Mechanical and electrical response of carbon nanotube-based fabric composites to Hopkinson bar loading

The numerous structural applications of composites, coupled with their complex, rate-dependent mechanical behavior necessitate research into their mechanical response under dynamic loading scenarios. While the damage mechanisms of composites under dynamic compression loading are well-understood, measuring the occurrence of damage in a non-invasive manner is challenging. Toward this end, we investigate the electrical response of an embedded percolating carbon nanotube network in woven fabric/epoxy composites to dynamic compression loading. The percolating network is established through the use of a non-uniform dispersion of carbon nanotubes, achieved using a fiber sizing agent. The resulting conductive network is sensitive to delamination and damage occurring near the fiber surfaces. The dynamic mechanical response of the composite specimens is explored using Hopkinson bar methodology. Definite increases in baseline resistance of the conductive composite specimens are seen after repeated impacts demonstrating the ability of the carbon nanotube network of these conductively modified composites to respond electrically to damage induced during dynamic loading.     

Processing and electrical properties of carbon nanotube/vinyl ester nanocomposites

Vinyl ester resins are often utilized in advanced naval composite structures due to the relatively low viscosity of the resin and the capability to cure at ambient temperatures. These qualities facilitate the production of large naval composite structures using resin infusion techniques. Vinyl ester monomer was synthesized from the epoxy resin to overcome processing challenges associated with volatility of the styrene monomer in vinyl ester resin. In this research we have investigated the use of a calendering approach for dispersion of multi-walled carbon nanotubes in vinyl ester monomer, and the subsequent processing of nanotube/vinyl ester composites. The high aspect ratios of the carbon nanotubes were preserved during processing and enabled the formation of a conductive percolating network at low nanotube concentrations. An electrical percolation threshold below 0.1 wt.% carbon nanotubes in vinyl ester was observed. Formation of percolating carbon nanotube networks at low concentration holds promise for the utilization of carbon nanotubes as in situ sensors for detecting deformation and damage in advanced naval composites.     

Delamination detection with carbon nanotube thread in self-sensing composite materials

Laminated composite materials can reach high mechanical properties at low weight. Composite materials, however, are susceptible to damage due to their low interlaminar mechanical properties and poor heat and charge transport in the transverse direction to the laminate. Moreover, methods to inspect and ensure the reliability of composites are expensive and labor intensive. Recently carbon nanotube forests were spun into thread that is tough and electrically conductive. The thread was integrated into composite materials and used for the first time as a sensor to monitor strains and detect damage including delamination in the material. These self-sensing composites were found to be very sensitive to damage and will help to revolutionize the maintenance of composite structures, which will now be based on their condition and not their amount of use.     

Dispersion control and characterization in multiwalled carbon nanotube and phenylethynyl-terminated imide composites

The inherent multifunctional properties of carbon nanotubes provide an opportunity to create novel composites, but their dispersion into a polymer matrix is challenging due to nanotube dimensions, interparticle forces, and poor interaction with the polymer. In this study, we used melt mixing to disperse multiwalled carbon nanotubes (MWNTs) in a polyimide resin under various process conditions to understand the efficacy of the process and the energy required to achieve dispersion and distribution. Through controlled variation of process conditions, we achieved various degrees of nanotube dispersion and distribution. The different dispersion and distribution states were observed by microscopy and correlated with the magnitude of the changes seen in the glass transition temperature and viscosity when compared to the neat resin. The results of these studies will be used to assess the compatibility of nanocomposite resins with composite fabrication methods and predict appropriate processing conditions for producing multiscale composites.     

Damage mode characterization of mechanically fastened composite joints using carbon nanotube networks

The practice of upgrading metal parts with composites in large structures has led to an increased use of composite joints, particularly mechanical fastenings, due to ease of assembly and replacement. A drawback of mechanical joints is that damage is difficult to detect visually. In this research, an embedded carbon nanotube network has been used to modify the conductivity of bolted composite joints. In situ electrical resistance measurements in conductive composites have potential to provide quantitative evidence of damage as well as insight into the type of damage which occurs during tensile loading. We demonstrate that the electrical resistance of a bolted composite joint is more sensitive to certain modes of damage (e.g., matrix cracking and delamination) than others (bearing and shear-out), due mostly to the varying amount of void space created, thus proving the potential of an embedded carbon nanotube network in the health monitoring of mechanically fastened cross-ply composites.     

碳化硅颗粒填充的碳纳米管/环氧树脂复合材料的吸波性能

以碳纳米管、碳化硅颗粒为原料制备环氧树脂复合吸波材料,并对其吸波性能进行测试,研究了碳纳米管、碳化硅颗粒含量与复合材料吸波性能的关系.结果表明碳纳米管、碳化硅颗粒的含量对复合材料的吸波性能有较大影响.随碳纳米管含量的增加,碳纳米管/环氧树脂复合材料的吸波性能先提高后降低,碳纳米管含量存在最佳值(12%,质量分数).将碳...     

Carbon nanotube-based polymer composites: A trade-off between manufacturing cost and mechanical performance

A methodology was devised to evaluate the newly-developed carbon nanotube reinforced polymer composites by means of mechanical performance and manufacturing cost. Glass fibre reinforced-epoxy composite plates were produced having different parameters: (a) three manufacturing processes, (b) geometrical dimensions, (c) carbon nanotubes concentration in the epoxy resin and finally (d) modified resin infusion temperature. Tensile coupons were machined out of the manufactured plates and their quasi-static mechanical properties were evaluated. Three cost models were developed to assess plates and tensile coupons manufacturing cost for each different case. Optimal values were evaluated for major manufacturing parameters, driving force being the mechanical properties of interest (quality) as well as their low manufacturing cost. It is demonstrated that the added cost to manufacture such nano-reinforced composites is attributed to increase strength on the expense of ductility; the main benefit of the carbon nanotube-based polymer composites seems to be their ability to be monitored. Almost 20% added cost is paid to attain this new function of piezo-resistivity for the RTM process, while this amount further increases for the non-automated processes such as the Hand Lay-up.     

Effects of carbon nanotubes and carbon fiber reinforcements on thermal conductivity and ablation properties of carbon/phenolic composites

The ablation properties and thermal conductivity of carbon nanotube (CNT) and carbon fiber (CF)/phenolic composites were evaluated for different filler types and structures. It was found that the mechanical and thermal properties of phenolic-polymer matrix composites were improved significantly by the addition of carbon materials as reinforcement. The concentrations of CF and CNT reinforcing materials used in this study were 30 vol% and 0.5 wt%, respectively. The thermal conductivity and thermal diffusion of the different composites were observed during ablation testing, using an oxygen–kerosene (1:1) flame torch. The thermal conductivity of CF mat/phenolic composites was higher than that of random CF/phenolic composites. Both CF mat and CNT/phenolic composites exhibited much better thermal conductivity and ablation properties than did neat phenolic resin. The more conductive carbon materials significantly enhanced the heat conduction and dissipation from the flame location, thereby minimizing local thermal damage.     

A facile method for preparing CNT-grafted carbon fibers and improved tensile strength of their composites

Carbon nanotube (CNT)-grafted carbon fibers (CFs) have emerged as new reinforcements for improving the mechanical properties of CF-reinforced composites but such enhancement in macroscale composites has not been realized. This paper reports a facile method for preparing CNT-grafted CFs and improving the tensile strength of their composites. A CNT/polyacrylonitrile solution was sprayed onto the surface of the CF woven fabrics, and the CNTs were grafted by a thermal treatment at 300 °C. CNT-grafted CF composites were fabricated using the CNT-grafted CF woven fabrics using a vacuum-assisted resin transfer molding process with epoxy resin. The CNT-grafted CF composite exhibited 22% enhancement in the tensile strength compared to that of the pristine CF composite. Fracture surfaces of the CNT-grafted CF composites showed that the grafted CNTs obstructed the propagation of micro-cracks and micro-delamination around the CFs and also yarn boundaries, resulting in improved tensile strength of CNT-grafted CF composites.     

Percolation behaviour of multiwalled carbon nanotubes of altered length and primary agglomerate morphology in melt mixed isotactic polypropylene-based composites

The effect of ball milling on the structural characteristics and further on the dispersion and percolation behaviour of multiwalled carbon nanotubes (MWCNTs) in melt mixed composites using a maleic anhydride modified isotactic polypropylene as matrix was investigated. TEM and SEM revealed that ball milled nanotubes were considerably shorter and showed a compact primary agglomerate morphology compared to the as-synthesised MWCNTs. At macro scale ball milled MWCNTs were found to be better dispersed, whereas at sub-micron scale the states of dispersion of both nanotube materials were comparable. The differences in the composite morphologies as well as in the composites electrical and rheological percolation behaviour were assigned to the altered MWCNT structure due to ball milling treatment. The dispersibility of ball milled MWCNTs was restricted due to their more compact agglomerate morphology. Furthermore, the ability to form percolated network structures was restrained by their shorter length and, again, their compact primary agglomerates. An effective agglomerate interaction radius depending on the nanotube structural characteristics, length and agglomerate morphology, is suggested in order to explain the experimental findings.     

Novel method for functionalising and patterning textile composites: Liquid resin print

The paper reports a novel method of integrating resin into continuous textile reinforcement. The method presents a print of liquid reactive resin into textile preforms. A series of targeted injections forms a patch which upon consolidation and curing transforms into a stiff region continuously spanning through preform thickness. Enhancing the injected resin with conductive phase allows creating a pattern of patches with controlled dimensions and added functionalities. Patterned composites reveal features which are not typical for conventional composites such as fibre bridged interfaces, regular thickness variation, and gradient matrix properties. The presented study explores the role of these features in (a) the mechanical behaviour of these materials, focusing on their deformation and failure mechanisms in tension, and (b) the feasibility of adding functionality by printing electrically conductive resins containing carbon nano-tubes (CNT). It was shown that resin print is a promising method for local functionalization of structural composites.     

The impact of fluorinated MWCNT additives on the enhanced dynamic mechanical properties of e-beam-cured epoxy

Multi-walled carbon nanotubes were embedded into e-beam-cured epoxy resin to improve the mechanical properties of epoxy resin. The surfaces of these carbon nanotubes were modified using a fluorination treatment to improve their dispersion and adhesion in epoxy resin. The dynamic mechanical properties of epoxy/carbon nanotube composites were investigated at various heating rates and frequencies. As an effect of fluorination treatment, the semi-ionic bond of C–F on the surface of multi-walled carbon nanotubes played an important role in the improved dispersion and adhesion of carbon nanotubes into the epoxy resin. The storage modulus and loss modulus of the composites increased with higher applied frequency. The activation energy of the composites was increased by the effects of a higher heating rate due to the slow heat transfer in the epoxy/carbon nanotube composites. Eventually, the dynamic mechanical properties of the investigated epoxy were significantly improved by the carbon nanotubes dispersed therein via the fluorination treatment.     

Highly strong and conductive carbon nanotube/cellulose composite paper

Carbon nanotube (CNT)/cellulose composite materials were fabricated in a paper making process optimized for a CNT network to form on the cellulose fibers. The measured electric conductivity was from 0.05 to 671 S/m for 0.5–16.7 wt.% CNT content, higher than that for other polymer composites. The real permittivities were the highest in the microwave region. The unique CNT network structure is thought to be the reason for these high conductivity and permittivity values. Compared to other carbon materials, our carbon CNT/cellulose composite material had improved parameters without decreased mechanical strength. The near-field electromagnetic shielding effectiveness (EMI SE) measured by a microstrip line method depended on the sheet conductivity and qualitatively matched the results of electromagnetic field simulations using a finite-difference time-domain simulator. A high near-field EMI SE of 50-dB was achieved in the 5–10 GHz frequency region with 4.8 wt.% composite paper. The far-field EMI SE was measured by a free space method. Fairly good agreement was obtained between the measured and calculated results. Approximately 10 wt.% CNT is required to achieve composite paper with 20-dB far-field EMI SE.     

Effects of stretching on mechanical properties of aligned multi-walled carbon nanotube/epoxy composites

Composites based on epoxy resin and differently aligned multi-walled carbon nanotube (MWCNT) sheets have been developed using hot-melt prepreg processing. Aligned MWCNT sheets were produced from MWCNT arrays using the drawing and winding technique. Wavy MWCNTs in the sheets have limited reinforcement efficiency in the composites. Therefore, mechanical stretching of the MWCNT sheets and their prepregs was conducted for this study. Mechanical stretching of the MWCNT sheets and hot stretching of the MWCNT/epoxy prepregs markedly improved the mechanical properties of the composites. The improved mechanical properties of stretched composites derived from the increased MWCNT volume fraction and the reduced MWCNT waviness caused by stretching. With a 3% stretch ratio, the MWCNT/epoxy composites achieved their best mechanical properties in this study. Although hot stretching of the prepregs increased the tensile strength and modulus of the composites considerably, its efficiency was lower than that of stretching the MWCNT sheets.     

In situ assessment of carbon nanotube flow and filtration monitoring through glass fabric using electrical resistance measurement

Filtration of nanofillers into porous fabric media is still an issue during the preparation of advanced fiber-reinforced composites. The assessment of resin/multiwall carbon nanotube (MWCNT) flow, MWCNT filtration, and the cure monitoring of glass fiber/carbon nanotube-polyester composites by means of the measurement of the electrical resistance was introduced. The vacuum-assisted resin transfer molding technique was used. The electrical resistances measured over the span of a composite were qualitatively correlated with MWCNT flow and the degree of MWCNT filtration. It was found that while the complexity of the fabrics could likely introduce preferential deposition of MWCNTs, their filtration is mainly affected by their dispersion state in the resin suspension. Relationships among critical parameters such as the lengths and diameters of MWCNTs, the inter- and intra-tow dimensions of glass fabrics, the dispersion level of MWCNTs, and the viscosity of nanocomposite samples are discussed and correlated to the filtration, cure, and flow phenomena. We showed that our method can also serve as an early warning to obviate defects in the resulting composite.     

Comparison of the thermal properties between composites reinforced by raw and amino-functionalized carbon materials

Carbon materials, such as graphite oxides, carbon nanotubes and graphenes, have exceptional thermal conductivity, which render them excellent candidates as fillers in advanced thermal interface materials for high density electronics. In this paper, these carbon materials were functionalized with 4,4′-diaminodiphenyl sulphone (DDS), to enhance the bonding between the carbon materials and the resin matrix. Their visibly different properties were investigated. It seems that DDS-functionalization can obviously improve the interfacial heat transfer between the carbon materials and the epoxy matrix. The thermal conductivity enhancement of D-Graphene composites (0.493 W/m K) was about 30% higher than that of D-MWNTs composites (0.387 W/m K) at 0.5 vol.% loading. The different effects among EGO, D-EGO, MWNTs, D-MWNTs and D-Graphene in polymer composites were also discussed. It was demonstrated that DDS-functionalized carbon materials had an obvious effect on the thermal performances of composite materials and were more effective in thermal conductivity enhancement.     

Damage sensing of adhesively-bonded hybrid composite/steel joints using carbon nanotubes

Carbon nanotube networks have been used previously for in situ sensing of matrix damage in fiber-reinforced composites. In this research, the ability of carbon nanotube networks to sense and distinguish different types of damage in adhesively-bonded hybrid composite-to-metal joints is evaluated. Toward this end, conductive networks of carbon nanotubes are introduced to the composite substrate as well as the epoxy adhesive. By altering the geometry and chemically treating the steel substrate surface, different failure mechanisms of the single-lap shear joints are achieved. It is demonstrated that these failure mechanisms each possess a distinct resistance response, therefore proving the ability to not only sense failure in situ, but also to distinguish the extent and nature of damage which occurs.     

Development of a carbonization-in-nitrogen method for measuring the fiber content of carbon fiber reinforced thermoset composites

Carbon fiber reinforced thermoset composites such as carbon fiber epoxy composites are widely used in aircraft and aerospace, and are being increasingly used in automotive applications because of their lightweight characteristics, high specific strength, and stiffness. The carbon fiber content in the composite plays a critical role in enhancing structural performance. The carbon fibers contribute to the strength and stiffness; therefore, the mechanical properties of the composite are greatly influenced by the carbon fiber content. Measurement of carbon fiber content is essential for product quality control and process optimization. In this work, a novel carbonization-in-nitrogen (CIN) method is developed to characterize the fiber content in carbon fiber thermoset composites. A carbon fiber composite sample is carbonized in a nitrogen environment at elevated temperatures, alongside a neat resin sample. The carbon fibers are protected from oxidization while the resin (the neat resin and the resin matrix in the composite sample) is carbonized under nitrogen environment. The neat resin sample is used to calibrate the resin carbonization rate and calculate the amount of the resin matrix in the composite sample. The new method has been validated on several thermoset resin systems, and found to yield accurate estimation of fiber content in carbon fiber thermoset composites.     

Study of the mechanical,electrical and morphological properties of PU/MWCNT composites obtained by two different processing routes

A comparative study of the influence of processing route on polyurethanes (PUs)/multiwalled carbon nanotube (MWCNT) composites mechanical and electrical properties and also morphology was undergone employing two differentiated processing methods, solvent casting and buckypaper infiltration, for producing PU composites with low, medium and high mass fractions of acid treated MWCNT, and with no covalent linkages between the matrix and the nanotubes. As for example, with a MWCNT mass fraction of ∼18 wt.% the second method produced stiffer (270 MPa), lighter (948 kg m⁻³) and more electrically conductive (1.8 S cm⁻¹) composite while the first one gave softer (111 MPa) and more ductile (141%) materials. These properties differences are related to the different PU/MWCNT dispositions obtained through each synthesis route. Nanotubes percolating concentration is found to be crucial on composite properties evolution and a preferential interaction of MWCNT with PU hard segments is observed for solvent cast composites.     

Mechanical properties of aligned multi-walled carbon nanotube/epoxy composites processed using a hot-melt prepreg method

This study examined the mechanical properties of aligned multi-walled carbon nanotube (CNT)/epoxy composites processed using a hot-melt prepreg method. Vertically aligned ultra-long CNT arrays (forest) were synthesized using chemical vapor deposition, and were converted to horizontally aligned CNT sheets by pulling them out. An aligned CNT/epoxy prepreg was fabricated using hot-melting with B-stage cured epoxy resin film. The resin content in prepreg was well controlled. The prepreg sheets showed good drapability and tackiness. Composite film specimens of 24-33 μm thickness were produced, and tensile tests were conducted to evaluate the mechanical properties. The resultant composites exhibit higher Young’s modulus and tensile strength than those of composites produced using conventional CNT/epoxy mixing methods. For example, the maximum elastic modulus and ultimate tensile strength (UTS) of a CNT (21.4 vol.%)/epoxy composite were 50.6 GPa and 183 MPa. These values were, respectively, 19 and 2.9 times those of the epoxy resin.