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The shape memory (SM) capabilities of nanocomposites based on two photocurable acrylated/methacrylated resins, doped with carbon nanotubes (CNTs), and manufactured by digital light processing 3D printing were investigated. The mechanical properties and glass transition temperature (Tg) can be tailored in a broad range by varying the weight ratio of the two resins (Tg ranging from 15 to 190 °C; Young’s modulus from 1.5 to 2500 MPa). Shape fixity (SF) and recovery (SR) ratios are strongly influenced by the temperature being significantly higher at temperatures close to the Tg. The results confirm that the SF strongly depends on the stiffness of chain segments between cross-linking points, whereas the SR mainly depends on the cross-link density of the network. CNT addition barely affects the SF and SR in the conventional oven, whereas the recovery speed using IR heating is significantly increased for the doped nanocomposites due to their higher IR absorbance.

The importance of proper CNT dispersion is still the main challenge in CNTs doped epoxy nanocomposites. Therefore, this study was aimed to investigate the effect of toroidal stirring-assisted sonic...

Self-heating GNPs-based electrical networks were successfully obtained for their use in electrothermal applications. The electrical resistance of the GNPs-networks created on glass fiber fabrics was strongly dependent on fiber direction. The electrothermal response was fast and the maximum temperature was achieved in the system after ∼20 s. Increments in temperature above 80°C were obtained at the surface of the coated glass fiber fabric at relatively low intensity currents. Cyclic self-heating did not cause appreciable diminution in performance. Additionally, their potential application in evaluation of the quality of dispersion of the nanoreinforcement was demonstrated, as regions with lower contents in GNPs showed higher temperature due to weak links between GNPs located in highly conductive paths forming the electrical network.

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.

This paper presents a process where carbon fibers and hydrogen can be recovered simultaneously through a two-stage thermal treatment of an epoxy-carbon fiber composite. For this purpose, some pieces of epoxy resin reinforced with carbon fiber fabrics have been fabricated and, after curing, have been pyrolyzed in an installation consisting of two reactors. In the first one, the thermal decomposition of the resin takes place, and in the second one, the gases and vapors coming from the first reactor are thermally treated. Once this process is completed, the solid generated is oxidized with air to eliminate the resin residues and carbonaceous products from the fibers surface. The recovered carbon fiber fabrics have been reused to make new cured parts and their electrical and mechanical properties have been measured. The results show that it is possible to obtain carbon fiber fabrics that can be processed as they leave the recycling process and that retain 80% of the tensile modulus, 70% of the flexural strength, and 50% of the interlaminar shear strength. At the same time, a gaseous stream with more than 66% by volume of hydrogen can be obtained, reaching a maximum of 81.7%. This research was funded by the Ministry of Science and Innovation of the Spanish Government through the project with reference PID2019-110770RB-I00 and by the Basque Government through the project with reference KK-2020/00107 (ELKARTEK program). Besides, the Basque Government also contributed to this work by means of the regular funding granted to consolidated research teams (IT993-16) and the researcher training grant awarded to Naia Gastelu.

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

Smart coatings based on polymer matrix doped with carbon nanoparticles, such as carbon nanotubes or graphene, are being widely studied. The addition of carbon nanofillers into organic coatings usually enhances their performance, increasing their barrier properties, corrosion resistance, hardness, and wear strength. Moreover, the developed composites provide a new generation of protective organic coatings, being able to intelligently respond to damage or external stimuli. Carbon nanoparticles induce new functionalities to polymer coatings, most of them related to the higher electrical conductivity of nanocomposite due to the formation of percolation network. These coatings can be used as strain sensors and gauges, based on the variation of their electrical resistance (structural health monitoring, SHM). In addition, they act as self-heaters by the application of electrical voltage associated to resistive heating by Joule effect. This opens new potential applications, particularly deicing and defogging coatings. Superhydrophobic and self-cleaning coatings are inspired from lotus effect, designing micro- and nanoscaled hierarchical surfaces. Coatings with self-healable polymer matrix are able to repair surface damages. Other relevant smart capabilities of these new coatings are flame retardant, lubricating, stimuli-chromism, and antibacterial activity, among others.

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 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.