impact response of carbon fibre … · resin [13].therefore to maintain mechanical prop- ... 344....

Advanced Composites Letters, Vol. 26, Iss. 3, 2017 82

1. INTRODUCTIONCarbonfibre-reinforced polymer (CFRP) compos-ites are used in various engineering fields with their excellent mechanical properties and lightweight [1,2]. An improvement in matrix (thermoplastics and thermosets) properties results in better me-chanical properties of CFRP composites. Hence, the conventional structural materials such as aluminum and titanium alloys have been largely replaced by CFRP composites [3].Epoxy resins are almost uti-lized in thermoset composites. Even though carbon fibre-reinforced epoxy composite offers superior specific strength and specific modulus compared to many metallic materials. However, epoxy resin has low fracture energy because it is generally brittle in nature.In addition, the delamination occurred on the laminated composite material to impact. Insertion of an interleaf layer [4-6], reinforcing with three-dimensional braided or woven fabrics [7], stitching of reinforcements [8], surface treatment of rein-forcements [9,10], and the insertion of fillers, such as carbon nanotubes, into the matrix [11]was used to prevent the delamination. Aircraft construction manufactured currently using epoxy composites, but in the case of wings of high-speed aircraft, brit-tle epoxy resin system is leading to laminated struc-tures with a poor tolerance to low-energy impact

caused by continuous impact such as drizzle and dust, that caused deterioration during cyclic loading. Although the newer toughened epoxy systems is de-veloping for solving these problems, but they do not still reach as thermoplastic materials [3,12].On the other hand, fibre-reinforced thermoplastic polymer (FRTP) composites have good fracture toughness, high damage tolerance, ease of shape-forming be-fore consolidation, significantly faster manufactur-ing, and the ability to be reshaped and reused. Ac-cording to develop of engineering plastics such as polyetheretherketone (PEEK), polyethersulphone (PES), polyphenylene sulphide (PPS), polyethyl-eneterephthalate (PET), polycarbonate (PC), poly-amide (PA) and polypropylene (PP), FRTP compos-ites have been improved the mechanical properties. However, they are still not as mechanical properties as thermoset composites. And making processes are constrained by high melt viscosity of thermoplastic resin [13].Therefore to maintain mechanical prop-erties of fibre-reinforced thermoset composites and to improve fracture toughness, it used currently to add fibre-reinforced thermoplastic filler in the ep-oxy resin [14,15], insert high damage tolerance re-inforcement [16-18] and insert fibre-reinforced ther-moplastic film [4,19]. But thermoset/thermoplastic material are not compatible, thus interface strength

IMPACT RESPONSE OF CARBON FIBRE FABRIC/THERMOSET-THERMO-PLASTIC COMBINED POLYMER COMPOSITES

Joon Seok Lee and Jong Won Kim*

Department of Textile Engineering and Technology, Yeungnam University, Gyeongsan 38541, Republic of Korea

Received 7 December 2016; accepted 7 March 2017

*Author to whom correspondence should be addressed: e-mail: [email protected]

ABSTRACTCarbon fibre-reinforced polymer (CFRP) composites are used in various engineering fields with their excellent mechanical properties and lightweight. However, thermoset epoxy composites with brittle epoxy resin system are leading low fracture toughness that caused deterioration during cyclic loading.In addition, the delamination occurred on the thermoset laminated composite material to impact. In this study, to improve these problems, thermoplastic resin added on the top and bottom of thermoset composites and to impregnate two resins that are not compatible on reinforcement, combined prepregs were manufactured for co-curing process. As a result, the formation of thermoplastic layers on the top and bottom of thermoset composites was not significantly af-fecting much of impact load. However, this improved greatly the energy absorption so that improved fracture toughness.

Key Words: Carbon fibre-reinforced composites; co-curing process; fracture toughness; thermoset-thermoplas-tic combined polymer composites

Joon Seok Lee, Jong Won Kim Full Article

83 Advanced Composites Letters, Vol. 26, Iss. 3, 2017

is low between each materials.

In this study,fibre-reinforced composites was manu-factured using thermoset and thermoplastic resins to produce laminated composite materials with good fracture toughness and damage tolerance which is a fault of thermoset laminated composites. The pres-ent studies utilize the idea of combining of thermo-set and thermoplastic resins to impregnate on carbon fibre fabric.To impregnate two resins that are not compatible on carbon fibre fabric, combined pre-pregs (CBP) were manufactured and PP resin added on the top and bottom of fibre-reinforced composite with applying co-curing process for forming of two resins unification. Consequently, the present stud-ies utilize the idea of improving fracture toughness without significantly affecting much of impact load of fibre-reinforced thermoset composites.

2. EXPERIMENTAL2.1. MaterialsIn this study, a woven fabric (SNC-1242R, Plain, density (count /25 mm) = 6.4, weight = 420 g/m2, Seanal Tech-tex Co., Korea) with carbon fibre (Toray T-300, 12K, Japan) was used as the reinforcement. A PP film (SFI-740P, MFI = 5.5 g/10 min, molecular weight = 270,000, Lotte Chemical Co., Republic of Korea) with a thickness of 30 µm was used as the thermoplastic resin. Epoxy resin (YD-128, Kukdo, Korea) and the curing agent (H-4065, Kukdo, Ko-rea) were mixed in a 4:1 ratio.

2.2. Preparation of the combined compositesThe thermoplastic prepregs (TPP), which were man-ufactured using five stacked PP films, were laminat-

ed inside a mold (295 mm × 295 mm) both above and under the carbon fibre fabric by the procedure described in a previous study [20]. Once the mold was preheated inside a hot press at 230 °C for 10 min, the pressure was increased slowly to 21 MPa over a period of 10 min and then a dwell time of 10 min was used. The mold was then cooled to room temperature and the pressure was relieved. The pre-preg was then detached from the mold. In the hand lay-up process, the thermoset prepregs (TSP) were used along with the carbon fibre fabric impregnat-ed with epoxy resin. The thermoset-thermoplastic combined prepregs (CBP) were manufactured us-ing five stacked PP films and were laminated under the carbon fibre fabric following the same proce-dure used in the case of the thermoplastic prepregs. In this case, a pressure of 7 MPa was used. High pressure makes the impregnation of the melted PP films into the upper portion of the carbon fibre fab-ric. Also impregnated PP resin in the bottom portion with only low pressure can prevent the impregna-tion of epoxy resin into the lower portion during up-per hand lay-up process.Fig. 1 shows cross-section image of combined prepreg before the impregnation with epoxy resin.

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properties of thermoplastic composites”, Materials,9/6(2016), 448. 14. Nash, N.H., Young, T.M., Mcgrail, P.T., and Stanley, W.F., “Inclusion of a thermoplastic phase to

improve impact and post-impact performances of carbon fibre reinforced thermosetting composites-A review”, Mater. Design, 85 (2015), 582-597.

15. Hogg, P.J., “Toughening of thermosetting composites with thermoplastic fibres”, Mat. Sci. Eng. A-Struct., 412 (2005), 97-103.

16. Sarasini, F., Tirillo, J., Valente, M., Valente, T., Cioffi, S., Iannace, S., and Sorrentino, L., “Effect of basalt fibre hybridization on the impact behaviour under low impact velocity of glass/basalt woven fabric/epoxy resin composites”, Compos. Part A, 47 (2013), 109-123.

17. Reis, R.N.B., Ferreira, J.A.M., Santos, P., Richardson, M.O.W., and Santos, J.B., “Impact response of Kevlar composites with filled epoxy matrix”, Compos. Struct.,94 (2012), 3520-3528.

18. Yadav, S.N., Kumar, V., and Verma, S.K., “Fracture toughness behaviour of carbon fibre epoxy composite with Kevlar reinforced interleave”, Mater. Sci. Eng. B, 132 (2006), 108-112.

19. Houshyar, S., Shanks, R.A., and Hodzic, A., “The effect of fibre concentration on mechanical and thermal properties of fibre-reinforced polypropylene composites”, J. Appl. Polym. Sci., 96 (2005), 2260-2272.

20. Kim, J.W. and Lee, J.S., “Influence of interleaved films on the mechanical properties of carbon fibre fabric/polypropylene thermoplastic composites”, Materials, 9/5 (2016), 344.

Fig. 1:cross-section image of combined prepreg before the impregnation with epoxy resin.

Fig. 1:cross-section image of combined prepreg before the impregnation with epoxy resin.

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Fig. 2: lay-up process for various composites; (a) Thermoset Composite (TSC); (b-e) Combined Composite

(CBC); (f) Thermoplastic Composite (TPC).

Table 1: Physical properties of the composites prepared using different lay-up processes

Properties TSC CBC-1 CBC-3 CBC-5 CBC-7 TPC

Thickness (mm) 2.65 2.73 2.87 2.95 3.05 3.11

Vf (%) 65.54 64.87 64.36 63.75 63.28 63.01

Density (g/cm3) 1.499 1.482 1.452 1.419 1.384 1.364

Vc (%) 6.81 6.87 6.93 6.95 6.96 7.02

Fig. 2: lay-up process for various composites; (a) Thermoset Composite (TSC); (b-e) Combined Composite (CBC); (f) Thermoplastic Composite (TPC).



As shown in Fig. 1, after the lamination, the upper portion of the carbon fibre fabric, which was not im-pregnated, was impregnated with epoxy resin using the hand lay-up method under the same conditions as those used for manufacturing the thermoset pre-pregs. Generally, the interface strength of both ther-moset and thermoplastic resins is poor, and a sepa-rate process is required for improving it. However, in this study, the carbon fibre fabric integrated the two resins that are not compatible.The three types of prepregs were laminated by different lay-up process-es, as shown in Fig. 2. Once the mold was preheated inside a hot press at 80 °C for 10 min, the pressure was increased slowly to 21 MPa over a period of 10 min. The temperature was increased to 230 °C at a heating rate of 5 °C/min. The mold was then cooled to room temperature and the pressure was relieved. The composite was then detached from the mold.

2.3. MeasurementThe specimen thickness (mm) was averaged by mea-suring 5 places, including 4 edges and the centre, of each 5 specimens using a thickness gauge. Fibre volume fraction (Vf) was calculated using a burn-off test according to ASTM D 2584. The compos-ites were placed in a furnace in an inert environment for 5 h at 500 °C. The Vf was then calculated using the initial weight of the carbon fibre fabric and the weight before and after the burn-off test. The actual density of the composites was calculated using the Archimedes principle according to ASTM D 792 by measuring the differences between their weights in air and water. Void Content (Vc) was calculated us-ing Eqns. (1) and (2) according to ASTM D 2734.

(1)

(2)

where ρ is the primary density, %m is the mass of each constituent, and x is the number of thermoset prepreg layers.

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The mold was then cooled to room temperature and the pressure was relieved. The composite was then detached from the mold. Fig. 2 2.3. Measurement The specimen thickness (mm) was averaged by measuring 5 places, including 4 edges and the centre, of each 5 specimens using a thickness gauge. Fibre volume fraction (Vf) was calculated using a burn-off test according to ASTM D 2584. The composites were placed in a furnace in an inert environment for 5 h at 500 °C. The Vf was then calculated using the initial weight of the carbon fibre fabric and the weight before and after the burn-off test. The actual density of the composites was calculated using the Archimedes principle according to ASTM D 792 by measuring the differences between their weights in air and water. Void Content (Vc) was calculated using Eqns. (1) and (2) according to ASTM D 2734.

(1)

(2) where is the primary density, %m is the mass of each constituent, and x is the number of thermoset prepreg layers. The viscoelastic properties of the composites were investigated by dynamic thermal mechanical analysis (DMTA, Q-800, TA Instruments, Newcastle, USA). The single cantilever method was used. The measurements were performed at a frequency of 1 Hz and an amplitude of 1.5 µm. The storage modulus was measured at temperatures varying from -30 to 170 °C.The interlaminar shear strength (ILSS) was measured according to ASTM D 2344 using a tensile tester (OTT-05, Oriental Co., Korea) with a span-to-depth ratio of 4 at a cross-head speed of 1 mm/min. The drop-weight impact property was investigated using a drop-weight impact testing machine (Ceast 9350, Instron, Norwood, USA) equipped with a 12.7 mm diameter hemispherical tip. The mass of the pendulum was 15.13 kg, impact velocity was 2.57 m/s2, and impact energy was 50 J. After the impact test, the specimens were analysed using C-scan equipment (Ez- Scan VII, Orient NDT, Korea). 3. RESULTS AND DISCUSSIONS 3.1. Physical properties Table 1 is physical properties of composites according to lay-up method. The thickness of composites is increased with an increase in the number of TPP layers and Vf of each composites is decreased. This is because high melting viscosity PP resin in forming process is less squeezed out than epoxy resin. The density of composites is decreased with an increase in the number of TPP layers. This is because the density of PP resin is lower than that of epoxy resin and carbon fibre. The void content (Vc) of the composites slightly increased with an increased in the number of TPP layers. It is because, according to Equation (1) and (2), the density of the composites resin decreased with PP resin despite a decrease in the density and volume fraction (Vf) of the composites. The reason Vc of TSC is a little lower than that of TPC, because thermoset resin, which has low viscosity, also shows better impregnation than thermoplastic resin. Therefore physical characteristic is not influenced by co-curing process. Table 1 3.2. DMTA

The viscoelastic properties of the composites were investigated by dynamic thermal mechanical anal-ysis (DMTA, Q-800, TA Instruments, Newcastle, USA). The single cantilever method was used. The measurements were performed at a frequency of 1 Hz and an amplitude of 1.5 µm. The storage modu-lus was measured at temperatures varying from -30 to 170 °C.The interlaminar shear strength (ILSS) was measured according to ASTM D 2344 using a tensile tester (OTT-05, Oriental Co., Korea) with a span-to-depth ratio of 4 at a cross-head speed of 1 mm/min. The drop-weight impact property was investigated using a drop-weight impact testing machine (Ceast 9350, Instron, Norwood, USA) equipped with a 12.7 mm diameter hemispherical tip. The mass of the pendulum was 15.13 kg, impact velocity was 2.57 m/s2, and impact energy was 50 J. After the impact test, the specimens were analysed using C-scan equipment (Ez- Scan VII, Orient NDT, Korea).

3. RESULTS AND DISCUSSIONS3.1. Physical propertiesTable 1 is physical properties of composites accord-ing to lay-up method. The thickness of composites is increased with an increase in the number of TPP layers and Vf of each composites is decreased. This is because high melting viscosity PP resin in form-ing process is less squeezed out than epoxy resin. The density of composites is decreased with an in-crease in the number of TPP layers. This is because the density of PP resin is lower than that of epoxy resin and carbon fibre. The void content (Vc) of the composites slightly increased with an increased in the number of TPP layers. It is because, according to Equation (1) and (2), the density of the compos-ites resin decreased with PP resin despite a decrease in the density and volume fraction (Vf) of the com-posites. The reason Vc of TSC is a little lower than that of TPC, because thermoset resin, which has low viscosity, also shows better impregnation than ther-moplastic resin. Therefore physical characteristic is not influenced by co-curing process.

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Fig. 2: lay-up process for various composites; (a) Thermoset Composite (TSC); (b-e) Combined Composite

(CBC); (f) Thermoplastic Composite (TPC).

Table 1: Physical properties of the composites prepared using different lay-up processes

Properties TSC CBC-1 CBC-3 CBC-5 CBC-7 TPC

Thickness (mm) 2.65 2.73 2.87 2.95 3.05 3.11

Vf (%) 65.54 64.87 64.36 63.75 63.28 63.01

Density (g/cm3) 1.499 1.482 1.452 1.419 1.384 1.364

Vc (%) 6.81 6.87 6.93 6.95 6.96 7.02



3.2. DMTAFig.3 is storage modulus of composites as measured by DMTA. All storage modulus of specimen was decreased with an increased in temperature because matrix soften at high temperature. A decreased is bigger with an increased in number of TPP layers. This is because adhesion with carbon fibre of PP res-in is lower than epoxy resin. The storage modulus of CBC-3, CBC-5, CBC-7, and TPC decreased sharply at temperatures greater than 160 °C. But the stor-age modulus of TSC and CBC-1 decreased slightly. This is because the CBC-1 which contained the ther-moplastic resin has thermoplastic resin only at the top and bottom of the surfaces. In other words, this is because the carbon fibre layers were constructed with epoxy resin only.

Fig. 3: Storage modulus of composites prepared using different lay-up processes as measured by DMTA.

3.3. ILSSFig. 4 shows the ILSS of the composites prepared by different lay-up processes. ILSS decreased with an increase in the number of TPP layers. Compared to TPC, TSC showed a sharp decreased of 77.56 % in the ILSS (from 43.77 MPa to 9.82 MPa). This is attributed to the decrease in the load to break the composites using PP resin, which has low mechani-cal properties than epoxy resin, and the increased in the thickness and Vc of the composites. However, CBC-1 showed only a slight change 7.44% (to 40.51 MPa) in the ILSS compared to TSC. ILSS is measure of the strength of composites when the interfaces of the carbon fibre layers are separated. Therefore, this is because the interfaces between carbon fibre layers were composed of epoxy resin only.

Fig. 5 shows the cross-section images of the com-

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Fig. 3: Storage modulus of composites prepared using different lay-up processes as measured by DMTA.

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Fig. 4:ILSS of the composites prepared by different lay-up processes.

Fig. 4: ILSS of the composites prepared by different lay-up processes.

posites prepared by different lay-up processes after the ILSS test. Fig. 5(a) shows matrix crack and de-lamination. This is because the interface of the car-bon fibre layer consisted only of the epoxy resin. Matrix crack which were generated by an external force, spread through the carbon fibre fabric layers vertically. Both matrix crack and delamination can be observed in Fig. 5(b). However these matrix and occurred on the thermoset resin. The crack propaga-tion was terminated by the thermoplastic resin. Fig. 5(c) shows the composites consisting only of the matrix crack. This is because interface of the car-bon fibre fabric layers are consisted only of the PP resin.

3.4. Drop weight impact propertyFig.6(a) shows the impact load-time diagrams ob-tained from the impact test of the composites pre-pared by different lay-up processes. The impact load tended to decreased with an increase in the number of TPP layers. Compared to TSC and TPC showed a significant decrease in the impact load (from 14265.26 to 9515.28 N). The CBC-1 also showed a decrease of 5.35% (to 13501.02 N) in the impact load. This is because the interface of composites in CBC-1 has few PP resins. The time of maximum impact load increased with an increase in the num-ber of TPP layers. This is because the load applied to the epoxy resins does not spread to all materials; thus, only the impacted area is affected. On the other hand, the load applied to the PP resins can spread to all materials. Fig.6(b) show the energy-time curves obtained after the impact test of the composites prepared by different lay-up processes. From the curves, it can be observed that the impact energy was decreased after reaching a maximum energy. This suggests that all the composites were not de-



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Fig. 5: Cross-section images of the composites after the ILSS test; (a) TSC; (b) CBC-3; and (c) TPC.

Fig. 5: Cross-section images of the composites after the ILSS test; (a) TSC; (b) CBC-3; and (c) TPC.

stroyed completely. Some of energy was reflected and some of it was absorbed. The energy absorption tended to increase with an increase in the number of the TPP layers. Compared TSC and TPC showed a significant increase of 29.98 % in energy absorption (from 24.78 to 32.21 J). This is because PP resin absorbed impact energy than epoxy resin. And com-pared CBC-1 and TSC showed a sharp increase of 15.29 % in energy absorption (to 28.57 J). But, im-pact load decreased by 5.35 %. Thus, the addition of

PP resins on the top and bottom improved the impact energy absorption of the epoxy resin composites.

Fig.7 shows the C-scan cross-section images of the composites after the impact test of the composites prepared by different lay-up processes. The delami-nation decreased with an increase in the number of TPP layers. The delamination can be only observed in TSC, CBC-1 and CBC-2. This should suggest


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Fig. 6: Impact load-time diagrams(a) and energy-time curves(b) from the impact test of the composites prepared

by different lay-up processes.

Fig. 6: Impact load-time diagrams(a) and energy-time curves(b) from the impact test of the composites prepared by different lay-up processes.

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Fig. 7: C-Scan cross-section images of the composites after the impact test: (a) TSC; (b) CBC-1; (c) CBC-3; (d)

CBC-5; (e) CBC-7; and (f) TPC.

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Fig. 6: Impact load-time diagrams(a) and energy-time curves(b) from the impact test of the composites prepared

by different lay-up processes.

Fig. 7: C-Scan cross-section images of the composites after the impact test: (a) TSC; (b) CBC-1; (c) CBC-3; (d) CBC-5; (e) CBC-7; and (f) TPC.



that delamination can be observed only in thermoset resin. Compared to CBC-1 and TSC which consist-ed of the thermoset resin, showed a lower damage area. (indicated by red in the figure).

4. CONCLUSIONSIn this study, PP resin was added on the top and bot-tom for improving defect of fibre-reinforced epoxy composites; low fracture toughness. To impregnate two resins that are not compatible on reinforcement, combined prepregs were manufactured for co-cur-ing process. As a result, CBC-1 showed a decrease of 5.35 % in impact load, but an increase of 15.29 % in energy absorption. In short, the energy absorp-tion was increased greatly without great difference of impact load. Therefore the composite which was improved fracture toughness by impact, that can be used for aerospace and air craft industry such as air-craft wings, fuselage and tail. And also can be used for automobile industry which requires preventions of delamination and fragment by impact.

ACKNOWLEDGEMENTSThis work was supported by the Technology Inno-vation Program (10063368) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). Also, authors sincerely appreciate Hae Jin Cho for his as-sistance on the experiments.

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17. Reis, R.N.B., Ferreira, J.A.M., Santos, P., Rich-ardson, M.O.W., and Santos, J.B., “Impact re-sponse of Kevlar composites with filled epoxy ma-trix”, Compos. Struct.,94 (2012), 3520-3528.

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19. Houshyar, S., Shanks, R.A., and Hodzic, A., “The effect of fibre concentration on mechanical and ther-mal properties of fibre-reinforced polypropylene composites”, J. Appl. Polym. Sci., 96 (2005), 2260-2272.

20. Kim, J.W. and Lee, J.S., “Influence of interleaved films on the mechanical properties of carbon fibre fabric/polypropylene thermoplastic composites”, Materials, 9/5 (2016), 344.


impact response of carbon fibre … · resin [13].therefore to maintain mechanical prop- ... 344....

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