- Email: [email protected]
epoxy composites under tensile and shear loading
Composites Communications 15 (2019) 30–33
Contents lists available at ScienceDirect
Composites Communications journal homepage: www.elsevier.com/locate/coco
Electro-mechanical characterization of three-dimensionally conductive graphite/epoxy composites under tensile and shear loading
T
Riley Shermana, Vijaya Chalivendraa,*, Asha Hallb, Mulugeta Haileb, Latha Natarajb, Michael Coatneyb, Yong Kimc a
Department of Mechanical Engineering, University of Massachusetts Dartmouth, Dartmouth, MA, 02747, USA U.S. Army Research Laboratory, Aberdeen Proving Ground, MD, 21005, USA c Department of Bioengineering, University of Massachusetts Dartmouth, Dartmouth, MA, 02747, USA b
A B S T R A C T
An experimental study was performed to compare the electro-mechanical response of three-dimensionally conductive woven carbon fiber/epoxy laminated composites under quasi-static uniaxial tensile and in-plane shear loading conditions. Three-dimensional (3D) electrical network was generated in these composites by embedding carbon nanotubes (CNTs) of 0.025 wt% in the epoxy and reinforcing short carbon fibers (150 μm and 350 μm long) with a fiber density of 1000 fibers/ mm2 between the laminates using electro-flocking process. CNTs are effectively dispersed in the epoxy matrix using a combination of ultrasonication and shear mixing techniques. A compression molding fabrication technique was employed to fabricate composite materials. For all composite types, in general, the in-situ electrical response showed a trend of initial decrease in resistance and later increase in resistance for tensile loading. However, no noticeable increase in electrical resistance was observed until failure of the composite under shear loading conditions. Both CNTs and short carbon fibers made new contacts with neighboring carbon fiber laminates for entire duration of shear loading even though the composite was undergoing progressive failure.
1. Introduction Fiber reinforced polymeric composite materials are currently on the cutting edge of specialty due to their ability to combine the low weight of polymers with the incredible strength of fibers [1]. To further specialize these composites, carbon nanotubes (CNTs) were embedded into them to significantly improve their mechanical properties [2,3]. Other studies are aimed to explore the possibility of multifunctional composites by utilizing the highly electrically conductive nature of CNTs [4–6]. An area that has been gaining a lot of attention in multifunctional CNT composites is in-situ damage sensing. The in-situ change of electrical resistance of CNTs conductive network in polymer composites under mechanical loads may serve as indicators of different damage mechanisms under various loading conditions [7–17]. Vadlamani et al. [8] investigated the effect of CNTs weight fraction on electro-mechanical response under tensile loading conditions. In their study, the authors reported a decreasing electrical resistance trend up to 45% due to matrix's molecular chains declustering and straightening at lower weight fractions of 0.1% and 0.3%. Later, the same authors [10] studied the electro-mechanical response of rubber toughened and rigid particles epoxy systems under quasi-static tensile loadings. They observed threshold strain value for damage propagation in these particulate polymer systems from change in electrical resistance
*
measurements. Heeder et al. [7], examined the electrical response of CNTs embedded epoxy materials under uniaxial compression at quasistatic, medium strain rate and high strain rate loading conditions. Although a monotonic decrease in electrical response was observed under quasi-static and high strain rate conditions, a decreasing and sudden increasing in resistance was noticed under medium strain rates. In case of laminated composites, Thostenson and Chou [18] employed 0.5% wt. of CNTs in an advanced fiber composite to study the electrical conductivity under fatigue loads. Saafi [19] used 1.5% wt. of CNTs in cement matrix to develop an in situ wireless and embedded sensor for damage detection in concrete structures. Gao et al. [20] also used 0.5% wt. of CNTs in fiber reinforced polymer composites to detect damage under fatigue loading conditions. Using 0.75% wt. of CNTs, Friedrich et al. [21] studied various damage mode characteristics such as matrix cracking delamination, bearing and shear-out in mechanically fastened composite joints. In the above studies of laminated composites, only CNTs were embedded to generate conductive network within the composites and hence higher amount of CNTs were used. In this study, for the first time, a combination of CNTs and short carbon fibers were used to generate three-dimensional network within the composites. Two different length short carbon fibers of 150 μm and 350 μm were reinforced between the graphite laminates using electro-flocking approach. A comparative
Corresponding author. E-mail address: [email protected] (V. Chalivendra).
https://doi.org/10.1016/j.coco.2019.05.010 Received 28 October 2018; Received in revised form 17 April 2019; Accepted 31 May 2019 Available online 01 June 2019 2452-2139/ © 2019 Elsevier Ltd. All rights reserved.
Composites Communications 15 (2019) 30–33
R. Sherman, et al.
study of electro-mechanical response is conducted under quasi-static tensile and in-plane shear loading conditions. 2. Experimental details 2.1. Materials A thermoset epoxy system 2000 resin with 2120 hardener with a mixing ratio of 100 to 27 was used as the matrix (Supplied by Fibre Glast Development Corporation, Brookville, OH, USA). A small amount, 0.025% by weight, multi-walled carbon nanotubes (MWCNTs) (supplied by Cheap Tubes Inc., Cambridgeport, VT) of purity > 99% were mixed via controlled shear mixing and ultrasonication into the resin prior to fabrication. 3 K (3000 filaments per fiber) twill woven carbon fiber fabric (Supplied by Fibre Glast Development Corporation, Brookville, OH, USA) was used as laminates. Short carbons fibers of length 150 μm and 350 μm with a diameter of 7 μm (supplied by Asbury Graphite Mills, INC. NJ, USA) were used. Short carbon fiber density of 1000 fibers/mm2 is considered in this study.
Fig. 1. Schematic showing the electro flocking procedure. Table 1 Carbon fabric information, initial and final resistivity of composites. Composites of carbon fiber length (μm)
None 150 350
Number of carbon fabric layers
20 13 10
Fiber volume fraction of carbon fabric
Resistivity (Ohms-mm)
Initial
Final at break
Initial
Final at break
61.9 38.5 32.8
0.952 0.134 0.447
0.992 0.148276 0.514
3.404 9.271 3.535
3.399 9.253 3.535
Tensile
2.2. Fabrication
Shear
The first step in the fabrication process is dispersion of CNTs into the epoxy matrix. A detailed description was discussed in our previous work [8–17]. Due to brevity of space, a brief description is provided here. For all composite types, 0.025 wt% of CNTs are first mechanically mixed with a stirrer into the thermoset resin (part A). This mixture is then simultaneously shear mixed and ultrasonicated for 1 h. The mixture is later degassed in a vacuum chamber for at least 2 h. Next, the
Fig. 2. (A) and (B) show the standard specimen configurations used for tensile and shear conditions respectively. 31
Composites Communications 15 (2019) 30–33
R. Sherman, et al.
Fig. 3. (A) Electro-mechanical response under tensile loading, and (B) electro-mechanical response under shear loading.
shown in Fig. 2. An Instron Materials Testing System was utilized to collect the load and displacement data for shear and tensile loading. Electrical data was collected by utilizing the standard four point circumferential ring probes along with two high impedance electrometers (Keithley 6514), a multimeter (Keithley DMM2000) and a precision DC current source (Keithley 6220). In all tests, a constant, 100 mA current is passed through outer probes of the specimen. The voltage difference was measured across the multimeter through a set of electrometers. LabView data acquisition was used to record the electrical data. From the measured voltage, percentage change in electrical resistance was measured. Both the electrical and tensile/shear data are collected simultaneously and then correlated with each other by plotting them on the same plot against axial/shear strain. A Set of 8–10 specimens are tested for each composite type under both tension and shear loading. A representative curve for each composite type under both tension and shear is used.
hardener (part B), is added and the shear mixing/ultrasonication process is repeated for 20 min. In both mixing phases, the ultrasonication is set to pulsate at 4 s on and 9 s off. The mixing container is also kept in an ice bath. This is done to ensure the CNTs are not damaged due to excessive local heat during sonication process. Once the epoxy mix with CNTs is prepared, the electro up-flocking process is performed [14–17]. Fig. 1 is a schematic of the electro flocking process, which consists of a 0–90 twill woven graphite fabric (thickness of 0.3 mm) coated with above mix of epoxy with CNTs (attached to the top conductive plate) and evenly distributed short carbon fibers placed on the bottom electrode. A high voltage difference was applied across the two electrodes and the electric field generated charges to the carbon fibers. The charged fibers travel vertically upwards while also creating a dipole moment in the fibers that embed vertically into the graphite fabric. The process is repeated for each laminate of the composite. The layers are then laid up in the compression mold. In between each reinforcement layer, extra amount of CNTs embedded epoxy was poured. Once all layers are stacked, the layup is compressed. Excess epoxy is forced out of the layup, leaving a very consistent and repeatable composite sheet. Table 1 provides number of layers of carbon fabrics and volume fraction of the carbon fabric in the composites.
3. Experimental results Fig. 3 shows the electro-mechanical response of composites under both tensile and shear loading conditions. For tensile loading conditions, as shown in Fig. 3(a), composites with no short carbon fibers showed superior tensile strength and elongation at break compared to other cases. Composites having short carbon fiber length of 150 μm showed tensile strength similar to that of composites having no short carbon fibers. However, the percentage elongation at break decreased
2.3. Electro-mechanical characterization The specimen configurations for determining electro-mechanical response of composites under tensile and in-plane shear testing are 32
Composites Communications 15 (2019) 30–33
R. Sherman, et al.
by around 20%. Composites having short carbon fiber length of 350 μm demonstrated lower tensile strength (about 45% decrease) compared to that of 150 μm, but they maintained similar percentage elongation at break of 8%. The reasons for these decreases are due to that short carbon fibers between the graphite laminates do not bond well with epoxy and more over they increase the amount of epoxy between the laminates. The increase in amount of epoxy makes the composite brittle and thus reduces the elongation at break. The longer carbon fiber length of 350 μm further increases the amount of epoxy, causing the significant decrease in tensile strength. The electrical response of the composites under tensile loading is divided into two zones. The first zone represents the decreasing trend of resistivity and second zone represents increasing trend of resistivity. Dotted vertical lines in Fig. 3(a) separate these zones. The initial decrease in resistivity is attributed to fact that both CNTs and short carbon fibers make new contacts under axial deformation. This initial decrease zone is further extended for composites having 350 μm short carbon fibers, because of their length, they were able to prolong the making of new contacts during axial deformation. With further increase in axial strain, all three composite types showed increasing trend. Composites having no short carbon fibers showed highest percentage of resistivity compared to other two cases. This is again due to fact that as the damage is progressing, the short carbon fiber composites continue to make new contacts and this is more prevalent in composites having 350 μm short carbon fibers. For in-plane shear loading conditions, as shown in Fig. 3(b), the shear strength of composites having no short carbon fibers is the highest compared to those of short carbon fibers. The addition of short carbon fibers of both cases decreased the shear strength significantly by about 30%. The shear strain at break of composites of short carbon fibers also decreased by about 10% (for 350 μm) and 25% (for 150 μm) compared to that of no short carbon fibers. The reason for decrease of both shear strength and shear strain at break is again attributed to the additional amount of epoxy that is generated between the laminates as discussed above. Electrical response under shear loading almost demonstrated only decrease in resistivity compared to tensile loading conditions. Although there is damage growth with increase in shear strain, the formation new electrical contacts by CNTs and short carbon fibers with neighboring graphite fabric competed against the increase in resistivity due to damage growth. The small decrease in percentage change in resistivity (less than 0.4%) also provides the fact, there is a stiff competition between damage growth and formation of new electrical contacts under shear loading compared to tensile loading. With short carbon fibers of 350 μm long, the composites showed almost no change in resistance for most of the shear loading process. Table 1 provides initial and final resistivity of composites under both tensile and shear loading conditions.
Acknowledgements The authors acknowledge the financial support of U.S. Army Research at Aberdeen Proving Ground collaborative agreement (W911NF-17-2-0198). References [1] I.M. Daniel, Engineering Mechanics of Composite Materials, second ed., Oxford University Press, New York, 2005. [2] A. Godara, L. Gorbatikh, G. Kalinka, A. Warrier, O. Rochez, L. Mezzo, F. Luizi, A.W. Van Vuure, S.V. Lomov, I. Verpoest, Interfacial shear strength of a glass fiber/ epoxy bonding in composites modified with carbon nanotubes, Compos. Sci. Technol. 70 (2010) 1346–1352 https://doi.org/10.1016/j.compscitech.2010.04. 010. [3] B. Yu, Z. Jiang, X.Z. Tang, C.Y. Yue, J. Yang, Enhanced interphase between epoxy matrix and carbon fiber with carbon nanotube-modified silane coating, Compos. Sci. Technol. 99 (2014) 31–140 https://doi.org/10.1016/j.compscitech.2014.05. 021. [4] F.H. Gojny, M.H.C. Wichmann, B. Fiedler, W. Bauhofer, K. Schulte, Influence of nano-modification on the mechanical and electrical properties of conventional fibre-reinforced composites, Compos Part A: Appl S. 36 (2005) 1525–1535 https:// doi.org/10.1016/j.compositesa.2005.02.007. [5] G.Y. Li, P.M. Wang, X. Zhao, Pressure-sensitive properties and microstructure of carbon nanotube reinforced cement composites, Cement Concr. Compos. 29 (2007) 377–382 https://doi.org/10.1016/j.cemconcomp.2006.12.011. [6] N. Yamamoto, R. Guzman de Villoria, B.L. Wardle, Electrical and thermal property enhancement of fiber-reinforced polymer laminate composites through controlled implementation of multi-walled carbon nanotubes, Compos. Sci. Technol. 72 (2015) 2009–2015 https://doi.org/10.1016/j.compscitech.2012.09.006. [7] N. Heeder, A. Shukla, V.B. Chalivendra, S.Z. Yang, Sensitivity and dynamic electrical response of CNT-reinforced nanocomposites, J. Mater. Sci. 47 (2012) 3808–3816 https://doi.org/10.1007/s10853-011-6235-8. [8] V. Vadlamani, V.B. Chalivendra, A. Shukla, S. Yang, Sensing of damage in carbon nanotubes and carbon black reinforced epoxy composites under tensile loading, polym. Comp 33 (2012) 1809–1815 https://doi.org/10.1002/pc.22326. [9] S. Cardoso, V.B. Chalivendra, A. Shukla, S. Yang, Damage detection in the fracture process zone of rubber toughened epoxy using carbon nanotube sensory network, Eng. Fract. Mech. 96 (2012) 380–391 https://doi.org/10.1016/j.engfracmech. 2012.08.012. [10] V.K. Vadlamani, V.B. Chalivendra, A. Shukla, S. Yang, In-situ sensing of non-linear deformation and damage in epoxy particulate composites, Smart Mater. Struct. 21 (2012) 075011 https://doi.org/10.1088/0964-1726/21/7/075011. [11] S.M. Cardoso, C.M. O'Connell, R. Pivonka, C. Mooney, V.B. Chalivendra, A. Shukla, S. Yang, Effect of external loads on damage detection of rubber-toughened nanocomposites using carbon nanotubes sensory network, Polym. Comp. 37 (2014) 360–369. [12] J. Stuckey, V.B. Chalivendra, M.A. Haile, A. Hall, Damage detection in epoxy embedded carbon nanotubes using electrical resistance and acoustic emission, J. Nanomech Micromech. 7 (2017) 06017001 https://doi.org/10.1061/(ASCE)NM. 2153-5477.0000122. [13] K. Shkolnik, V. Chalivendra, Numerical studies of electrical contacts of carbon nanotubes-embedded epoxy under tensile loading, Acta Mech. 229 (2018) 99–107 https://doi.org/10.1007/s00707-017-1955-8. [14] S. Yang, V. Chalivendra, E. Benjamin, Y.K. Kim, Electrical response of novel carbon nanotubes embedded and carbon fiber Z-axis reinforced jute/epoxy laminated composites, Polym. Compos. 42 (S2) (2018) E1189–E1198 https://doi.org/10. 1002/pc.24935. [15] S. Yang, V. Chalivendra, Y.K. Kim, Damage sensing in multi-functional hybrid natural fiber composites under shear loading, Smart Mater. Struct. 27 (2018) 115034 https://doi.org/10.1088/1361-665X/aae7f2. [16] J. O’Donnell, V. Chalivendra, A. Hall, M. Haile, L. Nataraj, M. Coatney, Y. Kim, Electro-mechanical studies of multi-functional glass fiber/epoxy reinforced composites, J. Reinf. Plast. Compos. 38 (11) (2019) 506–520 https://doi.org/10.1177/ 0731684419832796. [17] R. Sherman, V. Chalivendra, A. Hall, M. Haile, L. Nataraj, M. Coatney, Y. Kim, Characterization of electro-mechanical response in novel carbon fiber composite materials, J. Compos. Mater. (2019), https://doi.org/10.1177/ 0021998319839113. [18] E.T. Thostenson, T.W. Chou, Real-time in situ sensing of damage evolution in advanced fiber composites using carbon nanotube networks, Nanotechnology 19 (2008) 215713 https://doi.org/10.1088/0957-4484/19/21/215713. [19] M. Saafi, Wireless and embedded carbon nanotube networks for damage detection in concrete structures, Nanotechnology 20 (2009) 395502 https://doi.org/10. 1088/0957-4484/20/39/395502. [20] L. Gao, E.T. Thostenson, Z. Zhang, J.H. Byun, T.W. Chou, Damage monitoring in fiber-reinforced composites under fatigue loading using carbon nanotube networks, Philos. Mag. A 90 (2010) 4085–4099 https://doi.org/10.1080/ 14786430903352649. [21] S.M. Friedrich, A.S. Wu, E.T. Thostenson, T.W. Chou, Damage mode characterization of mechanically fastened composite joints using carbon nanotube networks, Compos Part A-Appl S 42 (2011) 2003–2009 https://doi.org/10.1016/j. compositesa.2011.09.006.
4. Conclusions A comparative study was performed to investigate the electro-mechanical response of graphite composites embedded with CNTs and short carbon fibers under quasi-static tensile and in-plane shear loading conditions. Addition of short carbon fibers decreased shear strength and elongation at break under both load conditions. The electrical resistivity change under axial loading showed a combination of decreasing and increasing trends. The length of decreasing resistivity zone is extended for the composite having 150 μm short carbon fibers. However, the electrical resistivity of all composite types showed almost only decreasing trend under shear loading conditions. The amount of decrease in resistivity under shear loading is very less compared to that of tensile loading. Hence, the electrical response of these novel threedimensionally conductive composites showed definite trend in tensile conditions compared to shear conditions indicting that the damage sensing through electrical response is not that useful under shear loading although most of the laminated composites undergo interlaminar shear failure in real-life conditions. 33
Contents lists available at ScienceDirect
Composites Communications journal homepage: www.elsevier.com/locate/coco
Electro-mechanical characterization of three-dimensionally conductive graphite/epoxy composites under tensile and shear loading
T
Riley Shermana, Vijaya Chalivendraa,*, Asha Hallb, Mulugeta Haileb, Latha Natarajb, Michael Coatneyb, Yong Kimc a
Department of Mechanical Engineering, University of Massachusetts Dartmouth, Dartmouth, MA, 02747, USA U.S. Army Research Laboratory, Aberdeen Proving Ground, MD, 21005, USA c Department of Bioengineering, University of Massachusetts Dartmouth, Dartmouth, MA, 02747, USA b
A B S T R A C T
An experimental study was performed to compare the electro-mechanical response of three-dimensionally conductive woven carbon fiber/epoxy laminated composites under quasi-static uniaxial tensile and in-plane shear loading conditions. Three-dimensional (3D) electrical network was generated in these composites by embedding carbon nanotubes (CNTs) of 0.025 wt% in the epoxy and reinforcing short carbon fibers (150 μm and 350 μm long) with a fiber density of 1000 fibers/ mm2 between the laminates using electro-flocking process. CNTs are effectively dispersed in the epoxy matrix using a combination of ultrasonication and shear mixing techniques. A compression molding fabrication technique was employed to fabricate composite materials. For all composite types, in general, the in-situ electrical response showed a trend of initial decrease in resistance and later increase in resistance for tensile loading. However, no noticeable increase in electrical resistance was observed until failure of the composite under shear loading conditions. Both CNTs and short carbon fibers made new contacts with neighboring carbon fiber laminates for entire duration of shear loading even though the composite was undergoing progressive failure.
1. Introduction Fiber reinforced polymeric composite materials are currently on the cutting edge of specialty due to their ability to combine the low weight of polymers with the incredible strength of fibers [1]. To further specialize these composites, carbon nanotubes (CNTs) were embedded into them to significantly improve their mechanical properties [2,3]. Other studies are aimed to explore the possibility of multifunctional composites by utilizing the highly electrically conductive nature of CNTs [4–6]. An area that has been gaining a lot of attention in multifunctional CNT composites is in-situ damage sensing. The in-situ change of electrical resistance of CNTs conductive network in polymer composites under mechanical loads may serve as indicators of different damage mechanisms under various loading conditions [7–17]. Vadlamani et al. [8] investigated the effect of CNTs weight fraction on electro-mechanical response under tensile loading conditions. In their study, the authors reported a decreasing electrical resistance trend up to 45% due to matrix's molecular chains declustering and straightening at lower weight fractions of 0.1% and 0.3%. Later, the same authors [10] studied the electro-mechanical response of rubber toughened and rigid particles epoxy systems under quasi-static tensile loadings. They observed threshold strain value for damage propagation in these particulate polymer systems from change in electrical resistance
*
measurements. Heeder et al. [7], examined the electrical response of CNTs embedded epoxy materials under uniaxial compression at quasistatic, medium strain rate and high strain rate loading conditions. Although a monotonic decrease in electrical response was observed under quasi-static and high strain rate conditions, a decreasing and sudden increasing in resistance was noticed under medium strain rates. In case of laminated composites, Thostenson and Chou [18] employed 0.5% wt. of CNTs in an advanced fiber composite to study the electrical conductivity under fatigue loads. Saafi [19] used 1.5% wt. of CNTs in cement matrix to develop an in situ wireless and embedded sensor for damage detection in concrete structures. Gao et al. [20] also used 0.5% wt. of CNTs in fiber reinforced polymer composites to detect damage under fatigue loading conditions. Using 0.75% wt. of CNTs, Friedrich et al. [21] studied various damage mode characteristics such as matrix cracking delamination, bearing and shear-out in mechanically fastened composite joints. In the above studies of laminated composites, only CNTs were embedded to generate conductive network within the composites and hence higher amount of CNTs were used. In this study, for the first time, a combination of CNTs and short carbon fibers were used to generate three-dimensional network within the composites. Two different length short carbon fibers of 150 μm and 350 μm were reinforced between the graphite laminates using electro-flocking approach. A comparative
Corresponding author. E-mail address: [email protected] (V. Chalivendra).
https://doi.org/10.1016/j.coco.2019.05.010 Received 28 October 2018; Received in revised form 17 April 2019; Accepted 31 May 2019 Available online 01 June 2019 2452-2139/ © 2019 Elsevier Ltd. All rights reserved.
Composites Communications 15 (2019) 30–33
R. Sherman, et al.
study of electro-mechanical response is conducted under quasi-static tensile and in-plane shear loading conditions. 2. Experimental details 2.1. Materials A thermoset epoxy system 2000 resin with 2120 hardener with a mixing ratio of 100 to 27 was used as the matrix (Supplied by Fibre Glast Development Corporation, Brookville, OH, USA). A small amount, 0.025% by weight, multi-walled carbon nanotubes (MWCNTs) (supplied by Cheap Tubes Inc., Cambridgeport, VT) of purity > 99% were mixed via controlled shear mixing and ultrasonication into the resin prior to fabrication. 3 K (3000 filaments per fiber) twill woven carbon fiber fabric (Supplied by Fibre Glast Development Corporation, Brookville, OH, USA) was used as laminates. Short carbons fibers of length 150 μm and 350 μm with a diameter of 7 μm (supplied by Asbury Graphite Mills, INC. NJ, USA) were used. Short carbon fiber density of 1000 fibers/mm2 is considered in this study.
Fig. 1. Schematic showing the electro flocking procedure. Table 1 Carbon fabric information, initial and final resistivity of composites. Composites of carbon fiber length (μm)
None 150 350
Number of carbon fabric layers
20 13 10
Fiber volume fraction of carbon fabric
Resistivity (Ohms-mm)
Initial
Final at break
Initial
Final at break
61.9 38.5 32.8
0.952 0.134 0.447
0.992 0.148276 0.514
3.404 9.271 3.535
3.399 9.253 3.535
Tensile
2.2. Fabrication
Shear
The first step in the fabrication process is dispersion of CNTs into the epoxy matrix. A detailed description was discussed in our previous work [8–17]. Due to brevity of space, a brief description is provided here. For all composite types, 0.025 wt% of CNTs are first mechanically mixed with a stirrer into the thermoset resin (part A). This mixture is then simultaneously shear mixed and ultrasonicated for 1 h. The mixture is later degassed in a vacuum chamber for at least 2 h. Next, the
Fig. 2. (A) and (B) show the standard specimen configurations used for tensile and shear conditions respectively. 31
Composites Communications 15 (2019) 30–33
R. Sherman, et al.
Fig. 3. (A) Electro-mechanical response under tensile loading, and (B) electro-mechanical response under shear loading.
shown in Fig. 2. An Instron Materials Testing System was utilized to collect the load and displacement data for shear and tensile loading. Electrical data was collected by utilizing the standard four point circumferential ring probes along with two high impedance electrometers (Keithley 6514), a multimeter (Keithley DMM2000) and a precision DC current source (Keithley 6220). In all tests, a constant, 100 mA current is passed through outer probes of the specimen. The voltage difference was measured across the multimeter through a set of electrometers. LabView data acquisition was used to record the electrical data. From the measured voltage, percentage change in electrical resistance was measured. Both the electrical and tensile/shear data are collected simultaneously and then correlated with each other by plotting them on the same plot against axial/shear strain. A Set of 8–10 specimens are tested for each composite type under both tension and shear loading. A representative curve for each composite type under both tension and shear is used.
hardener (part B), is added and the shear mixing/ultrasonication process is repeated for 20 min. In both mixing phases, the ultrasonication is set to pulsate at 4 s on and 9 s off. The mixing container is also kept in an ice bath. This is done to ensure the CNTs are not damaged due to excessive local heat during sonication process. Once the epoxy mix with CNTs is prepared, the electro up-flocking process is performed [14–17]. Fig. 1 is a schematic of the electro flocking process, which consists of a 0–90 twill woven graphite fabric (thickness of 0.3 mm) coated with above mix of epoxy with CNTs (attached to the top conductive plate) and evenly distributed short carbon fibers placed on the bottom electrode. A high voltage difference was applied across the two electrodes and the electric field generated charges to the carbon fibers. The charged fibers travel vertically upwards while also creating a dipole moment in the fibers that embed vertically into the graphite fabric. The process is repeated for each laminate of the composite. The layers are then laid up in the compression mold. In between each reinforcement layer, extra amount of CNTs embedded epoxy was poured. Once all layers are stacked, the layup is compressed. Excess epoxy is forced out of the layup, leaving a very consistent and repeatable composite sheet. Table 1 provides number of layers of carbon fabrics and volume fraction of the carbon fabric in the composites.
3. Experimental results Fig. 3 shows the electro-mechanical response of composites under both tensile and shear loading conditions. For tensile loading conditions, as shown in Fig. 3(a), composites with no short carbon fibers showed superior tensile strength and elongation at break compared to other cases. Composites having short carbon fiber length of 150 μm showed tensile strength similar to that of composites having no short carbon fibers. However, the percentage elongation at break decreased
2.3. Electro-mechanical characterization The specimen configurations for determining electro-mechanical response of composites under tensile and in-plane shear testing are 32
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by around 20%. Composites having short carbon fiber length of 350 μm demonstrated lower tensile strength (about 45% decrease) compared to that of 150 μm, but they maintained similar percentage elongation at break of 8%. The reasons for these decreases are due to that short carbon fibers between the graphite laminates do not bond well with epoxy and more over they increase the amount of epoxy between the laminates. The increase in amount of epoxy makes the composite brittle and thus reduces the elongation at break. The longer carbon fiber length of 350 μm further increases the amount of epoxy, causing the significant decrease in tensile strength. The electrical response of the composites under tensile loading is divided into two zones. The first zone represents the decreasing trend of resistivity and second zone represents increasing trend of resistivity. Dotted vertical lines in Fig. 3(a) separate these zones. The initial decrease in resistivity is attributed to fact that both CNTs and short carbon fibers make new contacts under axial deformation. This initial decrease zone is further extended for composites having 350 μm short carbon fibers, because of their length, they were able to prolong the making of new contacts during axial deformation. With further increase in axial strain, all three composite types showed increasing trend. Composites having no short carbon fibers showed highest percentage of resistivity compared to other two cases. This is again due to fact that as the damage is progressing, the short carbon fiber composites continue to make new contacts and this is more prevalent in composites having 350 μm short carbon fibers. For in-plane shear loading conditions, as shown in Fig. 3(b), the shear strength of composites having no short carbon fibers is the highest compared to those of short carbon fibers. The addition of short carbon fibers of both cases decreased the shear strength significantly by about 30%. The shear strain at break of composites of short carbon fibers also decreased by about 10% (for 350 μm) and 25% (for 150 μm) compared to that of no short carbon fibers. The reason for decrease of both shear strength and shear strain at break is again attributed to the additional amount of epoxy that is generated between the laminates as discussed above. Electrical response under shear loading almost demonstrated only decrease in resistivity compared to tensile loading conditions. Although there is damage growth with increase in shear strain, the formation new electrical contacts by CNTs and short carbon fibers with neighboring graphite fabric competed against the increase in resistivity due to damage growth. The small decrease in percentage change in resistivity (less than 0.4%) also provides the fact, there is a stiff competition between damage growth and formation of new electrical contacts under shear loading compared to tensile loading. With short carbon fibers of 350 μm long, the composites showed almost no change in resistance for most of the shear loading process. Table 1 provides initial and final resistivity of composites under both tensile and shear loading conditions.
Acknowledgements The authors acknowledge the financial support of U.S. Army Research at Aberdeen Proving Ground collaborative agreement (W911NF-17-2-0198). References [1] I.M. Daniel, Engineering Mechanics of Composite Materials, second ed., Oxford University Press, New York, 2005. [2] A. Godara, L. Gorbatikh, G. Kalinka, A. Warrier, O. Rochez, L. Mezzo, F. Luizi, A.W. Van Vuure, S.V. Lomov, I. Verpoest, Interfacial shear strength of a glass fiber/ epoxy bonding in composites modified with carbon nanotubes, Compos. Sci. Technol. 70 (2010) 1346–1352 https://doi.org/10.1016/j.compscitech.2010.04. 010. [3] B. Yu, Z. Jiang, X.Z. Tang, C.Y. Yue, J. Yang, Enhanced interphase between epoxy matrix and carbon fiber with carbon nanotube-modified silane coating, Compos. Sci. Technol. 99 (2014) 31–140 https://doi.org/10.1016/j.compscitech.2014.05. 021. [4] F.H. Gojny, M.H.C. Wichmann, B. Fiedler, W. Bauhofer, K. 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4. Conclusions A comparative study was performed to investigate the electro-mechanical response of graphite composites embedded with CNTs and short carbon fibers under quasi-static tensile and in-plane shear loading conditions. Addition of short carbon fibers decreased shear strength and elongation at break under both load conditions. The electrical resistivity change under axial loading showed a combination of decreasing and increasing trends. The length of decreasing resistivity zone is extended for the composite having 150 μm short carbon fibers. However, the electrical resistivity of all composite types showed almost only decreasing trend under shear loading conditions. The amount of decrease in resistivity under shear loading is very less compared to that of tensile loading. Hence, the electrical response of these novel threedimensionally conductive composites showed definite trend in tensile conditions compared to shear conditions indicting that the damage sensing through electrical response is not that useful under shear loading although most of the laminated composites undergo interlaminar shear failure in real-life conditions. 33