A comparative study of the load transfer mechanisms of the carbon nanotube reinforced polymer composites with interfacial crystallization

With the help of shear-lag theory, load transfer analysis is performed on the carbon nanotube reinforced polymer composites with interfacial crystallization of different morphologies, including transcrystallinity layer (TCL) and nanohybrid shish-kebab (NHSK) structures. By comparison, we find that the TCL structures can ease the burden of the CNT while the NHSK structures can lead to a fluctuating distribution of the axial stress in the CNT. Both structures can improve the effective elastic modulus of the composites, though the effect of the TCL structures is more pronounced. Besides, the enhancement of the load transfer efficiency of the composites is also observed, the study of the interfacial stress on different kinds of interfaces shows that the reinforcing effect of the TCL structures is sensitive to both the CNT/crystalline polymer interface and crystalline polymer/amorphous polymer interface, while the major decisive factor for the NHSK structures is confined to be the CNT/crystalline polymer interface because of the interlocking effect. Based on these features, some suggestions are given for tailoring the high-performance carbon nanotube reinforced polymer composites.     

Characterisation of inter-ply shear in uncured carbon fibre prepreg

Understanding the inter-ply shear behaviour of uncured carbon fibre prepreg is fundamental to avoiding process-induced defects during manufacturing of large-scale components. Shear tests for AS4/8552 are compared to a one-dimensional viscoelastic–plastic model for inter-ply shear. The paper presents a methodology capable of determining the parameters of temperature, rate and pressure required for minimum resistance to movement of a prepreg. Investigating the joint strength and friction values individually shows that friction increases with temperature, contrary to previous work, and that the new value of joint strength is predominant at lower temperatures. Rate dependent variables are strongly linked to the resin behaviour, confirming the need for a viscoelastic model. Simple application to industrial scenarios is discussed along with more complex process modelling.     

Thermal conductive and electrical properties of polyurethane/hyperbranched poly(urea-urethane)-grafted multi-walled carbon nanotube composites

Hyperbranched poly(urea-urethane)-grafted multi-walled carbon nanotubes (HPU-MWCNTs) were incorporated in a polyurethane (PU) matrix based on poly(ethylene oxide-tetrahydrofuran) and aliphatic polyisocyanate resin as curing agent. The 9–12 nm thick HPU shell formed on the MWCNTs improved the dispersion of MWCNTs and enhanced the interfacial adhesion between the PU matrix and MWCNTs, leading to improvements in storage modulus and T_g of the composites and enhancement of the thermal stability of PU. Thus, composites with 0.5–1 wt% MWCNTs increased the thermal conductivity by about 60–70% when compared to, and retained the high electrical resistivity of, neat PU.     

Optimum mixing ratio of epoxy for glass fiber reinforced composites with high thermal stability

The optimum condition of glass fiber/epoxy composites was investigated according to mixing ratio of two epoxy matrices. Novolac type epoxy and isocyanate modified epoxy were used as composites matrix. Based on chemical composition of mixing matrix, optimum mixing ratio of epoxy resins was obtained through FT-IR instrument. In order to investigate thermal stability and interface of epoxy resin, glass transition temperature was observed by DSC instrument, and static contact angle was measured by reflecting microscope. Change of IR peak and T_g was conformed according to different epoxy mixing ratios. After fabrication of glass fiber/epoxy composites, tensile, compression, and flexural properties were tested by UTM by room and high temperature. The composites exhibited best mechanical properties when epoxy mixing ratio was 1:1.     

Modelling the influence of microstructural morphology and triple junctions on hydrogen transport in nanopolycrystalline nickel

Hydrogen transport in nanopolycrystalline (NPC) face centred cubic (FCC) nickel has received considerable attention as a result of the material's unique structural embrittlement behaviour. Triple junctions, where three grain boundaries meet, play an important role in hydrogen diffusion. Experiments have indicated that hydrogen transport at a triple junction (TJ) is orders of magnitude greater. In this contribution, a multiphase NPC model is proposed and used to investigate the influence on hydrogen transport of TJs within the surface of the NPC nickel using finite nanostructural element analyses. This 2D multiphase NPC model increases the density of triple junctions as the grain size reduces. The multiphase NPC model consists of two phases comprising nano grain interiors (GI) and intergranular phases. The intergranular (Ig) phase is divided into grain boundary affected zones (GBAZ) regions and TJ regions. The results of this finite nanostructural analysis show that hydrogen transport is enhanced at TJs and the bulk diffusion of hydrogen in NPC material is faster as the volume fraction of TJ increases and nano grain size decreases. The accumulation of hydrogen in three phase (GI, GBAZ, and TJ) microstructures is higher than the two phase (GI and Ig) microstructure case. The accumulation of hydrogen in TJ and Ig are heterogeneous in NPC nickel. The importance of the microstructural morphology in terms of the presence of pores, fine grains in TJ, changes in the shape of TJ with changes in the density of TJ and a TJ effect related to hydrogen transport in NPC nickel is all evidenced. This means that the TJ and microstructural morphology cannot be neglected when predicting hydrogen transport in a NPC nickel.     

The thermal conductivity of Al(OH)3 covered MWCNT/epoxy terminated dimethyl polysiloxane composite based on analytical Al(OH)3 covered MWCNT

Aluminum-hydroxide-covered multi-walled carbon nanotubes (A–MWCNT) were fabricated as a thermally conductive material. The thermal conductivity of A–MWCNT was estimated based on Casimir theory. The effective thermal conductivity of A–MWCNT was estimated at about ∼26 W/mK. The thermal conductivity of A–MWCNT/epoxy-terminated polydimethylsiloxane (ETDS) composite was examined as a function of A–MWCNT loading, and the results showed the maximum value at 1.5 wt% of A–MWCNT loading, above which it decreased slightly. The effective medium approximation (EMA) developed by Maxwell–Garnett (M–G) was used to analyze the thermal conducting behavior of the composite. The experimental results showed negative deviation from the expected thermal conductivity, k_e, beyond 1.5 wt% of A–MWCNT loading, because the composites containing A–MWCNT were strongly affected by interfacial resistance. The interfacial resistance value calculated from M–G approximation increased when filler loading was higher than 1.5 wt% because of the folded and partially agglomerated A–MWCNT along with insufficient interfacial interactions.     

Effects of modified and non-modified clay on the rheological behavior of high density polyethylene

HDPE nanocomposites containing 2.5, 5 and 10 wt.% of non-modified and modified clays (NMC and MC) were prepared by melt extrusion in a twin screw extruder. Compression molded samples were prepared. Transmission electron microscopy (TEM) indicated a partial intercalation of the modified clay nanofiller within the HDPE matrix comparing to that of non-modified clay. The moduli of nanocomposites increased with increase in nanofiller concentration; but this increase was greater in the low frequency region. The non-modified clay had a greater increase in the elastic behavior, while the modified clay increased viscose behavior because of more interactions with the matrix and partial intercalation. The rheological behaviors of both HDPE/NMC and HDPE/MC nanocomposites are more sensitive to nanoparticles’ concentration at low frequencies. The HDPE/MC nanocomposites showed semi-circle shapes comparing to HDPE/NMC nanocomposites. While the Cole–Cole plot of HDPE/NMC nanocomposite had more departure of semi-circle shape. The agglomerated particles could concentrate the imposed stress so the yield stress reached at lower shear rates comparing to pure HDPE and HDPE filled 2.5 wt.% NMC nanocomposite. Study of suspension models showed that the Eilers-Van Dijck and Einsten models fitted to almost experimental data satisfactorily.     

Postbuckling of nanotube-reinforced composite cylindrical shells under combined axial and radial mechanical loads in thermal environment

A postbuckling analysis is presented for nanocomposite cylindrical shells reinforced by single-walled carbon nanotubes (SWCNTs) subjected to combined axial and radial mechanical loads in thermal environment. Two types of carbon nanotube-reinforced composite (CNTRC) shells, namely, uniformly distributed (UD) and functionally graded (FG) reinforcements, are considered. The material properties of FG-CNTRCs are assumed to be graded in the thickness direction, and are estimated through a micromechanical model. The governing equations are based on a higher order shear deformation shell theory with a von Kármán-type of kinematic nonlinearity. The thermal effects are also included and the material properties of CNTRCs are assumed to be temperature-dependent. A boundary layer theory and associated singular perturbation technique are employed to determine the buckling loads and postbuckling equilibrium paths. The numerical illustrations concern the postbuckling behavior of perfect and imperfect, FG-CNTRC cylindrical shells under combined action of external pressure and axial compression for different values of load-proportional parameters. The results for UD-CNTRC shell, which is a special case in the present study, are compared with those of the FG-CNTRC shell.     

Size-dependent free flexural vibrational behavior of functionally graded nanobeams using semi-analytical differential transform method

In this paper nonlocal Euler–Bernoulli beam theory is employed for vibration analysis of functionally graded (FG) size-dependent nanobeams by using Navier-based analytical method and a semi analytical differential transform method. Two kinds of mathematical models, namely, power law and Mori-Tanaka models are considered. The nonlocal Eringen theory takes into account the effect of small size, which enables the present model to become effective in the analysis and design of nanosensors and nanoactuators. Governing equations are derived through Hamilton's principle and they are solved applying semi analytical differential transform method (DTM). It is demonstrated that the DTM has high precision and computational efficiency in the vibration analysis of FG nanobeams. The good agreement between the results of this article and those available in literature validated the presented approach. The detailed mathematical derivations are presented and numerical investigations are performed while the emphasis is placed on investigating the effect of the several parameters such as small scale effects, different material compositions, mode number and thickness ratio on the normalized natural frequencies of the FG nanobeams in detail. It is explicitly shown that the vibration of a FG nanobeams is significantly influenced by these effects. Numerical results are presented to serve as benchmarks for future analyses of FG nanobeams.     

Pull-in instability of carbon nanotube-reinforced nano-switches considering scale,surface and thermal effects

In this paper, an analytical method is presented to investigate the effect of surface characteristic and temperature change on the pull-in instability of electrically actuated nano-switches reinforced by carbon nanotubes (CNTs) based on Eringen's nonlocal beam theory. An extremely nonlinear fourth-order governing equation for the doubly clamped nano-switches made of CNTs/Si composites nanobeam is derived and solved by using the principle of virtual work, where Van der Waals force as atomic interactions and Casimir force as macro effects of quantum field fluctuation of vacuum are combined as an electrostatic force with fringing field effects. The results show that both the pull-in voltage and pull-in deflection of CNTs/Si composite nanobeam increase with the increase of CNTs volume ratio but decrease with the increase of temperature change. The coupling influences of small scale parameter, geometric behavior, surface characteristic and thermal effect on the pull-in instability of electrostatically actuated CNTs/Si nanobeam are detailedly discussed.     

Bending stiffening of graphene and other 2D materials via controlled rippling

In this paper, the effects of rippling on the bending stiffness of a monolayer graphene are studied. The initial rippling of the surface is modeled by cosine functions with a hierarchical topology. Considering both large displacement and small scale effect, the governing equilibrium equations are determined and solved. Then an equivalent bending stiffness is calculated for a rippled graphene and the effects of rippling, material discreteness, and structural dimension on its stiffness are discussed in details. The results quantify how the rippling strongly increases the effective bending stiffness of graphene and interacts with the discrete nature of the material not only because of increase in the moment of inertia. This approach can be applied to ripples design of 2D materials in order to achieve stiffening in bending as required in specific applications.     

A developed model to assume the interphase properties in a ternary polymer nanocomposite reinforced with two nanofillers

In the present study, a model is developed for prediction of tensile modulus and interphase properties in ternary polymer nanocomposites containing two nanofillers. In this regard, Ji model which assumes the interphase in particulate filled nanocomposites is developed for ternary systems in which, the arrangement of two nanofillers in parallel and series orders, the role of interphase properties and the dispersion quality of nanoparticles are taken into account by a simple approach. The calculations by the developed model are compared with the experimental results obtained for polypropylene (PP)/montmorillonite nanoclay (MMT)/CaCO₃ ternary nanocomposite. It is found that the absence of interphase causes much dissimilarity between experimental and theoretical data. However, a good agreement is obtained between the experimental data and the predicted results by the developed model for ternary polymer nanocomposites.     

Comparison of mechanical and interfacial properties of kenaf fiber before and after rice-washed water treatment

Rice-washed water was used to treat kenaf fiber using a spray coating method. Untreated kenaf fiber was compared with rice-washed water treated kenaf fiber in order to evaluate the treatments effects on mechanical and interfacial properties. The tensile strength and interfacial shear strength of kenaf fiber were improved by the rice-washed water treatment. Differences in the surface morphology of treated and untreated kenaf fiber was observed using FE-SEM photograph. TGA testing indicated that rice-washed water treated kenaf fiber improved the fiber's thermal stability. Static contact angle measurements of wettability demonstrated that the surface treated kenaf fiber was hydrophilic than that of untreated kenaf fiber. This relatively simple and environmentally friendly rice-washed water treatment of Kenaf fibers resulted in improve mechanical and interfacial properties.     

Experimental and theoretical analyses of mechanical properties of PP/PLA/clay nanocomposites

Compatibilized and non-compatibilized blends of polypropylene (PP) and poly(lactic acid) (PLA) with various compositions containing nanoclay particles were prepared by one step melt compounding in a twin screw extruder. Two nanocomposite systems with different matrices i.e. PP-rich (75/25 composition) containing Cloisite 15A and PLA-rich (25/75 composition) containing Cloisite 30B were selected for investigation of effect of nanoclays and n-butyl acrylate glycidyl methacrylate ethylene terpolymers (PTW) as compatibilizer on mechanical properties of PP/PLA/clay nanocomposites. Tensile and impact properties of the nanocomposite systems were investigated and correlated with their microstructures. Tensile modulus and strength of the blends were increased while elongation at break decreased by increasing PLA content. There was an irregular relationship between impact strength of the blends and PLA content. Several proposed models for blends and nanocomposites were used for prediction of tensile modulus of the samples. Most of the proposed models for blends could predict the tensile modulus of the blends successfully at low content of PLA. Another notable point was that most of the micromechanical models for nanocomposites fitted well to experimental values at low content of the clays and showed deviations at high clay loadings.     

Numerical study of the effects of reinforcement/matrix interphase on stress-strain behavior of YAl2 particle reinforced MgLiAl composites

For the potential influence produced by the reinforcement/matrix interphase in particle reinforced metal matrix composites (PMMCs), a unit cell model with transition interphase was proposed. Uniaxial tensile loading was simulated and the stress/strain behavior was predicted. The results show that a transition interphase with both appropriate strength and thickness could affect the failure mode, reduce the stress concentration, and enhance the maximum strain value of the composite.     

Transient wave velocities in pre-stressed thin-walled beams of open profile with Cosserat-type micro-structure

In the present paper, a new wave theory of thin-walled beams of open profile with Cosserat-type micro-structure is suggested based on the approach proposed by Rossikhin and Shitikova [1] for pre-stressed spatially curved thin-walled beams of open profile. The aim of the authors is to create the theory of propagation of transient waves (surfaces of strong discontinuity) in thin-walled beams of open profile, which should be quite different from Timoshenko-like theories, resulting in the data comparable with those corresponding to transient wave propagation in the three-dimensional Cosserat continuum.     

Effect of humidity during manufacturing on the interfacial strength of non-pre-dried flax fibre/unsaturated polyester composites

The influence of moisture content in the environment during manufacture of a novel cobalt-free UP matrix reinforced with flax fibres, on the fibre–matrix adhesion was studied. Flax surface energy was experimentally determined by measuring contact angles on technical fibres, using the Wilhelmy technique and the acid–base theory. The mechanical strength of the interface under different humidity conditions was characterized by the critical local value of interfacial shear stress, τ_d, at the moment of crack initiation, which was assessed by single-fibre pull-out tests. Differential scanning calorimetry and X-ray photoelectron spectroscopy analysis gave further insight into the topic. The results suggest that the effect of humidity during manufacturing on the composite interface might be limited. However, longitudinal composite strength decreased somewhat for composites produced in humid conditions, showing that there is some detrimental effect of high levels of moisture during cure on the fibre mechanical performance, likely caused by some fibre degradation.     

Nanostructured interfaces for enhancing mechanical properties of composites: Computational micromechanical studies

Computational micromechanical studies of the effect of nanostructuring and nanoengineering of interfaces, phase and grain boundaries of materials on the mechanical properties and strength of materials and the potential of interface nanostructuring to enhance the materials properties are reviewed. Several groups of materials (composites, nanocomposites, nanocrystalline metals, wood) are considered with view on the effect of nanostructured interfaces on their properties. The structures of various nanostructured interfaces (protein structures and mineral bridges in biopolymers in nacre and microfibrils in wood; pores, interphases and nanoparticles in fiber/matrix interfaces of polymer fiber reinforced composites and nanocomposites; dislocations and precipitates in grain boundaries of nanocrystalline metals) and the methods of their modeling are discussed. It is concluded that nanostructuring of interfaces and phase boundaries is a powerful tool for controlling the material deformation and strength behavior, and allows to enhance the mechanical properties and strength of the materials. Heterogeneous interfaces, with low stiffness leading to the localization of deformation, and nanoreinforcements oriented normally to the main reinforcing elements can ensure the highest damage resistance of materials.