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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

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...

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 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

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.

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

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

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

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

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.

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

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

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

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

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

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

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

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

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

Carbon 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 electromechanical performance and reversibility of sensitive GNP/epoxy strain sensors were experimentally and theoretically analyzed. Under tensile loads, the strain sensors showed lower sensitivity and more linearity than bulk sensors, behavior attributed to a slight preferential orientation of the GNPs along the in-plane direction. The Gauge Factor (GF) obtained was 9.1 ± 0.9 and 11 ± 1 for strain values up to 0.005 mm mm−1 and above 0.015 mm mm−1, respectively. In contrast, the electromechanical response when subjected to compressive strain is more complex and three different regions are distinguished: (I) diminution, (II) stabilization and (III) increase of the normalized electrical resistance. Here, GF under compressive loads was negative at low strain values (region I), being −13 ± 2, and positive at high strain (region III), with a value of 8 ± 1. Theoretical analysis revealed that at low strain, there is prevalence of in-plane tunneling mechanisms whereas at higher strain, the out of plane mechanisms dominate, explaining the apparently anomalous behavior at compressive loads. Additionally, strain sensors showed high reversibility with cyclic load in the electromechanical response, but under compressive forces, the loading-unloading electrical resistance curve was asymmetric due to the opening and closing of microcavities and defects in the vicinities of the GNPs.

Polycaprolactone (PCL) is being researched as a self-healing agent blended with epoxy resins by several reasons: low melting point, differential expansive bleeding (DBE) of PCL, and reaction induced phase separation (RIPS) of PCL/epoxy blends. In this work, PCL/epoxy blends were prepared with different PCL ratios and two different epoxy networks, cured with aliphatic and aromatic amine hardeners. The curing kinetic affects to the blend morphology, varying its critical composition. The self-healing behavior is strongly affected by the blend morphology, reaching the maximum efficiency for co-continuous phases. Blends with dispersed PCL phase into epoxy matrix can also show high self-healing efficiency because of the low PCL domains that act as reservoir of self-healing agent. In this last case, it was confirmed that the most efficient self-healable blends are one whose area occupied by PCL phase is the largest. These blends remain the good thermal and mechanical behavior of epoxy matrix, in contrast to the worsened properties of blends with bicontinuous morphology. In this work, the self-healing mechanism of blends is studied in depth by scanning electron microscopy. Furthermore, the influence of the geometry of the initial surface damage is also evaluated, affecting to the measurement of self-healing efficiency.

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 the present study, a numerical model was built using LS-DYNA® to investigate the static and dynamic fracture behaviours of neat epoxy. Fracture and Izod tests were conducted experimentally for validation. The applicability of different numerical methods, i.e., finite element method and meshfree methods (element-free Galerkin, and smoothed particle Galerkin), was investigated considering different loading conditions (static and dynamic). When compared with the experimental data, the meshfree methods were found to replicate the dynamic fracture behaviour more precisely, while all the methods provided comparable results under static loading conditions. Besides, to foster the development of more accurate numerical methods focussed on the fracture mechanism in epoxy resin, the effects of pre-crack length and strain rate on modelling were investigated by exploiting the results from the fracture tests and Izod tests, respectively. The results showed that the pre-crack length had a strong influence on the fracture toughness obtained, while incorporating the strain rate effect in a material model noticeably influenced the damage caused by the impactor and the final value of energy absorbed.

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.

A structural analysis demonstrating how the manufacturing method of graphene nanoplatelets (GNPs) into a polymer matrix can strongly modify the GNPs morphology and, consequently, their properties, was carried out. Three different methods based on sonication and high shear forces were used to elucidate defects induction and possible size diminution. Manufacturing methods including high shear forces caused the extension of the GNPs while sonication induces wrinkling of the sheets. Residual stresses are induced in the nanoplatelets structure showing an increase in the Raman intensities ratios ID/IG and ID′/IG when a major cycles number of calendering are applied.

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.

Graphene nanoplatelets (GNPs), in different contents, are added to epoxy resin. This work mainly consists of a deep characterization of the composites in order to evaluate their behavior regarding neat epoxy resin. In fact, their main properties, such as their morphological features, their thermo-mechanical and mechanical properties, their electrical conductivity and thermal diffusivity, their hydrophobic behavior and their barrier properties have been studied. It is confirmed that the GNP addition induces an important stiffing of thermosetting resin. However, the rest of mechanical properties, strength and elongation, are diminished due to a weak interface formed between the epoxy matrix and non-functionalized GNPs. The electrical conductivity and thermal diffusivity show an important increase. Electrical conductivity increases several orders of magnitude from a minimum value of percolation while the thermal diffusivity increases proportionally to GNP content. Other advantage of the GNP addition is the increase of the hydrophobic behavior of the composites, determined by measurements of contact angle of water drops. Finally, GNP/epoxy composites present higher barrier properties in humid environments. The addition of GNP decreases the maximum water content absorbed and the diffusion coefficient. However, this enhancement is not too large due to the weak interface and therefore the presence of hollows in the GNP/epoxy composites.

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.

Graphene nanoplatelets/epoxy nanocomposites were prepared using a high shear toroidal mixer as dispersion technique. Suitable dispersions were obtained. Several graphene nanoplatelets, with different thickness and lateral dimensions, were added in order to analyze the influence of these parameters in the final properties. An important nanofiller concentration gradient was found from the top to the bottom in nanocomposites reinforced with large nanoplatelets due to a natural deposition by gravity. This phenomenon is not appreciable when the nanoplatelets size decreased. However, the small nanoplatelets have a greater tendency to agglomerate in packages of several parallel particles. In general, graphene nanoplatelets addition caused an increment in glass transition temperature, stiffness and thermal stability compared to the epoxy resin. However, it was also found that graphene nanoplatelets dimensions significantly affect to these enhancements. Nanocomposites reinforced with larger and thicker nanoplatelets presented lower glass transition temperature, higher modulus and higher decomposition temperature.

In recent years, the interest in monitoring the damage of composite materials by measuring the variation of electrical resistance is increasing because deformations and cracks generated in the structure produce changes in the electrical conductivity of the material. In this work, the structural integrity of glass fiber composite materials with epoxy matrix doped with graphene nanoparticles is evaluated under in plane tests (interlaminar shear tests and fracture propagation tests in mode I and II). The results demonstrated the ability of graphene nanoparticles to form conductive networks in the epoxy resin with auto-detection capability of deformation and damage. In the interlaminar shear tests, permanent changes in resistance were associated to strain and delamination produced during the test. In the case of fracture tests in mode I and II, the electrical resistance increased as the crack grew. The use of multiple electrodes has also allowed locating the area of damage generation in the material. Finally, the electrical response of discontinuities generated out of plane of the laminate was analyzed by means of cuts through the material.

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 use of mechanical processes, such as calendering, is an interesting method to disperse carbon nanofibres because this technique avoids the use of organic solvents needed in chemical methods. Variations of the process are used by researchers, and further study of the effect of the calendering parameters is still needed. For this reason, the effectiveness of the dispersion of several percentages of carbon nanofibres (0.5–3 wt.%) in epoxy resin using different parameters of the calendering process has been studied in this work. Carbon nanofibres/epoxy nanocomposites and non-cured nanoreinforced mixtures have been characterised. This study showed that the dispersion was enhanced with a Sequential method. Density and thermomechanical properties of nanocomposites manufactured by the selected calendering method were evaluated. The viscosity of the CNF/epoxy mixtures at different percentages of nanoreinforcement was adequate to use them as a matrix in multiscale reinforced composites.

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.

The oxidation behavior of a metal matrix composite based on a high-speed steel (HSS, M2 grade) as a matrix and 50 vol% of TiCN particles as reinforcement has been studied, as well as the M2 steel as a reference. The oxidation behavior of the materials were tested at 500 °C and 800 °C in static laboratory air. The values of the mass gain and the examination of the oxide scales by scanning electron microscopy and X-ray diffraction demonstrated that the composite material remains stable under oxidizing conditions to higher temperatures than the base steel. The wear tests were carried out at room temperature under dry sliding wear condition by the pin-on-disk technique, under two different loads (5 N and 10 N). The mass loss of the samples were measured after different sliding distances (up to 8000 m), and the wear tracks were examined by SEM. It was found that the predominant wear mechanism is abrasion for both systems, and that the interaction between TiCN particles and metal matrix leads to improve the wear resistance of composite material in comparison with the high speed steel.