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Fracture Toughness (Mode-II) of Nanostitched Composites
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Available online at www.sciencedirect.com Structural Integrity Procedia 00 (2019) 000–000
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1st International Workshop on Plasticity, Damage and Fracture of Engineering Materials Procedia Structural Integrity 21 (2019) 146–153
Fracture Toughness (Mode-II) of Nanostitched Composites
1st International Workshop on Plasticity, Damage and Fracture of Engineering Materials
Kadir Bilisik1, ⃰ Gulhan Erdogan1 Erdal Sapanci2 and Sila Gungor2
Fracture Toughness (Mode-II) of Nanostitched Composites
Nano/Micro Fiber Preform Design and Composite Laboratory, Department of Textile Engineering, Faculty of Engineering, Erciyes University, 38039 Talas-Kayseri, Turkey 1, 2ROKETSAN Industries, 06780, 1 Elmadag-Ankara, Turkey 2 2
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Kadir Bilisik ⃰ Gulhan Erdogan Erdal Sapanci and Sila Gungor
Nano/Micro Fiber Preform Design and Composite Laboratory, Department of Textile Engineering, Faculty of Engineering, Erciyes University, 38039 Talas-Kayseri, Turkey 2 ROKETSAN Industries, 06780, Elmadag-Ankara, Turkey
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Abstract
The properties of fracture toughness (mode-II) of nanostitched carbon/epoxy composites were investigated. The mode-II of the nanostitched structure demonstrated extraordinary enhancement considering to the control sample. It was proved that carbon nanostitching yarn in the though-the-thickness of the preform structure was adequate. The fundamental mechanism for raising the Abstract mode-II strength in the nanostitched composite was the interlayer matrix breakages predominantly as a form of shear hackle marks in whereofnanostitching suppressed the layer to layer opening in stitching region during crack growth.The Multiwall The properties fracture toughness (mode-II) of nanostitched carbon/epoxy composites were investigated. mode-IIcarbon of the nanotubes in structure the matrix and filament also diminished the stress clusteringtoperhaps as a sample. form ofItfriction, filaments/matrix nanostitched demonstrated extraordinary enhancement considering the control was proved that carbon debonding in yarn the interlaced unit cell and filament the failed matrix zone.The So,fundamental nanostitchedmechanism carbon/epoxy composite nanostitching in the though-the-thickness of theslippage preforminstructure was adequate. for raising the showed interlainar shear properties. mode-II better strength in the nanostitched composite was the interlayer matrix breakages predominantly as a form of shear hackle marks in where nanostitching suppressed the layer to layer opening in stitching region during crack growth. Multiwall carbon nanotubes in the matrix and filament also diminished the stress clustering perhaps as a form of friction, filaments/matrix debonding in the interlaced unit cell and filament slippage in the failed matrix zone. So, nanostitched carbon/epoxy composite showed better interlainar shear properties. © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/ © 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the 1st International Workshop on Plasticity, Damage and Fracture of Engineering Materials This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) organizers under responsibility of the 1st International Workshop on Plasticity, Damage and Fracture of Engineering Materials organizers Peer-review © 2019 The Authors. Published by Elsevier B.V. Keywords: nanotubes; nanoprepreg; nanostitching; mode‒II toughness; shear hackle. This is an Carbon open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/ Peer-review under responsibility of the 1st International Workshop on Plasticity, Damage and Fracture of Engineering Materials organizers Keywords: Carbon nanotubes; nanoprepreg; nanostitching; mode‒II toughness; shear hackle.
Corresponding author. Tel.: +90352 207 66 66/Ext. 32875 or 32851; fax: +90352 437 5784. E-mail address: [email protected] ⃰
2452-3216 © 2019 The Authors. Published by Elsevier B.V. ⃰ Corresponding Tel.: +90352 207CC 66BY-NC-ND 66/Ext. 32875 or 32851; fax: +90352 437 5784. This is an open author. access article under the license (http://creativecommons.org/licenses/by-nc-nd/4.0/ E-mail address: [email protected]
Peer-review under responsibility of the 1st International Workshop on Plasticity, Damage and Fracture of Engineering Materials organizers 2452-3216 © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/
Peer-review under responsibility of the 1st International Workshop on Plasticity, Damage and Fracture of Engineering Materials organizers
2452-3216 © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the 1st International Workshop on Plasticity, Damage and Fracture of Engineering Materials organizers 10.1016/j.prostr.2019.12.096
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1. Introduction Carbon fiber preform and composites have been used in space and aero vehicle industries as well as defense sectors as structural parts due to their extraordinary thermo-mechanic, electro-magnetic and damage resistance properties Kamiya et al. (2000). However, traditional carbon/matrix structures exhibited inferior delamination and damage tolerance resistances. Hence, binder fiber in fabric architecture were introduced by using the traditional textile technologies such as stitching Tong et al. (2002), Bilisik and Yolacan (2014), Bilisik and Yolacan (2012), carpet and velvet as well as flocking. Other innovative techniques were considered as three dimensional weaving Bilisik (2010), Bilisik et al. (2013), three dimensional braiding Bilisik and Sahbaz (2012), Bilisik (2011) and z-pin anchoring Pingkarawat and Mouritz (2014). In recent past, nano materials as a form of nanoparticle, nanotubes and rods as well as nanofibers were used to make nanofibrous composites. Nano materials were added in the resin by using several techniques as mixing, or ultrasonication, spraying and transfer-printing before composite process Garcia et al. (2008). The nano materials were placed onto two dimensional fiber substrate or uniaxial/biaxial prepreg as nano/resin mixture or sprayed forms. They can also be attached or grafted to the substrates Khan and Kim (2011). Typically, the in-plane properties of the 3D woven composites have low due to the binder fiber and total fiber volume fraction in which higher degree of interlacements at three fiber sets effected preform fiber volume during fabric formation. Z-pin in the preform was not continuous due to lack of loop section. The nano materials in the fibrous composite were also discrete form and they were not continuous in the fabric architecture. The resin properties influenced the toughening mechanism of mode˗II interlaminar shear fracture Kuwata and Hogg (2011). It was obtained that fiber surface treatment in laminated composite enhanced the interlaminar strength and the mode‒II shear fracture Madhukar and Drazal (1992). But, the interfacial modification was not effective compare to the toughened resin. The matrix and fiber/resin interface mostly influenced the interlaminar fracture behavior of composites Deng and Ye (1999). Several surface modification techniques which were fiber surface roughening and plasma treatments were employed to improve the adhesion between matrix and filaments Li et al. (2015). Nonetheless, it was reported that the high modulus fibres was damaged by these surface modifications Wu and Cheng (2006). One of the studies demonstrated that 3D orthogonal carbon/epoxy composite demonstrated extraordinarily better fracture toughness performance compared to the classical laminated composites Guenon et al. (1989). The stitching yarn in the through-the-thickness of composite improved the mode‒II toughness due to using the high areal stitching density Herwan et al. (2014). Mouritz identified that the fracture resistance was improved due to tortoise crack path which was generated by stitched fiber Mouritz (2004). It was discovered that interlaminar toughness of stitched carbon structure was greater than the neat sample Jain et al. (1998). The crack growth in stitched structure was impeded via bridging and arresting. The fractured surface had stitched fiber/resin debonding and intra/interlayer opening Tan et al. (2012). It was demonstrated that the GII was enhanced by accumulative effect of the nanofibers/z‒pins in the composite Ravindran et al. (2019). It was obtained that the high z-pin density composite demonstrated improved mode‒II toughness Huang and Waas (2014). The z-pin length/diameter ratio and density were the critical parameters Partridge and Cartie (2005). Recently, it was reported that the mode‒II toughness of the nanostitched carbon/epoxy nanocomposite showed several fold increase compared to the pristine Bilisik et al. (2019 a; DOI: 10.1177/0021998319857462). It was found that nanoprepreg carbon/epoxy composite exhibited threefold increase for mode‒II and average two fold increase for mode‒I as compared to the control sample Garcia et al. (2008). The reason could be the complex accumulative interaction of resin/nanotubes/filaments. These interactions were probably bridging, pull-out, friction, and stick-slip. Another research study reported that the nanocomposite exhibited one and half fold increase on fracture toughness (mode‒II) compared to the base sample Falzon et al. (2013). The crack propagation and plastic deformation in the crack tip region caused nonlinear mode‒II failure Carlsson et al. (1986). The failure in the plastic damaged zone of the mode‒II crack front was micro tearing and minor fiber bridging Hashemi et al. (1990). On the other hand, the carack growth and coalescence of micro tensile cracks on the mode‒II crack tip was identified Xia and Hutchinson (1994). Therefore, the purpose of this research was to investigate the mode‒II interlaminar fracture toughness of nanostitched carbon/epoxy composite structures via the end notched flexure. Grafting CNT on the filament bundle surfaces led to critical enhancement in flexural and mode‒II properties but it did not affect the tensile properties Khan et al. (2018). It was reported that the carbon/epoxy nanocomposite exhibited 27% increase in the GII because of addition of fluorine functional group to the carbon nanotubes (f-CNTs)
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Davis and Whelan (2011). Amino treated nanofiber mat and plain nanofiber mat also improved the mode‒II toughness and mechanical properties of nanocomposites, respectively Beckerman and Pickering (2015). The fiber shear in the resin and resin tensile fracture were dominant parameters for the mode‒II failure of composite Russel (1987). CNT dispersion to carbon/epoxy nanocomposite via spraying and film transferred technique illustrated that nanotubes significantly increased the fiber bridging to improve the fracture toughness Joshi and Dikshit (2012). Graphene nanoplatelets (GnPs) and the MWCNTs additions in carbon/epoxy composites enhanced its interlayer shear and fracture toughness properties because of strong bonding at nano materials/resin/filaments Srivastava et al. (2017). A model on fracture toughness was introduced based on bridging and strain energy during delamination crack growth Sun and Jin (2006). The mode‒II failure was quantitatively modeled. Lee found that resin properties dominated microcrack shear initiation stress and coalescence process. Resin yielding and plastic deformations were found as important parameters to control the mode‒II toughness Lee (1997). The fracture process during mode‒II loading was well defined by cohesive zone model especially for thermoplastic composites Reis et al. (2019). It was also reported that 3D orthogonal architecture provided a higher strength, whereas 3D angle interlock provided more energy absorption and prevented mode-II crack propagation probably due to fiber interlacement patterns Pankow et al. (2011). One of another study showed that crack growth of GII exhibited a low and unstable propagation (ductile and slow) for initial state and it showed a high and stable propagation (brittle and fast) for latter state when the infrared thermography technique was employed Perez et al. (2019). The interlacement pattern influenced the crack propagation in woven composite in particular, instable crack growth was obtained in mode‒II fractured composite Blake et al. (2012). The nanoparticle property transfer was studied by using the energy and mechanic models Batra and Sears (2007). By applying the Cottrell–Kelly–Tyson (CKT) model, nanocomposite fracture toughness was determined considering the nanotube specifications and pull-out energy Wagner et al. (2013). The restraint of stress distribution of the nanotubes in the composite was defined by using the two-scale model Romanov et al. (2015). 2. Materials and methods 2.1. Nanostitched carbon/epoxy composites Carbon substrates (Polyacrylonitrile (PAN), Spinteks A.S., TR) were utilized to make stitched prepreg preform. The carbon and para‒aramid stitching yarns, epoxy (Araldite LZ 5021, Biesterfeld GmbH, DE), multiwall carbon nanotubes (MWCNTs) and carbon fabric were used to design the preform composites. Two dimensional (2D) carbon fabric was weaved from 12K filaments. The substrate was low order interlaced satin (1/4) patterns. The multiwall carbon nanotubes (MWCNTs, Nanothinx, GR) were considered and its specification of MWCNTs is given in Table 1. The mode II toughness test samples were manufactured as four kind preform structures. First one was base (CSU) preform which was four layer carbon fabrics. Second type was stitched (CS-CS and CS-TS) preforms. The CS-CS was four layer satin fabrics, and stitched with carbon fiber. However, the CS-TS was four layers satin fabrics, and stitched with Twaron CT yarn. Third type was base nano (CSU-N) preform. The CSU-N was four layer satin fabrics with added multiwall carbon nanotubes.
Fig. 1. Carbon/epoxy carbon nanostitched composites, actual and schematic, respectively.
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Table 1. Specifications of multiwall carbon nanotubes (www.nanothinx.com (2019). Surface Molecular Particle dimensions Purity Nanomaterial area weight (m2/g) (g/mol) (nm x micron x nm) (%) MWCNTs, (Nanothinx,GR)
15-35 x 10≥ x 1-2≥
12.01
>100
≥97
149
(g/cm3)
Tensile strength (GPa)
Tensile modulus (TPa)
Melting temperature (°C)
1.74
200
1.0
3550
Density
Fourth type was nanostitched preforms. The CS-CS-N was carbon nanostitched, nanoprepreg four layer preforms. The CS-TS-N was Twaron CT nanostitched, nanoprepreg four layers satin preform. Stitched yarns in all preforms were placed at their through-the-thickness (Fig. 1) and their properties are provided in Table 2. Nanostitched composites were created by utilizing the nanostitched preforms. More detail explanation on the composite fabrication was provided in the reference Bilisik et al. (2019 b). Table 2. Specifications of untwisted stitching yarns. Fiber type Fiber diameter Twaron CT (Para-aramid fiber, Teijin, JP) Polyacrylonitrile (PAN) Carbon (Carbon fiber, Aksaca, TR)
Fiber density
Tensile strength
Tensile modulus
Elongation at break
Yarn linear density
(µm)
(g/cm3)
(GPa)
(GPa)
(%)
(dtex)
12
1.45
3.2
115
2.9
6
1.78
4.2
240
1.8
3360 6K
2.2. Mode‒II interlaminar fracture toughness (GIIC) test Mode-II properties of all developed carbon/epoxy composites were determined via end notch flexure (ENF) method. ASTM Standard D7905‒14 (2014) was followed. Fig. 2 shows the mode‒II testing by using the 3-point flexture fixture. The test was conducted to obtain GIIC for both non-precrack (NPC) and precrack (PC) cases.
Fig. 2. ENF sample during mode-II testing (left) and failed samples (right) (digital image).
3. Results and discussion 3.1. Mode‒II fracture toughness results Fig. 3(a‒b) illustrated the load(P)‒displacement(δ) curves of composites for NPC and PC cases. The carbon and para-aramid nanostitched and stitched composites showed greater values considering to the nano and base composites. The load‒displacement curves of all composites become almost linear vertical slope.
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Fig. 3. Mode-II load(P)‒displacement(δ) curves (30 cm). (a) NPC case; (b) PC case (Bilisik et al (2019 a).
Fig. 4(a‒b) exhibited compliances (mm/N) versus the cube root of crack length (mm 3) for all satin 1/4 interlaced composites for non-precrack and precrack cases. All satin interlaced composites for precrack condition were lightly steeper considering to the non-precrack. Their data were perfectly fitted regression line as they were implied by their R‒squared values (0.9998‒0.9963). After the compliance calibration coefficients (A and m) were obtained, they were considered to find the GIIC toughness values of all the composites.
Fig. 4. Compliances versus crack length (30 mm) curves for all composites. (a) PC case; (b) NPC case.
Fig. 5 presents the average GIIC outcome in all composites for NPC and PC conditions. In PC case, the GIIC of the carbon and para-aramid nanostitched structures (CS-CS-N and CS-TS-N) were better compared to the base (CSU) due to addition of the nanostitched filament TOW and MWCNTs. Further, the GIIC of the carbon and para-aramid stitched structures (CS-CS and CS-TS) were lightly enhanced considering the base (CSU) due to addition of the stitched filament TOW. The carbon stitched fiber was more effective compared to the Twaron ® filament TOW due to stiffness properties of the PAN fiber and fiber-matrix adhesion in interface regions, in particular interlacement zone for each predetermined unit cell. It was analyzed that nanostitching and stitching enhanced the interlayer toughness of all the base and nano composites. Nevertheless, the mode-II values of nano (CSU-N) and base (CSU) composites were close to each other due to local MWCNTs agglomeration in the resin or filament TOW in the axisymmetric fabric surface. On the other hand, the mode‒II of PC for all composites were higher compared to the NPC cases. This was because of the nonlinearities as a form of heterogonous resin distribution in the starting of the crack region. The principle mode-II toughness behavior of the nanostitched composite was the interlayer resin failure (almost symmetric). The failure type was predominantly shear hackles around the cohesive matrix zone and filamentary region. The crack propagated between warp layers as a form of matrix layer fracture and it moved through the cross‒section of the filling filament TOW. The MWCNTs probably slow down the crack propagation and they suppressed the stress clustering around the fractured matrix. Concurrently, nanostitching suppressed the layer separation in stitching district during the through-the-thickness crack growth. The nanostiched filament bundle failure was not obtained. This was probably because of nonlinear interlaminar shear transfer during stress distribution. Thus, this was caused to severe matrix breakages. In the crack tip, carbon nanotubes in the fiber and matrix diminished the stress clustering as a form of filament-matrix debonding; filament or nanotubes pull-out; filament to filament or filament-nanotube stick-slip or sliding action and friction.
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3.2. SEM micrograph for failure analysis The scanning electron microscope images show the warp side fracture of the base (CSU) composite in Fig. 6(a‒b). As depicted in Fig. 6, severe resin breakages in the matrix/filament boundaries were identified. Minor multiple filament bundle fractures were observed as a form of tensile and shear breakages (Fig. 6a). Kink form brittle multiple warp filament bundle breakages was obtained. After the multiple warp filaments were broken, they were skewed during mode-II loading. This led to the crack branching near to the blunt crack tip region (Fig. 6b). Thus, delamination was spread from interlayer to the angular layer-to-layer zones. 6.0 5.5
Gııc (NPC)
Gııc (PC)
5.0
GIIC (kj/m2 )
4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 CSU Base
CS-CS
CS-TS Stitched
CSU-N Nano
CS-CS-N
CS-TS-N
Stitched/Nano
Composite structures
Fig. 5. Mode-II values of carbon/epoxy composites for PC and NPC cases.
Fig. 6. (a) SEM views of fractured warp side structure (CSU); (b) fractured warp side structure near to the blunt crack tip (CSU).
The SEM images of mode‒II toughness of warp side fractured nanostitched (CS-CS-N) composite are depicted in Fig. 7(a‒b). As depicted in Fig. 7, tensile filament failures were observed. Further, multiple filaments-matrix debonding around the fractured matrix was found. Angular severe matrix breakages near to the crack tip were identified (Fig.7a). Multiple resin fracture and some agglomerated MWCNTs in the matrix was found at the boundary of the filament to filament interface region in where several tensile filament bundle breakages and delaminated filaments were also obtained (Fig 7b). It was realized that nanostitching responded as a crack growth barrier neighbouring the stitching region and complete nanostitching breakages was not identified because of high modulus and high strength carbon and p-aramid fibres. 4. Conclusions The mode-II fracture toughness of the nanostitched and stitched composites exhibited several fold improvement considering to the control. Therefore, the nanostitch addition to the base enhanced its fracture toughness properties. Carbon stitch yarn was adequate for enhancing the fracture toughness considering to the para-aramid due to its fiber stiffness and robust filaments and resin adhesion properties. The fundamental mechanism of the mode-II behavior of the nanostitched composite was predominant the interlayer matrix failure as a form of shear hackles. The MWCNTs probably slow down the stress clustering in the
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vicinity of the matrix plastic deformation region. At the same time, nanostitching suppressed the layer separation around the stitching region during the crack growth. In the crack tip, carbon nanotubes in the matrix and filament diminished the stress clustering as a form of filament-matrix debonding; filament or nanotubes pull-out; filament to filament or filament-nanotube sliding or stick-slip action and friction.
Fig. 7. (a) SEM image of warp side fractured region; (b) fractured CS-CS-N composite around the crack tip.
In base composite, resin breakages in the matrix-filament boundaries and minor multiple filament bundle breakages were identified as a form of tensile and shear failures. In nanostitched composite, multiple resin fracture and a few tensile filament bundle breakages were obtained. In addition, minor nanostitching fracture was identified due to using the high modulus filament TOWs. It was realized that nanostitching reponded as a ply-separation barrier around the stitching region. Acknowledgements This work was supported by Roketsan Industries Grant No. RS/ERCİYES DSM-76301-14-01N/R. References ASTM Standard D7905‒14, 2014. Standard test method for determination of the mode II interlaminar fracture toughness of unidirectional fiberreinforced polymer matrix composites. West Conshohocken, PA. Batra, R. C., Sears, A., 2007. Continuum models of multi-walled carbon nanotubes. International Journal of Solids and Structures 44, 7577–7596. Beckermann, G. W., Pickering, K. L., 2015. Mode‒I and Mode‒II interlaminar fracture toughness of composite laminates interleaved with electrospun nanofibre veils. Composites Part A: Applied Science and Manufacturing 72, 11–21. Bilisik, K., 2010. Dimensional stability of multiaxis 3D woven carbon preform. Journal of the Textile Institute 101, 380–388. Bilisik, K., 2011. Three dimensional axial braided preforms: Experimental determination of effects of structure-process parameters on unit cell. Textile Research Journal 81, 2095‒2116. Bilisik, K., Sahbaz, N., 2012. Structure-unit cell base approach on three dimensional representative braided preforms from 4-step braiding: experimental determination of effect of structure-process parameters on predetermined yarn path. Textile Research Journal 82, 220‒241. Bilisik, K., Yolacan, G., 2012. Experimental determination of bending behavior of multilayered and multistitched E-glass fabric structures. Textile Research Journal 82, 1038‒1049. Bilisik, K., Sahbaz, N., Bilisik, N. E., Bilisik, H. E., 2013. Three dimensional (3D) fully interlaced woven preforms for composites. Textile Research Journal 83, 2060‒2084. Bilisik, K., Yolacan, G., 2014. Experimental characterization of multistitched two dimensional (2D) woven E-glass/polyester composites under low velocity impact load. Journal of Composite Materials 48, 2145‒2162. Bilisik, K., Erdogan, G., Sapanci, E., Gungor, S., 2019 a. Mode-II toughness of nanostitched carbon/epoxy multiwall carbon nanotubes prepreg composites: Experimental investigation by using end notched flexure. Journal of Composite Materials Epub ahead of print 17 June DOI: 10.1177/0021998319857462. Bilisik, K., Karaduman, N. S., Sapanci, E., 2019 b. Flexural characterization of 3D prepreg/stitched carbon/epoxy/multiwall carbon nanotube preforms and composites. Journal of Composite Materials 53, 563-577. Blake, S. P., Berube, K. A., Lopez-Anido, R. A., 2012. Interlaminar fracture toughness of woven E-glass fabric composites. Journal of Composite Materiels 46, 1583–1592. Carlsson, L. A., Gillespie, J. W., Trethewey, B. R., 1986. Mode‒II interlaminar fracture of graphite/epoxy and Graphite/PEEK. Journal of Reinforced Plastics and Composites 5, 170‒187. Davis, D. C., Whelan, B. D., 2011.An experimental study of interlaminar shear fracture toughness of a nanotube reinforced composite. Composites Part B: Engineering 42, 105–116.
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1st International Workshop on Plasticity, Damage and Fracture of Engineering Materials Procedia Structural Integrity 21 (2019) 146–153
Fracture Toughness (Mode-II) of Nanostitched Composites
1st International Workshop on Plasticity, Damage and Fracture of Engineering Materials
Kadir Bilisik1, ⃰ Gulhan Erdogan1 Erdal Sapanci2 and Sila Gungor2
Fracture Toughness (Mode-II) of Nanostitched Composites
Nano/Micro Fiber Preform Design and Composite Laboratory, Department of Textile Engineering, Faculty of Engineering, Erciyes University, 38039 Talas-Kayseri, Turkey 1, 2ROKETSAN Industries, 06780, 1 Elmadag-Ankara, Turkey 2 2
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Kadir Bilisik ⃰ Gulhan Erdogan Erdal Sapanci and Sila Gungor
Nano/Micro Fiber Preform Design and Composite Laboratory, Department of Textile Engineering, Faculty of Engineering, Erciyes University, 38039 Talas-Kayseri, Turkey 2 ROKETSAN Industries, 06780, Elmadag-Ankara, Turkey
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Abstract
The properties of fracture toughness (mode-II) of nanostitched carbon/epoxy composites were investigated. The mode-II of the nanostitched structure demonstrated extraordinary enhancement considering to the control sample. It was proved that carbon nanostitching yarn in the though-the-thickness of the preform structure was adequate. The fundamental mechanism for raising the Abstract mode-II strength in the nanostitched composite was the interlayer matrix breakages predominantly as a form of shear hackle marks in whereofnanostitching suppressed the layer to layer opening in stitching region during crack growth.The Multiwall The properties fracture toughness (mode-II) of nanostitched carbon/epoxy composites were investigated. mode-IIcarbon of the nanotubes in structure the matrix and filament also diminished the stress clusteringtoperhaps as a sample. form ofItfriction, filaments/matrix nanostitched demonstrated extraordinary enhancement considering the control was proved that carbon debonding in yarn the interlaced unit cell and filament the failed matrix zone.The So,fundamental nanostitchedmechanism carbon/epoxy composite nanostitching in the though-the-thickness of theslippage preforminstructure was adequate. for raising the showed interlainar shear properties. mode-II better strength in the nanostitched composite was the interlayer matrix breakages predominantly as a form of shear hackle marks in where nanostitching suppressed the layer to layer opening in stitching region during crack growth. Multiwall carbon nanotubes in the matrix and filament also diminished the stress clustering perhaps as a form of friction, filaments/matrix debonding in the interlaced unit cell and filament slippage in the failed matrix zone. So, nanostitched carbon/epoxy composite showed better interlainar shear properties. © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/ © 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the 1st International Workshop on Plasticity, Damage and Fracture of Engineering Materials This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) organizers under responsibility of the 1st International Workshop on Plasticity, Damage and Fracture of Engineering Materials organizers Peer-review © 2019 The Authors. Published by Elsevier B.V. Keywords: nanotubes; nanoprepreg; nanostitching; mode‒II toughness; shear hackle. This is an Carbon open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/ Peer-review under responsibility of the 1st International Workshop on Plasticity, Damage and Fracture of Engineering Materials organizers Keywords: Carbon nanotubes; nanoprepreg; nanostitching; mode‒II toughness; shear hackle.
Corresponding author. Tel.: +90352 207 66 66/Ext. 32875 or 32851; fax: +90352 437 5784. E-mail address: [email protected] ⃰
2452-3216 © 2019 The Authors. Published by Elsevier B.V. ⃰ Corresponding Tel.: +90352 207CC 66BY-NC-ND 66/Ext. 32875 or 32851; fax: +90352 437 5784. This is an open author. access article under the license (http://creativecommons.org/licenses/by-nc-nd/4.0/ E-mail address: [email protected]
Peer-review under responsibility of the 1st International Workshop on Plasticity, Damage and Fracture of Engineering Materials organizers 2452-3216 © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/
Peer-review under responsibility of the 1st International Workshop on Plasticity, Damage and Fracture of Engineering Materials organizers
2452-3216 © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the 1st International Workshop on Plasticity, Damage and Fracture of Engineering Materials organizers 10.1016/j.prostr.2019.12.096
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1. Introduction Carbon fiber preform and composites have been used in space and aero vehicle industries as well as defense sectors as structural parts due to their extraordinary thermo-mechanic, electro-magnetic and damage resistance properties Kamiya et al. (2000). However, traditional carbon/matrix structures exhibited inferior delamination and damage tolerance resistances. Hence, binder fiber in fabric architecture were introduced by using the traditional textile technologies such as stitching Tong et al. (2002), Bilisik and Yolacan (2014), Bilisik and Yolacan (2012), carpet and velvet as well as flocking. Other innovative techniques were considered as three dimensional weaving Bilisik (2010), Bilisik et al. (2013), three dimensional braiding Bilisik and Sahbaz (2012), Bilisik (2011) and z-pin anchoring Pingkarawat and Mouritz (2014). In recent past, nano materials as a form of nanoparticle, nanotubes and rods as well as nanofibers were used to make nanofibrous composites. Nano materials were added in the resin by using several techniques as mixing, or ultrasonication, spraying and transfer-printing before composite process Garcia et al. (2008). The nano materials were placed onto two dimensional fiber substrate or uniaxial/biaxial prepreg as nano/resin mixture or sprayed forms. They can also be attached or grafted to the substrates Khan and Kim (2011). Typically, the in-plane properties of the 3D woven composites have low due to the binder fiber and total fiber volume fraction in which higher degree of interlacements at three fiber sets effected preform fiber volume during fabric formation. Z-pin in the preform was not continuous due to lack of loop section. The nano materials in the fibrous composite were also discrete form and they were not continuous in the fabric architecture. The resin properties influenced the toughening mechanism of mode˗II interlaminar shear fracture Kuwata and Hogg (2011). It was obtained that fiber surface treatment in laminated composite enhanced the interlaminar strength and the mode‒II shear fracture Madhukar and Drazal (1992). But, the interfacial modification was not effective compare to the toughened resin. The matrix and fiber/resin interface mostly influenced the interlaminar fracture behavior of composites Deng and Ye (1999). Several surface modification techniques which were fiber surface roughening and plasma treatments were employed to improve the adhesion between matrix and filaments Li et al. (2015). Nonetheless, it was reported that the high modulus fibres was damaged by these surface modifications Wu and Cheng (2006). One of the studies demonstrated that 3D orthogonal carbon/epoxy composite demonstrated extraordinarily better fracture toughness performance compared to the classical laminated composites Guenon et al. (1989). The stitching yarn in the through-the-thickness of composite improved the mode‒II toughness due to using the high areal stitching density Herwan et al. (2014). Mouritz identified that the fracture resistance was improved due to tortoise crack path which was generated by stitched fiber Mouritz (2004). It was discovered that interlaminar toughness of stitched carbon structure was greater than the neat sample Jain et al. (1998). The crack growth in stitched structure was impeded via bridging and arresting. The fractured surface had stitched fiber/resin debonding and intra/interlayer opening Tan et al. (2012). It was demonstrated that the GII was enhanced by accumulative effect of the nanofibers/z‒pins in the composite Ravindran et al. (2019). It was obtained that the high z-pin density composite demonstrated improved mode‒II toughness Huang and Waas (2014). The z-pin length/diameter ratio and density were the critical parameters Partridge and Cartie (2005). Recently, it was reported that the mode‒II toughness of the nanostitched carbon/epoxy nanocomposite showed several fold increase compared to the pristine Bilisik et al. (2019 a; DOI: 10.1177/0021998319857462). It was found that nanoprepreg carbon/epoxy composite exhibited threefold increase for mode‒II and average two fold increase for mode‒I as compared to the control sample Garcia et al. (2008). The reason could be the complex accumulative interaction of resin/nanotubes/filaments. These interactions were probably bridging, pull-out, friction, and stick-slip. Another research study reported that the nanocomposite exhibited one and half fold increase on fracture toughness (mode‒II) compared to the base sample Falzon et al. (2013). The crack propagation and plastic deformation in the crack tip region caused nonlinear mode‒II failure Carlsson et al. (1986). The failure in the plastic damaged zone of the mode‒II crack front was micro tearing and minor fiber bridging Hashemi et al. (1990). On the other hand, the carack growth and coalescence of micro tensile cracks on the mode‒II crack tip was identified Xia and Hutchinson (1994). Therefore, the purpose of this research was to investigate the mode‒II interlaminar fracture toughness of nanostitched carbon/epoxy composite structures via the end notched flexure. Grafting CNT on the filament bundle surfaces led to critical enhancement in flexural and mode‒II properties but it did not affect the tensile properties Khan et al. (2018). It was reported that the carbon/epoxy nanocomposite exhibited 27% increase in the GII because of addition of fluorine functional group to the carbon nanotubes (f-CNTs)
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Davis and Whelan (2011). Amino treated nanofiber mat and plain nanofiber mat also improved the mode‒II toughness and mechanical properties of nanocomposites, respectively Beckerman and Pickering (2015). The fiber shear in the resin and resin tensile fracture were dominant parameters for the mode‒II failure of composite Russel (1987). CNT dispersion to carbon/epoxy nanocomposite via spraying and film transferred technique illustrated that nanotubes significantly increased the fiber bridging to improve the fracture toughness Joshi and Dikshit (2012). Graphene nanoplatelets (GnPs) and the MWCNTs additions in carbon/epoxy composites enhanced its interlayer shear and fracture toughness properties because of strong bonding at nano materials/resin/filaments Srivastava et al. (2017). A model on fracture toughness was introduced based on bridging and strain energy during delamination crack growth Sun and Jin (2006). The mode‒II failure was quantitatively modeled. Lee found that resin properties dominated microcrack shear initiation stress and coalescence process. Resin yielding and plastic deformations were found as important parameters to control the mode‒II toughness Lee (1997). The fracture process during mode‒II loading was well defined by cohesive zone model especially for thermoplastic composites Reis et al. (2019). It was also reported that 3D orthogonal architecture provided a higher strength, whereas 3D angle interlock provided more energy absorption and prevented mode-II crack propagation probably due to fiber interlacement patterns Pankow et al. (2011). One of another study showed that crack growth of GII exhibited a low and unstable propagation (ductile and slow) for initial state and it showed a high and stable propagation (brittle and fast) for latter state when the infrared thermography technique was employed Perez et al. (2019). The interlacement pattern influenced the crack propagation in woven composite in particular, instable crack growth was obtained in mode‒II fractured composite Blake et al. (2012). The nanoparticle property transfer was studied by using the energy and mechanic models Batra and Sears (2007). By applying the Cottrell–Kelly–Tyson (CKT) model, nanocomposite fracture toughness was determined considering the nanotube specifications and pull-out energy Wagner et al. (2013). The restraint of stress distribution of the nanotubes in the composite was defined by using the two-scale model Romanov et al. (2015). 2. Materials and methods 2.1. Nanostitched carbon/epoxy composites Carbon substrates (Polyacrylonitrile (PAN), Spinteks A.S., TR) were utilized to make stitched prepreg preform. The carbon and para‒aramid stitching yarns, epoxy (Araldite LZ 5021, Biesterfeld GmbH, DE), multiwall carbon nanotubes (MWCNTs) and carbon fabric were used to design the preform composites. Two dimensional (2D) carbon fabric was weaved from 12K filaments. The substrate was low order interlaced satin (1/4) patterns. The multiwall carbon nanotubes (MWCNTs, Nanothinx, GR) were considered and its specification of MWCNTs is given in Table 1. The mode II toughness test samples were manufactured as four kind preform structures. First one was base (CSU) preform which was four layer carbon fabrics. Second type was stitched (CS-CS and CS-TS) preforms. The CS-CS was four layer satin fabrics, and stitched with carbon fiber. However, the CS-TS was four layers satin fabrics, and stitched with Twaron CT yarn. Third type was base nano (CSU-N) preform. The CSU-N was four layer satin fabrics with added multiwall carbon nanotubes.
Fig. 1. Carbon/epoxy carbon nanostitched composites, actual and schematic, respectively.
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Table 1. Specifications of multiwall carbon nanotubes (www.nanothinx.com (2019). Surface Molecular Particle dimensions Purity Nanomaterial area weight (m2/g) (g/mol) (nm x micron x nm) (%) MWCNTs, (Nanothinx,GR)
15-35 x 10≥ x 1-2≥
12.01
>100
≥97
149
(g/cm3)
Tensile strength (GPa)
Tensile modulus (TPa)
Melting temperature (°C)
1.74
200
1.0
3550
Density
Fourth type was nanostitched preforms. The CS-CS-N was carbon nanostitched, nanoprepreg four layer preforms. The CS-TS-N was Twaron CT nanostitched, nanoprepreg four layers satin preform. Stitched yarns in all preforms were placed at their through-the-thickness (Fig. 1) and their properties are provided in Table 2. Nanostitched composites were created by utilizing the nanostitched preforms. More detail explanation on the composite fabrication was provided in the reference Bilisik et al. (2019 b). Table 2. Specifications of untwisted stitching yarns. Fiber type Fiber diameter Twaron CT (Para-aramid fiber, Teijin, JP) Polyacrylonitrile (PAN) Carbon (Carbon fiber, Aksaca, TR)
Fiber density
Tensile strength
Tensile modulus
Elongation at break
Yarn linear density
(µm)
(g/cm3)
(GPa)
(GPa)
(%)
(dtex)
12
1.45
3.2
115
2.9
6
1.78
4.2
240
1.8
3360 6K
2.2. Mode‒II interlaminar fracture toughness (GIIC) test Mode-II properties of all developed carbon/epoxy composites were determined via end notch flexure (ENF) method. ASTM Standard D7905‒14 (2014) was followed. Fig. 2 shows the mode‒II testing by using the 3-point flexture fixture. The test was conducted to obtain GIIC for both non-precrack (NPC) and precrack (PC) cases.
Fig. 2. ENF sample during mode-II testing (left) and failed samples (right) (digital image).
3. Results and discussion 3.1. Mode‒II fracture toughness results Fig. 3(a‒b) illustrated the load(P)‒displacement(δ) curves of composites for NPC and PC cases. The carbon and para-aramid nanostitched and stitched composites showed greater values considering to the nano and base composites. The load‒displacement curves of all composites become almost linear vertical slope.
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Fig. 3. Mode-II load(P)‒displacement(δ) curves (30 cm). (a) NPC case; (b) PC case (Bilisik et al (2019 a).
Fig. 4(a‒b) exhibited compliances (mm/N) versus the cube root of crack length (mm 3) for all satin 1/4 interlaced composites for non-precrack and precrack cases. All satin interlaced composites for precrack condition were lightly steeper considering to the non-precrack. Their data were perfectly fitted regression line as they were implied by their R‒squared values (0.9998‒0.9963). After the compliance calibration coefficients (A and m) were obtained, they were considered to find the GIIC toughness values of all the composites.
Fig. 4. Compliances versus crack length (30 mm) curves for all composites. (a) PC case; (b) NPC case.
Fig. 5 presents the average GIIC outcome in all composites for NPC and PC conditions. In PC case, the GIIC of the carbon and para-aramid nanostitched structures (CS-CS-N and CS-TS-N) were better compared to the base (CSU) due to addition of the nanostitched filament TOW and MWCNTs. Further, the GIIC of the carbon and para-aramid stitched structures (CS-CS and CS-TS) were lightly enhanced considering the base (CSU) due to addition of the stitched filament TOW. The carbon stitched fiber was more effective compared to the Twaron ® filament TOW due to stiffness properties of the PAN fiber and fiber-matrix adhesion in interface regions, in particular interlacement zone for each predetermined unit cell. It was analyzed that nanostitching and stitching enhanced the interlayer toughness of all the base and nano composites. Nevertheless, the mode-II values of nano (CSU-N) and base (CSU) composites were close to each other due to local MWCNTs agglomeration in the resin or filament TOW in the axisymmetric fabric surface. On the other hand, the mode‒II of PC for all composites were higher compared to the NPC cases. This was because of the nonlinearities as a form of heterogonous resin distribution in the starting of the crack region. The principle mode-II toughness behavior of the nanostitched composite was the interlayer resin failure (almost symmetric). The failure type was predominantly shear hackles around the cohesive matrix zone and filamentary region. The crack propagated between warp layers as a form of matrix layer fracture and it moved through the cross‒section of the filling filament TOW. The MWCNTs probably slow down the crack propagation and they suppressed the stress clustering around the fractured matrix. Concurrently, nanostitching suppressed the layer separation in stitching district during the through-the-thickness crack growth. The nanostiched filament bundle failure was not obtained. This was probably because of nonlinear interlaminar shear transfer during stress distribution. Thus, this was caused to severe matrix breakages. In the crack tip, carbon nanotubes in the fiber and matrix diminished the stress clustering as a form of filament-matrix debonding; filament or nanotubes pull-out; filament to filament or filament-nanotube stick-slip or sliding action and friction.
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3.2. SEM micrograph for failure analysis The scanning electron microscope images show the warp side fracture of the base (CSU) composite in Fig. 6(a‒b). As depicted in Fig. 6, severe resin breakages in the matrix/filament boundaries were identified. Minor multiple filament bundle fractures were observed as a form of tensile and shear breakages (Fig. 6a). Kink form brittle multiple warp filament bundle breakages was obtained. After the multiple warp filaments were broken, they were skewed during mode-II loading. This led to the crack branching near to the blunt crack tip region (Fig. 6b). Thus, delamination was spread from interlayer to the angular layer-to-layer zones. 6.0 5.5
Gııc (NPC)
Gııc (PC)
5.0
GIIC (kj/m2 )
4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 CSU Base
CS-CS
CS-TS Stitched
CSU-N Nano
CS-CS-N
CS-TS-N
Stitched/Nano
Composite structures
Fig. 5. Mode-II values of carbon/epoxy composites for PC and NPC cases.
Fig. 6. (a) SEM views of fractured warp side structure (CSU); (b) fractured warp side structure near to the blunt crack tip (CSU).
The SEM images of mode‒II toughness of warp side fractured nanostitched (CS-CS-N) composite are depicted in Fig. 7(a‒b). As depicted in Fig. 7, tensile filament failures were observed. Further, multiple filaments-matrix debonding around the fractured matrix was found. Angular severe matrix breakages near to the crack tip were identified (Fig.7a). Multiple resin fracture and some agglomerated MWCNTs in the matrix was found at the boundary of the filament to filament interface region in where several tensile filament bundle breakages and delaminated filaments were also obtained (Fig 7b). It was realized that nanostitching responded as a crack growth barrier neighbouring the stitching region and complete nanostitching breakages was not identified because of high modulus and high strength carbon and p-aramid fibres. 4. Conclusions The mode-II fracture toughness of the nanostitched and stitched composites exhibited several fold improvement considering to the control. Therefore, the nanostitch addition to the base enhanced its fracture toughness properties. Carbon stitch yarn was adequate for enhancing the fracture toughness considering to the para-aramid due to its fiber stiffness and robust filaments and resin adhesion properties. The fundamental mechanism of the mode-II behavior of the nanostitched composite was predominant the interlayer matrix failure as a form of shear hackles. The MWCNTs probably slow down the stress clustering in the
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vicinity of the matrix plastic deformation region. At the same time, nanostitching suppressed the layer separation around the stitching region during the crack growth. In the crack tip, carbon nanotubes in the matrix and filament diminished the stress clustering as a form of filament-matrix debonding; filament or nanotubes pull-out; filament to filament or filament-nanotube sliding or stick-slip action and friction.
Fig. 7. (a) SEM image of warp side fractured region; (b) fractured CS-CS-N composite around the crack tip.
In base composite, resin breakages in the matrix-filament boundaries and minor multiple filament bundle breakages were identified as a form of tensile and shear failures. In nanostitched composite, multiple resin fracture and a few tensile filament bundle breakages were obtained. In addition, minor nanostitching fracture was identified due to using the high modulus filament TOWs. It was realized that nanostitching reponded as a ply-separation barrier around the stitching region. Acknowledgements This work was supported by Roketsan Industries Grant No. RS/ERCİYES DSM-76301-14-01N/R. References ASTM Standard D7905‒14, 2014. Standard test method for determination of the mode II interlaminar fracture toughness of unidirectional fiberreinforced polymer matrix composites. West Conshohocken, PA. Batra, R. C., Sears, A., 2007. Continuum models of multi-walled carbon nanotubes. International Journal of Solids and Structures 44, 7577–7596. Beckermann, G. W., Pickering, K. L., 2015. Mode‒I and Mode‒II interlaminar fracture toughness of composite laminates interleaved with electrospun nanofibre veils. Composites Part A: Applied Science and Manufacturing 72, 11–21. Bilisik, K., 2010. Dimensional stability of multiaxis 3D woven carbon preform. Journal of the Textile Institute 101, 380–388. Bilisik, K., 2011. Three dimensional axial braided preforms: Experimental determination of effects of structure-process parameters on unit cell. Textile Research Journal 81, 2095‒2116. Bilisik, K., Sahbaz, N., 2012. Structure-unit cell base approach on three dimensional representative braided preforms from 4-step braiding: experimental determination of effect of structure-process parameters on predetermined yarn path. Textile Research Journal 82, 220‒241. Bilisik, K., Yolacan, G., 2012. Experimental determination of bending behavior of multilayered and multistitched E-glass fabric structures. Textile Research Journal 82, 1038‒1049. Bilisik, K., Sahbaz, N., Bilisik, N. E., Bilisik, H. E., 2013. Three dimensional (3D) fully interlaced woven preforms for composites. Textile Research Journal 83, 2060‒2084. Bilisik, K., Yolacan, G., 2014. Experimental characterization of multistitched two dimensional (2D) woven E-glass/polyester composites under low velocity impact load. Journal of Composite Materials 48, 2145‒2162. Bilisik, K., Erdogan, G., Sapanci, E., Gungor, S., 2019 a. Mode-II toughness of nanostitched carbon/epoxy multiwall carbon nanotubes prepreg composites: Experimental investigation by using end notched flexure. Journal of Composite Materials Epub ahead of print 17 June DOI: 10.1177/0021998319857462. Bilisik, K., Karaduman, N. S., Sapanci, E., 2019 b. Flexural characterization of 3D prepreg/stitched carbon/epoxy/multiwall carbon nanotube preforms and composites. Journal of Composite Materials 53, 563-577. Blake, S. P., Berube, K. A., Lopez-Anido, R. A., 2012. Interlaminar fracture toughness of woven E-glass fabric composites. Journal of Composite Materiels 46, 1583–1592. Carlsson, L. A., Gillespie, J. W., Trethewey, B. R., 1986. Mode‒II interlaminar fracture of graphite/epoxy and Graphite/PEEK. Journal of Reinforced Plastics and Composites 5, 170‒187. Davis, D. C., Whelan, B. D., 2011.An experimental study of interlaminar shear fracture toughness of a nanotube reinforced composite. Composites Part B: Engineering 42, 105–116.
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