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Through-thickness electric conductivity of toughened carbon-fibre-reinforced polymer laminates with resin-rich layers
Composites Science and Technology 122 (2016) 67e72
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Through-thickness electric conductivity of toughened carbon-fibrereinforced polymer laminates with resin-rich layers Yoshiyasu Hirano a, Takuya Yamane b, Akira Todoroki c, * a
Advanced Composite Research Center, Institute of Aeronautical Technology, Japan Aerospace Exploration Agency, 6-13-1 Osawa, Mitaka, Tokyo 1810015, Japan b Graduate student of Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro, Tokyo 1528552, Japan c Department of Mechanical Sciences and Engineering, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro, Tokyo 1528552, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 July 2015 Received in revised form 3 October 2015 Accepted 12 November 2015 Available online 19 November 2015
Self-sensing carbon-fibre-reinforced polymer (CFRP) that use changes in electrical resistance has been applied to monitor delamination cracks in CFRP composite structures. CFRP laminated composites have orthotropic electric conductivity. Despite the existence of insulator resin-rich layers between each ply, the toughened CFRP laminates have electric conductivity in the thickness direction, which plays important role in delamination monitoring with changes in electrical resistance. Further more, it is also quite important to understand the lightning damage behaviour of CFRP laminate and to perform numerical simulations estimating the current flow in CFRP laminate when lightning event. However, the measured electric conductance has not been confirmed to be a material property for toughened CFRP. The objective of the present study is thus to clarify the mechanism of realizing electric conductivity in the through-thickness direction. During the curing process, alternating current is applied, and impedance is measured for toughened CFRP. It is found that conductivity in the through-thickness direction of toughened CFRP laminates is due to chance carbon fibre contact between plies during the curing process, and is not a material property. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Carbon fibres Polymer-matrix composites Electrical properties
1. Introduction Carbon-fibre-reinforced polymer (CFRP) composite structures are widely employed as aerospace and automobile components to reduce structural weight. However, it is difficult to find delamination cracks in laminated CFRP visually. There is thus a demand for a delamination monitoring system. Several researchers have proposed self-sensing CFRPs that are based on electrical resistance changes [1e10]. Todoroki et al. published articles on self-sensing CFRPs where delamination cracking and matrix cracking were monitored using the electrical resistance changes of multiple segments on the CFRP plate surface [11e16]. The previous papers revealed that electric conductivity in the fibre direction is proportional to the carbon fibre volume fraction [13]. Contact between carbon fibres provide electric conductivity in the transverse direction, while contact between plies provides
* Corresponding author. E-mail addresses: [email protected] (Y. Hirano), [email protected]. titech.ac.jp (T. Yamane), [email protected] (A. Todoroki). http://dx.doi.org/10.1016/j.compscitech.2015.11.018 0266-3538/© 2015 Elsevier Ltd. All rights reserved.
electric conductivity in the through-thickness direction for normal CFRP composites. Fig. 1 shows the cross-sectional view of normal CFRP composites (Mitsubishi Rayon, Tokyo, Japan, PYROFIL TR-30/ epoxy #380) having a fibre volume fraction of 62%. As seen in the figure, it is difficult to find the interlaminar boundary of each ply. To improve interlaminar strength, a resin-rich layer with toughener particle has recently been introduced for toughened CFRP composites. Fig. 2 shows the cross-sectional view of toughened CFRP composites (Toho Tenax, Tokyo, Japan, IMS60/133). Resin-rich layers are clearly observed at each interlaminar boundary in the figure. Although the toughened CFRP has an electrically insulating resin-rich layer, the toughened CFRP has electric conductivity in the through-thickness direction as shown in Ref. [17]. The conductivity in the through-thickness direction is, however, much lower than that in the fibre direction. For IMS60/133 composite laminates, the conductivity ratio in the thickness direction is only 10 9 that in the fibre direction. As shown in Fig. 1, carbon fibres in different plies have contact points in the case of a normal CFRP. For the toughened CFRP, however, carbon fibre contact is not observed in the cross-sectional view, as shown in Fig. 2. Electric conductivity in the through-
Y. Hirano et al. / Composites Science and Technology 122 (2016) 67e72
Fig. 1. Typical cross sectional view of TR30/380.
Fig. 2. Typical cross sectional view of toughened CFRP.
thickness direction plays important role in performing delamination monitoring with changes in electrical resistance, and numerical simulation predicting electrical current inside the CFRP laminate in the event of lightning strike. For example, without knowledge of the electric conductivity in the through-thickness direction, the result of numerical simulation of electric current in the event of a lightning strike may differ from that of actual toughened CFRP laminated structures. However, the measured electric conductance of the toughened CFRP has not been confirmed to be a fixed material property for the fixed fibre volume fraction. The objective of the present study is thus to clarify the mechanism by which electric conductivity is realized in the thickness direction of toughened CFRP with resin-rich layers. During the curing process, alternating current was applied and the impedance change was measured. High electric current was applied after the curing and the temperature change was observed employing infrared thermography to investigate the electrical contact points between plies. A cross-sectional observation was made to investigate the fibre contact between plies. Electric conductivities in three directions of the fibre direction, the transverse direction, and the thickness direction were measured and the data scatter was compared for the different directions to confirm the validity of the modelled mechanism.
Tenax Co Ltd., Tokyo, Japan). The recommended curing temperature is 180 C for IMS60/133 composites. However, this curing temperature is too high for the normal lead wires that are required to measure electric impedance during the curing process. To prevent damage to the lead wires at high temperature, the maximum curing temperature was set to 135 C in the present study. Measurements of complex viscosity from room temperature to 250 C indicated that resin started to transform to a liquid state at 120 C (the measurements were performed by DSC method with DHR-2 produced by TA Instrument at heating rate 2.5 C/min). At a temperature of 135 C, therefore, the resin was in an almost completely liquid state. This means the temperature was high enough to observe the effect of resin flow even though the specimens were not completely cured. Fig. 3 shows the curing cycle in the present study. To prevent a non-uniform temperature distribution, the temperature was held for 1 h at 80 C. Afterward, the temperature rose by 2.6 C/min to 135 C. At 135 C, the temperature was held constant for 3 h to investigate the effect of resin flow on electric conductivity in the through-thickness direction. Fig. 4 shows the specimen used in the present study. A prepreg sheet of IMS60/133 was cut into two rectangular sheets each having width of 20 mm and length of 40 mm. The two rectangular prepreg sheets were overlapped as shown in Fig. 4(b); the length of overlap was 20 mm. To prevent carbon fibre contact at the specimen edge between the upper and the lower prepreg sheets, polytetrafluoroethylene films of 0.05 mm thickness (usually referred to as Teflon) were placed at the four sides as shown in Fig. 4(a). Each film overlapped an edge by 2 mm. To make electric contact with the specimen, copper tape of 0.08 mm thickness was attached at the end of each specimen as shown in Fig. 4. Before attaching the copper tape, the prepreg sheet specimen surface was cleaned and surface resin was removed using acetone to ensure good electric contact. As large changes in impedance and phase angle were measured in the present study, the contact resistance at electrodes can be neglected. A two-probe method, therefore, was adopted even though the measured impedance includes contact resistance at the copper tape electrodes. The impedance and phase angle were measured using an LCR meter (model 3522, Hioki Co. Ltd., Japan). To measure the specimen impedance and phase angle between current and voltage, alternating current of 1 kHz and 30 mA was used. For the completely cured CFRP, the phase angle of the electrical impedance is 0 [14]. There is no frequency dependency of alternating current for CFRP around 1.0 kHz. That is the reason why the alternating current of 1 kHz is selected in the present study. After the curing process, using specimens that had electric conductance in the thickness direction, greater direct electric
140 120 Temperature, oC
68
100 80 60 40 20 0
2. Specimens and testing methods The material used in the present study was IMS60/133 (Toho
0 5000 10000 15000 20000 25000 30000 35000 Time, s Fig. 3. Curing process of electric short investigation.
Y. Hirano et al. / Composites Science and Technology 122 (2016) 67e72
69
160 or 40
140 or 20
5
(a) Specimen for longitudinal and transverse direction. 15
Fig. 4. Specimen configuration to investigate electrical short during curing.
current of 0.1 A was applied to measure the temperature rise due to resistive heating. The surface temperature of the specimen was measured employing infrared thermography (MobIR M3, Wuhan Guide Infrared Technology Co. Ltd., China). For the measurements of the infrared thermography, the emissivity of 0.9 is used [18]. After the resistive heating test, the high-temperature region was cut out and the cross section of the specimen was observed to investigate carbon fibre contact between plies using a video microscope. To measure the scatter of measured electric conductivity IMS60/ 133 of completely cured at 180 C, a unidirectional plate comprising 20 plies that were 200 mm long, 100 mm wide and 4.5 mm thick was fabricated. From this plate, three specimens that were 160 mm long and 10 mm wide were obtained to measure electric conductivity in the fibre direction (s0), three specimens that were 40 mm long and 10 mm wide were obtained to measure electric conductivity in the transverse direction (s90), and 14 specimens that were 15-mm squares were obtained to measure electric conductivity in the through-thickness direction (st). To measure the electric conductivity exactly without the effect of contact resistance at electrodes, the four-probe method was used for all specimens employing the LCR meter described before. A copper plating method was applied to make electric contact with the specimens after polishing the surfaces. Fig. 5 shows the configuration of the four-probe method specimen. To measure s0 and s90 for the specimens, electric current was applied from the specimen edge (see Fig. 5 (a)). The inner couple of electrodes were used to measure voltage. To measure st for the specimens (see Fig. 5(b)), dual square electrodes were mounted on the specimen surfaces. The outer square electrode was used to apply electric current and the inner square electrode to measure voltage. Wang and Chung used these four-probe method [19]. As the electric conductance in the through-thickness direction is negligibly small compared to that of fibre direction or transverse direction, the outer square electrode makes equal voltage surface inside the outer square electrode. The inner square electrode detects the voltage without the effect of electric contact at the electrode.
1.5 (b) Specimen for thickness direction. Fig. 5. Specimen configuration to investigate electrical conductivity after curing.
Fig. 6. Measured electric impedance change during curing process of IMS60/133. (without initial electrical short between the two plies).
curing process. Fig. 7 shows the measured phase angle. The ordinate is the phase angle of the alternating electric current and the abscissa is time. The figures reveal that the specimen before curing had high impedance and the phase angle was almost 90 . This
3. Results and discussion Fig. 6 presents the change in the measured electric impedance. The ordinate is the impedance and the abscissa is time during the
Fig. 7. Measured electric phase angle change during curing process of IMS60/133. (without initial electrical short between the two plies).
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Y. Hirano et al. / Composites Science and Technology 122 (2016) 67e72
means the two prepreg sheets were completely electrically insulated and acted as parallel plates of an electric capacitor. The resinrich layer of the toughened CFRP thus had no electric conductivity. After the temperature rose to 135 C, the impedance suddenly decreased and the phase angle almost reached zero. The two plies therefore had electric contact when the temperature reached 135 C. This suggests that resin flow has a large effect on electric contact in the though-thickness direction. To investigate the effect of static electrical charge that may be generated during the prepreg layup process, similar experiments were conducted with an electrical bonding between plies. The electrical bonding was simply applied by connecting the two electrodes of the specimen (connecting two plies electrically). Using this process, the two plies have electrically the same potential before curing. Fig. 8 presents the measured impedance change with the electrical short. Fig. 9 presents the measured results of the phase angle with the electrical short. These figures show results similar to those in Figs. 6 and 7. Therefore, the static electric charge had no effect on the electric contact between the two plies. As shown in Refs. [17], the conductivity of IMS60/133 in the thickness direction is approximately 0.0017 Sm (resistivity of 5.6 Ucm). Achieving this conductivity using multiwall carbon nanotubes requires a 0.1% weight of carbon nanotubes [20], and this is almost the percolation limit for obtaining high electric conductivity as shown in Ref. [21]. In the case of a vapour-grown carbon nanofiber, the required volume fraction of the fibre is 5%. If the electrically conductive filler comprises particles such as fragments of carbon fibres, a greater volume fraction is required. As there is no such amount of electric conductive filler in the IMS60/133, small electric conductive particles do not affect electric conductivity in the thickness direction even if a small amount of such particles are added. After the curing process, direct electric current of 0.1 A was applied to the specimens. The specimen surface temperature was observed employing infrared thermography. The results are presented in Fig. 10. The temperature was measured for the 15 s following the start of application of electric current. Higher temperature regions are shown in red (in the web version) in the figure. It is seen that the specimen surface was not uniformly heated; i.e., the region of higher temperature was not uniformly distributed. The highest point is 90 C as shown in white area. The other green area is approximately 40 C in Fig. 10. As the specimen initially had an insulator layer before curing (phase angle of 90 ) and the direct current did not flow though the condenser, the region of higher temperature indicates existence of electric contact between plies. Therefore, a cross-sectional observation was made at the contact point and another point of lower temperature as a reference. Fig. 11 shows the cross-sectional view of the normal cross section where the higher temperature was
Fig. 9. Measured electric phase angle change during curing process of IMS60/133. (with initial electrical short between the two plies before curing).
Fig. 10. Measured image of thermography when direct current is applied between the two plies.
Fig. 11. Cross-sectional view of IMS60/133 of typical interlayer.
Fig. 8. Measured electric impedance change during curing process of IMS60/133. (with initial electrical short between the two plies before curing).
not observed (green area in Fig. 10). Fig. 11(a) shows the cross section in the fibre direction while Fig. 11 (b) shows that perpendicular to the fibre direction. In both figures, a thick resin-rich layer can be observed. It seems that there was no carbon fibre contact
Y. Hirano et al. / Composites Science and Technology 122 (2016) 67e72
between the upper and lower plies. Fig. 12 shows the cross-sectional view where the highest resistive heating was observed (white area in Fig. 10). Fig. 12(a) shows the cross section in the fibre direction while Fig. 12(b) shows that in the transverse direction. It is seen that the distance between the upper and lower layers are closer than in the normal cross section presented in Fig. 11. Fig. 12(b) shows fibre contacts between the upper and lower layers. The results reveal that electric conductance in thickness direction of toughened CFRP is due to fibre contact between plies due to resin flow during cure process. As previously mentioned, the two prepreg sheet specimens did not have electric contact between plies before curing. After resin flow started at 135 C, the two plies had electric contact. The conductivity in the thickness direction was higher than the conductivity of the threshold value of the percolation model of conductive particles. Therefore, many conductive particles must be mixed if the conductivity is due to conductive particles. Of course, such a high percentage weight of conductive particles was not mixed in the prepreg of the toughened CFRP. The observation of the point with high resistive heating revealed carbon fibre contacts between plies. It is thus concluded that the electric conductivity of the toughened CFRP in thickness direction is due to carbon fibre contact between plies caused by resin flow during the curing process. Therefore, the conductivity of toughened CFRP in the thickness direction is not a material property, but may depend on, for example, the stacking sequence or thickness or fabrication process. To confirm the electric conductivity was due to fibre contact between plies, electric conductivity was measured in three directions. Table 1 gives the measurement results. The fibre volume fraction of was measured for seven specimens as 0.64 (with standard deviation of 0.02). The coefficient of variation (the standard deviation divided by the mean value) represents the relative scatter of the measurement data. Table 1 reveals that the scatter of the electric conductivity in the though-thickness direction st was much
71
Table 1 Measured statistical results of electric conductivity.
s0 s90 st
Mean (S/m)
S.D. (S/m)
C.V.
4.18 104 4.68 1.03 10 2
0.14 104 0.70 0.55 10 2
0.033 0.15 0.53
S.D.: Standard deviation. C.V.: Coefficient of variation.
larger than that of the electric conductivity in other directions. The highest measured conductivity was 2.1 10 2 S/m and the lowest was 6.07 10 3 S/m. This large scatter indicates that st depends on the chance of fibre contact between plies and that the mean value of electric conductivity may change depending on the fabrication process and stacking sequence. Further experimental research is needed to clarify the variation of the mean value of electric conductivity in the though-thickness direction. 4. Conclusions To investigate the mechanism of electric conductance in the thickness direction of toughened CFRP having a thick resin-rich layer, the electric impedance and phase angle of a toughened CFRP prepreg sheet during the curing process were experimentally measured by using a LCR meter. The temperature distribution due to resistive heating with direct electric current of the specimen surface was measured employing infrared thermography after curing, and a cross-sectional view of a greatly heated point was observed. The following results were obtained. (1) Before curing, the prepreg sheet had no electric conductivity in the thickness direction. After the start of resin flow, electric conductivity in the thickness direction was appeared. (2) Applying directional current between plies showed that resistive heating was not uniform at the surface. An area undergoing great resistive heating showed that the two plies were much less separated than before curing. (3) Fibre contacts between plies due to resin flow during the curing process were the electric contacts between plies. Therefore, the electric conductivity of toughened CFRP is not a material property but depends on resin flow during fabrication. References
Fig. 12. Cross-sectional view of IMS60/133 at the high temperature area observed using thermography.
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Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Through-thickness electric conductivity of toughened carbon-fibrereinforced polymer laminates with resin-rich layers Yoshiyasu Hirano a, Takuya Yamane b, Akira Todoroki c, * a
Advanced Composite Research Center, Institute of Aeronautical Technology, Japan Aerospace Exploration Agency, 6-13-1 Osawa, Mitaka, Tokyo 1810015, Japan b Graduate student of Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro, Tokyo 1528552, Japan c Department of Mechanical Sciences and Engineering, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro, Tokyo 1528552, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 July 2015 Received in revised form 3 October 2015 Accepted 12 November 2015 Available online 19 November 2015
Self-sensing carbon-fibre-reinforced polymer (CFRP) that use changes in electrical resistance has been applied to monitor delamination cracks in CFRP composite structures. CFRP laminated composites have orthotropic electric conductivity. Despite the existence of insulator resin-rich layers between each ply, the toughened CFRP laminates have electric conductivity in the thickness direction, which plays important role in delamination monitoring with changes in electrical resistance. Further more, it is also quite important to understand the lightning damage behaviour of CFRP laminate and to perform numerical simulations estimating the current flow in CFRP laminate when lightning event. However, the measured electric conductance has not been confirmed to be a material property for toughened CFRP. The objective of the present study is thus to clarify the mechanism of realizing electric conductivity in the through-thickness direction. During the curing process, alternating current is applied, and impedance is measured for toughened CFRP. It is found that conductivity in the through-thickness direction of toughened CFRP laminates is due to chance carbon fibre contact between plies during the curing process, and is not a material property. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Carbon fibres Polymer-matrix composites Electrical properties
1. Introduction Carbon-fibre-reinforced polymer (CFRP) composite structures are widely employed as aerospace and automobile components to reduce structural weight. However, it is difficult to find delamination cracks in laminated CFRP visually. There is thus a demand for a delamination monitoring system. Several researchers have proposed self-sensing CFRPs that are based on electrical resistance changes [1e10]. Todoroki et al. published articles on self-sensing CFRPs where delamination cracking and matrix cracking were monitored using the electrical resistance changes of multiple segments on the CFRP plate surface [11e16]. The previous papers revealed that electric conductivity in the fibre direction is proportional to the carbon fibre volume fraction [13]. Contact between carbon fibres provide electric conductivity in the transverse direction, while contact between plies provides
* Corresponding author. E-mail addresses: [email protected] (Y. Hirano), [email protected]. titech.ac.jp (T. Yamane), [email protected] (A. Todoroki). http://dx.doi.org/10.1016/j.compscitech.2015.11.018 0266-3538/© 2015 Elsevier Ltd. All rights reserved.
electric conductivity in the through-thickness direction for normal CFRP composites. Fig. 1 shows the cross-sectional view of normal CFRP composites (Mitsubishi Rayon, Tokyo, Japan, PYROFIL TR-30/ epoxy #380) having a fibre volume fraction of 62%. As seen in the figure, it is difficult to find the interlaminar boundary of each ply. To improve interlaminar strength, a resin-rich layer with toughener particle has recently been introduced for toughened CFRP composites. Fig. 2 shows the cross-sectional view of toughened CFRP composites (Toho Tenax, Tokyo, Japan, IMS60/133). Resin-rich layers are clearly observed at each interlaminar boundary in the figure. Although the toughened CFRP has an electrically insulating resin-rich layer, the toughened CFRP has electric conductivity in the through-thickness direction as shown in Ref. [17]. The conductivity in the through-thickness direction is, however, much lower than that in the fibre direction. For IMS60/133 composite laminates, the conductivity ratio in the thickness direction is only 10 9 that in the fibre direction. As shown in Fig. 1, carbon fibres in different plies have contact points in the case of a normal CFRP. For the toughened CFRP, however, carbon fibre contact is not observed in the cross-sectional view, as shown in Fig. 2. Electric conductivity in the through-
Y. Hirano et al. / Composites Science and Technology 122 (2016) 67e72
Fig. 1. Typical cross sectional view of TR30/380.
Fig. 2. Typical cross sectional view of toughened CFRP.
thickness direction plays important role in performing delamination monitoring with changes in electrical resistance, and numerical simulation predicting electrical current inside the CFRP laminate in the event of lightning strike. For example, without knowledge of the electric conductivity in the through-thickness direction, the result of numerical simulation of electric current in the event of a lightning strike may differ from that of actual toughened CFRP laminated structures. However, the measured electric conductance of the toughened CFRP has not been confirmed to be a fixed material property for the fixed fibre volume fraction. The objective of the present study is thus to clarify the mechanism by which electric conductivity is realized in the thickness direction of toughened CFRP with resin-rich layers. During the curing process, alternating current was applied and the impedance change was measured. High electric current was applied after the curing and the temperature change was observed employing infrared thermography to investigate the electrical contact points between plies. A cross-sectional observation was made to investigate the fibre contact between plies. Electric conductivities in three directions of the fibre direction, the transverse direction, and the thickness direction were measured and the data scatter was compared for the different directions to confirm the validity of the modelled mechanism.
Tenax Co Ltd., Tokyo, Japan). The recommended curing temperature is 180 C for IMS60/133 composites. However, this curing temperature is too high for the normal lead wires that are required to measure electric impedance during the curing process. To prevent damage to the lead wires at high temperature, the maximum curing temperature was set to 135 C in the present study. Measurements of complex viscosity from room temperature to 250 C indicated that resin started to transform to a liquid state at 120 C (the measurements were performed by DSC method with DHR-2 produced by TA Instrument at heating rate 2.5 C/min). At a temperature of 135 C, therefore, the resin was in an almost completely liquid state. This means the temperature was high enough to observe the effect of resin flow even though the specimens were not completely cured. Fig. 3 shows the curing cycle in the present study. To prevent a non-uniform temperature distribution, the temperature was held for 1 h at 80 C. Afterward, the temperature rose by 2.6 C/min to 135 C. At 135 C, the temperature was held constant for 3 h to investigate the effect of resin flow on electric conductivity in the through-thickness direction. Fig. 4 shows the specimen used in the present study. A prepreg sheet of IMS60/133 was cut into two rectangular sheets each having width of 20 mm and length of 40 mm. The two rectangular prepreg sheets were overlapped as shown in Fig. 4(b); the length of overlap was 20 mm. To prevent carbon fibre contact at the specimen edge between the upper and the lower prepreg sheets, polytetrafluoroethylene films of 0.05 mm thickness (usually referred to as Teflon) were placed at the four sides as shown in Fig. 4(a). Each film overlapped an edge by 2 mm. To make electric contact with the specimen, copper tape of 0.08 mm thickness was attached at the end of each specimen as shown in Fig. 4. Before attaching the copper tape, the prepreg sheet specimen surface was cleaned and surface resin was removed using acetone to ensure good electric contact. As large changes in impedance and phase angle were measured in the present study, the contact resistance at electrodes can be neglected. A two-probe method, therefore, was adopted even though the measured impedance includes contact resistance at the copper tape electrodes. The impedance and phase angle were measured using an LCR meter (model 3522, Hioki Co. Ltd., Japan). To measure the specimen impedance and phase angle between current and voltage, alternating current of 1 kHz and 30 mA was used. For the completely cured CFRP, the phase angle of the electrical impedance is 0 [14]. There is no frequency dependency of alternating current for CFRP around 1.0 kHz. That is the reason why the alternating current of 1 kHz is selected in the present study. After the curing process, using specimens that had electric conductance in the thickness direction, greater direct electric
140 120 Temperature, oC
68
100 80 60 40 20 0
2. Specimens and testing methods The material used in the present study was IMS60/133 (Toho
0 5000 10000 15000 20000 25000 30000 35000 Time, s Fig. 3. Curing process of electric short investigation.
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69
160 or 40
140 or 20
5
(a) Specimen for longitudinal and transverse direction. 15
Fig. 4. Specimen configuration to investigate electrical short during curing.
current of 0.1 A was applied to measure the temperature rise due to resistive heating. The surface temperature of the specimen was measured employing infrared thermography (MobIR M3, Wuhan Guide Infrared Technology Co. Ltd., China). For the measurements of the infrared thermography, the emissivity of 0.9 is used [18]. After the resistive heating test, the high-temperature region was cut out and the cross section of the specimen was observed to investigate carbon fibre contact between plies using a video microscope. To measure the scatter of measured electric conductivity IMS60/ 133 of completely cured at 180 C, a unidirectional plate comprising 20 plies that were 200 mm long, 100 mm wide and 4.5 mm thick was fabricated. From this plate, three specimens that were 160 mm long and 10 mm wide were obtained to measure electric conductivity in the fibre direction (s0), three specimens that were 40 mm long and 10 mm wide were obtained to measure electric conductivity in the transverse direction (s90), and 14 specimens that were 15-mm squares were obtained to measure electric conductivity in the through-thickness direction (st). To measure the electric conductivity exactly without the effect of contact resistance at electrodes, the four-probe method was used for all specimens employing the LCR meter described before. A copper plating method was applied to make electric contact with the specimens after polishing the surfaces. Fig. 5 shows the configuration of the four-probe method specimen. To measure s0 and s90 for the specimens, electric current was applied from the specimen edge (see Fig. 5 (a)). The inner couple of electrodes were used to measure voltage. To measure st for the specimens (see Fig. 5(b)), dual square electrodes were mounted on the specimen surfaces. The outer square electrode was used to apply electric current and the inner square electrode to measure voltage. Wang and Chung used these four-probe method [19]. As the electric conductance in the through-thickness direction is negligibly small compared to that of fibre direction or transverse direction, the outer square electrode makes equal voltage surface inside the outer square electrode. The inner square electrode detects the voltage without the effect of electric contact at the electrode.
1.5 (b) Specimen for thickness direction. Fig. 5. Specimen configuration to investigate electrical conductivity after curing.
Fig. 6. Measured electric impedance change during curing process of IMS60/133. (without initial electrical short between the two plies).
curing process. Fig. 7 shows the measured phase angle. The ordinate is the phase angle of the alternating electric current and the abscissa is time. The figures reveal that the specimen before curing had high impedance and the phase angle was almost 90 . This
3. Results and discussion Fig. 6 presents the change in the measured electric impedance. The ordinate is the impedance and the abscissa is time during the
Fig. 7. Measured electric phase angle change during curing process of IMS60/133. (without initial electrical short between the two plies).
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means the two prepreg sheets were completely electrically insulated and acted as parallel plates of an electric capacitor. The resinrich layer of the toughened CFRP thus had no electric conductivity. After the temperature rose to 135 C, the impedance suddenly decreased and the phase angle almost reached zero. The two plies therefore had electric contact when the temperature reached 135 C. This suggests that resin flow has a large effect on electric contact in the though-thickness direction. To investigate the effect of static electrical charge that may be generated during the prepreg layup process, similar experiments were conducted with an electrical bonding between plies. The electrical bonding was simply applied by connecting the two electrodes of the specimen (connecting two plies electrically). Using this process, the two plies have electrically the same potential before curing. Fig. 8 presents the measured impedance change with the electrical short. Fig. 9 presents the measured results of the phase angle with the electrical short. These figures show results similar to those in Figs. 6 and 7. Therefore, the static electric charge had no effect on the electric contact between the two plies. As shown in Refs. [17], the conductivity of IMS60/133 in the thickness direction is approximately 0.0017 Sm (resistivity of 5.6 Ucm). Achieving this conductivity using multiwall carbon nanotubes requires a 0.1% weight of carbon nanotubes [20], and this is almost the percolation limit for obtaining high electric conductivity as shown in Ref. [21]. In the case of a vapour-grown carbon nanofiber, the required volume fraction of the fibre is 5%. If the electrically conductive filler comprises particles such as fragments of carbon fibres, a greater volume fraction is required. As there is no such amount of electric conductive filler in the IMS60/133, small electric conductive particles do not affect electric conductivity in the thickness direction even if a small amount of such particles are added. After the curing process, direct electric current of 0.1 A was applied to the specimens. The specimen surface temperature was observed employing infrared thermography. The results are presented in Fig. 10. The temperature was measured for the 15 s following the start of application of electric current. Higher temperature regions are shown in red (in the web version) in the figure. It is seen that the specimen surface was not uniformly heated; i.e., the region of higher temperature was not uniformly distributed. The highest point is 90 C as shown in white area. The other green area is approximately 40 C in Fig. 10. As the specimen initially had an insulator layer before curing (phase angle of 90 ) and the direct current did not flow though the condenser, the region of higher temperature indicates existence of electric contact between plies. Therefore, a cross-sectional observation was made at the contact point and another point of lower temperature as a reference. Fig. 11 shows the cross-sectional view of the normal cross section where the higher temperature was
Fig. 9. Measured electric phase angle change during curing process of IMS60/133. (with initial electrical short between the two plies before curing).
Fig. 10. Measured image of thermography when direct current is applied between the two plies.
Fig. 11. Cross-sectional view of IMS60/133 of typical interlayer.
Fig. 8. Measured electric impedance change during curing process of IMS60/133. (with initial electrical short between the two plies before curing).
not observed (green area in Fig. 10). Fig. 11(a) shows the cross section in the fibre direction while Fig. 11 (b) shows that perpendicular to the fibre direction. In both figures, a thick resin-rich layer can be observed. It seems that there was no carbon fibre contact
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between the upper and lower plies. Fig. 12 shows the cross-sectional view where the highest resistive heating was observed (white area in Fig. 10). Fig. 12(a) shows the cross section in the fibre direction while Fig. 12(b) shows that in the transverse direction. It is seen that the distance between the upper and lower layers are closer than in the normal cross section presented in Fig. 11. Fig. 12(b) shows fibre contacts between the upper and lower layers. The results reveal that electric conductance in thickness direction of toughened CFRP is due to fibre contact between plies due to resin flow during cure process. As previously mentioned, the two prepreg sheet specimens did not have electric contact between plies before curing. After resin flow started at 135 C, the two plies had electric contact. The conductivity in the thickness direction was higher than the conductivity of the threshold value of the percolation model of conductive particles. Therefore, many conductive particles must be mixed if the conductivity is due to conductive particles. Of course, such a high percentage weight of conductive particles was not mixed in the prepreg of the toughened CFRP. The observation of the point with high resistive heating revealed carbon fibre contacts between plies. It is thus concluded that the electric conductivity of the toughened CFRP in thickness direction is due to carbon fibre contact between plies caused by resin flow during the curing process. Therefore, the conductivity of toughened CFRP in the thickness direction is not a material property, but may depend on, for example, the stacking sequence or thickness or fabrication process. To confirm the electric conductivity was due to fibre contact between plies, electric conductivity was measured in three directions. Table 1 gives the measurement results. The fibre volume fraction of was measured for seven specimens as 0.64 (with standard deviation of 0.02). The coefficient of variation (the standard deviation divided by the mean value) represents the relative scatter of the measurement data. Table 1 reveals that the scatter of the electric conductivity in the though-thickness direction st was much
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Table 1 Measured statistical results of electric conductivity.
s0 s90 st
Mean (S/m)
S.D. (S/m)
C.V.
4.18 104 4.68 1.03 10 2
0.14 104 0.70 0.55 10 2
0.033 0.15 0.53
S.D.: Standard deviation. C.V.: Coefficient of variation.
larger than that of the electric conductivity in other directions. The highest measured conductivity was 2.1 10 2 S/m and the lowest was 6.07 10 3 S/m. This large scatter indicates that st depends on the chance of fibre contact between plies and that the mean value of electric conductivity may change depending on the fabrication process and stacking sequence. Further experimental research is needed to clarify the variation of the mean value of electric conductivity in the though-thickness direction. 4. Conclusions To investigate the mechanism of electric conductance in the thickness direction of toughened CFRP having a thick resin-rich layer, the electric impedance and phase angle of a toughened CFRP prepreg sheet during the curing process were experimentally measured by using a LCR meter. The temperature distribution due to resistive heating with direct electric current of the specimen surface was measured employing infrared thermography after curing, and a cross-sectional view of a greatly heated point was observed. The following results were obtained. (1) Before curing, the prepreg sheet had no electric conductivity in the thickness direction. After the start of resin flow, electric conductivity in the thickness direction was appeared. (2) Applying directional current between plies showed that resistive heating was not uniform at the surface. An area undergoing great resistive heating showed that the two plies were much less separated than before curing. (3) Fibre contacts between plies due to resin flow during the curing process were the electric contacts between plies. Therefore, the electric conductivity of toughened CFRP is not a material property but depends on resin flow during fabrication. References
Fig. 12. Cross-sectional view of IMS60/133 at the high temperature area observed using thermography.
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