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After successfully demonstrating the spray deposited CNTs as sensing tools for internal damage monitoring in GFRPs, carbon fibre prepregs were used as substrates for in-situ damage sensing test. Due to the conducting nature of carbon fibre which can be used as electrical sensor for fibre breakage, special effort has also been made to emphasis the effect of deposited CNT network for sensing delaminations.

In-situ damage sensing tests are performed under Mode-I loading conditions, using

electrical methods to detect crack propagation and internal damage. Although electrically conductive carbon fibres by themselves could be used as sensor for damage detection, the existence of localized CNTs is expected to improve not only the through-thickness conductivity, but also sensitivity via changing of sensing

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mechanism. During deformation, the percolated CNT networks within the insulating matrix are acting as sensors to detect early stage failure modes such as matrix cracking or delaminations, rather than late stage failure modes such as fibre breakage which could be better directly sensed by the carbon fibres.

To evaluate the effect of the introduction of CNTs into the composites, we compared the electrical property enhancement, as well as the sensing signal stability with different CNT loadings (0.02 and 0.047 wt.% / 0.019 and 0.044 vol.%). The electrical resistance of laminates was reduced from 1.430 Ohm to 0.984 Ohm with the introduction of 0.047 wt.% CNTs. In real applications, a safety threshold is probably required for necessary repair or replacement, which should not vary significantly between different composite laminates or structures. Therefore, the standard deviation of the electrical signals has been compared for the elastic zone among all specimens, in order to better understand the effect of CNTs within the composites during their early stage damage sensing and failure monitoring (Fig. 6.7). The larger the scatter, the less repeatable the sensing signals between different specimens. For each CNT loading, ten specimens are tested and analysed.

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Fig. 6.7 Standard deviation curves for reference, 0.02 wt.% (0.019 vol.%) CNT, and 0.047 wt.% (0.044 vol.%) CNT specimens. The lower the standard deviation value, the more stable the sensing signals. Illustrations on right show the mechanism of electrical sensing.

For the complete elastic deformation zone with various loadings, the displacement is plotted up to 12.5 mm for the sensing signal variation analysis (Fig. 6.7). The average displacement of all specimens became non-linear between 10 and 12 mm. In Fig. 6.7, at a displacement of 10 mm, the sensing signal variation for the reference specimen is about four times compared to that of specimens with CNTs introduced. When the specimen becomes non-linear, the slopes of the variation curve between three concentrations are obviously different. Without CNTs, the standard deviation curve starts to increase dramatically, indicating a large signal variation between different specimens. However, with 0.05 wt.% CNT loaded specimen, the value was only one fourth of the reference, confirming significant improvement in repeatability and stability of the tested electrical sensing signals among different specimens.

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This stability improvement is due to the introduction of the spray coated percolated CNT network within the insulating epoxy matrix between conducting carbon fibre fabric plies (as illustrated in Fig. 6.7 inset). During initial elastic deformation, resin rich regions clearly do not contribute to the electrical sensing signals due to their insulating nature. It is well known that CFRPs can be directly used for electrical damage sensing [20], especially with respect to surface resistance changes under flexural loadings [71]. The mechanism is based on the change of degrees of current penetration upon tension or compression [20]. Other damage sensing studies on CFRP were mainly performed in fibre direction, using carbon fibre breakage induced resistance changes [72-74]. For the reference specimen, through-thickness electrical signals are therefore completely attributed to the conductive carbon fibre fabrics and their physical contacts, which results in large sensing signal variability. However, with spray coated CNT networks in between the plies, sensitive CNT networks are introduced in these resin rich regions and reduce the reliance on local physical contacts between carbon fibres, significantly improving the stability and consistence of the sensing signals, allowing them to be used for strain or damage detection, even at small deformations, i.e. elastic deformation.

The in-situ damage sensing tests were established using the DCB tests. In Fig. 6.8a, a typical load and displacement curve of a 0.047 wt.% CNT coated specimen is shown. The applied load is increasing linearly to a maximum, followed by a sudden load drop as the crack propagates in a rather unstable manner. This unstable fracture is typical for carbon fabric/epoxy laminates, exhibiting fibre rich and resin rich regions, leading to differences in local toughness and consequently unstable crack propagation. After crack initiation, the force builds up again, until the crack propagates which results in a load drop. This process is carried out until the crack

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length of the specimen reaches a certain value. The load-displacement curves can also been analysed to obtain the exact displacement value for each load drop, to be used for further analysis of the damage sensing signals.

Fig. 6.8 In-situ damage sensing graph of 0.047 wt.% (0.044 vol.%) CNT laminates: (a) Load-displacement curve (black) with electrical resistance (red) in absolute

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values; (b) Load-displacement curve (black) with electrical resistance (red) in relative percentage change.

As the crack opens, the overall trend of electrical resistance is increasing, which is due to the longer distance for the electrons to “pass-through” the sample. Each load drop is associated with a sudden increase in resistance and indicates a breakdown of the CNT conductive network due to crack propagation. It can be clearly seen that with each load drop, the electrical resistance increases from a starting level to a higher level, which is maintained until the next load drop. Importantly, the increment of the electrical resistance signal attributed to crack initiation and propagation can be correlated to each associated load drop.

Instead of plotting measured volumetric resistance as sensing signals, the resistance change over the original resistance (ΔR/Ro) in percentage is also plotted (Fig. 6.8b),

in order to overcome variability between specimens. This way, similar resistance changes are observed for different specimens, confirming the improved stability and consistency of the sensing signals for the laminates with deposited CNTs.

To develop a further understanding of the relationship between measured electrical resistance and internal damage of the specimen, the derivative of the force curves is plotted. From each negative value on these derivative curves, the exact point of cracking can be identified and with this, the resistance change and force change at those locations can be calculated (Fig. 6.9). Only data points with a force change above 0.25 N were collected in order to minimize instrumentation error. Apart from the correlation of force change and resistance which is shown in Fig. 6.9a, the relative change of those two parameters was also plotted as well (Fig. 6.9b).

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Fig. 6.9 Correlation between applied load and measured resistance of 0.047 wt.% (0.044 vol.%) CNT specimen: (a) in absolute value; (b) in relative percentage change (normalized value).

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Fig. 6.9 shows the relationship between force and resistance change at the same location throughout the experiment. The trend of these two parameters matches exactly, especially for the normalized value (Fig. 6.9b), confirming a correlation between the load change and the electrical resistance change as a result of damage. Most of the internal damage can be quantified through the measured change in resistance. Although some data points are not completely consistent with the general trend, this is probably due to the complex CNT network within the composites. For instance, certain connected pathways in a CNT network are weaker than others, leading to a significant resistance change at relatively small load changes. In fact, according to previous studies on CNT based damage sensing networks [156-158], even more diluted networks could lead here to greater sensitivity with the ability to detect even smaller internal damage such as microscopic matrix cracks and/or interfacial debonds. Although in real application, optimum CNT amounts should be employed to provide not only percolated but more sensitive network for internal damage monitoring.