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This paper reports on an investigation of the critical parameters which determine the electrical and electromechanical properties of carbon nanotube (CNT) nanocomposites. For this purpose, a novel analytical model, based on the tunnelling mechanisms of CNTs, is proposed. Three dispersion parameters are introduced in the model to reflect the CNT aggregation state. Microscopy analysis and electrical and strain monitoring tests were carried out on CNT nanocomposites manufactured by toroidal stirring and three roll milling. It is observed that electrical conductivity is greatly affected by dispersion procedure as well as strain sensitivity, measured by the gauge factor (GF). Generally, well dispersed materials have higher conductivities and GF. In this regard, the aggregate ratio has a prevalent effect. Experimental data and theoretical predictions allow the correlation of dispersion parameters given by manufacturing procedures with electrical properties to develop highly sensitive nanocomposites. This demonstrates the potential and applicability of the proposed model.

The capabilities of Digital Light Processed CNT doped nanocomposites for electrothermally triggered selective shape memory have been explored. Here, selective shape memory enables fixing the temporary shape and recovering the original one of different regions of a specimen individually, without affecting the other regions. In this regard, the shape memory capabilities of different matrices based on mixtures of rigid and elastic resins, consisting in hard segments (HS) and soft segments (SS), respectively, doped with CNTs have been investigated. A decrease in the HS:SS ratio leads to a decrease in the glass transition temperature, which constitutes the transformation temperature needed for fixing the temporary shape and recovering the original one. However, the electrical conductivity experiences a sudden drop, reaching values below the percolation threshold at 0:100 samples. Therefore, only the 100:0 and 80:20 samples were valid for shape memory triggered by Joule's effect. Furthermore, selective shape memory tests were conducted by placing different electrodes throughout the material by dividing the specimen in one, two or three individual regions that could be activated selectively. Here, it has been observed that the shape recovery ratio was above 75% in every case, proving the potential of the proposed materials and technique for selective shape memory activation by Joule's heating.

The effect of the addition of carbon nanotubes (CNTs) to adhesive films in composite joints has been studied due to their potential in structural health monitoring (SHM) applications. Carbon nanotube dispersion has been prepared by sonication in an aqueous media using sodium dodecyl sulfate (SDS) as surfactant. A dispersion procedure has been developed to promote a homogeneous dispersion of CNTs within an adhesive. Studies of aqueous dispersions have been carried out by FEG-SEM analysis, proving the disaggregation effect that SDS has on the CNT dispersion and the breakage induced by sonication. By mechanical testing of adhesive joints, it has been observed that the addition of CNTs does not have a significant effect on lap shear strength (LSS), with the best results being achieved in the case of dispersions made with 0.25 wt%. A microstructural study of fracture surfaces shows that the main failure mode is cohesive although in some areas adhesive failure has been observed. In this regard, there is no significant difference between doped and non-doped joints. In addition, electrical conductivity through the bonding line is highly improved, making the adhesive joint electrically conductive, which proves its potential for SHM applications.

Functionalized graphene nanoplatelets networks created through glass fiber fabrics were used to detect and locate damage in multiscale composite materials. The electrical behavior of multiscale composite materials was strongly influenced by microstructural features. Coated fabrics presented high sensitivity to breakage of fibers due to the preferential orientation of f-GNPs through fibers. This sensitivity was higher when damage was induced perpendicular to the fiber direction and the region where damage could be detected was bigger in the case of locating the measuring electrical channels perpendicular to fiber direction. These phenomena are related again to the morphology of the electrical network through the coating. Due to the insulating character through thickness of composites, detection and location was limited to layers of fabric of the composite. Nevertheless, self-sensors had the capacity of detecting and locating damage with high sensitivity by means of abrupt increases in electrical resistance induced by breakage.

The addition of functionalized graphene nanoplatelets (f-GNPs) into the epoxy resin of glass fiber multiscale composite materials creates an electrically conductive network. This electrical network is highly sensitive to strain induced in the material. For this reason, it is a competitive material to be used in structural health monitoring (SHM). Sensitivity of multiscale composite materials is ∼50 under tensile loads and the behavior under compression loads differs, making possible the detection of different load conditions. Independency of the strain rate is related to the enhanced interface between f-GNPs and the epoxy matrix due to functionalization. Additionally, the electrical behavior is reversible without the loss of efficiency in sensorial properties.

Sensing capabilities of carbon nanotube (CNT) epoxy materials have been studied in order to develop multifunctional coatings for damage detection. For that purposes CNT doped epoxy nmixtures have been manufacturing using toroidal stirring. The microstructural characterization of CNT dispersion has been carried out by optical microscopy, showing the good homogenization effect induced by toroidal stirring. Then, electromechanical tests have been carried out on bulk nanocomposites and strain gauges. From tensile tests, it has been noticed that the Gauge Factor (GF) of strain gauges is much higher than those of bulk nanocomposites and conventional metallic gauges. Bending tests, which gauges made over tensile or compressive faces of the samples show that strain gauges have different behaviors while bulk materials show similar ones due to electrical volume interactions, proving the potential of strain gauges for monitoring complex load states. Damage sensing of CNT/epoxy materials has been also proved by inducing artificial defects on a monitored coating as changes in electrical resistance related to damage size. Thus damage detection, location and quantification have been achieved.

Hybrid multi-material joints of titanium alloy (Ti6Al4V) bonded to carbon fibre reinforced polymer (CFRP) are really interesting for lightweight high-performance structures, i.e., aeronautical and marine applications. Co-bonding technique, in which the joining takes place at the same time as the manufacturing of the CFRP, could be a great solution for easy manufacturing fibre metal laminate (FML) and complex structures, or a time saving process. One of the biggest challenges in this type of multi-material structures is to be able to confirm the structural health in service. The present work deals with the use, as matrix, of an epoxy resin reinforced with multiwalled carbon nanotubes (MWCNT) for manufacturing of Ti-CFRP joints by co-bonding. The aim is the structural health monitoring (SHM) of these hybrid structures, enhancing their mechanical properties. The obtained results show an improvement of more than 140% in fracture resistance of the multi-material joints with optimal surface preparation, by incorporating CNT. Furthermore, electrical monitoring makes it possible to discern the quality of the manufactured joints, increasing their reliability and performance. Besides, the ability to localize crack propagation and stresses prior to joint failure has been confirmed, so CNT addition results in a powerful tool for SHM multi-material joints.

The incorporation of graphene nanoplatelets (GNPs) as a coating on glass fibre fabrics for strain monitoring applications was analysed. Non-functionalized and functionalized GNPs were used in order to study the effect of the functionalization in the morphology of the coating and the electrical behaviour of the material. In contrast with non-functionalized GNPs, when NH 2 -functionalized GNPs were used, the nanoparticles adapted to the surface of the fibres causing a major effectiveness of the electrical network created along the fibres, achieving an electrical conductivity in the order of 10 −2 S/m. The sensitivity values, obtained under tensile loads, reached values in the order of 840 up to 16400.

The mechanical and crack sensing capabilities of CFRP-metal Mode-II coupons and skin-stringer T-joints under bending conditions with carbon nanotube (CNT) doped adhesive films are explored. Different surface treatments of the aluminium substrate are tested (grit blasting and sandpaper brushing). Mode-II energy fracture slightly increases with CNT addition due to the nanoparticle toughening effect when the surface treatment is adequate. However, in the case of brushed specimens, the enhanced lubricant properties of CNTs leads to the promotion of weak interfaces between adhesive and metal substrate. The electrical resistance increases with the load nose displacement with a good correspondence to the crack length. A crack opening effect is herein noticed in the last stages of the Mode-II tests due to the difference between the stiffness of metal and CFRP adherents, which is reflected in a sudden increase of the electrical resistance. These results have been also validated by theoretical analysis. Electromechanical response of T-joints shows a similar behaviour with an increasing sensitivity when placing a CFRP stringer, due to a more prevalent crack opening observed in this type of specimens. Therefore, crack sensing capabilities are demonstrated at both coupon and sub-element level.

This paper studies the development and characterization of 3D printed conductive circuits based on epoxy inks doped with carbon nanotubes (CNTs) and/or graphene nanoplatelets (GNPs), which could be used in applications such as electronic circuits, heated seats, Anti-icing and De-Icing Systems (ADIS) or Structural Health Monitoring (SHM) systems, among others. A design of experiments (DOE) has been carried out by the Taguchi method to develop and optimize an ADIS based on a 3D printed circuit, using a conductive ink doped with CNTs/GNPs to achieve Joules heating. The target of the DOE is based on reaching the highest average temperature, which was measured by infrared thermographic analysis, taking also into account the quality of the 3D printed circuits. The DOE factors were the CNT and GNP content, the voltage applied to the circuit and its geometry, proposing two alternative designs. Finally, a de-icing test was carried out with the optimal parameters obtained from the DOE. Results evince the viability of the system developed, being able to melt an ice layer of 2.5 mm thick in 3 min and 30 s.

Many studies were performed to improve CNT dispersion into epoxy using different mechanical dispersion methods as well as CNT functionalization. In this study, a novel method is introduced to enhance CNT dispersion using 2D nanoclay as a secondary filler. Hence, this study was aimed to investigate the effect of nanoclay platelets on electrical, mechanical, and piezoresistive characteristics of the SWCNTs doped epoxy nanocomposites. Two different types of nanocomposites were prepared for comparison including binary (SWCNT/epoxy) and hybrid (SWCNT-nanoclay/epoxy) states. CNT content of 0.1 wt% was used for the binary and hybrid states while two different nanoclay loadings (0.5 wt% and 1.0 wt%) were employed in the hybrid nanocomposites. Tensile and mode I fracture tests were performed for the mechanical and electromechanical characterization using two probe techniques while electron microscopy and X-ray diffraction were used for microstructural analysis. Results showed severe CNT agglomeration in the binary state whilst a homogenous CNT dispersion was achieved in the ternary states. The binary nanocomposite showed weak performance in terms of electrical, mechanical and piezoresistive properties caused by severe CNT aggregates. On the other hand, addition of nanoclay into CNTs doped epoxy manifested a significant increase in the electrical, mechanical and piezoresistive-sensitivity performance of the hybrid nanocomposites compared to the binary one. The best performance was achieved at 0.5 wt% nanoclay loading where electrical conductivity increased by six orders of magnitude, UTS increased by 37%, KIC and GIC increased by 34% and 64%, respectively, with respect to the binary nanocomposite. Crack-pinning and crack deflection were accounted for the fracture toughness increase in ternary composites. Nonlinear piezoresistivity resulting from the predominate effect of tunneling resistance ruled piezoresistivity in the hybrid nanocomposites. A sensitivity of 2.1 and 2.0 at strain of 0.01 were obtained for 0.5 wt% and 1.0 wt% nanoclay contents, respectively, whereas no sensitivity was achieved for the binary composite.

The electrical behaviour and mechanical properties of functionalized graphene nanoplatelets (GNPs) reinforced epoxy-based composites were evaluated. These properties were analysed scaling up from the nanocomposite to the multiscale composite material. The electrical conductivity of 12 wt % GNPs reinforced nanocomposites was in the order of 10 −4 S/m, in contrast to the in-plane conductivity of multiscale composite materials with the same GNPs content, which was found to be ∼10 −3 S/m. This was attributed to microstructural features that also provoke the electrical conductivity through the thickness diminish down to ∼10 −4 S/m in multiscale composites. Additionally, mechanical properties were enhanced by the dispersion of GNP in nanocomposites but experienced a reduction in glass fibre composites because of the formation of a weak interface between the GNPs filled matrix and fibres.

The addition of graphene nanoplatelets (GNPs) into the epoxy adhesives has been studied in order to increase their thermal conductivity. Thermally conductive adhesives are often used as thermal interface materials (TIMs). The incorporation of 8 and 10 wt% GNPs reinforcement caused a thermal conductivity enhancement of ~206 and ~306%, respectively. The wettability seems to decrease with low GNPs content (2–3 wt%) in comparison with the neat epoxy adhesive but the contact angle remains constant for higher GNPs contents. Lap shear strength remains constant for neat adhesives and resins doped with GNPs. The lack of enhancement of adhesive properties of doped resins is due to a weak interface reinforcement-matrix. In fact, the joint failure is in the adhesive except for high GNPs content (10 wt%) where a cohesive failure mode is observed.

Fabrication of multiscale composites by the addition of CNT in epoxy matrices as previous step to the impregnation process leads to several manufacturing difficulties associated to filtration effects and viscosity increments. These effects have been analyzed finding a relation between the type of CNT and the dispersion degree with the change in the rheological behavior and the heterogeneity of the mechanical properties. This heterogeneity is associated to filtration effects of the nanoreinforcement. Modification of the carbon fibers by a nanoreinforced sizing has been presented as an alternative, obtaining homogeneous composites and avoiding the filtration effects. These multiscale composites have higher electrical and interlaminar mechanical properties.

The study carried out analyses the strain sensor capability of nanocomposites reinforced with different graphene nanoplatelets (GNPs) contents. Nanocomposites with GNPs content around the percolation threshold have shown high gauge factor values, around 65 at high strain. The dominant mechanism of the sensors, based on changes induced in the tunnel and contact resistance, was evaluated. Monitored GNP/epoxy based sensors have shown to be capable of discriminating between tensile and flexural strain modes as the configuration of the GNPs during the tests behaves differently. Flexural strains makes possible the creation of new electrically conductive paths meaning an apparent lower sensitivity while strain produced in a tensile test provokes the breakage of the tunnel or contact based conductive paths.

The reversibility and failure detection and localization capabilities of sensors based on graphene nanoplatelets (GNPs) were tested. Results have shown that the strain sensing capability is reversible presenting no hysteresis and has been demonstrated to endure up to 50 cycles. It was elucidated that if a stationary load is applied, normalized electrical resistance increases and remains mainly constant while load is maintained. Once this load has been completely removed, the structure recovers the initial electrical resistance what reinforced the reversibility of the sensor. Additionally, developed GNP/epoxy sensors were demonstrated to detect and locate damage in the sample due to a breakage of the electrical network when failure occurs that could be differentiated from changes due to strain as they caused an abrupt increase.

A new manufacturing process based on applying weak magnetic field during the curing treatment is reported in order to induce a selective mobility and alignment of the carbon nanotubes on epoxy resins. Depending on experimental conditions, this procedure allows manufacturing different materials: (a) anisotropic composites with aligned carbon nanotubes and (b) composites with selected nanoreinforced areas and neat epoxy areas. Different composites have been manufactured, such as dielectric materials with insulating inside and electrically doped surfaces, or doped composites with insulating ring. The procedure basically consists on the anchorage of magnetite nanoparticles in the surface of carbon nanotubes and the application of magnetic field during the curing treatment. The mobility of nanofillers can be directly observed through the appearance of black and transparent areas on the composites, while the alignment of carbon nanotubes is analyzed by scanning electron microscopy of high resolution. The anisotropy and different behavior of selected areas for the manufactured composites have been determined by electrical measurements.

The present study investigates, through experimental and numerical approaches, the incorporation effect of different weight concentrations of multi-walled carbon nanotubes (MWCNTs) on the mechanical properties of epoxy. Tensile and mode I fracture tests were performed to investigate the effect of the addition of MWCNTs on Young’s modulus, Ultimate Tensile Strength (UTS), critical stress intensity factor (KIC) and critical strain energy release rate (GIC). Different carbon nanotubes (CNTs) contents were employed to compare the effect of the resulting microstructures (well-dispersed and agglomerated CNTs) on the mechanical properties. Field Emission Scanning Electron Microscopy (FESEM) and Scanning Electron Microscopy (SEM) were used for microstructural analysis and fractography. Experimental results showed that UTS was improved (28%) by incorporation of MWCNTs while the KIC and GIC were substantially increased by 192% and 614%, respectively. CNT pullout and crack bridging were the main contributing mechanisms in toughening the epoxy at low CNT contents (0.1 and 0.25 wt%). In contrast, a combination of crack bridging and crack branching was responsible for the resin toughening in the case of 0.5 wt% loading. The significant increase for KIC and GIC demonstrated the excellent performance of the dispersion approach used in this study. Finite Element modelling was used to provide a more robust analysis of the effect of CNT incorporation in tension tests and of the toughening mechanism of the nanocomposites in mode-I fracture tests.

Carbon fibers reclamation from waste composites by pyrolysis is developing fast. Pyrolysis allows the reclamation of the fibers by thermal decomposition of the polymeric resin, but the material value of these resins is not yet recovered. In this research work, the possibility of obtaining high-value gaseous fractions by treating the vapors generated in the decomposition of a poly(benzoxazine) containing carbon fiber waste composite has been studied. The experiments have been carried out in a lab-scale facility consisting of two reactors in series. In the first reactor, the pyrolysis of the waste is carried out at 500 °C, while in the second reactor the vapors coming from the first reactor are treated at 900 °C. Such treatment enables to triple the amount of gas in comparison to conventional pyrolysis. Besides, this gas contains 56% by volume of H2, which means the recovery of approximately 60% of the hydrogen contained in the resin. On the other hand, clean carbon fibers with mechanical properties within commercial values have been also obtained.

The influence of dispersion procedure and nanofiller geometry on thermal and electrical properties of graphene nanoplatelet (GNP) based composites has been investigated. A theoretical model, based on the contacts between adjacent nanoparticles, has been proposed aiming to connect thermal and electrical properties. It has been observed that GNP overlapping (type I) induces a decrease on thermal conductivity. Its effect on electrical conductivity is more complex and depends on the areas of overlap and in-plane contacts (type II). A higher type I area in comparison to type II implies an increase of electrical conductivity with overlapping whereas the opposite effect is found when type II area is higher than type I. The predicted results of the theoretical model fitted experimental measurements at different GNP contents and three roll milling processing conditions, giving a better overview of the influence of GNP geometry and interactions on electrical and thermal properties of nanocomposites.

Carbon nanotubes (CNT) were added to epoxy/polycaprolactone (PCL) blends in order to get electrically conductive blends. As it was expected, an increase of CNT content promotes a higher electrical conductivity due to the creation of more electrical pathways. Furthermore, it was observed that the enhancement is also observed with increasing PCL content, reaching values of around 0.2 S/m for 0.2 wt % CNT and 20 wt % PCL samples, due to its effect on rheological properties of the mixture, which affect positively to the CNT dispersion process, thus leading to more homogeneous and well dispersed nanocomposites, as observed by microstructural analysis. Thermal conductivity also increases with CNT content but decreases with PCL addition due to its insulating properties. A study of the voltage required to reach the above-mentioned self-healing temperature for each material has been performed by thermography monitoring. It has been observed that the samples with the highest amount of both CNT and PCL, have the best resistive heating capabilities due to a higher thermoresistive efficiency of the electrical network, reaching temperatures above 100 °C, at voltages below 150 V.

Standard Mode-I and skin-stringer sub-elements were manufactured using novel adhesive films reinforced with carbon nanotubes. Peeling tests were conducted to analyse the different crack propagation mechanisms. In this context, the influence of manufacturing methods and artificial defects is deeply explored. It was observed that the electrical resistance increased with crack length due to a breakage of electrical pathways, depending on manufacturing and induced defects. Co-bonded specimens showed a more stable behaviour due to a better interface between the adhesive and substrate than joints manufactured by secondary bonding. Moreover, by analysing the influence of artificial defects, it was observed that larger discontinuities induced more unstable electromechanical behaviours as there is a more prevalent breakage of electrical pathways. In this regard, samples with Teflon inserts showed sharper increases of electrical resistance than those previously treated with a liquid agent simulating a kissing bond. Therefore, the proposed technique shows a high potential and applicability for Structural Health Monitoring (SHM) of integrated composite structures.