)under quasi- static tension loading in an effort to better understand the UF E-glass231 A265124 Kevlar1291211 CSF Twaron Kevlar2 role of defects on CNT fibers.On average,strength decreases with an increase in gage length(Fig.5);this behavior is consis- Fig.4-Tensile strength of various high performance fibers tent with that of other high-performance fibers reported in under quasi-static and high strain rate loadings. the literature [24-26].Gage length dependency is typically attributed to strength-limiting defects within a fiber.In the case of CNT fibers,which are characterized by their disrupted internal microstructure and lack of intertube bonding,it is in linear density and CNT alignment through additional draw- expected that this dependency would occur.In this case, ing and twisting.In addition to providing a comparative study CNT length and entanglement may play a more signifcant of the two CNT fibers,these experiments aim to evaluate the role in the length dependency.All future comparisons will effect of specimen gage length and applied strain rate on be made at the same gage length to provide a higher level measured mechanical properties. of consistency

in linear density and CNT alignment through additional draw￾ing and twisting. In addition to providing a comparative study of the two CNT fibers, these experiments aim to evaluate the effect of specimen gage length and applied strain rate on measured mechanical properties. 3.1. Gage length effect Due to a greater availability of the untreated fiber, these spec￾imens were evaluated at gage lengths of l1 = 3.18 mm (1/8 in), l2 = 4.76 mm (3/16 in) and l3 = 7.94 mm (5/16 in) under quasi￾static tension loading in an effort to better understand the role of defects on CNT fibers. On average, strength decreases with an increase in gage length (Fig. 5); this behavior is consis￾tent with that of other high-performance fibers reported in the literature [24–26]. Gage length dependency is typically attributed to strength-limiting defects within a fiber. In the case of CNT fibers, which are characterized by their disrupted internal microstructure and lack of intertube bonding, it is expected that this dependency would occur. In this case, CNT length and entanglement may play a more significant role in the length dependency. All future comparisons will be made at the same gage length to provide a higher level of consistency. 345678 0.0 0.2 0.4 0.6 0.8 1.0 1.2 σUTS (GPa) l (mm) Fig. 5 – Effect of gage length on untreated CNT fiber ultimate tensile strength under quasi-static loading. The 8 mm gage length UF specimen behavior has been measured previously [17]. Table 1 – Experimental parameters and mechanical properties of CNT fibers. Specimen type Quantity e_ (s1 ) l (mm) ef (%) rUTS (N/tex) rUTS (GPa) E (GPa) UF [17] 55 2.1 (103 ) l3 10 ± 2 0.4 ± 0.1 0.70 ± 0.2 9 ± 3 UF 8 3.5 (103 ) l2 9 ± 1 0.49 ± 0.03 0.97 ± 0.08 32 ± 4 UF 12 5.2 (103 ) l1 9 ± 2 0.49 ± 0.04 0.99 ± 0.08 50 ± 10 UF 22 450 ± 60 l3 5 ± 2 0.6 ± 0.1 1.2 ± 0.2 80 ± 20a UF 11 1300 ± 300 l1 4 ± 1 0.7 ± 0.1 1.5 ± 0.3 70 ± 30a UF 13 790 ± 30 l1 3 ± 1 0.9 ± 0.1 1.8 ± 0.2 90 ± 20a CSF 5 2.1 (103 ) l3 Pull-out Pull-out 120 ± 10 CSF 3 5.2 (103 ) l1 5.59 2.22 4.45 90 ± 20 CSF 17 490 ± 50 l3 2.3 ± 0.4 1.7 ± 0.2 3.5 ± 0.5 140 ± 30a CSF 10 760 ± 50 l1 6 ± 1 2.0 ± 0.3 3.9 ± 0.5 110 ± 20a a Elastic modulus is measured during the strain rate ramping up period. 0 50 100 150 200 0 1 2 3 4 5 Q-S 450 s-1 1300 s-1 790 s-1 Q-S 490 s-1 760 s-1 σUTS (GPa) E (GPa) Fig. 3 – Quasi-static and dynamic mechanical performance comparison of untreated (UF, hollow points) and chemically treated and stretched (CSF, filled points) CNT fibers. UF E-glass A265 Kevlar129 CSF Twaron Kevlar 0 1 2 3 4 5 6 σ (GPa) quasi-static 700-1500 s-1 [23] [22] [ [21] 21] [21] Fig. 4 – Tensile strength of various high performance fibers under quasi-static and high strain rate loadings. 3878 CARBON 50 (2012) 3876 – 3881

CARB0N50(2012)3876-388I 3879 250 15 0 UF 200 CSF 150 14 (eds) 0 100 00 d 13 0 6 12 -11 60 2 46810 e(%) 0 形 0 0 8 Fig.7-Electromechanical response to dynamic tension 0 0 200 400 600 800 1000120014001600 loading for two 3.18 mm gage CSF specimens;the most brittle and ductile fiber results are presented. &(s") Fig.6-Strain rate effect on ultimate tensile strength and gage factors,these calculations demonstrate that definite, elastic moduli at different gage lengths.The 8 mm gage measurable electrical resistance changes occur within a rea- length UF specimen behavior has been measured previously sonable strain range,providing evidence that the CNT fibers 17小. can perform as strain sensors. Failed fibers exhibit kinking along the fiber length.Upon failure,tensile-induced recoil results in compression loading along the fiber ends.The compression strength of these fibers 3.2. Strain rate effect is generally lower than their tensile strength(especially so for CNT fibers are known for shear or drafting failure,rather than the case of the CSF specimens)[17].Therefore,it stands to reason that compression-induced kinking can be observed CNT breakage-induced fracture [7,17].During fiber failure, after tensile failure. CNTs must disentangle and slip,both of which are rate- The electromechanical behavior of two CSF specimens are dependent behaviors.It is,therefore,expected that these plotted in Fig.9;this time we present a case of fiber pull-out fibers would exhibit rate-dependent mechanical properties. compared with fiber breakage during loading.In the case of Through quasi-static and dynamic tension experiments,we observe that the ultimate tensile strength of both untreated fiber pull-out (an unfortunately common phenomenon in and acid-treated CNT fibers increases with applied strain rate the stronger CSF specimens),electrical resistance increases (Fig.6).This behavior is exhibited by other fibers [27,28].Rate- only slightly due to permanent strain rendering post-exami- dependency appears to have the most notable effect at strain nation redundant.The ability to sense strain and damage rates below 800s-1 in the case of the untreated fibers. within a CNT fiber under dynamic tensile loading provides a solid proof of concept for CNT fiber-based sensing in high-rate 3.3. Dynamic electromechanical behavior composite applications. CNTs are well-known for their electrical conductivity;this Conclusions property is somewhat transferrable to CNT fibers.In this study,the electrical conductivity of the UF and CSF specimens A study into the response of untreated and chemically treated is7.2±1.4(102-1m-1and2.4±1.2(10)2-1m-1,respec- and stretched CNT fibers to dynamic tensile loading reveals tively [17].Previously,piezoresistivity was observed during new insight into the rate-dependent behavior and high-rate quasi-static tensile loading of UF specimens [17].Here,we electromechanical response of these materials.These CNT fi- demonstrate piezoresistivity during dynamic tensile loading bers exhibit rate dependency at strain rates <800 s-1.Further- (Fig.7)and the potential for sensing of strain and damage dur- more,a brief examination of gage length dependency ing high-rate loading scenarios.Electrical resistance in- indicates a measurable decrease in strength at gage lengths creases with applied tension loading.Upon failure,electrical above 5 mm which is likely a result of defects within the fiber resistance becomes infinite due to complete seperation of bundle. the broken fiber ends(Fig.8).A gage factor estimate for UF The additional drawing,spinning and acid-treatment re- and CSF (3.03-4.37)specimens can be made by comparing sulted in a significant increase in fiber strength and modulus changes in electrical resistance with applied strain thus placing these aerogel-spun CNT fibers on par with other (GF=(AR/R)/e).It must be noted that this factor only accounts high-performance wet-spun and forest-spun fibers.Electrical for electrical resistance changes and does not account for po- resistance measurements acquired from dynamic tension tential thermal influences.Despite the significant range in experiments provide clear correlations between the high-rate

3.2. Strain rate effect CNT fibers are known for shear or drafting failure, rather than CNT breakage-induced fracture [7,17]. During fiber failure, CNTs must disentangle and slip, both of which are rate￾dependent behaviors. It is, therefore, expected that these fibers would exhibit rate-dependent mechanical properties. Through quasi-static and dynamic tension experiments, we observe that the ultimate tensile strength of both untreated and acid-treated CNT fibers increases with applied strain rate (Fig. 6). This behavior is exhibited by other fibers [27,28]. Rate￾dependency appears to have the most notable effect at strain rates below 800 s1 in the case of the untreated fibers. 3.3. Dynamic electromechanical behavior CNTs are well-known for their electrical conductivity; this property is somewhat transferrable to CNT fibers. In this study, the electrical conductivity of the UF and CSF specimens is 7.2 ± 1.4 (104 ) X1m1 and 2.4 ± 1.2 (105 ) X1m1 , respec￾tively [17]. Previously, piezoresistivity was observed during quasi-static tensile loading of UF specimens [17]. Here, we demonstrate piezoresistivity during dynamic tensile loading (Fig. 7) and the potential for sensing of strain and damage dur￾ing high-rate loading scenarios. Electrical resistance in￾creases with applied tension loading. Upon failure, electrical resistance becomes infinite due to complete seperation of the broken fiber ends (Fig. 8). A gage factor estimate for UF and CSF (3.03–4.37) specimens can be made by comparing changes in electrical resistance with applied strain (GF = (DR/R)/e). It must be noted that this factor only accounts for electrical resistance changes and does not account for po￾tential thermal influences. Despite the significant range in gage factors, these calculations demonstrate that definite, measurable electrical resistance changes occur within a rea￾sonable strain range, providing evidence that the CNT fibers can perform as strain sensors. Failed fibers exhibit kinking along the fiber length. Upon failure, tensile-induced recoil results in compression loading along the fiber ends. The compression strength of these fibers is generally lower than their tensile strength (especially so for the case of the CSF specimens) [17]. Therefore, it stands to reason that compression-induced kinking can be observed after tensile failure. The electromechanical behavior of two CSF specimens are plotted in Fig. 9; this time we present a case of fiber pull-out compared with fiber breakage during loading. In the case of fiber pull-out (an unfortunately common phenomenon in the stronger CSF specimens), electrical resistance increases only slightly due to permanent strain rendering post-exami￾nation redundant. The ability to sense strain and damage within a CNT fiber under dynamic tensile loading provides a solid proof of concept for CNT fiber-based sensing in high-rate composite applications. 4. Conclusions A study into the response of untreated and chemically treated and stretched CNT fibers to dynamic tensile loading reveals new insight into the rate-dependent behavior and high-rate electromechanical response of these materials. These CNT fi- bers exhibit rate dependency at strain rates <800 s1 . Further￾more, a brief examination of gage length dependency indicates a measurable decrease in strength at gage lengths above 5 mm which is likely a result of defects within the fiber bundle. The additional drawing, spinning and acid-treatment re￾sulted in a significant increase in fiber strength and modulus, thus placing these aerogel-spun CNT fibers on par with other high-performance wet-spun and forest-spun fibers. Electrical resistance measurements acquired from dynamic tension experiments provide clear correlations between the high-rate 0 50 100 150 200 250 0 200 400 600 800 1000 1200 1400 1600 0 1 2 3 4 5 6 7 8 UF CSF E (GPa) σUTS (GPa) ε (s-1) . Fig. 6 – Strain rate effect on ultimate tensile strength and elastic moduli at different gage lengths. The 8 mm gage length UF specimen behavior has been measured previously [17]. 0246 0 1 2 3 4 0 2 4 6 8 10 σ (GPa) σ ε (%) 11 12 13 14 15 R R ( Ω) Fig. 7 – Electromechanical response to dynamic tension loading for two 3.18 mm gage CSF specimens; the most brittle and ductile fiber results are presented. CARBON 50 (2012) 3876 – 3881 3879

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