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Sequential dual-cured epoxy composites, based on off-stoichiometric thiol–epoxy mixtures catalysed by 1-methylimidazole, have been developed by adding carbon nanotubes (CNT). The epoxy curing process initially consists in two thermally activated curing stages: a first thiol–epoxy reaction and later homopolymerization at a higher temperature. This system presents easy shaping/conforming and shape memory properties through thermo-mechanical treatments. Addition of the electrical CNT network into the epoxy matrix allows electrical switching, which increases its performance and in-situ applicability. The obtained results confirmed that CNTs catalyse the homopolymerization epoxy reaction, hindering the sequential curing process, due to the π–π anchoring of imidazole catalyser over the CNTs surface, enhanced by donor–acceptor interaction. Non-doped off-stoichiometric resins present relatively high thermal strength, with a glass transition temperature in the range of 73–109 °C, and high stiffness, with a storage modulus close to 2–3 GPa. They can be easily conformed at low temperature, 60 °C, before their second curing stage, showing a high shaping efficiency (around 90%) and full fixing efficiency (>98%). Nanocomposites with 0.2% CNT present efficient Joule heating, triggering the shape memory at low voltage

In this paper, a novel carbon nanotube (CNT) polycaprolactone (PCL), epoxy, and glass fiber (GF) composite is reported. Here, the nanoreinforced composites show a flexural strength increase of around 30%, whereas the interlaminar shear strength increases by 10–15% in comparison to unenhanced samples. This occurs because the addition of the CNTs induces a better PCL/epoxy/GF interaction. Furthermore, the nanoparticles also give novel functionalities to the multiscale composite, such as strain and damage monitoring. Here, the electrical response of the tensile- and compressive-subjected faces was simultaneously measured during flexural tests as well as the transverse conductivity in interlaminar tests, showing an exceptional capability for damage detection. Moreover, it was observed that the electrical sensitivity increases with PCL content due to a higher efficiency of the dispersion process that promotes the creation of a more uniform electrical network.

This work consists of the development of an easy strategy to transform any structure into an efficient surface heater by the application of a low voltage over 3D printed nanocomposite circuits. To this end, the electrical conductivity and self-heating capabilities of UV-Assisted Direct Write 3D printed circuits doped with carbon nanotubes were widely explored as a function of the number of printed layers. Moreover, an optimization of the printing process was carried out by comparing the accuracy and printability obtained when printing with two different configurations: extruding and curing the ink in the same stage or curing the extruded ink in a second stage, after the whole layer was deposited. In this regard, the great homogeneity and repeatability of the heating showed by the four-layer printed circuits, together with their excellent performance for long heating times, proved their applicability to convert any structure to a surface heater. Finally, the deicing capability of the four-layer circuit was demonstrated, being able to remove a 2.5 mm thick ice layer in 4 min and 4 s.

Electrical and electromechanical properties of hybrid graphene nanoplatelet (GNP)/carbon nanotube (CNT)-reinforced composites were analyzed under two different sonication conditions. The electrical conductivity increases with increasing nanofiller content, while the optimum sonication time decreases in a low viscosity media. Therefore, for samples with a higher concentration of GNPs, an increase of sonication time of the hybrid GNP/CNT mixture generally leads to an enhancement of the electrical conductivity, up to values of 3 S/m. This means that the optimum sonication process to achieve the best performances is reached in the longest times. Strain sensing tests show a higher prevalence of GNPs at samples with a high GNP/CNT ratio, reaching gauge factors of around 10, with an exponential behavior of electrical resistance with applied strain, whereas samples with lower GNP/CNT ratio have a more linear response owing to a higher prevalence of CNT tunneling transport mechanisms, with gauge factors of around 3–4.

Co-cured multi-material metal–polymer composites joints are recent interesting structural materials for locally reinforcing a structure in specific areas of high structural requirements, in fibre metal laminates and lightweight high-performance structures. The influence of manufacturing processes on the morphological quality and their mechanical behaviour has been analysed on joints constituted by sol-gel treated Ti6Al4V and carbon fibre reinforced composites (CFRP). In addition, carbon nanotubes (CNT) have been added to an epoxy matrix to develop multiscale CNT reinforced CFRP, increasing their electrical conductivity and allowing their structural health monitoring (SHM). Mechanical behaviour of manufactured multi-material joints is analysed by the measurement of lap shear strength (LSS) and Mode I adhesive fracture energy (GIC) using double cantilever beam specimens (DCB). It has been proven that the addition of MWCNT improves the conductivity of the multi-material joints, even including surface treatment with sol-gel, allowing structural health monitoring (SHM). Moreover, it has been proven that the manufacturing process affects the polymer interface thickness and the porosity, which strongly influence the mechanical and SHM behaviour. On the one hand, the increase in the adhesive layer thickness leads to a great improvement in mode I fracture energy. On the other hand, a lower interface thickness enhances the SHM sensibility due to the proximity between MWCNT and layers of conductive substrates, carbon woven and titanium alloy.

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 use of graphene nanoplatelets for several purposes such as barrier properties and structural health monitoring is widely studied in polymer-based coating technology. Nevertheless, their extremely good effect on wear resistance makes them particularly interesting for coating technology. The effect of graphene nanoplatelets addition on the wear behaviour of an epoxy resin was analysed. The effects of graphene nanoplatelets content (0–8 wt.%), morphology and functionalization in wear behaviour were analysed by evaluating the main wear parameters of the nanocomposites studied. The morphology of the wear surfaces was analysed by scanning electron microscopy (SEM) and 3D optical profilometry in order to evaluate the mechanism and severity of wear. The results showed that the wear behaviour of the epoxy resin improved considerably with the addition of graphene, especially for higher contents. Moreover, as the wear mechanism depends on the morphology of graphene nanoplatelets, it was concluded that the proper selection of graphene can be made depending on the chosen application.

Carbon fiber-reinforced plastic bonded joints with novel carbon nanotube (CNT) adhesive films were manufactured and tested under different aging conditions by varying the surfactant content added to enhance CNT dispersion. Single lap shear (SLS) tests were conducted in their initial state and after 1 and 2 months immersed in distilled water at 60 &deg, C. In addition, their electrical response was measured in terms of the electrical resistance change through thickness. The lap shear strength showed an initial decrease due to plasticization of weak hydrogen bonds, and then a partial recovery due to secondary crosslinking. This plasticization effect was confirmed by differential scanning calorimetry analysis with a decrease in the glass transition temperature. The electrical response varied with aging conditions, showing a higher plasticity region in the 1-month SLS joints, and a sharper increase in the case of the non-aged and 2-month-aged samples, these changes were more prevalent with increasing surfactant content. By adjusting the measured electrical data to simple theoretical calculations, it was possible to establish the first estimation of damage accumulation, which was higher in the case of non-aged and 2-month-aged samples, due to the presence of more prevalent brittle mechanisms for the CNT-doped joints.

Mechanical and strain sensing capabilities of carbon nanotube (CNT) reinforced composites manufactured by digital light processing (DLP) 3D printing technology have been studied. Both CNT content and a post-curing treatment effects have been analyzed. It has been observed that post-curing treatment has a significant influence on mechanical properties, with an increase of Young&rsquo, s modulus and glass transition temperature whereas their effect in electrical properties is not so important. Furthermore, the strain sensing tests show a linear response of electrical resistance with applied strain, with higher values of sensitivity when decreasing CNT content due to a higher interparticle distance. Moreover, the electrical sensitivity of bending tests is significantly lower than in tensile ones due to the compression subjected face effect. Therefore, the good gauge factor values (around 2&ndash, 3) and the high linear response proves the applicability of the proposed nanocomposites in structural health monitoring applications.

This study presents an investigation of the influence of Single-Walled Carbon Nanotubes (SWCNTs) and Double-Walled Carbon Nanotubes (DWCNTs) on tensile and impact strengths of epoxy based nanocomposites. Two different CNTs weight percentages were employed including 0.5 and 0.75 wt.% in order to verify their effects on microstructures and mechanical properties. Scanning Electron Microscopy (SEM) and Field Emission Scanning Electron Microscopy (FESEM) were used to analyze the microstructural characteristics of the CNT doped epoxy including presence of defects such as air bubbles, tiny pores and aggregates. DWCNTs/epoxy nanocomposites presented higher amount of pores due to lack of functionalization, while SWCNTs/epoxy ones showed more entanglement mostly at high CNT loading due to its larger aspect ratio. Results showed that tensile strength was slightly enhanced (6 %) for 0.5 wt.% SWCNT, whereas for other nanocomposites it reduced in comparison with the neat epoxy, due to presence of manufacturing defects in the material. Impact strength significantly improved by 51 % and 31 % at 0.5 wt.% SWCNTs and 0.5 wt.% DWCNTs, respectively, resulting from activation of crack bridging and CNTs pull-out mechanisms. Different behaviours between tensile and impact tests are related to homogeneous stress distribution and localized stress concentration in tensile and impact tests, respectively.

Since the invention of Carbon Nanotubes (CNTs), the self-sensing capability of Multi-walled Carbon Nanotubes (MWCNT) based nanocomposites has been widely analyzed, whilst Single-Walled Carbon Nanotubes (SWCNTs) have been studied less extensively. Moreover, brittleness of epoxy based materials made them vulnerable to crack propagation, necessitating the monitoring of crack initiation and propagation. While research activities on strain monitoring of CNT based epoxy subject to tensile and flexural tests are present in the literature, works on the crack growth sensing capability under fracture toughness tests have not been reported. The present paper was therefore aimed to investigate the piezoresistive characteristics of epoxy-based nanocomposite loaded at different SWCNTs contents i.e. 0.25, 0.5 and 0.75 wt%, under tensile and mode I fracture toughness tests in order to compare and correlate the developed microstructures, including well-dispersed CNTs and aggregates, with the electromechanical properties of the epoxy based nanocomposites. SEM and FESEM analysis were conducted to investigate the fracture surface morphology and the state of the CNTs dispersions. A uniformly dispersed CNTs as well as proper cohesive bonding were obtained at CNTs content of 0.25 and 0.5 wt% whereas CNTs loading of 0.75 wt% resulted in excessive amount of aggregates accompanied with weak CNTs/epoxy bonding. Tunneling effect between neighboring CNTs was the dominant conductive mechanism, responsible for achieving proper sensitivity. For tensile tests, at a relatively low strain (less than 1%), the highest sensitivity was obtained at a SWCNTs content of 0.25 wt%, however showing a nonlinear trend in normalized resistance versus strain, i.e. the sensitivity increased by increasing strain. In fracture test, the SWCNT-doped materials were found to be capable of detecting damage initiation and extension by showing an abrupt increase in normalized electrical resistance upon the onset of crack growth while the piezo-resistivity before failure was either linear or nonlinear depending on the CNTs loading.

Many studies performed on multifunctional properties of epoxy based nanocomposites reinforced with carbon nanotubes (CNTs) and nanoclay (NC) whereas their synergetic effects on piezoresistive behaviour of ternary state nanocomposites still remains unaddressed. Therefore, the hybrid effects of double-walled CNTs (DWCNTs) and NC on the mechanical, electrical and piezoresistive performances of the epoxy were addressed in this study. Nanocomposites were prepared in two different states, i.e. the binary state (DWCNTs/epoxy) and the ternary states (DWCNTs-NC/epoxy). SEM, FESEM, and XRD were used for the microstructural analysis of the materials while tensile and mode I fracture tests were performed for mechanical and piezoresistive characterizations. The addition of NC to CNTs doped epoxy resulted in a better CNT dispersion, hindering CNT re-agglomeration. A significant increase in KIC (94%) and GIC (254%) compared to the neat epoxy was obtained for the hybrid nanocomposites loaded at 1 wt% NC due to crack bridging and crack deflection. The electrical conductivity of the ternary state materials increased by 700% and 400% with respect to the binary nanocomposite, for 0.5 wt% and 1 wt% NC loadings, respectively. The hybrid nanocomposites also manifested higher piezoresistivity and a more robust signal in tensile and fracture tests, respectively.

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.

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.

This work aims to investigate the crack propagation monitoring capacity of carbon nanotube (CNT) reinforced adhesive films in joints with dissimilar materials. To achieve this purpose, Mode-II standard tests have been carried out while the electrical response has been determined in terms on electrical resistance measurements. It has been observed that electrical and mechanical response show a good agreement, with an increase of the electrical resistance with crack length due to the breakage of electrical pathways. Brushed and grit-blasted metallic substrates were tested and some significant differences between electrical response have been observed. Samples with poorer surface treatment showed a more unstable response due to a continuous creation and breakage of electrical pathways while grit-blasted specimens present a uniform electrical resistance increase. These results were also stated by microstructural analysis, showing the presence of weak areas in case of brushed substrates. In addition to that, skin-stringer elements were also tested at static conditions, showing a good electrical continuity. Therefore, the potential and applicability of the proposed technique for SHM of bonded joints have been demonstrated

Electromechanical sensing devices, based on resins doped with carbon nanotubes, were developed by digital light processing (DLP) 3D printing technology in order to increase design freedom and identify new future and innovative applications. The analysis of electromechanical properties was carried out on specific sensors manufactured by DLP 3D printing technology with complex geometries: a spring, a three-column device and a footstep-sensing platform based on the three-column device. All of them show a great sensitivity of the measured electrical resistance to the applied load and high cyclic reproducibility, demonstrating their versatility and applicability to be implemented in numerous items in our daily lives or in industrial devices. Different types of carbon nanotubes—single-walled, double-walled and multi-walled CNTs (SWCNTs, DWCNTs, MWCNTs)—were used to evaluate the effect of their morphology on electrical and electromechanical performance. SWCNT- and DWCNT-doped nanocomposites presented a higher Tg compared with MWCNT-doped nanocomposites due to a lower UV light shielding effect. This phenomenon also justifies the decrease of nanocomposite Tg with the increase of CNT content in every case. The electromechanical analysis reveals that SWCNT- and DWCNT-doped nanocomposites show a higher electromechanical performance than nanocomposites doped with MWCNTs, with a slight increment of strain sensitivity in tensile conditions, but also a significant strain sensitivity gain at bending conditions.

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.

A performance mapping of GNP/epoxy composites was developed according to their electromechanical and electrothermal properties for applications as strain sensors and Joule heaters. To achieve this purpose, a deep theoretical and experimental study of the thermal and electrical conductivity of nanocomposites has been carried out, determining the influence of both nanofiller content and sonication time. Concerning dispersion procedure, at lower contents, higher sonication times induce a decrease of thermal and electrical conductivity due to a more prevalent GNP breakage effect. However, at higher GNP contents, sonication time implies an enhancement of both electrical and thermal properties due to a prevalence of exfoliating mechanisms. Strain monitoring tests indicate that electrical sensitivity increases in an opposite way than electrical conductivity, due to a higher prevalence of tunneling mechanisms, with the 5 wt.% specimens being those with the best results. Moreover, Joule heating tests showed the dominant role of electrical mechanisms on the effectiveness of resistive heating, with the 8 wt.% GNP samples being those with the best capabilities. By taking the different functionalities into account, it can be concluded that 5 wt.% samples with 1 h sonication time are the most balanced for electrothermal applications, as shown in a radar chart.

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.

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.

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.

To date, most analytical models used to calculate electrical conductivity in carbon nanotube (CNT) reinforced nanocomposites are not able to predict electrical properties for contents much higher than the percolation threshold. This is because these models do not take into account many critical factors, such as nanotube waviness, dispersion state and process parameters. In the present paper, a novel analytical model based on an equivalent percolation threshold concept, valid for all CNT contents, is developed for this approach. To achieve this, the influence of all these factors has been investigated and several experimental tests have been conducted in order to validate the model. The electrical conductivity varies by several orders of magnitude depending on the value of these parameters, increasing with carbon nanotube content and aspect ratio and decreasing with its waviness. From experimental data, it is found that the waviness increases with carbon nanotube content. Besides, functionalization also causes a local distortion of CNTs, producing more entanglement. When comparing two different dispersion procedures, calendering and toroidal milling, it is noticed that the first method has a greater stretching effect because the shear forces induced are much higher, causing the breakage of carbon nanotubes.

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.

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.

Mechanical and strain sensing behavior of polycaprolactone (PCL) epoxy blends reinforced with carbon nanotubes (CNTs) has been studied. It was observed that glass transition temperature generally decreases with PCL content while the effect of CNT is more complex, leading to an initial increase and then a sudden decrease when comparing to neat PCL/epoxy samples. It is explained by the increasing miscibility of PCL in the epoxy system, being more significant in presence of properly dispersed nanofillers, promoting a decrease of cross-linking degree. Concerning the mechanical behavior, CNT addition does not have an effective reinforcing effect, as expected, because of the increasing miscibility of PCL. In addition, PCL nanocomposites have a linear electrical response to applied strain, showing a high potential as sensors. Moreover, electrical sensitivity decreases with PCL content due to a higher deformation of PCL in comparison to epoxy matrix, corroborated by in situ microstructural characterization during mechanical tests, leading to a reduction in the tunneling distance between adjacent CNTs.

In this study, the hydrothermal resistance of an epoxy resin (aircraft quality) reinforced with graphene is analyzed. Different geometries and aspect ratios (thickness and lateral dimensions) of graphene nanoplatelets were studied. The addition of these graphene nanoplatelets induces important advantages, such as an increase of the glass transition temperature and stiffness and an enhancement of barrier properties of the epoxy matrix, in spite of the excellent behavior of pristine resin. The effectiveness of graphene nanoplatelets increases with their specific surface area while their dispersion degree is suitable. Thinner nanoplatelets tend to wrinkle, decreasing their efficiency as nanofillers. Graphene used as reinforcement not only reduces the absorbed moisture content but also decreases its effect on the thermal and mechanical properties related to the matrix.

Single lap shear tests are carried out at the joints of similar and dissimilar materials, and their electrical resistance through thickness is monitored to evaluate their sensing capabilities. Al-Al and carbon fibre reinforced polymer (CFRP)-Al joints are manufactured with two types of surface treatments of the Al substrates: brushing (B) and grit blasting (GB). The lap shear strength (LSS) is higher with grit blasting due to the optimum surface roughness and the creation of an active surface. Carbon nanotube (CNT)-doped samples are detrimental to the LSS in Al-Al joints compared to the neat adhesive, while a slight increase in LSS for CFRP-Al and CFRP-CFRP is observed due to a better CNT-CFRP interaction and good load transfer between the substrates. Electrical monitoring tests are strongly influenced by the surface treatment with better adhesive-substrate interfaces leading to a more stable response and higher sensitivity, proving the high applicability of the proposed technique.

In this work the wear behaviour of epoxy matrix composites with different types and percentages of multiwalled carbon nanotubes (MWCNTs) has been studied. Three different types (NC3100, NC3150 and NC3152) and percentages (0.1 and 0.5 wt%) of MWCNTs were dispersed into an epoxy resin by a calendering process. The tribological properties of epoxy-MWCNTs nanocomposites were investigated using “pin-on disc” wear testing machine under different conditions (counterpart material, distances and sliding speeds test). Scanning electron microscopy and 3D optical profilometer were used to observe the worn surfaces of the samples. Compared with neat epoxy, the composites with MWCNTs showed a lower mass loss, friction coefficient and wear rate, and these parameters decreased with the increase of MWCNT percentage. Also, the results demonstrated that the epoxy composites with 0.5 wt% of amino-MWCNT (NC3152) have the best tribological properties.

A novel nanoreinforced adhesive film has been developed to detect adhesive deformation and crack propagation along the bonding line by means of the electrical response of the material. Adhesive films were doped by spraying an aqueous dispersion of carbon nanotubes (CNTs) over the surface. To determine the sensitivity of bonded joints, single lap shear (SLS) and mode-I fracture energy tests have been carried out while their electrical response has been measured. It has been found that CNT-doped adhesive films are able to detect adhesive deformation and final failure for SLS specimens and crack initiation and propagation along the bonding line for mode-I specimens with a high sensitivity. Sudden increases on electrical resistance are correlated to a rapid growing of the crack length due to instability on crack propagation in a tick-slip case, whereas specimens with a more uniform crack propagation are linked to a steadier increase on electrical resistance, and both of them are properly correlated to the mechanical response. By analyzing more in detail the electrical response and comparing with theoretical approaches, the stick-slip behavior is associated with the presence of porosity and lack of adhesives because of possible manufacturing issues such as adhesive overflowing. These statements are also validated by microstructural analysis. Therefore, the potential and applicability of the proposed adhesive films for evaluating the structural integrity has been demonstrated.

Dynamic measurements on carbon nanotube (CNT) multiscale glass fiber reinforced polymers (GFRPs) were performed to assess the usage of self-sensing composite structures in vibration environment. To achieve that purpose, a composite cantilever beam was tested in static and dynamic conditions. Applied strain was measured by means of strain gauges mounted on the tested specimen. Static measurements were performed to investigate linearity and repeatability of the CNT sample response. The dynamic behavior was evaluated with step sine excitation at different forcing frequencies. Preliminary results finally proved the validity and the applicability of carbon nanotubes composites to measure dynamic forcing and vibration.

Electrical measurements of carbon nanotube multiscale GFRPs have been carried out for the monitoring of low velocity impact dynamics. To achieve that purpose, several plates have been fed by a power supply and a high frequency acquisition system has been used. Electrical measurements show that there is an initial decrease of electrical resistance due to plate compression, followed by an increase due to tunneling effect of carbon nanotubes. Finally, the effect of mechanical rebound is correlated to drop rise cycles of the electrical resistance. The sensitivity of the measured signals is also correlated with the impact energy and the electrodes disposition. Thus, the proposed method proves the validity and applicability of carbon nanotubes to characterize the low-velocity impact dynamics of a composite laminate.

Thermal interface materials (TIMs) are extensively used to improve thermal conduction across two mating parts. In this study, the viability of using epoxy resins reinforced with graphene nanoplatelets (GNPs) as TIMs is evaluated. Different GNPs contents are added using a mini-calander to achieve a homogeneous dispersion. The addition of GNPs induces an increase of the glass transition temperature and the storage modulus referenced to the thermosetting matrix. Furthermore, the introduction of high GNPs contents (10 wt%) causes a dramatic increase of the thermal diffusivity (300%) and electrical conductivity (∼10−2 S/m). GNP/epoxy adhesives present enhanced wettability upon aluminum adherends, compared with the strength of joints bonded with neat epoxy adhesives. The introduction of high GNPs contents induces a change of the failure mechanism of joints, from adhesive for neat epoxy resin to cohesive mode.

Sensing capabilities of carbon nanotube (CNT) doped adhesive films under bending conditions were investigated. Standard Mode-II coupons and skin-stringer sub-elements were manufactured while their electrical resistance was monitored. Mode-II energy fracture was improved when adding CNTs in comparison to neat adhesive joints due to their toughening effect. Electrical monitoring showed a stable increase of the electrical resistance with crack length with lower sensitivity than Mode-I testing due to a lower crack opening effect. A good agreement was observed by comparing to theoretical estimations, indicating a uniform adhesive distribution inside the joint, confirmed by SEM analysis . Monitoring tests on skin-stringer sub-elements subjected to bending load showed a similar behavior but some sharp increases were observed, especially in the last stages of the test, due to a less uniform adhesive distribution in the central region. Therefore, the proposed technique shows a high potential and applicability for Structural Health Monitoring of composite structures.

The use of carbon nanostructures for epoxy matrices modification has been widely studied, nevertheless there are several alternative methods for manufacturing that try to avoid difficulties related to their tendency to keep entangled. The use of the calendering approach and high shear mixing alternatives is common for dispersing these nanoreinforcements. The present article compares these two methods as well as possible synergies from the use of the two alternatives together. It has been found that the dispersion technique used modifies the final dispersion level reached as well as on the final properties of the different nanocomposites. Nevertheless, this effect depends on the type of nanoreinforcement (structure and functionalization) and the property measured. Results suggest that each carbon nanostructure requires an individual design of the dispersion stage to get the optimum properties. Thus, the optimum technique may be different depending on the final desired properties, and the dispersion cycle should be designed carefully depending of the final material aim and the nanostructure used. Nevertheless, typical dispersion cycles are currently applied for different type of nanoreinforcements.

The influence of geometry and structure of different commercial multi-walled carbon nanotubes (MWCNTs) have been analysed. MWCNTs have been characterized by transmission and high-resolution scanning electron microscopy, measurement of specific surface area by nitrogen isotherm and X-ray diffraction. The behaviour of carbon nanotube/epoxy composites has been studied by dynamic mechanical thermal analysis, differential scanning calorimetry, measurements of density and electrical conductivity. Composites manufactured at the same experimental conditions and with the same nanofiller content presented different thermal, mechanical and electrical properties. Despite using MWCNTs with similar aspect ratio, the presence of surface defects on the nanotube structure induces an important decrease in storage modulus and electrical conductivity of composites. The functionalization of MWCNTs leads to composites with lower electrical conductivity due to the breaking of sp2 carbon delocalization and also due to the insulating polymer film wrapping the nanotube. In contrast, resin reinforced with long MWCNTs present higher modulus and electrical conductivity than those filled with shorter nanotubes.

Several conventional techniques for nanofiller dispersion (high shear mixing, calendering, or a combination of the two) based on applying shear mechanical forces were studied in order to analyze their efficiency in manufacturing epoxy resin reinforced with graphene nanoplatelets (GNPs). Both provide composites with uniform GNP distributions although the calendering process also induces partial exfoliation of graphene. High-shear mixing could induce the reduction of flakes’ lateral size due to breakage and the rolling of nanoplatelets. Several physical properties of manufactured composites were measured. The density and glass transition temperature increase for composites manufactured by the calendering method due to the steric hindrance of exfoliated graphene nanosheets. Finally, the thermal stability of resin is increased by the addition of GNPs. The degradation temperature is increased up to 20 °C for composites manufactured by the calendering method.

In this work, carbon fiber composites with epoxy matrices modified with different carbon nanotubes (CNTs) contents (0.1, 0.2 and 0.3 wt.%) and with different surface conditions (non-functionalized and amino-functionalized ones) have been manufactured by vacuum assisted resin infusion molding. The dispersion of the CNTs in the epoxy resin has been made using a calendering method. Multiscale composites showed an improved mechanical behavior and this effect was more evident in the case of composites filled with functionalized CNTs, mainly in the case of the mechanical properties dominated by the matrix, such as flexural strength and interlaminar shear strength (ILSS). The formation of a stronger interface between the amino-functionalized CNT and the epoxy matrix favors the stress transfer and the participation of more effective strengthening mechanisms. However, the dispersion of amino-CNTs was less effective and the presence of micro-agglomerates reduced their potential strengthening effect.

The use of a three roll machine to disperse nanoreinforcements in polymer matrix has been proved as an alternative to chemical routes of dispersion. Nevertheless, the degree of dispersion achieved depends on the nanoreinforcement type and the sequences used in the calendering process. This research work focuses in the evaluation of the differences in the dispersion effectiveness of functionalized and non-functionalized carbon nanotubes (CNTs) in an epoxy resin. This aspect has influence not only on the final properties of the nanocomposites but also on the infiltration process due to changes in the rheological behavior and possible filtering of bigger aggregates. Samples of both types of nanoreinforcements up to percentages of 0.5% in weight were characterized using several techniques before and after the curing stage to evaluate variations in the dispersion of the nanoreinforcement and in the final properties of the two types of nanocomposites. The possibility of using CNT/epoxy mixtures as matrix of multiscale composites has been also evaluated fabricating panels by vacuum assisted resin infusion molding (VARIM).

The electrical response of NH2-functionalized graphene nanoplatelets composite materials under strain was studied. Two different manufacturing methods are proposed to create the electrical network in this work: (a) the incorporation of the nanoplatelets into the epoxy matrix and (b) the coating of the glass fabric with a sizing filled with the same nanoplatelets. Both types of multiscale composite materials, with an in-plane electrical conductivity of ~10-3 S/m, showed an exponential growth of the electrical resistance as the strain increases due to distancing between adjacent functionalized graphene nanoplatelets and contact loss between overlying ones. The sensitivity of the materials analyzed during this research, using the described procedures, has been shown to be higher than commercially available strain gauges. The proposed procedures for self-sensing of the structural composite material would facilitate the structural health monitoring of components in difficult to access emplacements such as offshore wind power farms. Although the sensitivity of the multiscale composite materials was considerably higher than the sensitivity of metallic foils used as strain gauges, the value reached with NH2 functionalized graphene nanoplatelets coated fabrics was nearly an order of magnitude superior. This result elucidated their potential to be used as smart fabrics to monitor human movements such as bending of fingers or knees. By using the proposed method, the smart fabric could immediately detect the bending and recover instantly. This fact permits precise monitoring of the time of bending as well as the degree of bending.

Self-heating of conductive nanofilled resins due to the Joule effect is interesting for numerous applications, including computing, self-reparation, self-post-curing treatment of resins, fabrication of adhesive joints, de-icing coatings and so on. In this work, we study the effect of the nature and amount of graphitic nanofiller on the self-heating of epoxy composites. The addition of graphitic nanofillers induced an increase in the thermal conductivity of the epoxy resins, directly proportional to the nanofiller content. Percolation was not observed because of the heat transport through phonons. In contrast, the electrical conductivity curves present a clear percolation threshold, due to the necessity of an electrical percolation network. The electrical threshold is much lower for composites reinforced with carbon nanotubes (CNTs, 0.1 wt.%) than for the resin filled with graphene nanoplatelets (GNPs, 5 %). This fact is due to their very different specific areas. The composites filled with CNTs reach higher temperatures than the ones reinforced with GNPs, applying low electrical voltage because of their higher electrical conductivity. In contrast, the self-heating is more homogeneous for the GNP/epoxy resins due to their higher thermal conductivity. It was also confirmed that the self-heating is repetitive in several cycles, reaching the same temperature when the same voltage is applied.

The interest in structural health monitoring of carbon fiber-reinforced polymers using electrical methods to detect damage in structures is growing because once the material is fabricated the evaluation of strain and damage is simple and feasible. In order to obtain the conductivity, the polymer matrix must be conductive and the use of nanoreinforcement seems to be the most feasible method. In this work, the behavior of nanoreinforced polymer with carbon nanotubes (CNTs) and composites with glass and carbon fibers with nanoreinforced matrices was investigated. These composites were evaluated in tensile tests by simultaneously measuring stress, strain and resistivity. During elastic deformation, a linear increase in resistance was observed and during fracture of the composite fibers, stronger and discontinuous changes in the resistivity were observed. Among other factors, the percentage of nanotubes incorporated in the matrix turned out to be an important factor in the sensitivity of the method.