Carbon Nanotube Pull-out Research Articles

Ultra-strong collagen-mimic carbon nanotube bundles

In spite of worldwide research, carbon nanotubes (CNTs) still have not fully realized their original promise as ideal reinforcements for composite materials due to a number of challenging issues such as weak interface, poor dispersion, misalignment and lack of optimized design. Here we propose a bio-inspired structure of CNT bundles with controllable crosslink density and staggered pattern of organization that mimic the architecture of natural collagen fiber. Molecular mechanics (MM) simulations show that, under tensile loading, the bio-inspired CNT bundles undergo a transition in failure mode from CNT pull-out to CNT break as the crosslink density increases, with strength an order of magnitude higher than that of the existing strongest conventional carbon fibers. Based on the MM simulations, a generalized tension-shear chain model with four dimensionless parameters is developed to guide the design of CNT bundles for optimized mechanical properties.

Multi-scale interlaminar fracture mechanisms in woven composite laminates reinforced with aligned carbon nanotubes

Abstract Aligned nanoscale fibers have been shown to provide inter and intralaminar reinforcement of fiber-reinforced polymer composites. In one hierarchical architecture, aligned carbon nanotubes (CNTs) grown on advanced microfibers in a woven ply creates a ‘fuzzy fiber’ reinforced plastic (FFRP) laminate. Here, the mechanisms of Mode I fracture toughness enhancement for such laminates are elucidated experimentally by varying the type of epoxy and aligned CNT length. Reinforcement effectiveness is found to vary from reduced initiation toughness to over 100% increase in steady-state fracture toughness, depending upon the interlaminar fracture mechanisms. Fracture-surface morphology investigations reveal that interlaminar toughness enhancement is significantly less for an aerospace infusion resin than that for a much tougher hand lay-up marine epoxy. Long (∼20 micron) aligned CNTs add significant toughness in steady state (>1 kJ/m2 increase for marine epoxy) via CNT pullout and by driving the crack through tortuous paths around and through microfiber bundles/tows, whereas shorter CNTs produce less toughening (or even reduced toughness in aerospace epoxy), which is attributed to increased microfiber–matrix cohesive failure. These findings reveal the multi-scale nature of the aligned-CNT reinforced composite ply interface, and the mechanisms at work at the micro and nanoscales that influence the macroscopic behavior. These new insights provide impetus for using aligned CNTs to tailor microfiber polymer composites by adjusting microfiber orientation, steering cracks through tortuous paths, and maximizing fracture surface area through both microfiber and nanofiber pullout.

High strength micron size carbon fibers from polyacrylonitrile–carbon nanotube precursors

An improved, high strength, carbon fiber derived from islands-in-a-sea bi-component gel spun polyacrylonitrile (PAN)–carbon nanotube (CNT) precursor fibers containing 1 wt% mixture of single, double, and few walled CNTs was developed. Microscale experiments with properly designed MEMS tools provided the mechanical properties of individual, 1-μm diameter carbon filaments, which were isolated from bundles of 407 fibers. The statistics of the mechanical strength were described well by the cumulative Weibull probability density function that resulted in characteristic strength of 6.2GPa and a Weibull modulus of 4.5, while the highest tensile strength and Young’s modulus values were 7.3GPa and 318GPa, respectively. At the lower end of the spectrum, the strength values correlated well with predictions based on an effective flaw size obtained from fracture cross-sections. On the other hand, the failure cross-sections of the high strength carbon fibers contained a large number of long and oriented CNTs but no discernible flaws. The high interfacial strength between the CNTs and the surrounding carbon resulted in fracture and telescopic pull-out of the CNTs, which was corroborated by individual CNT pull-out experiments with MEMS tools inside an SEM, and in situ fiber failure observations of telescopic pull-out of CNTs inside a TEM.

Morphological and Microstructural Property Comparison of Bulk and Aligned Cvd-Grown Carbon Nanotubes

Carbon nanotubes are one-dimensional materials found in various forms, the most important of which are bulk unordered carbon nanotubes of lengths of few microns and aligned tubes regularly a few millimeters long. Differences in length, orientation, alignment, nanotube graphitization and purity will influence the eventual performance of the material. Herein four characterization methods are used to characterize and compare bulk and aligned carbon nanotubes. Results show that the diameters of the millimeter-long aligned carbon nanotubes are approximately 60 ∼ 80 nm with thick nanotube walls of about 25–30 nm, while the diameters of the bulk carbon nanotubes are about 15–20 nm with much thinner walls. The aligned carbon nanotubes have significantly larger graphitization degrees, higher purity and greater orientation than the bulk carbon nanotubes that tend to self-agglomerate under no external stimulus. Silicon carbide matrix nanocomposites reinforced by the aligned carbon nanotubes were found to be denser that those reinforced by the bulk carbon nanotubes and also exhibit extensive, uniform, and long pullout of carbon nanotubes.

Finite element simulation of single carbon nanotube pull-outs from a cementitious nanocomposite material using an elastic-plastic-damage and cohesive surface models

Carbon nanotubes (CNTs) have recently been integrated in the most widely used material in the world 'concrete' for improving its mechanical properties and fracture resistance. This paper computationally investigates the pull-out behaviour of a single CNT from cement. The effects of: 1) CNT-cement interfacial shear strength, stiffness, and fracture energy; 2) the cement mechanical properties; 3) CNT's mechanical properties, aspect ratio, and surface area to volume ratio on the pull-out strength from a cement matrix are investigated through simulating the single straight CNT pull-out. A coupled elastic-plastic-damage constitutive model is adopted to simulate the behaviour of the cement matrix, whereas the continuum shell model is used to simulate the elastic behaviour of CNT. The surface-based cohesive behaviour is employed for modelling the interface between CNT and cement matrix. It is shown that CNT pull-out is mainly governed by the interfacial fracture energy, and not the interfacial shear strength.

Molecular dynamic simulation of oblique pullout of carbon nanotube from resin

In this study, a molecular dynamic model was employed to simulate oblique extraction of a multi-walled carbon nanotube (MWNT) from resin and to calculate the carbon nanotube’s (CNTs) deformation and stress distribution. Numerical results reveal the occurrence of elongation and necking in a CNT prior to the movement of its embedded end and of local buckling in a segment near where a CNT exits resin under large angle oblique extraction. Numerical results for the 0°, 15°, 30°, 45°, 60° and 75° oblique extractions reveal the effects of the oblique angle on the force–displacement curve and the tensile stress distribution over selected CNT cross-sections. Based on the simulation results, it is noted that CNT breakage in large-angle oblique extraction is mainly attributed to stress concentration caused by local buckling and it has a detrimental effect on the CNT toughening capacity.

A numerical study on carbon nanotube–hybridized carbon fibre pullout

Carbon nanotube (CNT)–hybridized carbon fibre (CF) composite is a new generation composite, where CNTs grow radially on carbon fibres to form a hybrid reinforcing phase. To evaluate the bridging effect of this new reinforcing phase, a numerical method is proposed to theoretically investigate the pullout of a hybrid fibre. There are two finite element models developed in this method, which are applied to simulate a single CNT pullout from the matrix at microscale and the pullout of the hybrid fibre at macroscale. The bridging effect of the CNTs during the hybrid fibre pullout is simulated by spring elements in the macroscale finite element model, where the properties of spring elements are obtained from the microscale finite element simulation. The numerical results indicate that the apparent interfacial shear strength of the hybrid fibre and the specific pullout energy can be significantly increased due to the additional bonding of the CNT–matrix interface. A parametric study indicates that the bridging effect of the hybrid fibre can be further enhanced by improving the interfacial bonding between CNT and matrix and increasing the size or length of CNTs. This study provides a new numerical method to simulate the multiscale CNT/CF hybrid fibre pullout.

Toughness Enhancement by Aligned Multi-Walled Carbon Nanotubes Pullout from Polymer Matrix of Multi-Scale Composites

There have been tremendous researches on carbon nanotubes (CNTs) and CNTs reinforced composites due to their excellent mechanical, electromagnetical and thermal properties. Radially aligned multi-walled CNTs grown in situ on the surface of carbon fiber or its woven fabric by the chemical vapor deposition method not only provide the significant three-dimensional reinforcement, but also overcome the poor dispersion of nanostructures. In order to investigate how the aligned multi-walled CNTs affect the mechanical properties of the multi-scale composites, a representative volume element is used to simulate the multi-walled CNT pullout process by means of the finite element method. Related influencing factors are taken into consideration: CNT length, CNT Youngs modulus, and the shear strength of the interface between multi-walled CNTs and epoxy matrix, which is characterized by a cohesive zone model. The results reveal that the aligned multi-walled CNTs make a significant contribution to the fracture toughness of the multi-scale composites, and the CNT/epoxy composites with high toughness require relatively high interfacial shear strength, reasonable CNT length and Youngs modulus. The study is helpful for the optimal design of multi-scale composites.

Process and mechanical properties of carbon/carbon–silicon carbide composite reinforced with carbon nanotubes grown in situ

A hierarchical Cf/C–SiC composite was fabricated via in situ growth of carbon nanotubes (CNTs) on fiber cloths following polymer impregnation and pyrolysis process. The effects of CNTs grown in situ on mechanical properties of the composite, such as flexural strength, fracture toughness, crack propagation behavior and interfacial bonding strength, were evaluated. Fiber push-out test showed that the interfacial bonding strength between fiber and matrix was enhanced by CNTs grown in situ. The propagation of cracks into and in fiber bundles was impeded, which results in decreased crack density and a “pull-out of fiber bundle” failure mode. The flexural strength was increased while the fracture toughness was not improved significantly due to the decreased crack density and few interfacial debonding between fiber and matrix, although the local toughness can be improved by the pull-out of CNTs.

Interlaminar fracture toughness of composite laminates with CNT-enhanced nonwoven carbon tissue interleave

Abstract A nonwoven carbon tissue (NWCT), composed of conventional carbon fibers, was coated with carbon nanotubes (CNT) to compose a multi-scale reinforcing interleave layer which can be directly incorporated into the composite laminate layup process. This CNT-enhanced NWCT layer was found to significantly improve the Modes I and II interlaminar fracture toughness of a carbon fiber reinforced polymer (CFRP) laminate, as measured by double-cantilever-beam (DCB) and end-notched-flexure (ENF) tests. G IC and G IIC of the CNT-enhanced NWCT specimens were compared with baseline CFRP specimens having no interleave, and also specimens having NWCT interleave with no CNT. The mean G IC of the CNT-enhanced NWCT specimens was measured to be 353% higher than the CFRP specimens, a great improvement over the NWCT specimens which showed a 5% reduction relative to CFRP. The mean G IIC of the CNT-enhanced NWCT specimens was 246% higher than the CFRP specimens, which is a significant improvement over the 194% increase measured for the NWCT specimens. Increased G IIC was mainly achieved via crack bridging provided by the NWCT carbon fiber morphology. Additional improvement in G IIC , and the increase in G IC (rather than decrease) of the CNT-enhanced NWCT specimens was due to local CNT reinforcement of the resin near the NWCT fiber surfaces. Scanning electron microscopy shows evidence of improved resin-to-NWCT interface adhesion, short fiber breakage, and CNT pullout at the fracture surfaces.

On the aspect ratio effect of multi-walled carbon nanotube reinforcements on the mechanical properties of cementitious nanocomposites

For their novel mechanical, thermal, chemical, and electrical properties, carbon nanotubes (CNTs) are widely used in many fields of nanocomposite materials. In cementitious nanocomposites, CNTs can act as effective bridges to minimize and limit the propagation of micro-cracks through the matrix, under the conditions of well dispersion of the CNTs within the matrix and good bonding between the CNTs and the surrounding hydrated cement matrix. This study focuses on the effect of different concentrations of long multi-walled carbon nanotubes (MWCNTs) – high length/diameter aspect ratios of 1250–3750 – and short MWCNTs – aspect ratio of about 157 – in cement paste. Flexural bending tests were performed to evaluate four major mechanical properties of the cement/CNTs composites at ages of 7, 14, and 28days. Results show that the flexural strength of short 0.2wt.% MWCNT and long 0.1wt.% MWCNT increased by 269% and 65%, respectively, compared to the plain cement sample at 28days. The highest increase in ductility at 28days for the short 0.1wt.% MWCNT and the short 0.2wt.% MWCNT was 86% and 81%, respectively. It is concluded that nanocomposites with low concentration of long MWCNTs give comparable mechanical performance to the nanocomposites with higher concentration of short MWCNTs. Clear evidence was obtained from scanning electron microscope images for micro-crack bridging; many of the MWCNTs were stretching across the micro-cracks showing CNTs breakage and pull-out.

Theoretical model of double-walled carbon nanotube pullout from a composite matrix

A micromechanical model of double-walled carbon nanotube (DWCNT) pullout from a composite matrix is presented with the interfacial residual stress and van der Waals (vdW) forces taken into consideration. The interfacial residual stress induced by thermal expansion coefficient (TEC) mismatch is introduced via thermo-elastic constitutive relations. The influence of vdW interactions between two layers of DWCNT on the interfacial stress distributions of DWCNT and matrix is analyzed. The analytical expressions of interfacial shear stress and the axial stresses of DWCNT and matrix are derived, respectively. Furthermore, the influences of temperature change, interfacial friction coefficient, DWCNT aspect ratio, DWCNT volume fraction and the relative modulus between DWCNT and matrix are illustrated and discussed.

MD simulation of carbon nanotube pullout behavior and its use in determining mode I delamination toughness

This study presents a multi scale approach of understanding the influence of multi-walled carbon nanotube (MWNT) on delamination toughness. Molecular dynamic (MD) simulation was used to investigate the influence of diameter and length of a MWNT and the boundary and loading conditions on its pullout behavior from polymer matrix. Numerical results reveal that the MWNT experiences non-uniform stretching and necking prior to the movement of its embedded end and pullout, and the force–displacement curve features with four distinct regions. A quadruple-linear model was proposed via a parametric study and validated by comparing with existing experimental result, and then used as a traction law in deriving the formula for determining the enhancement in mode I delamination toughness due to MWNT presence.

Enhancement of fatigue life of polyurethane composites containing carbon nanotubes

The effects of carbon nanotube (CNT) inclusion on cyclic fatigue behavior and the tensile properties of polyurethane (PU) composites have been studied. Tension–tension cyclic fatigue tests were conducted at various load levels (30–50MPa) to establish the relationship between stress and the number of cycles to failure (S–N curves). The tensile energy to break PU composites was enhanced up to 38% by using a low amount of CNTs. In addition, the incorporation of CNTs increased the fatigue life of PU in the high-stress amplitude, low-cycle regime by up to 248%. Micrographs show highly dispersed CNTs and indicate the key mechanisms for enhancement in fatigue life such as CNT crack-bridging and pull-out. The tensile-tensile fatigue properties obtained in this work show that PU systems can outperform epoxy systems widely used in structural applications.

In situ growth carbon nanotube reinforced SiC f/SiC composite

Carbon nanotube (CNT) reinforced SiC f/SiC composite was prepared by in situ chemical vapor deposition (CVD) growth of CNTs on SiC fibers then following polymer impregnation pyrolysis (PIP) process. The nature of CNTs and the microstructure of the as prepared CNT–SiC f/SiC composite were investigated. The mechanical properties of the as prepared CNT–SiC f/SiC composite were measured. The results reveal that the in situ CVD growth of CNTs on SiC fibers remarkably promotes the mechanical properties of SiC f/SiC composite. The secondly pull-out of CNTs from matrix during the pull-out of the SiC fibers from matrix consumes the deformation energies, resulting in promotion of the mechanical properties for composite.

Modelling of the carbon nanotube bridging effect on the toughening of polymers and experimental verification

The effect of aligned and randomly oriented carbon nanotube (CNT), with respect to the crack growth plane, on the fracture toughness of polymers is modelled in this paper using the Elastic Plastic Fracture Mechanics. According to a critical length, two dominant toughening mechanisms for CNT-modified polymers are presented, i.e. CNT pull-out and CNT rupture. The model is then used to identify the effect of CNTs geometrical and mechanical properties on the enhancement of fracture toughness in CNT-modified polymers. The key CNT properties are the CNT radius, average length, ultimate strength, elongation before failure, interfacial shear strength between CNTs and the polymer and nanotube volume fraction. Finally, experimental results are compared with the model predictions. The correlation shows that processing of long, aligned CNTs remains the main barrier in achieving major fracture toughness enhancement.

Failure analysis and the optimal toughness design of carbon nanotube-reinforced composites

The combined analysis of the fracture toughness enhancement of carbon nanotube (CNT)-reinforced composites is herein carried out on the basis of atomistic simulation, shear-lag theory and facture mechanics. It is found that neither longer reinforced CNTs nor stronger CNT/matrix interfaces can definitely lead to the better fracture toughness of these composites. In contrast, the optimal interfacial chemical bond density and the optimal CNT length are those making the failure mode just in the transition from CNT pull-out to CNT break. To verify our theory, an atomic/continuum finite element method (FEM) is applied to investigate the fracture behavior of CNT-reinforced composites with different interfacial chemical bond densities. Our analysis shows that the optimal interfacial chemical bond density for (6,6) CNTs is about 5–10% and that increasing the CNT length beyond 100 nm does not further improve fracture toughness, but can easily lead to the self-folding and clustering of the CNTs. The proposed theoretical model is also applicable to short fiber-reinforced composites.

Enhancement of delamination fatigue resistance in carbon nanotube reinforced glass fiber/polymer composites

We show that the addition of small volume fractions of multi-walled carbon nanotubes (CNTs) to the matrix of glass–fiber composites reduces cyclic delamination crack propagation rates significantly. In addition, both critical and sub-critical inter-laminar fracture toughness values are increased. These results corroborate recent experimental evidence that the incorporation of CNTs improve fatigue life by a factor of two to three in in-plane cyclic loading. We show that in both the critical and sub-critical cases, the degree of delamination suppression is most pronounced at lower levels of applied cyclic strain energy release rate, Δ G. High-resolution scanning electron microscopy of the fracture surfaces suggests that the presence of the CNTs at the delamination crack front slows the propagation of the crack due to crack bridging, nanotube fracture, and nanotube pull-out. Further examination of the sub-critical fracture surfaces shows that the relative proportion of CNT pull-out to CNT fracture is dependent on the applied cyclic strain energy, with pull-out dominating as Δ G is reduced. The conditions for crack propagation via matrix cracking and nanotube pull-out and fracture are studied analytically using fracture mechanics theory and the results compared with data from the experiments. It is believed that the shift in the fracture behavior of the CNTs is responsible for the associated increase in the inter-laminar fracture resistance that is observed at lower levels of Δ G relative to composites not containing CNTs.

Effects of Vacancy Defects on the Interfacial Shear Strength of Carbon Nanotube Reinforced Polymer Composite

We investigate the effects of the vacancy defects (i.e., missing atoms) in carbon nanotubes (CNTs) on the interfacial shear strength (ISS) of the CNT-polyethylene composite with the molecular dynamics simulation. In the simulation, the crystalline polyethylene matrix is set up in a hexagonal array with the polymer chains parallel to the CNT axis. Vacancy defects in the CNT are introduced by removing the corresponding atoms from the pristine CNT (i.e., CNT without any defect). Three patterns of vacancy defects with three different sizes are considered. Two types of interfaces, with and without cross-links between the CNT and the matrix are also considered here. Polyethylene chains are used as cross-links between the CNT and the matrix. The Brenner potential is used for the carbon-carbon interaction in the CNT, while the polymer is modeled by a united-atom potential. The nonbonded van der Waals interaction between the CNT and the polymer matrix and within the polymer matrix itself is modeled with the Lennard-Jones potential. To determine the ISS, we conduct the CNT pull-out from the polymer matrix and the ISS has been estimated with the change of total potential energy of the CNT-polymer system. The simulation results reveal that the vacancy defects significantly influence the ISS. Moreover, the simulation clarifies that CNT breakage occurs during the pull-out process for large size vacancy defect which ultimately reduces the reinforcement.

Pressureless sintering of carbon nanotube–Al 2O 3 composites

Alumina ceramics reinforced with 1, 3, or 5 vol.% multi-walled carbon nanotubes (CNTs) were densified by pressureless sintering. Commercial CNTs were purified by acid treatment and then dispersed in water at pH 12. The dispersed CNTs were mixed with Al 2O 3 powder, which was also dispersed in water at pH 12. The mixture was freeze dried to prevent segregation by differential sedimentation during solvent evaporation. Cylindrical pellets were formed by uniaxial pressing and then densified by heating in flowing argon. The resulting pellets had relative densities as high as ∼99% after sintering at 1500 °C for 2 h. Higher temperatures or longer times resulted in lower densities and weight loss due to degradation of the CNTs by reaction with the Al 2O 3. A CNT/Al 2O 3 composite containing 1 vol.% CNT had a higher flexure strength (∼540 MPa) than pure Al 2O 3 densified under similar conditions (∼400 MPa). Improved fracture toughness of CNT–Al 2O 3 composites was attributed to CNT pullout. This study has shown, for the first time, that CNT/Al 2O 3 composites can be densified by pressureless sintering without damage to the CNTs.