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Real-time in situ monitoring of manufacturing process and CFRP quality by relative resistance change measurement
Polymer Testing 85 (2020) 106416
Contents lists available at ScienceDirect
Polymer Testing journal homepage: http://www.elsevier.com/locate/polytest
Real-time in situ monitoring of manufacturing process and CFRP quality by relative resistance change measurement Changyoon Jeong a, Tae Hee Lee b, c, Seung Man Noh c, **, Young-Bin Park a, * a
Department of Mechanical Engineering, Ulsan National Institute of Science and Technology, UNIST-gil 50, Ulju-gun, Ulsan, 44919, Republic of Korea Department of Chemical Engineering, Ulsan National Institute of Science and Technology, UNIST-gil 50, Ulju-gun, Ulsan, 44919, Republic of Korea c Research Center for Green Fine Chemicals, Korea Research Institute of Chemical Technology, Ulsan, 44412, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Keywords: Carbon fibres Cure behaviour Process monitoring Resin flow
In situ monitoring of resin flow, impregnation of carbon fiber fabrics, and curing during composite manufacturing are very important for determining the quality of composite parts. In conventional methods, sensors, such as optical fibers and strain gages, are bonded to or embedded in the composites for measuring the changes in mechanical and chemical properties. Although they can detect resin curing behavior and impregnation of carbon fibers, they may adversely affect the manufacturing process or structural integrity of the composites. In this study, carbon fiber itself was used as a sensor that minimizes the degradation of mechanical properties and increases the efficiency of monitoring the manufacturing process. The change in the electrical resistance of carbon fiber fabrics was monitored during the various manufacturing processes when the resin flowed through the carbon fiber fabric and curing progressed. The effectiveness of this monitoring method was confirmed, and it is expected to be applicable in monitoring the quality of the finished composite parts.
1. Introduction Carbon fiber reinforced plastics have been extensively used in many industrial applications, such as aircraft, automobiles, electronics, and civil structures, and have high specific strength and stiffness that render it potential in many other applications. The quality of the finished product strongly influences the mechanical and thermal properties and is strongly affected by several factors such as the curing and impreg nation of carbon fiber fabrics. To monitor these processes, extensive research has been conducted using sensors such as optical fibers [1–7], ultrasonic waves [8], dielectric systems [9–11], matrix sensors [12,13], and carbon nanotubes [14,15]. These methods typically require the sensor or monitoring system to be installed or embedded in a composite during manufacturing process, which may adversely affect the structural integrity in the final part. In addition, point sensors need to be installed in a large number for accurate measurement of the curing behavior. Several methods are used for curing behavior monitoring processes, such as differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), and Raman spectroscopy. However, set-up time for cure behavior analysis is required in these methods, which may defeat the purpose of exact estimation of the curing
behavior, especially when the resin formulation is complex [16–18]. Thus, a novel monitoring method that can measure resin flow, curing, and impregnation of carbon fiber fabric layers without embedding sensor needs to be developed. In this study, carbon fiber itself is proposed as a sensor, wherein multiple electrodes are readily installed and assigned to desirable po sitions. To monitor the resin flow and curing behavior in composite, thiol-epoxy polymers were used, which are characterized by rapid re action rate at a low temperature, high degree of conversion, and mul tiple crosslinking pathways and compared them with the outcome of other reaction conditions with different viscosities and curing reaction rates [19]. For precise monitoring of the curing and impregnation of carbon fibers, thiol-epoxy with different crosslinkers (SH3, SH4, and SH6) were used as resins where the type of the crosslinker influences rate of the crosslinking reaction and viscosity of the resins [17]. Resins with different viscosities show different flow velocities, and hence, this difference leads to varying degrees of impregnation and rate of cure during vacuum-assisted resin transfer molding (VARTM), resulting in hydrodynamic effects and compressive stresses in carbon fiber lay-ups. When the curing process proceeds, compressive strain is applied to the carbon fiber tows. The tows are radially compacted inducing decreases
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (Y.-B. Park). https://doi.org/10.1016/j.polymertesting.2020.106416 Received 27 September 2019; Received in revised form 11 January 2020; Accepted 8 February 2020 Available online 9 February 2020 0142-9418/© 2020 Elsevier Ltd. All rights reserved.
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Polymer Testing 85 (2020) 106416
in their relative resistances [20]. Resin flow and impregnation of carbon fiber fabrics are detected by the change in the electric signal of carbon fiber fabrics due to its hydrodynamic effects [7,21–23]. All the dynamic movements of the resin, including resin flow, impregnation, and curing, were evaluated.
polymer networks were reacted with stoichiometric amounts of epoxy groups and thiol groups, as shown in Scheme 1. 2.3. Characterization 2.3.1. Oscillational rheometer The shear viscosity and crosslinking kinetics of neat polymer net works were correlated with the rheological properties measured by a oscillational rheometer (Scientific Inc., MARS III). Steady shear mode rheometer was used for obtaining the shear viscosities of curing mix tures without a tertiary amine catalyst. The shear rate ranged from 0.1 to 1000 s 1 at 25 � C. The viscoelastic properties of the neat polymer net works during the crosslinking reactions were characterized using a small amplitude oscillatory shear mode. A parallel aluminum plate of 8 mm was used under a constant strain of 0.5% at 25 � C, frequency of 1 Hz, and gap of 500 μm.
2. Experimental 2.1. Materials Bisphenol-A type epoxy resin (EPOKUKDO, YD-128) with an equiv alent weight of 184–190 g eq 1 was purchased from Kukdo Chemical Co., Ltd. Trimethyl propane tri (3-mercaptopropionate) (SH3), pen taerythritol tetra(3-mercaptopropionate) (SH4), and dipentaerythritol hexa(3-mercaptopropionate) (SH6) were purchased from Bruno Bock Thiolchemical. 2,4,6-Tris(dimetylaminomethyl)phenol (DMP30) was purchased from Sigma Aldrich. Fig. 1 shows the molecular structures of the resins used to monitor the manufacturing processes. The woven carbon fibers (T300, density: 1.76 g/cm3, 3K plain weave) were supplied by Amoco Corporation (Chicago, IL, USA).
2.3.2. FT-IR analysis The chemical reactions of the epoxy/thiol groups were analyzed using an FT-IR spectrometer (Thermo Fisher Scientific Inc., Nico let6700/Nicolet Continuum). The temperature during the experiments was controlled with a Peltier module.
2.2. Preparation of measurement set-up
2.3.3. Differential scanning calorimetry The exothermic reactions between the epoxy groups and thiol groups were confirmed by isothermal DSC analysis (TA Instruments, DSC Q2000). A flow of N2 at 90 mL/min was used, and the weight of the samples for analysis was 5 mg. For the isothermal measurement, a temperature of 25 � C was used.
In situ method of monitoring the resin flow, curing, and degree of carbon fiber fabric layers’ impregnation was developed using carbon fiber resistance change during VARTM process, which is cost-effective and environmentally friendly, as compared to prepregs technologies, which require an autoclave for curing (Fig. 2). In a typical experiment, a woven carbon fiber fabric was laid on a Teflon sheet, which was used for easy demolding. The electric wires were attached to the carbon fiber fabric along the tows (at the ends of the tows) perpendicular to the resin flow direction and were positioned at the front, middle and end of the fabric with respect to the resin flow direction. In order to minimize the electrical resistance interference of the electric wire, Ag paste is on the wire. The VARTM system was used to make a pressure difference, which initiated the resin flow. Before infusion of the resin, the hardeners (SH3, SH4, and SH6) were activated by dissolving them in 5 wt % of a tertiary amine catalyst (DMP30) at a high-speed with overhead stirrer (Ocean Science, DISPERMAT), at 1500 rpm for 20 min. The mixture of the hardener and catalyst was further mixed with the epoxy resin. The
3. Results and discussion 3.1. Rheological properties of resins during composite manufacturing process The relation between the viscosities and flow velocities of the resins during VARTM is shown in Fig. 3. The flow front position was detected by the change in relative resistance of the carbon fiber fabric layer horizontally connected to several electrode pairs and by identifying the times at which the resistance changes at each position. The viscosity of
Fig. 1. Molecular structures of the materials used in this study. 2
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Polymer Testing 85 (2020) 106416
Fig. 2. Schematics of VARTM (vacuum assisted resin transfer molding) for monitoring resin flow, impregnation of fibers, and curing behavior.
which compacts the carbon fibers inducing resistance changes. That is, as the resin is transferred through the fiber lay-up, the net pressure applied to the carbon fibers decreases as the local resin pressure in creases. In addition, the resin flow induces the rearrangement of the fiber network, which also leads to resistance changes, as shown in Fig. 3 (a). Depending on the velocity of the resin, the time at which the resistance increases is different. Therefore, the position of the resin flow front can be found, as the time at which the resistance between the electrodes placed at the respective position increases, and the flow ve locity of the resin can be determined by taking into consideration both the time at which the resistance increase is observed and the distance traveled by the resin up to that point (Fig. 3(b)). Fig. 4(a) shows the shear viscosity of the resins (SH3, SH4, and SH6) with increasing shear rate. Therefore, the volume flow rate, which influences the relative resistance change of carbon fibers during resin infusion, can be calcu lated [15]. The volume flow rate of a resin passing through hose V_ is expressed by Eq. (1). ΔP V_ ¼ R
(1)
Here, ΔP is the pressure difference, and R is the resistance to flow.R (of fluids) is given by Eq. (2),
Scheme 1. Mechanism of thiol-epoxy click reaction.
R¼
the resin increases with increasing number of functional groups (SH), and it determines the flow velocity of the resin during VARTM. This is the reason that the flow velocity of a resin with high viscosity is lower than that of a resin with low viscosity under the same pressure difference during VARTM process. As the resin flows into carbon fiber fabric, a sharp increase in resistance is observed in the low viscosity resin. When the resin flows into carbon textiles, the hydrodynamic forces – with varying degrees depending on different volume flow rates – develop,
8ηL ; πr4
(2)
where η is the viscosity, L is the length of resin flow, and r is the radius of resin flow line. The volume flow rate and impregnation of carbon fiber fabrics increase when low viscosity resin flows, as the pressure differ ence, length, and radius of the resin flow line remain constant during VARTM. This means a large volume flow rate and easy impregnation induce movements and scattering between the carbon fiber fabrics on
Fig. 3. Resin (SH3, SH4 and SH6) flow front position through resin distribution medium as a function of time during VARTM, as detected by the change in the electric signals of carbon fiber fabric (a) and showing flow front position at different time point depending on resin flow velocity (b). 3
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Fig. 4. Comparison of viscosities of SH3, SH4, and SH6 resins using different methods: (a) Rheometer and (b) relative change in resistance of carbon fiber fabric.
the electrode pair’s line. That is, impregnation between fibers due to low viscosity leads to a greater resistance change. Therefore, the relative resistance between the carbon fiber fabrics sharply increases with decreasing viscosity and the point where the resistance does not change and constantly maintained is when gel point begins in Fig. 4(b).
measured to understand the crosslinking behaviors of three different multi-functional thiol hardeners (SH3, SH4, and SH6). Fig. 5(a) shows a steep increase in the storage modulus, indicating that the polymer mixture formed the network, setting the conditions which are strain, frequency, gap, and temperature constant. The hardeners with several functionalities have higher reaction rates (SH6 < SH4 < SH3) due to autocatalysis, which is influenced by the number of functional groups. FT-IR measurements were conducted at 25 � C with a Peltier temperature control module. Fig. 5(b) shows the conversion of thiol group (-SH,
3.2. Characterization of neat polymer network crosslinking reaction The rheological properties of neat polymer network mixtures were
Fig. 5. Conventional methods for characterizing crosslinking reaction of stoichiometric thiol/epoxy mixtures. a) Change in storage modulus (G0 ) during crosslinking reaction. b) Conversion of thiol groups as a function of time. c) Isothermal DSC scans of exothermic reactions. 4
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2560 cm 1), indicating the crosslinking reaction. The intensity was – C, 1608 cm 1). normalized with those of the aromatic double bonds (C– The time when conversion of the thiol group takes place is similar to position where the storage modulus begins to level off. There is no prominent change in the storage modulus during the initial stages. However, after a certain time point, the storage modulus increases abruptly. In contrast, the conversion of the thiol groups proceeds grad ually from the beginning itself. DSC is a common method for analyzing the crosslinking behavior of polymeric materials. Fig. 5(c) shows the plot of the isothermal heat flow as a function of the reaction time at 25 � C. The shapes of the graphs correspond to exothermic reactions un dergoing typical autocatalysis. The exothermic peaks reach their maximum when the effect of autocatalysis is maximum and then decrease as the crosslinking reaction gradually approaches completion.
carbon fiber fabric layers, but also inside them, to detect electric signals. Fig. 7 shows that each electrode pair located at the top and bottom of the carbon fiber fabric layers exhibits a sharp increase in resistance when the resin was soaked onto the carbon fiber fabrics. The sharp increase in resistance between the top and bottom of the carbon fiber fabrics implied that the SH3 resin with low viscosity easily permeated the carbon fiber fabrics as compared to other resins (Fig. 7). To verify the degree of impregnation of thick 8 carbon fiber fabrics layers that were greatly influenced by resin viscosity, we examined the interior of the cured composites depending on different viscosities. The scanning electron microscopy (SEM) images (Fig. 8) of the cross-sectional com posites show clear difference in the degree of impregnation. The resin with the lowest viscosity shows perfect impregnation of carbon fiber fabrics of 8 layers (Fig. 8(a)). The resins with high viscosity do not permeate the interior of the carbon fiber fabric layers, and we can even observe that the carbon fibers protrude as strands (Fig. 8(b) and (c)). Therefore, SH3 resin with low viscosity permeates the internal carbon fiber fabric layers, and this is strongly stimulated by the hydrodynamic resin flow. This emerges as an outstanding change in resistance as resin flows into carbon fabrics, although the change in resistance in resins with high viscosity (SH4 and SH6) was small. To further investigate the internal impregnation of resins, direction and position of resin flow, and curing, we observed the electric signals in various positions of the electrode pairs. Fig. 9 shows that 11–14 elec trode pairs are connected to the lowermost carbon fiber fabric, 12–15 electrodes are connected to the intermediate carbon fiber fabric, and 13–16 electrodes are connected to the topmost carbon fiber fabric. Generally, the composite manufacturing process can be divided into three stages: (i) vacuum state, (ii) infusion state, and (iii) curing [15]. We observed all the stages via the change in the electric signal of the surface and interior of the carbon fiber fabrics (Fig. 9(a)). In the vacuum state, the electric network of the carbon fiber fabrics is compact; dense conductive paths are created upon applying vacuum, and the relative resistance decreases. In the infusion state, a sharp increase in the relative resistance is observed, reaching its maximum value. Fig. 9(b) shows the magnified infusion state. Impregnation and direction and position of resin flow can be explained by the time points where relative resistances of the carbon fiber fabrics change. The time point of the increase in resistance of path 11–14 is close to that of path 13–16, followed by path 12–15. It means that the resin flows from the top and bottom first and penetrates the interior of 8 layers of carbon fiber fabric. After the resin permeates into the carbon fiber fabric, its relative resistance decreases and levels off during curing process. To observed the curing process more precisely, we compared the change in relative resistance of a sample with full impregnation with that of a sample with incomplete impregnation. The change in relative resistance of path 14–16 was two-fold higher than that of paths 14–15 and 15–16 (Fig. 10(a)). As the low viscosity resin flows into carbon fiber fabric layers that are completely wet inside, the change in relative resistance of path 14–16 shows a large increase compared to that of paths 14–15 and 15–16. This explains that there is no difference in the initial resistance of paths 14–16, 14–15, and 15–16 of the extremely conductive carbon fiber fabric layers. Therefore, the extent of impreg nation of the carbon fabric layers strongly influenced the relative resistance in carbon fiber. Contrary to this, Fig. 10(b) shows that the relative change in resistance of the sample with lesser impregnation is low due to the high viscosity of the resin, and there is only a slight change in the internal resistance from resin flow. This was the reason it did not penetrate the interior of the carbon fiber fabric layers. Therefore, there was no significant difference in relative resistance change depending on the path. It was effectively demonstrated that the impregnation of carbon fiber fabric layers can be detected by the extent of resistance changes and by comparing the resistance change of each electrode path in the carbon fiber fabric layers. The in situ monitoring of the resistance of respective paths can be regarded as a powerful tool for estimating the impregnation
3.3. Monitoring of curing process during composite manufacturing Fig. 6 shows relative resistances of fully impregnated carbon fiber fabric decrease during curing process after the resin infusion is com plete. When the polymer crosslinking process proceeds, the volumetric shrinkage of the resin due to the high level crosslinking induces the alignment and immobilization of carbon fibers, thus easing into a more stable conductive network in carbon fibers. Consequently, the relative resistance of carbon fibers connected with electrodes decreased and leveled off during solidification. Change in resistance of electrode pairs decreases with increasing time regardless of resin types during curing process. This means that the curing process can be monitored, if the electrode pairs are connected to the carbon fiber fabrics where curing is to be proceeded. The levelling off in relative resistance change of the carbon fiber fabrics explains that the resin is completely cured, and hence, the composite can be demolded. Fig. 6 shows that there is almost no change in the relative resistance around 1600s, showing that SH3 resin was completely cured by observing time point. Likewise, the curing process of SH4 and SH6 finished at 1100 and 800s. These were similar to curing times measured by the conventional curing behavior analysis methods (Fig. 5(a)–(c)). 3.4. Monitoring of internal impregnation and curing behavior of carbon fiber fabric layers To monitor the impregnation of epoxy resin into thick carbon fiber fabrics, we planted the electrodes not only at the top and bottom of the
Fig. 6. Typical curing behavior of resins (SH3, SH4 and SH6) monitored by the electrical resistance response from electrode pairs in the middle of carbon fiber fabric. 5
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Fig. 7. Impregnation of carbon fiber fabrics with resins (SH3, SH4 and SH6) using change in relative resistance between carbon fiber fabric layers.
Fig. 8. Cross sectional SEM images of carbon fiber reinforced plastics: (a) fully impregnated sample. (b) and (c) samples with incomplete impregnation.
Fig. 9. Electric signal changes of the carbon fiber fabric on the surface and in the interior of carbon fiber fabrics during composite manufacturing: electric signals of the overall (a) and resin infusion (b) process.
and curing of the carbon fiber fabric layers, which are important steps in determining the quality of the final composite. It is expected that the monitoring method can also be applied to prepregs using similar
principles. When a bleeder is used in conjunction with prepreg vacuum bagging or autoclave, the electrical-resistance-based monitoring method will allow to capture the compressive stresses induced by the
6
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Fig. 10. The degree of impregnation of the carbon fiber fabric layers is represented by an electric signal, which is comparable with the degree of resistance changes between carbon fiber fabric layers. (a) Fully impregnated sample, (b) sample with incomplete impregnation.
hydrodynamic forced and the resulting “bleeding” of the excess resin into the bleeder. The volumetric shrinkage in the prepreg lay-up as curing progresses can also be monitored. The method develop is appli cable to other types of composites consisting of conductive fiber re inforcements, and the fibers can be continuous or discontinuous as long as they warrant a conductive network.
Lee: Formal analysis. Seung Man Noh: Methodology, Supervision, Funding acquisition, Project administration. Young-Bin Park: Meth odology, Supervision, Funding acquisition, Project administration, Writing - review & editing.
4. Conclusions
This work was financially supported by the Human Resources Development grant through the Korea Institute of Energy Technology Evaluation and Planning funded by the Ministry of Trade, Industry and Energy of Korea (Grant No. 20174030201430) and the 2019 Research Fund (1.190009) of UNIST.
Acknowledgments
In situ monitoring of the flow, viscosity, and curing of resin and the impregnation of carbon fabrics was investigated using carbon fiber itself during composite manufacturing process. The electrical resistances in crease sharply when the resin flows into the transverse and longitudinal directions of the carbon fiber fabrics. As the viscosities of the resins decrease, the relative resistance of carbon fiber increases due to the volume flow rate and hydrodynamic effects. In 8 layers of carbon fiber fabrics used in composite manufacturing, it was possible to monitor the extent of resin impregnation in the carbon fiber fabrics, enabling assessment of the quality of composites. In addition, the curing within the multiple layers of carbon fiber fabrics could be monitored. The relative resistance of carbon fiber decreases gradually during curing process because the volumetric shrinkage and compressive strain compact carbon fibers, and their alignment and immobilization in fluences their relative resistance. The methods presented can be refined by optimizing the number, position, and distance of electrodes, which can improve the defect-detection ability and structural integrity of the composites.
References [1] K.E.C.R.E. Lyon, S.M. Angel, In situ cure monitoring of epoxy resins using fiberoptic Raman spectroscopy, J. Appl. Polym. Sci. 53 (1994) 1805–1812. [2] P.Y.C.V.M. Murukeshan, L.S. Ong, L.K. Seah, Cure monitoring of smart composites using Fiber Bragg Grating based embedded sensors, Sensor. Actuator. 79 (2000) 153–161. [3] D.-H.K. Hyun-Kyu Kang, Hyung-Joon Bang, Chang-Sun Hong1, Chun-Gon Kim, Cure monitoring of composite laminates using fiber optic sensors, Smart Mater. Struct. 11 (2002) 279–287. [4] Y. Ito, S. Minakuchi, T. Mizutani, N. Takeda, Cure monitoring of carbon–epoxy composites by optical fiber-based distributed strain–temperature sensing system, Adv. Compos. Mater. 21 (2012) 259–271. [5] D.-H.K. Hyun-Kyu Kang, Chang-Sun Hong, Chun-Gon Kim1, Simultaneous monitoring of strain and temperature during and after cure of unsymmetric composite laminate using fibre-optic sensors, Smart Mater. Struct. 12 (2003) 29–35. [6] R. Montanini, L. D’Acquisto, Simultaneous measurement of temperature and strain in glass fiber/epoxy composites by embedded fiber optic sensors: I. Cure monitoring, Smart Mater. Struct. 16 (2007) 1718–1726. [7] V. Antonucci, M. Giordano, L. Nicolais, A. Calabr� o, A. Cusano, A. Cutolo, S. Inserra, Resin flow monitoring in resin film infusion process, J. Mater. Process. Technol. 143–144 (2003) 687–692. [8] E.Q.A. Maffezzoli, V.A.M. Luprano, G. Montagna, L. Nicolais, Cure monitoring of epoxy matrices for composites by ultrasonic wave propagation, J. Appl. Polym. Sci. 73 (1998) 1969–1977. [9] D.G.L. Hyoung Geun Kim, Dielectric cure monitoring for glass polyester prepreg composites 57 (2002) 91–99. [10] D.B.A. McIlhagger*, B. Hill, The development of a dielectric system for the on-line cure monitoring of the resin transfer moulding process, Composites Part A 31 (2000) 1373–1381.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Changyoon Jeong: Investigation, Writing - original draft. Tae Hee 7
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Polymer Testing 85 (2020) 106416 [18] L. Merad, M. Cochez, S. Margueron, F. Jauchem, M. Ferriol, B. Benyoucef, P. Bourson, In-situ monitoring of the curing of epoxy resins by Raman spectroscopy, Polym. Test. 28 (2009) 42–45. [19] T.H. Lee, Y.I. Park, S.M. Noh, J.C. Kim, In-situ visualization of the kinetics of low temperature thiol-epoxy crosslinking reactions by using a pH-responsive epoxy resin, Prog. Org. Coating 104 (2017) 20–27. [20] J.S. Leng1, A. Asundi3, Real-time cure monitoring of smart composite materials using extrinsic Fabry-Perot interferometer and fiber Bragg grating sensors, Smart Mater. Struct. 11 (2002) 249–255. [21] Q. Govignon, S. Bickerton, P.A. Kelly, Simulation of the reinforcement compaction and resin flow during the complete resin infusion process, Compos. Appl. Sci. Manuf. 41 (2010) 45–57. [22] D.E.D. Kranbuehl, T. Hamilton, R. Clark, In-situ monitoring of the resin transfer molding impregnation and cure process, Polym. Eng. Sci. (1991) 31. [23] C.L.R.A. Saunders, M.G. Bader, Compression in the processing of polymer composites 2. Modelling of the viscoelastic compression of resin-impregnated fibre networks, Compos. Sci. Technol. 59 (1999) 1483–1494.
[11] D.G.L. Jin Soo Kim, Analysis of dielectric sensors for the cure monitoring of resin matrix composite materials, Sensor. Actuator. B 30 (1996) 159–164. [12] S. Kobayashi, R. Matsuzaki, A. Todoroki, Multipoint cure monitoring of CFRP laminates using a flexible matrix sensor, Compos. Sci. Technol. 69 (2009) 378–384. [13] S.D. Schwab, Sensor system for monitoring impregnation and cure during resin transfer molding, Polym. Compos. (1996). April 17. [14] J.R.N. Gnidakouong, H.D. Roh, J.-H. Kim, Y.-B. Park, In situ assessment of carbon nanotube flow and filtration monitoring through glass fabric using electrical resistance measurement, Compos. Appl. Sci. Manuf. 90 (2016) 137–146. [15] J.R.N. Gnidakouong, H.D. Roh, J.-H. Kim, Y.-B. Park, In situ process monitoring of hierarchical micro-/nano-composites using percolated carbon nanotube networks, Compos. Appl. Sci. Manuf. 84 (2016) 281–291. [16] R. Hardis, J.L.P. Jessop, F.E. Peters, M.R. Kessler, Cure kinetics characterization and monitoring of an epoxy resin using DSC, Raman spectroscopy, and DEA, Compos. Appl. Sci. Manuf. 49 (2013) 100–108. [17] R.M. Loureiro, T.C. Amarelo, S.P. Abuin, E.R. Soul�e, R.J.J. Williams, Kinetics of the epoxy–thiol click reaction initiated by a tertiary amine: calorimetric study using monofunctional components, Thermochim. Acta 616 (2015) 79–86.
8
Contents lists available at ScienceDirect
Polymer Testing journal homepage: http://www.elsevier.com/locate/polytest
Real-time in situ monitoring of manufacturing process and CFRP quality by relative resistance change measurement Changyoon Jeong a, Tae Hee Lee b, c, Seung Man Noh c, **, Young-Bin Park a, * a
Department of Mechanical Engineering, Ulsan National Institute of Science and Technology, UNIST-gil 50, Ulju-gun, Ulsan, 44919, Republic of Korea Department of Chemical Engineering, Ulsan National Institute of Science and Technology, UNIST-gil 50, Ulju-gun, Ulsan, 44919, Republic of Korea c Research Center for Green Fine Chemicals, Korea Research Institute of Chemical Technology, Ulsan, 44412, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Keywords: Carbon fibres Cure behaviour Process monitoring Resin flow
In situ monitoring of resin flow, impregnation of carbon fiber fabrics, and curing during composite manufacturing are very important for determining the quality of composite parts. In conventional methods, sensors, such as optical fibers and strain gages, are bonded to or embedded in the composites for measuring the changes in mechanical and chemical properties. Although they can detect resin curing behavior and impregnation of carbon fibers, they may adversely affect the manufacturing process or structural integrity of the composites. In this study, carbon fiber itself was used as a sensor that minimizes the degradation of mechanical properties and increases the efficiency of monitoring the manufacturing process. The change in the electrical resistance of carbon fiber fabrics was monitored during the various manufacturing processes when the resin flowed through the carbon fiber fabric and curing progressed. The effectiveness of this monitoring method was confirmed, and it is expected to be applicable in monitoring the quality of the finished composite parts.
1. Introduction Carbon fiber reinforced plastics have been extensively used in many industrial applications, such as aircraft, automobiles, electronics, and civil structures, and have high specific strength and stiffness that render it potential in many other applications. The quality of the finished product strongly influences the mechanical and thermal properties and is strongly affected by several factors such as the curing and impreg nation of carbon fiber fabrics. To monitor these processes, extensive research has been conducted using sensors such as optical fibers [1–7], ultrasonic waves [8], dielectric systems [9–11], matrix sensors [12,13], and carbon nanotubes [14,15]. These methods typically require the sensor or monitoring system to be installed or embedded in a composite during manufacturing process, which may adversely affect the structural integrity in the final part. In addition, point sensors need to be installed in a large number for accurate measurement of the curing behavior. Several methods are used for curing behavior monitoring processes, such as differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), and Raman spectroscopy. However, set-up time for cure behavior analysis is required in these methods, which may defeat the purpose of exact estimation of the curing
behavior, especially when the resin formulation is complex [16–18]. Thus, a novel monitoring method that can measure resin flow, curing, and impregnation of carbon fiber fabric layers without embedding sensor needs to be developed. In this study, carbon fiber itself is proposed as a sensor, wherein multiple electrodes are readily installed and assigned to desirable po sitions. To monitor the resin flow and curing behavior in composite, thiol-epoxy polymers were used, which are characterized by rapid re action rate at a low temperature, high degree of conversion, and mul tiple crosslinking pathways and compared them with the outcome of other reaction conditions with different viscosities and curing reaction rates [19]. For precise monitoring of the curing and impregnation of carbon fibers, thiol-epoxy with different crosslinkers (SH3, SH4, and SH6) were used as resins where the type of the crosslinker influences rate of the crosslinking reaction and viscosity of the resins [17]. Resins with different viscosities show different flow velocities, and hence, this difference leads to varying degrees of impregnation and rate of cure during vacuum-assisted resin transfer molding (VARTM), resulting in hydrodynamic effects and compressive stresses in carbon fiber lay-ups. When the curing process proceeds, compressive strain is applied to the carbon fiber tows. The tows are radially compacted inducing decreases
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (Y.-B. Park). https://doi.org/10.1016/j.polymertesting.2020.106416 Received 27 September 2019; Received in revised form 11 January 2020; Accepted 8 February 2020 Available online 9 February 2020 0142-9418/© 2020 Elsevier Ltd. All rights reserved.
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in their relative resistances [20]. Resin flow and impregnation of carbon fiber fabrics are detected by the change in the electric signal of carbon fiber fabrics due to its hydrodynamic effects [7,21–23]. All the dynamic movements of the resin, including resin flow, impregnation, and curing, were evaluated.
polymer networks were reacted with stoichiometric amounts of epoxy groups and thiol groups, as shown in Scheme 1. 2.3. Characterization 2.3.1. Oscillational rheometer The shear viscosity and crosslinking kinetics of neat polymer net works were correlated with the rheological properties measured by a oscillational rheometer (Scientific Inc., MARS III). Steady shear mode rheometer was used for obtaining the shear viscosities of curing mix tures without a tertiary amine catalyst. The shear rate ranged from 0.1 to 1000 s 1 at 25 � C. The viscoelastic properties of the neat polymer net works during the crosslinking reactions were characterized using a small amplitude oscillatory shear mode. A parallel aluminum plate of 8 mm was used under a constant strain of 0.5% at 25 � C, frequency of 1 Hz, and gap of 500 μm.
2. Experimental 2.1. Materials Bisphenol-A type epoxy resin (EPOKUKDO, YD-128) with an equiv alent weight of 184–190 g eq 1 was purchased from Kukdo Chemical Co., Ltd. Trimethyl propane tri (3-mercaptopropionate) (SH3), pen taerythritol tetra(3-mercaptopropionate) (SH4), and dipentaerythritol hexa(3-mercaptopropionate) (SH6) were purchased from Bruno Bock Thiolchemical. 2,4,6-Tris(dimetylaminomethyl)phenol (DMP30) was purchased from Sigma Aldrich. Fig. 1 shows the molecular structures of the resins used to monitor the manufacturing processes. The woven carbon fibers (T300, density: 1.76 g/cm3, 3K plain weave) were supplied by Amoco Corporation (Chicago, IL, USA).
2.3.2. FT-IR analysis The chemical reactions of the epoxy/thiol groups were analyzed using an FT-IR spectrometer (Thermo Fisher Scientific Inc., Nico let6700/Nicolet Continuum). The temperature during the experiments was controlled with a Peltier module.
2.2. Preparation of measurement set-up
2.3.3. Differential scanning calorimetry The exothermic reactions between the epoxy groups and thiol groups were confirmed by isothermal DSC analysis (TA Instruments, DSC Q2000). A flow of N2 at 90 mL/min was used, and the weight of the samples for analysis was 5 mg. For the isothermal measurement, a temperature of 25 � C was used.
In situ method of monitoring the resin flow, curing, and degree of carbon fiber fabric layers’ impregnation was developed using carbon fiber resistance change during VARTM process, which is cost-effective and environmentally friendly, as compared to prepregs technologies, which require an autoclave for curing (Fig. 2). In a typical experiment, a woven carbon fiber fabric was laid on a Teflon sheet, which was used for easy demolding. The electric wires were attached to the carbon fiber fabric along the tows (at the ends of the tows) perpendicular to the resin flow direction and were positioned at the front, middle and end of the fabric with respect to the resin flow direction. In order to minimize the electrical resistance interference of the electric wire, Ag paste is on the wire. The VARTM system was used to make a pressure difference, which initiated the resin flow. Before infusion of the resin, the hardeners (SH3, SH4, and SH6) were activated by dissolving them in 5 wt % of a tertiary amine catalyst (DMP30) at a high-speed with overhead stirrer (Ocean Science, DISPERMAT), at 1500 rpm for 20 min. The mixture of the hardener and catalyst was further mixed with the epoxy resin. The
3. Results and discussion 3.1. Rheological properties of resins during composite manufacturing process The relation between the viscosities and flow velocities of the resins during VARTM is shown in Fig. 3. The flow front position was detected by the change in relative resistance of the carbon fiber fabric layer horizontally connected to several electrode pairs and by identifying the times at which the resistance changes at each position. The viscosity of
Fig. 1. Molecular structures of the materials used in this study. 2
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Fig. 2. Schematics of VARTM (vacuum assisted resin transfer molding) for monitoring resin flow, impregnation of fibers, and curing behavior.
which compacts the carbon fibers inducing resistance changes. That is, as the resin is transferred through the fiber lay-up, the net pressure applied to the carbon fibers decreases as the local resin pressure in creases. In addition, the resin flow induces the rearrangement of the fiber network, which also leads to resistance changes, as shown in Fig. 3 (a). Depending on the velocity of the resin, the time at which the resistance increases is different. Therefore, the position of the resin flow front can be found, as the time at which the resistance between the electrodes placed at the respective position increases, and the flow ve locity of the resin can be determined by taking into consideration both the time at which the resistance increase is observed and the distance traveled by the resin up to that point (Fig. 3(b)). Fig. 4(a) shows the shear viscosity of the resins (SH3, SH4, and SH6) with increasing shear rate. Therefore, the volume flow rate, which influences the relative resistance change of carbon fibers during resin infusion, can be calcu lated [15]. The volume flow rate of a resin passing through hose V_ is expressed by Eq. (1). ΔP V_ ¼ R
(1)
Here, ΔP is the pressure difference, and R is the resistance to flow.R (of fluids) is given by Eq. (2),
Scheme 1. Mechanism of thiol-epoxy click reaction.
R¼
the resin increases with increasing number of functional groups (SH), and it determines the flow velocity of the resin during VARTM. This is the reason that the flow velocity of a resin with high viscosity is lower than that of a resin with low viscosity under the same pressure difference during VARTM process. As the resin flows into carbon fiber fabric, a sharp increase in resistance is observed in the low viscosity resin. When the resin flows into carbon textiles, the hydrodynamic forces – with varying degrees depending on different volume flow rates – develop,
8ηL ; πr4
(2)
where η is the viscosity, L is the length of resin flow, and r is the radius of resin flow line. The volume flow rate and impregnation of carbon fiber fabrics increase when low viscosity resin flows, as the pressure differ ence, length, and radius of the resin flow line remain constant during VARTM. This means a large volume flow rate and easy impregnation induce movements and scattering between the carbon fiber fabrics on
Fig. 3. Resin (SH3, SH4 and SH6) flow front position through resin distribution medium as a function of time during VARTM, as detected by the change in the electric signals of carbon fiber fabric (a) and showing flow front position at different time point depending on resin flow velocity (b). 3
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Fig. 4. Comparison of viscosities of SH3, SH4, and SH6 resins using different methods: (a) Rheometer and (b) relative change in resistance of carbon fiber fabric.
the electrode pair’s line. That is, impregnation between fibers due to low viscosity leads to a greater resistance change. Therefore, the relative resistance between the carbon fiber fabrics sharply increases with decreasing viscosity and the point where the resistance does not change and constantly maintained is when gel point begins in Fig. 4(b).
measured to understand the crosslinking behaviors of three different multi-functional thiol hardeners (SH3, SH4, and SH6). Fig. 5(a) shows a steep increase in the storage modulus, indicating that the polymer mixture formed the network, setting the conditions which are strain, frequency, gap, and temperature constant. The hardeners with several functionalities have higher reaction rates (SH6 < SH4 < SH3) due to autocatalysis, which is influenced by the number of functional groups. FT-IR measurements were conducted at 25 � C with a Peltier temperature control module. Fig. 5(b) shows the conversion of thiol group (-SH,
3.2. Characterization of neat polymer network crosslinking reaction The rheological properties of neat polymer network mixtures were
Fig. 5. Conventional methods for characterizing crosslinking reaction of stoichiometric thiol/epoxy mixtures. a) Change in storage modulus (G0 ) during crosslinking reaction. b) Conversion of thiol groups as a function of time. c) Isothermal DSC scans of exothermic reactions. 4
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2560 cm 1), indicating the crosslinking reaction. The intensity was – C, 1608 cm 1). normalized with those of the aromatic double bonds (C– The time when conversion of the thiol group takes place is similar to position where the storage modulus begins to level off. There is no prominent change in the storage modulus during the initial stages. However, after a certain time point, the storage modulus increases abruptly. In contrast, the conversion of the thiol groups proceeds grad ually from the beginning itself. DSC is a common method for analyzing the crosslinking behavior of polymeric materials. Fig. 5(c) shows the plot of the isothermal heat flow as a function of the reaction time at 25 � C. The shapes of the graphs correspond to exothermic reactions un dergoing typical autocatalysis. The exothermic peaks reach their maximum when the effect of autocatalysis is maximum and then decrease as the crosslinking reaction gradually approaches completion.
carbon fiber fabric layers, but also inside them, to detect electric signals. Fig. 7 shows that each electrode pair located at the top and bottom of the carbon fiber fabric layers exhibits a sharp increase in resistance when the resin was soaked onto the carbon fiber fabrics. The sharp increase in resistance between the top and bottom of the carbon fiber fabrics implied that the SH3 resin with low viscosity easily permeated the carbon fiber fabrics as compared to other resins (Fig. 7). To verify the degree of impregnation of thick 8 carbon fiber fabrics layers that were greatly influenced by resin viscosity, we examined the interior of the cured composites depending on different viscosities. The scanning electron microscopy (SEM) images (Fig. 8) of the cross-sectional com posites show clear difference in the degree of impregnation. The resin with the lowest viscosity shows perfect impregnation of carbon fiber fabrics of 8 layers (Fig. 8(a)). The resins with high viscosity do not permeate the interior of the carbon fiber fabric layers, and we can even observe that the carbon fibers protrude as strands (Fig. 8(b) and (c)). Therefore, SH3 resin with low viscosity permeates the internal carbon fiber fabric layers, and this is strongly stimulated by the hydrodynamic resin flow. This emerges as an outstanding change in resistance as resin flows into carbon fabrics, although the change in resistance in resins with high viscosity (SH4 and SH6) was small. To further investigate the internal impregnation of resins, direction and position of resin flow, and curing, we observed the electric signals in various positions of the electrode pairs. Fig. 9 shows that 11–14 elec trode pairs are connected to the lowermost carbon fiber fabric, 12–15 electrodes are connected to the intermediate carbon fiber fabric, and 13–16 electrodes are connected to the topmost carbon fiber fabric. Generally, the composite manufacturing process can be divided into three stages: (i) vacuum state, (ii) infusion state, and (iii) curing [15]. We observed all the stages via the change in the electric signal of the surface and interior of the carbon fiber fabrics (Fig. 9(a)). In the vacuum state, the electric network of the carbon fiber fabrics is compact; dense conductive paths are created upon applying vacuum, and the relative resistance decreases. In the infusion state, a sharp increase in the relative resistance is observed, reaching its maximum value. Fig. 9(b) shows the magnified infusion state. Impregnation and direction and position of resin flow can be explained by the time points where relative resistances of the carbon fiber fabrics change. The time point of the increase in resistance of path 11–14 is close to that of path 13–16, followed by path 12–15. It means that the resin flows from the top and bottom first and penetrates the interior of 8 layers of carbon fiber fabric. After the resin permeates into the carbon fiber fabric, its relative resistance decreases and levels off during curing process. To observed the curing process more precisely, we compared the change in relative resistance of a sample with full impregnation with that of a sample with incomplete impregnation. The change in relative resistance of path 14–16 was two-fold higher than that of paths 14–15 and 15–16 (Fig. 10(a)). As the low viscosity resin flows into carbon fiber fabric layers that are completely wet inside, the change in relative resistance of path 14–16 shows a large increase compared to that of paths 14–15 and 15–16. This explains that there is no difference in the initial resistance of paths 14–16, 14–15, and 15–16 of the extremely conductive carbon fiber fabric layers. Therefore, the extent of impreg nation of the carbon fabric layers strongly influenced the relative resistance in carbon fiber. Contrary to this, Fig. 10(b) shows that the relative change in resistance of the sample with lesser impregnation is low due to the high viscosity of the resin, and there is only a slight change in the internal resistance from resin flow. This was the reason it did not penetrate the interior of the carbon fiber fabric layers. Therefore, there was no significant difference in relative resistance change depending on the path. It was effectively demonstrated that the impregnation of carbon fiber fabric layers can be detected by the extent of resistance changes and by comparing the resistance change of each electrode path in the carbon fiber fabric layers. The in situ monitoring of the resistance of respective paths can be regarded as a powerful tool for estimating the impregnation
3.3. Monitoring of curing process during composite manufacturing Fig. 6 shows relative resistances of fully impregnated carbon fiber fabric decrease during curing process after the resin infusion is com plete. When the polymer crosslinking process proceeds, the volumetric shrinkage of the resin due to the high level crosslinking induces the alignment and immobilization of carbon fibers, thus easing into a more stable conductive network in carbon fibers. Consequently, the relative resistance of carbon fibers connected with electrodes decreased and leveled off during solidification. Change in resistance of electrode pairs decreases with increasing time regardless of resin types during curing process. This means that the curing process can be monitored, if the electrode pairs are connected to the carbon fiber fabrics where curing is to be proceeded. The levelling off in relative resistance change of the carbon fiber fabrics explains that the resin is completely cured, and hence, the composite can be demolded. Fig. 6 shows that there is almost no change in the relative resistance around 1600s, showing that SH3 resin was completely cured by observing time point. Likewise, the curing process of SH4 and SH6 finished at 1100 and 800s. These were similar to curing times measured by the conventional curing behavior analysis methods (Fig. 5(a)–(c)). 3.4. Monitoring of internal impregnation and curing behavior of carbon fiber fabric layers To monitor the impregnation of epoxy resin into thick carbon fiber fabrics, we planted the electrodes not only at the top and bottom of the
Fig. 6. Typical curing behavior of resins (SH3, SH4 and SH6) monitored by the electrical resistance response from electrode pairs in the middle of carbon fiber fabric. 5
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Fig. 7. Impregnation of carbon fiber fabrics with resins (SH3, SH4 and SH6) using change in relative resistance between carbon fiber fabric layers.
Fig. 8. Cross sectional SEM images of carbon fiber reinforced plastics: (a) fully impregnated sample. (b) and (c) samples with incomplete impregnation.
Fig. 9. Electric signal changes of the carbon fiber fabric on the surface and in the interior of carbon fiber fabrics during composite manufacturing: electric signals of the overall (a) and resin infusion (b) process.
and curing of the carbon fiber fabric layers, which are important steps in determining the quality of the final composite. It is expected that the monitoring method can also be applied to prepregs using similar
principles. When a bleeder is used in conjunction with prepreg vacuum bagging or autoclave, the electrical-resistance-based monitoring method will allow to capture the compressive stresses induced by the
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Fig. 10. The degree of impregnation of the carbon fiber fabric layers is represented by an electric signal, which is comparable with the degree of resistance changes between carbon fiber fabric layers. (a) Fully impregnated sample, (b) sample with incomplete impregnation.
hydrodynamic forced and the resulting “bleeding” of the excess resin into the bleeder. The volumetric shrinkage in the prepreg lay-up as curing progresses can also be monitored. The method develop is appli cable to other types of composites consisting of conductive fiber re inforcements, and the fibers can be continuous or discontinuous as long as they warrant a conductive network.
Lee: Formal analysis. Seung Man Noh: Methodology, Supervision, Funding acquisition, Project administration. Young-Bin Park: Meth odology, Supervision, Funding acquisition, Project administration, Writing - review & editing.
4. Conclusions
This work was financially supported by the Human Resources Development grant through the Korea Institute of Energy Technology Evaluation and Planning funded by the Ministry of Trade, Industry and Energy of Korea (Grant No. 20174030201430) and the 2019 Research Fund (1.190009) of UNIST.
Acknowledgments
In situ monitoring of the flow, viscosity, and curing of resin and the impregnation of carbon fabrics was investigated using carbon fiber itself during composite manufacturing process. The electrical resistances in crease sharply when the resin flows into the transverse and longitudinal directions of the carbon fiber fabrics. As the viscosities of the resins decrease, the relative resistance of carbon fiber increases due to the volume flow rate and hydrodynamic effects. In 8 layers of carbon fiber fabrics used in composite manufacturing, it was possible to monitor the extent of resin impregnation in the carbon fiber fabrics, enabling assessment of the quality of composites. In addition, the curing within the multiple layers of carbon fiber fabrics could be monitored. The relative resistance of carbon fiber decreases gradually during curing process because the volumetric shrinkage and compressive strain compact carbon fibers, and their alignment and immobilization in fluences their relative resistance. The methods presented can be refined by optimizing the number, position, and distance of electrodes, which can improve the defect-detection ability and structural integrity of the composites.
References [1] K.E.C.R.E. Lyon, S.M. Angel, In situ cure monitoring of epoxy resins using fiberoptic Raman spectroscopy, J. Appl. Polym. Sci. 53 (1994) 1805–1812. [2] P.Y.C.V.M. Murukeshan, L.S. Ong, L.K. Seah, Cure monitoring of smart composites using Fiber Bragg Grating based embedded sensors, Sensor. Actuator. 79 (2000) 153–161. [3] D.-H.K. Hyun-Kyu Kang, Hyung-Joon Bang, Chang-Sun Hong1, Chun-Gon Kim, Cure monitoring of composite laminates using fiber optic sensors, Smart Mater. Struct. 11 (2002) 279–287. [4] Y. Ito, S. Minakuchi, T. Mizutani, N. Takeda, Cure monitoring of carbon–epoxy composites by optical fiber-based distributed strain–temperature sensing system, Adv. Compos. Mater. 21 (2012) 259–271. [5] D.-H.K. Hyun-Kyu Kang, Chang-Sun Hong, Chun-Gon Kim1, Simultaneous monitoring of strain and temperature during and after cure of unsymmetric composite laminate using fibre-optic sensors, Smart Mater. Struct. 12 (2003) 29–35. [6] R. Montanini, L. D’Acquisto, Simultaneous measurement of temperature and strain in glass fiber/epoxy composites by embedded fiber optic sensors: I. Cure monitoring, Smart Mater. Struct. 16 (2007) 1718–1726. [7] V. Antonucci, M. Giordano, L. Nicolais, A. Calabr� o, A. Cusano, A. Cutolo, S. Inserra, Resin flow monitoring in resin film infusion process, J. Mater. Process. Technol. 143–144 (2003) 687–692. [8] E.Q.A. Maffezzoli, V.A.M. Luprano, G. Montagna, L. Nicolais, Cure monitoring of epoxy matrices for composites by ultrasonic wave propagation, J. Appl. Polym. Sci. 73 (1998) 1969–1977. [9] D.G.L. Hyoung Geun Kim, Dielectric cure monitoring for glass polyester prepreg composites 57 (2002) 91–99. [10] D.B.A. McIlhagger*, B. Hill, The development of a dielectric system for the on-line cure monitoring of the resin transfer moulding process, Composites Part A 31 (2000) 1373–1381.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Changyoon Jeong: Investigation, Writing - original draft. Tae Hee 7
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Polymer Testing 85 (2020) 106416 [18] L. Merad, M. Cochez, S. Margueron, F. Jauchem, M. Ferriol, B. Benyoucef, P. Bourson, In-situ monitoring of the curing of epoxy resins by Raman spectroscopy, Polym. Test. 28 (2009) 42–45. [19] T.H. Lee, Y.I. Park, S.M. Noh, J.C. Kim, In-situ visualization of the kinetics of low temperature thiol-epoxy crosslinking reactions by using a pH-responsive epoxy resin, Prog. Org. Coating 104 (2017) 20–27. [20] J.S. Leng1, A. Asundi3, Real-time cure monitoring of smart composite materials using extrinsic Fabry-Perot interferometer and fiber Bragg grating sensors, Smart Mater. Struct. 11 (2002) 249–255. [21] Q. Govignon, S. Bickerton, P.A. Kelly, Simulation of the reinforcement compaction and resin flow during the complete resin infusion process, Compos. Appl. Sci. Manuf. 41 (2010) 45–57. [22] D.E.D. Kranbuehl, T. Hamilton, R. Clark, In-situ monitoring of the resin transfer molding impregnation and cure process, Polym. Eng. Sci. (1991) 31. [23] C.L.R.A. Saunders, M.G. Bader, Compression in the processing of polymer composites 2. Modelling of the viscoelastic compression of resin-impregnated fibre networks, Compos. Sci. Technol. 59 (1999) 1483–1494.
[11] D.G.L. Jin Soo Kim, Analysis of dielectric sensors for the cure monitoring of resin matrix composite materials, Sensor. Actuator. B 30 (1996) 159–164. [12] S. Kobayashi, R. Matsuzaki, A. Todoroki, Multipoint cure monitoring of CFRP laminates using a flexible matrix sensor, Compos. Sci. Technol. 69 (2009) 378–384. [13] S.D. Schwab, Sensor system for monitoring impregnation and cure during resin transfer molding, Polym. Compos. (1996). April 17. [14] J.R.N. Gnidakouong, H.D. Roh, J.-H. Kim, Y.-B. Park, In situ assessment of carbon nanotube flow and filtration monitoring through glass fabric using electrical resistance measurement, Compos. Appl. Sci. Manuf. 90 (2016) 137–146. [15] J.R.N. Gnidakouong, H.D. Roh, J.-H. Kim, Y.-B. Park, In situ process monitoring of hierarchical micro-/nano-composites using percolated carbon nanotube networks, Compos. Appl. Sci. Manuf. 84 (2016) 281–291. [16] R. Hardis, J.L.P. Jessop, F.E. Peters, M.R. Kessler, Cure kinetics characterization and monitoring of an epoxy resin using DSC, Raman spectroscopy, and DEA, Compos. Appl. Sci. Manuf. 49 (2013) 100–108. [17] R.M. Loureiro, T.C. Amarelo, S.P. Abuin, E.R. Soul�e, R.J.J. Williams, Kinetics of the epoxy–thiol click reaction initiated by a tertiary amine: calorimetric study using monofunctional components, Thermochim. Acta 616 (2015) 79–86.
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