CNT Pullout Research Articles

Effect of nitrogen-doped type on fracture toughness improvement and crack growth resistance of carbon nanotube/epoxy nanocomposites: Combined multiscale analysis approach

Recently, nitrogen-doped carbon nanotubes (N-doped CNTs) have received great attention in nanocomposite design. It has become highly necessary to develop predictive models to elucidate their toughning behavior. In this study, the effects of CNTs with three different types of N-doped functional groups (quaternary, pyrrolic, and pyridinic) on the fracture toughness (FT) and crack growth of polymer nanocomposites are predicted using a multiscale analysis approach. To scale up from the nanoscale to the macroscale, a multiscale analysis approach integrating molecular dynamics, micromechanics theory, linear fracture mechanics theory, and a phase-field fracture model (PFFM) is adopted. The toughness enhancement trends of the three different types of N-doped functional groups were quantified by considering four toughening mechanisms (CNT debonding, plastic nanovoid growth, CNT pull-out, and CNT rupture), and compared with experimental result. The results show that the excellent interphase and interfacial properties of quaternary and pyridinic functional groups significantly improve the FT and crack growth resistance of N-doped CNT/epoxy nanocomposites. Our study provides high-performance solutions for experimental studies pertaining to the FT and crack growth of N-doped CNT/epoxy nanocomposites.

Strain-rate sensitivity analysis of microwave processed polypropylene-carbon nanotube composites

The study investigates the strain rate sensitivity of microwave-processed polypropylene nanocomposites containing a 10% volume fraction of multi-walled carbon nanotubes (MWCNTs), synthesized using a domestic microwave applicator (DMA). Through experimental analysis under various loading conditions including tensile, flexural, and multistep relaxation loads, the mechanical strength of these nanocomposites is assessed alongside destructive testing and scanning electron microscopy (SEM) results. Mathematical models are proposed to elucidate the mechanical response under different strain rate loadings. The nanocomposite exhibits elastoplastic behavior, with maximum elongation observed at a strain rate of 3 mm/min. During uniaxial tension tests, the ultimate strength increases significantly from 5.74 MPa at 1.52 mm/min to 13.8 MPa and 15.7 MPa at 1 mm/min and 2 mm/min respectively during multistep relaxation tests. The true stress-true strain curve follows a power law of strain hardening under high strain rate tensile tests. Flexural tests reveal maximum flexural strength and modulus at a strain rate of 1.25 mm/min, with elongation peaking at 1.50 mm/min. Mechanical properties, including modulus and ultimate strength, show an increasing trend with strain rate until a limiting value is reached, beyond which thermal softening occurs. SEM fractography illustrates the random and spatial distribution of CNTs at low strain rates, whereas agglomeration and CNT pull-out are observed at higher strain rates. This comprehensive analysis sheds light on the intricate mechanical behavior of microwave-processed polypropylene nanocomposites, offering insights into their potential applications and optimization strategies.

Stochastic multiscale multimode interlaminar fracture toughness of buckypaper nanocomposites

Delamination is one of the primary damage mechanisms in laminated composites that affects the lifetime integrity of the composite systems. The present study examines the effects of Buckypaper (BP) nano-reinforcement on the multimodal interlaminar fracture properties of the carbon fiber reinforced polymer (CFRP) composites reinforced with BP membranes. The novelty and the focus of this study is the effects of multi-wall carbon nanotube (MWCNT) network size and interphase in the interlaminar fracture toughness of BP nanocomposites. Dry and pre-infused non-functionalized MWCNT BP were integrated at the mid layer of the composite before laminate curing. The Atomic Force Microscopy Peak Force Quantitative Nanomechanics Mapping (AFM PFQNM) technique and the Weibull model were applied to study the effects of random CNT network size and interphase on mode I, II, and mixed-mode I-II fracture properties. Weibull analysis of 150 dry and pre-infused BP data confirms a high scatter of CNT network size in dry BP compared to pre-infused BP. Weibull modulus of CNT network size (2.33 for large size and 5.18 for small size) and interphase thickness (3.46) show a robust, consistent entangled structure of CNTs in pre-infused BP. Dry BP improves the propagation energy release rate of the CFRP laminates up to 54%. However, it does not provide additional toughening in the initiation crack energy for modes I, II, and I-II. Mode I initiation and propagation, mode II initiation and mixed-mode I/II fracture energy of pre-infused BP nanocomposites are ∼33%, ∼52.5%, ∼14%, and ∼28% higher than the reference. The interlaminar crack either moves exclusively through the BP layer by circumventing the rigid CNT networks or moves forward through the interface of the carbon fiber monofilament above and below the reinforcing BP layer. This saw-like interlaminar crack path triggers numerous toughening mechanisms such as nano-toughened epoxy, nanoscale CNT bridging and pullout, and crack deflection, and it increases the total crack propagation path.

Combining effect of annealing and reinforcement content on mechanical behavior of multi-walled CNT reinforced AA5052 composites

The large requirement of materials to enhance the overall performance of aerospace and automobile components focused the researchers on developing aluminum metal matrix composite (MMC) materials. This work deals with the fabrication of aluminum MMCs reinforcing with multi-walled carbon nanotubes (MWCNT) produced by a low-cost stir casting process. The tensile strength and micro-hardness tests were carried out to study the mechanical behavior of the composites. Moreover, the microstructural observation was also analysed with the help of FESEM and XRD analysis. The composites were subjected to annealing heat treatment process to investigate the mechanical strength. It was observed that with the reinforcement of CNT, the aluminum alloy composites exhibited superior mechanical properties as compared with the base alloy whereas annealing reduced the mechanical properties of the composites as compared to the as-casted condition for each composition of CNT content. Moreover, the tensile fractography was also characterized in terms of equiaxed dimples, CNT pull-out, matrix cracking on the fracture surfaces.

Reinforcing effect of multi-walled carbon nanotubes on microstructure and mechanical behavior of AA5052 composites assisted by in-situ TiC particles

In this research, the synergistic effects of multi-walled carbon nanotubes and in-situ synthesized titanium carbide (TiC) on the mechanical performance of aluminum hybrid composites were studied. The microstructural characterization involving the influence of both titanium carbide and carbon nanotubes was investigated. The reinforcing effects of both titanium carbide and multi-walled carbon nanotubes on the micro-hardness, tensile strength, and impact strength of the composites were also investigated. The microstructures of the fractured tensile and impact test surfaces were examined through FESEM (Field Emission Scanning Electron Microscope). Clear peaks of titanium carbide (TiC) and aluminum carbide (Al4C3) were observed in the X-Ray Diffraction (XRD) analysis. The micro-hardness of the aluminum composites was significantly improved after the reinforcement with CNT and TiC. The highest ultimate tensile strength and yield strength were found with Al–9%TiC-1%MWCNT and are equal to 206.44 MPa and 136.15 MPa, respectively, whereas a 16% decrease in the impact strength of Al–9%TiC-1%MWCNT was witnessed when compared to base alloy. The effects, such as CNT pull-out, CNT bridging was seen from tensile fractography of the composites. Further, the crack initiation from the pull-out cavity was also assumed to affect the fracture mechanism. Cleavage facets were associated both with the impact and tensile fracture surfaces of the composites. With the superior mechanical properties obtained, the aluminum hybrid composite can be replaced for different structural applications.

Comparative study of the effect of fiber surface treatments on the flexural and interlaminar shear strength of carbon fiber-reinforced composites

The aim of this study was to achieve enhanced flexure and interlaminar shear strength of carbon fiber-reinforced composites by grafting MWCNTs and depositing PyC layer on the fiber using CVD technique. The surface morphology of fiber prior and after treatment was examined by FESEM and TEM and the composites fabricated with surface modified fiber were tested by three-point bend test for comparative flexure and interlaminar shear behavior. A remarkable improvement of 32 % and 54 % was observed in the flexure strength of composites made of CNTs grafted and PyC-coated fiber respectively as compared to reference composites whereas significant augmentation of 35 % and 53 % respectively was found in the ILSS of composites. In-depth fractographic investigation was conducted to investigate the mechanisms responsible for this enhancement in flexure strength and ILSS of composites. The mechanical test results were supported and validated by the fractographic analysis. Individual CNT pullout, CNTs bundle pullout and crack bridging by CNTs were evidently observed as dominant mechanisms of fracture in these composites in addition to conventional interface debonding and fiber pullout.

Atomistic QM/MM simulations of the strength of covalent interfaces in carbon nanotube-polymer composites.

We investigate the failure of carbon-nanotube/polymer composites by using a recently-developed hybrid quantum-mechanical/molecular-mechanical (QM/MM) approach to simulate nanotube pull-out from a cross-linked polyethene matrix. Our study focuses on the strength and failure modes of covalently-bonded nanotube-polymer interfaces based on amine, carbene and carboxyl functional groups and a [2+1] cycloaddition. We find that the choice of the functional group linking the polymer matrix to the nanotube determines the effective strength of the interface, which can be increased by up to 50% (up to the limit dictated by the strength of the polymer backbone itself) by choosing groups with higher interfacial binding energy. We rank the functional groups presented in this work based on the strength of the resulting interface and suggest broad guidelines for the rational design of nanotube functionalisation for nanotube-polymer composites.

Static and fatigue interlaminar shear reinforcement in aligned carbon nanotube-reinforced hierarchical advanced composites

High densities (>10 billion fibers per cm2) of aligned carbon nanotubes (A-CNTs) are used to reinforce the interlaminar resin-rich region of aerospace-grade unidirectional carbon microfiber plies in a hierarchical carbon fiber reinforced plastic (CFRP) laminate architecture. Such nano-engineered interfaces have been shown to increase interlaminar fracture toughness and substructural in-plane strengths, and here we show a 115% average increase in fatigue life across all load levels (60–90% of static strength), with a larger increase of 249% in high-cycle (at 60% of static strength) fatigue, despite no statistically significant increase in static strength. These findings are in agreement with a numerical damage progression model developed to simulate both interlaminar and intralaminar damage in the laminates, which shows the relative insensitivity of short-beam shear (SBS) strength to the enhancement of interlaminar fracture toughness, e.g., a 50% increase in interlaminar toughness yields an SBS strength increase of less than 20%. Consistent with observations of other CNT-reinforced epoxy architectures, larger improvements in fatigue life are noted in low-stress regimes (e.g., high-cycle fatigue) vs. in high-stress regimes (e.g., static and low-cycle fatigue), indicating a transition in dominant mechanisms from high-energy dissipation caused by CNT pullout to low-energy dissipation caused by CNT fracture as stress increases.

Influence of carbon nanotubes as secondary phase addition on the mechanical properties and amorphization of boron carbide

The mechanical properties and amorphization response of a carbon nanotube (5 wt.%) boron carbide (CNT-B4C) composite with 1 μm grain size are investigated, and compared to those of coarse-grained (10 μm grain size) and ultrafine-grained (0.3 μm grain size) monolithic boron carbides. The quasi-static and dynamic uniaxial compressive strengths for CNT-B4C were statistically the same as those of the ultrafine-grained ceramic and higher than the coarse-grained material, contradicting the expected grain size hierarchy (Hall-Petch-type relationship). Addition of CNTs to B4C resulted in decreased quasi-static hardness compared to the large grain size material; however, dynamic hardness was substantially improved compared to quasi-static values. CNT pullout and crack bridging were observed to be possible toughening mechanisms. Finally, Raman spectroscopy was used to quantify amorphization, and it was concluded that addition of CNTs to boron carbide does not alter the propensity for amorphization, but does improve mechanical properties by enhanced toughening.

An Analytical Model of Interlaminar Fracture of Polymer Composite Reinforced by Carbon Fibres Grafted with Carbon Nanotubes.

An analytical model was developed to study the interlaminar fracture behaviour of polymer composite reinforced by carbon fibres grafted with carbon nanotubes. Delamination properties, such as load with displacement or crack (R-curve) and toughness with crack (GR-curve), can be obtained from this model. The bridging laws presented, based on the CNT pullout mechanism (CNT pullout from polymer matrix) and the CNT sword-in-sheath mechanism (CNT breakage), were incorporated into the proposed analytical model to investigate the influence of the structure of CNT growth onto CFs (CNT@CFs) on delamination properties. The numerical results showed that different toughening mechanisms led to different features of GR-curves, R-curves, and load with displacement curves. Parametric study demonstrated that strengthening the CNT@CF interface resulted in significant improvement in toughness. Further, it was found that elastic deformation of CNTs played an important role in the toughness improvement in the CNT sword-in-sheath mechanism, but no such role was evident in the CNT pullout mechanism.

Spark plasma sintering of ceramic matrix composite based on alumina, reinforced by carbon nanotubes

Alumina composites reinforced with 3 vol.% multi-walled carbon nanotubes (MWCNTs) were prepared by spark plasma sintering (SPS). The influence of sintering temperature (1400-1600 °C) on the composites microstructure and mechanical properties was investigated. Microstructure observations of the composite shows that some CNTs site along alumina grains boundary, while others embed into the alumina grains and shows that CNTs bonded strongly with the alumina matrix contributing to fracture toughness and microhardness increase. MWCNTs reinforcing mechanisms including CNT pull-out and crack deflection were directly observed by scanning electron microscope (SEM). For Al2O3/CNT composite sintered at 1600 °C, fracture toughness and microhardness are 4.93 MPa·m1/2 and 23.26 GPa respectively.

CNT/SiC composites produced by direct matrix infiltration of self-assembled CNT sponges

Carbon nanotube-reinforced silicon carbide composites (CNT/SiC) produced by direct infiltration of matrix into a porous CNT arrays have been demonstrated to possess a unique microstructure and excellent micromechanical properties. However, the thickness of the array preforms is usually very small, typically less than 2 mm. Therefore, fabrication of macroscopic centimeter-scale CNT/SiC composites by chemical vapor infiltration (CVI) process requires that the nanoscale fillers could form macroscopic architectures with an open pore network. Direct infiltration of matrix into porous CNT sponges consisting of a three-dimensional nanotube scaffold may provide a possible solution to this challenge. Here, the study reports on the fabrication of CNT/SiC composites by CVI of SiC matrix into the open pore network of CNT sponges while maintaining the original CNT network. The unique microstructure, mechanical properties, electrical conductivity, and electromagnetic shielding effectiveness of the resultant composites were systematically investigated. Energy dissipation toughening mechanism at the nanoscale such as CNT pullout is observed, and the phase composition of the fabricated materials includes β-SiC and CNTs.

Thermal resistant, mechanical and electrical properties of a novel ultrahigh-content randomly-oriented CNTs reinforced SiC matrix composite-sheet

Abstract Development of a thermal resistant CNT based sheet is important for high temperature engineering perspectives with multifunctional purposes. This work reports a novel CNT reinforced SiC matrix composite-sheet by polymer derived ceramics (PDC) processes using polycarbosilane as SiC precursor. The microstructure contains the amorphous SiC matrix (4 phases: SiC, SiC x O y , SiO 2 and free carbon), the chemical sharp CNT/SiC interfaces and the ultrahigh density multi-walled CNTs that are randomly oriented. The composite-sheet is thermally and mechanically stable in non-oxidizing atmosphere up to 1000 °C. The mechanical properties of the composite-sheet are significantly enhanced, with the Young's modulus increased by ∼7 times, and the tensile strength ∼4 times, compared to those of pure CNT sheet. The strengthening mechanism is a bridging effect of the SiC matrix to the CNTs to favor better load transfers during fracturing. Therefore, the composite-sheet shows an elastic-brittle fracture response, with limited CNT pull-out in the fracture surface after tensile test. Finally, the composite-sheet shows an excellent electrical conductivity that is also comparable to the pure CNT sheet, and is found enhanced after post heat-treatment at temperatures >800 °C in non-oxidizing atmosphere, presumably due to the more content free carbon formations in the SiC matrix.

Anisotropic compressive properties of CNT/SiC composites produced by direct matrix infiltration of vertically aligned CNT forests

Carbon nanotube-reinforced silicon carbide composites (CNT/SiC) produced by direct infiltration of matrix into a porous CNT arrays have been demonstrated to possess a unique microstructure and excellent micro-mechanical properties. Here, the study reports on the anisotropic compressive behavior of vertically aligned CNT-reinforced SiC composites (VACNT/SiC) produced by chemical vapor infiltration (CVI) of SiC matrix into the VACNT forests. Findings demonstrate that the longitudinal and transverse compressive strengths of VACNT/SiC specimens with a density of 1.54 g/cm3 are 160 ± 16 and 41 ± 3 MPa, respectively. No matter which compressive mode they all exhibit a typical ductile failure with a similar load-displacement behavior but the stress concentration level of the transverse compression is lower than that of the longitudinal. The mechanisms describing the enhanced strength of these composites were clarified by examining the morphologies of fracture surfaces under scanning electron microscope. CNT pull-out, bridging, sequential breaking and slippage of the walls of the CNT during failure were consistently observed in all fractured specimens.

Direct observation of carbon nanotube induced strengthening in aluminum composite via in situ tensile tests

In situ tensile tests inside a scanning electron microscope chamber are conducted on spark plasma sintered Al and Al-1vol.% CNT composites to understand the strengthening and deformation mechanisms due to long (25–30μm) CNT reinforcement addition. Al–CNT composite shows 40% higher tensile strength, and 65% higher stiffness for mere 1vol.% CNT addition. The failure occurs by CNT pullout from the matrix, which is directly imaged during the tensile testing. Telescopic sliding of CNT walls is also observed which aids to strengthening.

Effects of carbon nanotube alignment on electrical and mechanical properties of epoxy nanocomposites

This paper reports the alignment of multi-walled carbon nanotubes (MWCNTs) in an epoxy matrix as a result of DC electric fields applied during composite curing. Optical microscopy and polarized Raman spectroscopy are used to confirm the CNT alignment. The alignment of CNTs gives rise to much improved electrical conductivity, elastic modulus and quasi-static fracture toughness compared to those with CNTs of random orientation. An extraordinarily low electrical percolation threshold of about 0.0031vol% is achieved when measured along the alignment, which is more than one order of magnitude lower than 0.034vol% with random orientation or that measured perpendicular to the aligned CNTs. The examination of the fracture surfaces identifies pertinent toughening mechanisms in aligned CNT composites, namely crack tip deflection and CNT pullout. The significance of this paper is that the technique employed here can tailor the physical, mechanical and fracture properties of bulk nanocomposites even at a very low CNT concentration.

Effects of interfacial bonding in the Si-carbon nanotube nanocomposite: A molecular dynamics approach

We investigated the effects of interfacial bonding on the mechanical properties in the Si-carbon nanotube (CNT) nanocomposite by a molecular dynamics approach. To describe the system appropriately, we used a hybrid potential that includes Tersoff, AIREBO (adaptive intermolecular reactive empirical bond order), and Lennard–Jones potentials. With increasing bonding strength at the interface of Si matrix and CNT, toughness as well as Young’s modulus and maximum strength increased steadily. CNT pull-out and load transfer on the strong CNT were identified as the main mechanisms for the enhanced properties. At optimum bonding, crack tip was deflected around CNT and the fracture proceeded in plastic mode through Si matrix owing to the strong reinforcement of CNT, and resulted in a further enhancement of toughness. At maximum bonding, however, only load transfer is operative and the fracture returned to brittle mode. We concluded that a strong interface as long as the CNT maintains its structural integrity is desirable to realize the optimum result.

Carbon nanotubes: do they toughen brittle matrices?

The development of a model CNT-brittle matrix composite system, based on SiO2 glass containing well-dispersed CNTs at up to 15 wt%, allows a direct assessment of the effect of the nanoscale filler on fracture toughness (KIC). Samples were prepared by colloidal heterocoagulation followed by spark plasma sintering. Detailed KIC measurements, using both indentation and notched beam techniques, show a linear improvement with CNT content, with up to a twofold increase of fracture toughness at maximum loading. The results from the two methods used in this study show equivalent trends but differing absolute values; the relative merits of these two approaches to measuring nanocomposite toughness are compared. Possible toughening mechanisms associated with CNT pull-out, crack bridging, and crack deflection are identified, and discussed quantitatively, drawing on conventional short-fibre composite theory and the potential effects of scaling fibre diameter.

Fracture Toughness of Carbon Nanotube-Reinforced Metal- and Ceramic-Matrix Composites

Hierarchical analysis of the fracture toughness enhancement of carbon nanotube- (CNT-) reinforced hard matrix composites is carried out on the basis of shear-lag theory and facture mechanics. It is found that stronger CNT/matrix interfaces cannot definitely lead to the better fracture toughness of these composites, and the optimal interfacial chemical bond density is that making the failure mode just in the transition from CNT pull-out to CNT break. For hard matrix composites, the fracture toughness of composites with weak interfaces can be improved effectively by increasing the CNT length. However, for soft matrix composite, the fracture toughness improvement due to the reinforcing CNTs quickly becomes saturated with an increase in CNT length. The proposed theoretical model is also applicable to short fiber-reinforced composites.

The effect of carbon nanotubes microstructures on reinforcing properties of SWNTs/alumina composite

Alumina reinforced with 1wt% single-wall carbon nanotubes (SWNTs) was fabricated by hot-pressing. The fracture toughness of SWNTs/Al2O3 composite reaches 6.40±0.3MPam1/2, which is twice as high as that of unreinforced alumina. Nanoindentation introduced controlled cracks and the damage were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SWNTs reinforcing mechanisms including CNT pullout, CNT fracture, CNT bridging and crack deflection were directly observed, and the relationship between carbon nanotubes microstructures in the matrix and mechanical properties was also discussed in detailed.