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An adhesive based on a Fe

Carbon nanoparticle-reinforced adhesive films have been explored as surface sensors for the detection of small strains. It has been observed that graphene nanoplatelets, GNPs, promote a significant increase of the gauge factor when compared to carbon nanotubes, CNTs (5.6 to 0.6, respectively, at low strains), due to their intrinsic 2D nature. The application as surface sensors for the monitoring of the strain field in an aluminum plate has been proven to be successful, with a repeatable signal under consecutive cycles despite some irreversibility in the first one for GNPs. Furthermore, the electrical response given by the sensors under plastic deformation of the aluminum plate was in total agreement with the mechanical response validated by numerical analysis, proving the high potential of the proposed adhesive film for sensing purposes.

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.

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.

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.

A 3D printing system able to print circuits of conductive epoxy resin doped with carbon nanotubes (CNTs) is proposed. Different simple circuits, more specifically lines and strain gauge patterns, made of resins reinforced with 0.3, 0.5, 0.8 and 1 wt% of CNTs were printed on the surface of glass fiber laminates. It was observed that increasing the CNT content reduced the wettability of the printed circuits on the glass fiber substrate. In every case the contact angle was far below 90°. Furthermore, the strain sensing capabilities were analyzed under a flexural load. The results showed that the sensitivity increased with CNT content (with gauge factor values from 1.5 to 2.5) as a result of the prevalent effect of well-dispersed areas due to a reduction in the tunneling distance. On the other hand, the strain gauges showed a lower sensitivity (around 20%–40% less, depending on the condition) compared with line circuits due to localized compressive effects. Furthermore, good repeatability of the strain sensors was proved during cycling tests, with similar baseline and peak values for the electrical resistance in each cycle. Therefore, the proposed materials have a high potential for applications in structural health monitoring.

Graphene nanoplatelet (GNP) doped epoxy/polycaprolactone (PCL) blends have been proposed in the present work. It has been observed that the addition of GNPs induces an enhancement on the electrical conductivity with a prevalent exponential behavior due to the hopping conduction. In this regard, the presence of small insulating domains of PCL leads to an increase of the electrical conductivity due to the reduction of agglomerates. Furthermore, the addition of GNPs allows the thermal heating by Joule's Effect, leading to an autonomous self-healing due to the melting of the PCL. In this context, several samples where subjected to Joule's activated healing, leading to healing efficiencies from 40 to 70% under the presence of an artificial crack. Therefore, the proposed novel materials open a way for autonomous self-healable systems. S0266353821003067

Crack-sensing capabilities of a carbon nanotube (CNT) doped adhesive film have been analysed from coupon level to a composite repaired panel. Mechanical performance is not negatively affected by the addition of CNTs, due to their good distribution inside the adhesive. Moreover, the electrical resistance increases with displacement in both Single Lap Shear (SLS) and Mode-I joints, due to the combined effect of adhesive deformation and crack propagation . More specifically, Mode-I joints usually show a stick–slip behaviour with sharp increases and arrest phases in the electrical resistance, associated to changes in the failure modes. Furthermore, a structural health monitoring (SHM) test of a composite repair was carried out by different techniques such as acoustic emission , strain gauges and CNT adhesive monitoring. From electrical monitoring of the CNT adhesive, it is possible to identify some load thresholds from which the electrical resistance starts to increase. They coincide with sudden changes of the strain field monitored by the strain gauges, as well as, with sudden peaks of the sound level, being an indicative of crack nucleation, adhesive deformation and, finally, crack propagation. Therefore it highlights the great potential and robustness for on-line SHM purpose of the proposed CNT-doped adhesive films.

Tests on a double lap bonded joint, with transverse strips of randomly oriented carbon nanotubes (CNT) sprayed onto an epoxy adhesive film, showed a positive increment in electrical resistance under tensile load, even though the transverse strains were negative. Other experiments included in this work involved placing longitudinal and transversal CNT sensors in a tensile loaded aluminum plate, and, as reported by other authors, the results confirm that the resistance change is not only dependent on the strains oriented with the electrode line, while the other strain components also influence the response. This behavior is quite different to that of conventional strain gages which have a near zero sensitivity to strains not aligned to the sensor direction. The dependence of the electrical response on all the strain components makes it quite difficult, possibly unfeasible, to experimentally determine the individual strain components with this kind of sensors, however, the manufacturing of aligned CNT sensors could deal with this issue.

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.

An investigation on fatigue crack growth monitoring of bonded joints has been conducted employing a novel carbon nanotube (CNT) adhesive film, chosen for their proven exceptional properties for sensing purposes. Single lap shear fatigue tests have been carried out at a 7 kN load, load ratio 0.1 and 10 Hz frequency. A correlation between the electrical response of the adhesive joints and the fatigue crack growth has been established by means of voltage acquisition through the thickness in combination with crack length monitoring by microscopy analysis. Three phases can be distinguished during the fatigue tests: phase one without crack initiation with a steady electrical response; phase two in which cracks start propagating, with a slight increase of the electrical resistance, and phase three, corresponding to final propagation to failure with a drastic increase of the electrical resistance. A correlation between the electrical response, the estimated crack area and the measured edge-crack size is achieved, allowing to get a deeper understanding of crack growth phenomenon as a function of the number of cycles. Thus, the potential and the applicability of the proposed technique for monitoring crack growth in lap joint fatigue tests have been demonstrated.

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.

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.

A multifunctional supercapacitor based on a graphene nanoplatelet (GNP) coated woven carbon fiber (WCF) composite has been manufactured and its electrochemical and mechanical performance has been evaluated. Specific capacitance from voltammetry tests is about three times higher than the non-coated WCFs and several orders of magnitude above neat polymer WCF composites. Furthermore, an electrochemical impedance spectroscopy (EIS) analysis has been carried out in the coated and non-coated WCF capacitors. The equivalent circuit consisted on a series/parallel resistance/constant phase elements. EIS results show that the coated samples have superior capacitor properties, confirmed by chronoamperometry tests. The values of energy and peak power densities were also significantly higher in the coated WCFs, proving higher capabilities as supercapacitors. In addition, mechanical performance of structural supercapacitor is affected by the simultaneous addition of a polymer electrolyte and GNPs, with a reduction of mechanical strength when compared to neat polymer composites. However, and due to the lower viscosity of the electrolyte, there is a higher compaction of the material promoting an increase of WCF volume fraction on the LY-PEGDGE matrix samples, leading to similar values of Young Modulus. Despite the detriment of mechanical properties, they were far above other WCF-based structural supercapacitors. The proof of concept by illuminating a LED was highly successful, proving promising capabilities as structural supercapacitors.

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.

The strain-sensing, self-curing and self-heating capabilities of a 3D printed circuit made of a graphene nanoplatelet and carbon nanotube reinforced resin, have been widely explored. These materials exhibit high Joule’s heating effect capabilities that can be used for post-curing processes. More specifically, the values of glass transition temperature reached by Joule’s heating post-curing were very similar to those obtained by conventional oven heating. The temperature profile along each individual ribbon was relatively homogeneous, being an indicative of a good nanoparticle dispersion, confirmed by field emission gun scanning electron microscope analysis. Furthermore, the proposed printed circuits showed excellent strain-sensing capabilities with a much higher strain sensitivity, with a gauge factor of 6–8, in comparison to conventional metallic gauges or bulk nanocomposites, with a gauge factor of around 2, showing also good linearity. In addition, the breakage of individual ribbons can be easily detected by the strain-sensing system as a sharp increase of the electrical resistance. Finally, temperature compensation tests showed that, in case of printed ribbon breakage, it is possible to keep constant the average temperature of the circuit by raising the applied voltage to avoid ice accretion. Thus, with none to two broken ribbons, the 3D printed circuit can act as an efficient anti-icing and de-icing system.

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.

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.

The fracture behavior and crack sensing of mode-I joints with carbon nanotube (CNT)-reinforced adhesive films were explored in this paper under hydrothermal aging conditions. The measured fracture energy of CNT-reinforced joints in grit blasting conditions is higher for non-aged samples than for neat adhesive joints (around 20%) due to the nanofiller toughening and crack bridging effects. However, in the case of brushed surface-treated adherents, a drastic decrease is observed with the addition of CNTs (around 70%) due to the enhanced tribological properties of the nanofillers. Hydrothermal aging has a greater effect in the CNT-reinforced samples, showing a more prevalent plasticization effect, which is confirmed by the R-curves of the specimens. The effects of surface treatment on the crack propagation properties was observed by electrical resistance monitoring, where brushed samples showed a more unstable electrical response, explained by more unstable crack propagation and reflected by sharp increases of the electrical resistance. Aged specimens showed a very uniform increase of electrical resistance due to slower crack propagation, as induced by the plasticization effect of water. Therefore, the proposed adhesive shows a high applicability for crack detection and propagation without decreasing the mechanical properties.

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.

The effects of hydrothermal ageing on adhesive joints with surface treated aluminium substrates and a novel graphitic nanoparticle reinforced adhesive film have been explored. The adhesive doped with carbon nanotubes (CNTs) shows the highest diffusion coefficient whereas the adhesive film doped with graphene nanoplatelets (GNPs) presents the lowest one. This is explained by the combined effect of both barrier properties of graphitic nanoparticles, which is more prevalent in case of GNPs because of their 2D nature, and the amphiphilic behaviour of the surfactant used during the manufacture of the nanoreinforced adhesive film, which is more significant in case of CNTs because of their higher correlation nanoparticle to surfactant. Glass transition temperature shows a similar trend in every case, with a decrease with ageing time due to plasticization effect of hydrogen bonds. However, the lowest degradation takes place in CNT samples due to a reduction of the free volume induced by a higher porosity, so the plasticization effect is not as prevalent as in GNP or neat adhesive specimens, confirmed by DMTA analysis. Single lap shear (SLS) tests were also conducted using aluminium substrates. The results reveal a higher degradation on mechanical properties in GNP and neat adhesive joints, as expected and a stiffening effect from 15 days to 1 month because of a secondary crosslinking at water saturation. Structural health monitoring (SHM) capabilities were also analysed, indicating a good correspondence between interface properties and electrical measurements, confirming their high potential in SHM applications.

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.