Hong-Yuan Liu

Numerical study on the mode I delamination toughness of z-pinned laminates

A finite element (FE) model is developed to investigate mode I delamination toughness of z-pin reinforced composite laminates. The z-pin pullout process is simulated by the deformation of a set of non-linear springs. A critical crack opening displacement (COD) criterion is used to simulate crack growth in a double-cantilever-beam (DCB) made of z-pinned laminates. The toughness of the structure is quantified by the energy release rate, which is calculated using the contour integral method. The FE model is verified for both unpinned and z-pinned laminates. Predicted loading forces from FE analysis are compared to available test data. Good agreement is achieved. Our numerical results indicate that z-pins can greatly increase the mode I delamination toughness of the composite laminates. The influence of design parameters on the toughness enhancement of z-pinned laminates is also investigated, which provides important information to optimise and improve the z-pinning technique.

Effect of extended polymer chains on properties of transparent graphene nanosheets conductive film

This study examined the intercalation reaction of graphite oxide (GO) with poly(acryl amide)/poly(acrylic acid) (PMA) as a method to control the spacing between GOs. The interlayer spacing of GO was increased from 0.80 to 1.21 nm by grafting PMA on the GO surface. To fabricate transparent conductive films (TCFs), GOs must be reduced to graphene nanosheets (GNS) by a two-step chemical reduction with increased conductivity. The intercalated polymer chains of poly(acrylic acid) between GNS were extended as the carboxylic acid groups were deprotonated by the Na+ ions of NaBH4 on reduction, which efficiently inhibits GNS aggregation and restacking. The Na+ bonding on the polymer chains also facilitates electron transfer between the layers, yielding lower surface electrical resistance at the same GNS film thickness. The PMA grafted GNS (NE-PMA-GNS) composite films show the lowest sheet resistance of 2.11 × 102 Ω □−1, which is one order of magnitude less than that without grafting polymer (NE-GNS, 1.86 × 103 Ω □−1); moreover, instead of 0.22, the ratio of DC conductivity to optical conductivity (σDC/σOP) was 2.60. The higher σDC/σOP ratio indicates a higher TCFs performance.

Ultrafast Synthesis of Multifunctional N-Doped Graphene Foam in an Ethanol Flame

A hard template method to prepare N-doped graphene foams (NGF) with superfast template removal was developed through a pyrolyzing commercial polyurethane (PU) sponge coated with graphene oxide (GO) sheets in an ethanol flame. The removal of the template was fast and facile, and could be completed in less than 60 s in an open environment. The synthesized graphene foams consisted of a unique structure of 3D interconnected hollow struts with highly wrinkled surfaces, and the morphology of the hollow struts could be tuned by controlling the GO dispersion concentration. The foams showed high hydrophobicity and were used as absorbents for a variety of organic solvents and oils. The unique NGF structure afforded a high absorption rate and capacity, and a remarkable 98.7% pore volume of the foam could be utilized for absorption of hexane, exhibiting one of the highest capacity values among existing absorptive counterparts. The N-doping brought higher capacitive performance than conventional graphene foams prepared by chemical vapor deposition on nickel foam templates. The NGFs also displayed high elasticity and could recover completely after 50% compressive strain. Owing to easy availability and reduction environment of the flame, complete thermal decomposition of the PU sponge and highly porous open-cell structure, and flame resistance of the graphene foam, the present flame method was demonstrated to be a simple, effective, and ultrafast approach to fabricate ultra-low-density NGFs with good electromechanical response, excellent organic liquid absorption, and high-energy dissipation capabilities.

On steady-state fibre pull-outI The stress field

An improved theoretical model has been developed for describing the single-fibre pull-out process. With only a very few assumptions, solutions for the stress distribution can be obtained which satisfy all equilibrium equations, boundary conditions and continuity at the fibre/matrix. In the debonded region, a new friction law, in which the friction coefficient depends on the pull-out rate, is used to determine the interfacial shear stress. Numerical results for the stress distribution in both fully bonded and debonded regions are given for a typical carbon/epoxy composite system with different fibre pull-out rates, thermal residual stresses, material properties and the relative sizes of the fibre and matrix.

Evaluation of fibre tensile strength and fibre/matrix adhesion using single fibre fragmentation tests

Abstract Single fibre fragmentation tests were conducted to obtain a comprehensive understanding of the fibre fragmentation phenomenon in several carbon fibre/epoxy composite systems. Fibre tensile strength and fibre/matrix interfacial adhesion were evaluated, and some important experimental aspects associated with the evaluation of the test results were addressed. Fibre fragmentation in a matrix is a complex process with many factors interacting with each other, which influence the fibre fragmentation behaviour and, as a result, complicate the evaluation of the fibre/matrix interfacial properties. The failure modes of carbon fibres in epoxy resins were also greatly influenced by the variations of these factors. Without considering the special mechanisms of the fibre fragmentation behaviour for individual fibre/matrix systems, equivocal results may be obtained by simply adopting certain micromechanics model to define interfacial parameters.

On steady-state fibre pull-outII Computer simulation

Abstract On the basis of Part I of this paper (Zhang X, Liu, H-Y, Mai Y-W, Diaox X. On steady-state fibre pull-out Part I: stress field. Composites Science and Technology, 1999;59:2179–89) we present an extended analysis for the single-fibre pull-out process. The solutions of fibre axial stress, fibre displacement, and applied pull-out stress versus fibre displacement are obtained for the whole pull-out process. As distinct from previous work (Gao Y-C, Mai Y-W, Cottrell B. Fracture of fibre-reinforced materials. ZAMP 1988;39:550–72; Hutchinson JW, Jensen HN. Model of fibre debonding and pull-out in brittle composites with friction. Mechanics of Materials 1990;9:139–63; Hsueh C-H. Interfacial debonding and fibre pull-out stresses of fibre-reinforced composites. Materials Science and Engineering 1990;A123:1–11), a local shear strain criterion, in which the critical shear strain depends on the pull-out rate, is adopted as a more realistic interface debonding criterion. Load/displacement curves of the fibre pull-out process, which includes elastic deformation with a fully bonded interface, elastic deformation with a partially debonded interface and elastic deformation plus frictional sliding with a fully debonded interface, are obtained by computer simulations. The effects of fibre pull-out rate, thermal residual stress, friction coefficient and fibre volume fraction are also discussed.

On fibre pull-out with a rough interface

Abstract A theoretical analysis, based on the Fourier transformation approach, has been developed for the frictional pull-out of a single fibre with a rough interface from an elastic matrix. A Fourier series is used to express the amplitude function of the interfacial roughness, and solutions for the fibre pull-out stress are obtained for two different boundary conditions: one models a matrix containing an array of unidirectional fibres and the other a single fibre–matrix system. The interfacial roughness between the fibre and matrix is found to have a pronounced influence on the fibre sliding behaviour.

Self-assembly of graphene onto electrospun polyamide 66 nanofibers as transparent conductive thin films

A simple method was developed to assemble graphite oxide (GO) densely onto electrospun (ES) polyamide 66 (PA66) nanofibrous membranes, used as a guide for the deposition of graphene nanosheet (GNS) conductive networks for preparing transparent conductive thin film (TCF). The main advantage of this technique by comparison with previous methods is that graphene does not form a uniform coating, but a percolated conductive network, when guided by PA66 nanofiber templates. A low surface coverage of the transparent substrate by GNS resulted in high transmittance. Polyvinylpyrrolidone-stabilized GO (PVP-GO) was prepared as a modifier for improving the adsorption to the nanofibers. The resulting PVP-GO material could adsorb well on PA66 nanofibers due to stronger hydrogen bonds. Hence, a lower sufficient concentration of PVP-GO (0.050 wt%) solution was required than that for GO solution (0.100 wt%) to fabricate a complete conductive path through a possible enriched adsorption process. For TCF applications, a reduction step is essential because as-deposited GO is non-conductive. In this work, we reduced GO to GNS by a combination of chemical reduction and thermal annealing. The TCF optical transmittance also could be improved after thermal annealing at 350 °C above the PA66 melting point. Light scattering by PA66 nanofibers was found as the main cause of reduced transmittance. A fused film, obtained after electrospinning PA66 solution for 120 s, and immersing in 0.050 wt% PVP-GO solution, exhibits a surface resistance of 8.6 × 10³ Ω/square, while maintaining 88% light transmittance.

Stress transfer in the fibre fragmentation test: Part III Effect of matrix cracking and interface debonding

A theoretical stress analysis has been developed for the fibre fragmentation test in the presence of matrix cracks at sites of fibre breaks. The strain energy release rates for both matrix cracking and interface debonding are calculated for a carbon fibre/epoxy matrix composite. By comparing these strain energy release rates with the corresponding specific fracture resistances, the competition between matrix crack growth and interface debonding has been studied. The distributions of fibre axial stress and interfacial shear stress obtained from the present analysis show that the matrix crack substantially reduces the efficiency of stress transfer from the matrix to the fibre.

Improving the electrical conductivity and interface properties of carbon fiber/epoxy composites by low temperature flame growth of carbon nanotubes

Carbon nanotubes (CNTs) were grown in situ on carbon fibers (CFs) at low temperature (∼450 °C) in an ethanol combustion flame to develop multifunctional hierarchical reinforcements for epoxy resin matrices. Because of the low temperature, short duration and reducing atmosphere used in the flame growth process, there was no evident decrease of the tensile strength of the CFs. However, both the electrical conductivity and interfacial properties of the CFs were improved significantly after the CNTs were grown for only 3 minutes, resulting in >170% increase in in-plane electrical conductivity and ∼70% improvement in interfacial shear strength of the carbon fiber/epoxy composites. Electron microscopy studies revealed that both tip and root growth mechanisms were involved during the flame-induced synthesis. A good interfacial bonding strength between the CNTs and CFs was observed and could be attributed to the diffusion of metal catalyst particles into the CF surface and/or the carbon bonding between CNTs and CFs. Substantial improvements in electrical conductivity and interfacial properties without compromising the tensile strength of CFs after the flame growth of CNTs confirmed the efficiency and effectiveness of this method.