To utilize the effect of conductive CNT networks, and eliminate other conducting effect from system such as carbon fibre, the insulating glass fibre was first used as substrate for in-situ damage sensing works. Interlaminar fracture toughness and interlaminar shear strength tests were performed together with in-situ electrical measurement, and obtained damage sensing properties are presented and analysed in this section.
Interlaminar fracture toughness test
Surprisingly, at extremely low CNT loadings (0.012 wt.% / 0.013 vol.%), the GF/CNT composites showed already percolation with detectable electrical signals, indicating excellent network formation. This extremely low CNT loading was
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chosen due to our previous sensing experience based on percolated networks. Microscopy studies showed that the glass fibre surface was not completely covered by deposited CNTs, indicating potential scope for improved sensing results. The obtained electrical conductivity is believed to be not only attributed to an initial dispersion of CNTs onto the fibre surfaces, but also due to a “dynamic percolation” process [156, 213] to take place after the infusion process but before complete cross- linking of the epoxy resin.
Fig. 6.4a shows the damage sensing results together with a typical load-displacement curve from the DCB test. The successive drops in the load are due to crack propagation. The simultaneously measured resistivity increases with clear steps which directly correlate to the load drops and is due to the percolated CNT network being affected by crack propagation. This correlation confirms that the spray-coated conductive CNT network on the glass fibre fabrics can be used as a sensing tool to detect damage within the composite specimens during mechanical deformation. To further confirm the correlation between mechanical performance and electrical sensing signals, the relative force- and resistivity change (in percentage) is plotted (Fig. 6.4b). The relative change of force and resistance were taken from data separately, plotting with the displacement. Excellent correlation between these two signals is observed, indicating the reliability of using this technique to detect delamination damage in composite laminates.
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Fig. 6.4 In-situ damage sensing data for 0.012 wt.% (0.013 vol.%) CNT loaded GFRPs: (a) Normalized measured volume resistance change of the specimen during the DCB test, accompanied with L-D curve; (b) Correlation between resistance change and force change in normalized values.
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In order to better cover the interfacial regions within composites and provide better sensing properties, higher amounts of CNTs were deposited via the same procedures onto glass fibre substrates. 10 mg instead of 4 mg was deposited on each plies, and overall CNT loading was increased to 0.032 wt.% (0.034 vol.%). As shown in Fig. 6.5, unlike previous specimens with extremely low CNT loadings for which the electrical signals were rather unstable, here a much more stable electrical sensing signal is obtained as seen in the normalized graph for the higher amount of CNTs. Furthermore, with each force drop in the loading curve, a clear resistance jump can be seen from the sensing signals. The correlation between force change and resistance change was also presented in terms of a normalized percentage value. Good correlation was found between these two parameters, together with an increased range of changing values. This has led to a more obvious sensing signal when a crack was propagated. It is worth to mention that the measured electrical resistance was also significantly reduced by three orders of magnitude (from 1.097×107 to 8.409×104) between these two CNT loadings.
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Fig. 6.5 In-situ damage sensing results of 0.032 wt.% (0.034 vol.%) CNT deposited GFRPs.
This improvement in sensing signals was attributed to an improved percolating CNT network, which covered more interfacial regions and distributed in adjacent epoxy
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matrices. During the test, the crack as well as deformation of the specimen was affecting the CNT network, leading to changes in pathways for electrons which reflected as a change in measured resistance.
Interlaminar shear strength test
The in-situ damage sensing work was also performed for the SBS test to examine its effectiveness under flexure loadings. Both in-plane and through-thickness direction was used to apply electric field for sensing test; unfortunately, no electrical signals were obtained from through-thickness specimens. This was believed to be due to two main reasons: i) the relatively low loading of CNTs within composites (0.072 wt.% / 0.076 vol.%), and ii) the resin rich regions at the specimen surface which has a higher electrical resistance. Hence, only in-plane sensing results were successfully obtained and employed to analyse the internal damage of composites during the SBS test.
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Fig. 6.6 Damage sensing results for ILSS test: (a) load curve with sensing results; (b) zoom in graph at the failure point.
In Fig. 6.6, the load curves together with normalized electrical sensing signals are presented. For SBS tests, the force starts to build up at the beginning of the test, until
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interlaminar shear failure occurs which led to an obvious drop in force before the end of the test. Clearly, at the beginning of the test, the sensing signal was not much affected by the applied load but maintained at its starting level. It is worth to note that the electrical resistance was actually slightly reduced with increased loading, which was due to the flexural deformation which partly compresses the specimen and slightly reduces the gap between CNTs within the composites. This finding was consistent with other researcher’s results [172, 245]. When interlaminar shear failure takes place, the structure of the internal percolated CNT network is severely affected, leading to a dramatic change in sensing signals. From the enlarged graph at failure point (Fig. 6.6b), it can be seen very clearly that the recorded sensing signals correlate well with the applied load, and a highest relative resistance change was due to the final failure of the specimen.