Porous composites coated with hybrid nano carbon materials perform excellent electromagnetic interference shielding

This work reported preparation of porous composites using a simple dip-coating method, and the fabricated composites containing hybrid carbon nanomaterials performed excellent electromagnetic interference (EMI) shielding properties. A commercial sponge was coated with silver nanoparticles before being dip-coated with graphene (GP)/ink, multi-wall carbon nanotubes (MWCNTs)/ink, or hybrid GP/MWCNTs/ink to form Ag/carbon nanomaterial hybrid composites, and then the composites were subjected to EMI measurements in the frequency range of 0.45–1.5 GHz. For comparison, the sponges without Ag nanoparticle coating were also prepared. Herein, we found an insignificant difference in EMI SE among the porous composites without Ag nanoparticle coating, and the maximum values of approximately 14.4 dB was attained. Interestingly, the hybrid composites with Ag nanoparticle coating exhibited maximum EMI shielding of 24.33 dB. Due to their porous structure, the EMI SE measurements showed that reflection dominates the EMI SE for all the sponge composites studied in this work.     

Damage analysis of pseudo-ductile thin-ply UD hybrid composites – A new analytical method

A new simple analytical approach for predicting all possible damage modes of Uni-Directional (UD) hybrid composites and their stress–strain response in tensile loading is proposed. To do so, the required stress level for the damage modes (fragmentation, delamination and final failure) are assessed separately. The damage process of the UD hybrid can then be predicted based on the order of the required stress for each damage mode. Using the developed analytical method, a new series of standard-thickness glass/thin-ply carbon hybrid composites was tested and a very good pseudo-ductile tensile response with 1.0% pseudo-ductile strain and no load drop until final failure was achieved. The yield stress value for the best tested layup was more than 1130 MPa. The proposed analytical method is simple, very fast to run and it gives accurate results that can be used for designing thin-ply UD hybrid laminates with the desired tensile response and for conducting further parametric studies.     

Numerical and analytical modeling of aligned short fiber composites including imperfect interfaces

Finite element calculations were used to bound the modulus of aligned, short-fiber composites with randomly arranged fibers, including high fiber to matrix modulus ratios and high fiber aspect ratios. The bounds were narrow for low modulus ratio, but far apart for high ratio. These numerical experiments were used to evaluate prior numerical and analytical methods for modeling short-fiber composites. Prior numerical methods based on periodic boundary conditions were revealed as acceptable for low modulus ratio, but degenerate to lower bound modulus at high ratio. Numerical experiments were also compared to an Eshelby analysis and to an new, enhanced shear lag model. Both models could predict modulus for low modulus ratio, but also degenerated to lower bound modulus at high ratio. The new shear lag model accounts for stress transfer on fiber ends and includes imperfect interface effects; it was confirmed as accurate by comparison to finite element calculations.     

Analysis of effect of fiber orientation on Young’s modulus for unidirectional fiber reinforced composites

Young’s modulus of unidirectional glass fiber reinforced polymer (GFRP) composites for wind energy applications were studied using analytical, numerical and experimental methods. In order to explore the effect of fiber orientation angle on the Young’s modulus of composites, from the basic theory of elastic mechanics, a procedure which can be applied to evaluate the elastic stiffness matrix of GFRP composite as an analytical function of fiber orientation angle (from 0° to 90°), was developed. At the same time, different finite element models with inclined glass fiber were developed via the ABAQUS Scripting Interface. Results indicate that Young’s modulus of the composites strongly depends on the fiber orientation angles. A U-shaped dependency of the Young’s modulus of composites on the inclined angle of fiber is found, which agree well with the experimental results. The shear modulus is found to have significant effect on the composites’ Young’s modulus, too. The effect of volume content of glass fiber on the Young’s modulus of composites was investigated. Results indicate the relation between them is nearly linear. The results of the investigation are expected to provide some design guideline for the microstructural optimization of the glass fiber reinforced composites.     

Ply-interleaving technique for joining hybrid carbon/glass fibre composite materials

Efficiently joining materials with dissimilar mechanical and thermal properties is fundamental to the development of strong and lightweight load-bearing hybrid structures particularly for aerospace applications. This paper presents a ply-interleaving technique for joining dissimilar composite materials. The load-carrying capacity of such a joint depends strongly on several design parameters such as the distance between ply terminations, the spatial distribution of ply terminations, and the stiffness and coefficients of thermal expansion of the composites. The effects of these factors on the strength of quasi-isotropic hybrid carbon/glass fibre composite are investigated using combined experimental, analytical and computational methods. Through fractographic analyses significant insights are gained into the failure mechanism of the hybrid joints, which are then used to aid the development of predictive models using analytical and high fidelity computational methods. To characterise the interaction between transverse matrix cracking and delamination, continuum damage mechanics model and cohesive zone model are employed. The predictions are found to correlate well with experimental data. These modelling tools pave the way for optimising hybrid joint concepts, which will enable the structural integration of dielectric windows required for multifunctional load-bearing antenna aircraft structures.     

An analytical model for B-stage joining and co-curing of carbon fibre epoxy composites

The objective of this paper was to develop cure kinetic models to describe the B-stage curing and co-curing assembly of carbon fibre reinforced thermosetting polymer (CFRP) composites. Starting from the analytical model, temperature cycles and experimental procedures are developed to join a B-stage CFRP part to a reinforcing B-stage CFRP patch for local reinforcement. Our results show that by using the analytical model, one may precisely describe the cure reaction and join the composites without any additional adhesive. The co-cured composites were successfully manufactured with stable fibre volume fractions and glass transition temperatures between the two sub-components. Additionally, merits of the process, such as modifying reinforcing areas locally, or formation of net shape detail are discussed.     

The method of cells and the mechanical properties of textile composites

The method of cells has been gaining ground as a method for predicting the elastic properties of textile reinforced composite materials. This method deals away with the constant stress/strain assumption present in most analytical models allowing for a more accurate prediction of the elastic properties, with computational cost only a fraction in comparison with finite element analysis. A new implementation is presented here, which links this method to a mechanistic predictive geometry preprocessor allowing the presented model to deal with 2D and 3D textile reinforced composites. Numerical results for prediction of stiffness and strength of textile composites are generated and compared to other methods.     

An analytical method for predicting cutting forces in orthogonal machining of unidirectional composites

Since the chip-formation mechanism in orthogonal machining of composites is different from that of metals, the cutting theories developed for metals cannot be directly used for orthogonal machining of composites. The objective of this research was to develop a new analytical method using energy method to predict the machining forces for orthogonal machining of unidirectional polymer–matrix composites (PMCs) for fiber orientations ranging from 90° to 180° Experiments were conducted to verify the validity of the proposed model using tools with rake angles of 5°, 10°, 15°, and 20°.     

Development of a new constitutive model considering the shearing effect for anisotropic progressive damage in fiber-reinforced composites

A corrected Linde's criterion considering the shearing effect for anisotropic progressive damage is developed to describe the elastic-brittle behavior of fiber-reinforced composites. Based on this criterion, a new three-dimensional (3D) nonlinear finite element model for static damage of unidirectional fiber-reinforced composites is proposed within a framework of continuum mechanics. The model is validated by taking 3D braided composites as example to study the relationship between the damage of materials and the effective elastic properties. The impregnated unidirectional composites are treated as homogeneous and transversely isotropic materials, whose properties are calculated by the Chamis' equations. The more accurate failure mechanisms of composites are revealed in the simulation process, and the effects of braided parameters on the uniaxial tensile behavior of 3D braided composites are investigated. Comparison of numerical results and experimental data is also carried out, which shows a better agreement than that of former study using the 3D Hashin's criterion.     

Pseudo-ductility in intermingled carbon/glass hybrid composites with highly aligned discontinuous fibres

The aim of this research is to manufacture intermingled hybrid composites using aligned discontinuous fibres to achieve pseudo-ductility. Hybrid composites, made with different types of fibres that provide a balanced suite of modulus, strength and ductility, allow avoiding catastrophic failure that is a key limitation of composites. Two different material combinations of high strength carbon/E-glass and high modulus carbon/E-glass were selected. Several highly aligned and well dispersed short fibre hybrid composites with different carbon/glass ratios were manufactured and tested in tension in order to investigate the carbon ratio effect on the stress–strain curve. Good pseudo-ductile responses were obtained from the high modulus carbon/E-glass composites due to the fragmentation of the carbon fibres. The experimental results were also compared with an analytical solution. The intermingled hybrid composite with 0.25 relative carbon ratio gave the maximum pseudo-ductile strain, 1.1%, with a 110 GPa tensile modulus. Moreover, the initial modulus of the intermingled hybrids with 0.4 relative carbon ratio is 134 GPa, 3.5 times higher than that of E-glass/epoxy composites. The stress–strain curve shows a clear “yield point” at 441 MPa and a well dispersed and gradual damage process.     

Nonlinear numerical modelling of FRP reinforced glued laminated timber

Fibre-reinforced polymers (FRPs) are effective in the flexural stiffening and strengthening of structural members. Such systems can be optimised if accurate numerical models are developed. At present, limited information is available in the literature on numerical models that can predict with good accuracy the nonlinear behaviour of FRP reinforced low-grade glued laminated timber beams. This paper discusses the development of a finite element model, which incorporates nonlinear material modelling and nonlinear geometry to predict the load–deflection behaviour, stiffness, ultimate moment capacity and strain distribution of FRP plate reinforced glued laminated timber beams manufactured from mechanically stress graded spruce. Beams with and without sacrificial laminations are modelled and their performance is compared to unreinforced glued laminated timber beams. The model employed anisotropic plasticity theory for the timber in compression. The failure model used was the maximum stress criterion. Strong agreement was obtained between the predicted behaviour and the associated experimental findings. It was deduced from comparing the results from the numerical model with experimental findings that the FRP plate succeeds in increasing the performance of the adjacent timber significantly. The model is a useful tool for examination of the effect of reinforcement percentage and will be used for optimisation of the hybrid beam.     

Modeling and experimental verification of dielectric constants for three-dimensional woven composites

In order to determine the dielectric constants of 3D orthogonal woven single fiber type (SFT) and hybrid composites from their component dielectric properties, a theoretical model is proposed based on the rule of binary mixtures. The model shows that with the same fiber volume fraction, a component with a larger cross-sectional area perpendicular to the electric field has a greater contribution to the composite dielectric constant. For experimental verification, SFT basalt/epoxy and aramid (Kevlar 129)/epoxy as well as interply and intraply basalt/aramid/epoxy 3D orthogonal woven hybrid composites were fabricated and their dielectric properties were measured using the waveguide method at a frequency range of 8–12 GHz. At 10 GHz, the experimental results agreed well with the calculated results from the model for the SFT composites, while a positive hybrid effect on the dielectric constant was observed for the two hybrid composites.     

An efficient numerical scheme for the simulation of parallel-plate active magnetic regenerators

A one-dimensional model of a parallel-plate active magnetic regenerator (AMR) is presented in this work. The model is based on an efficient numerical scheme which has been developed after analysing the heat transfer mechanisms in the regenerator bed. The new finite difference scheme optimally combines explicit and implicit techniques in order to solve the one-dimensional conjugate heat transfer problem in an accurate and fast manner while ensuring energy conservation. The present model has been thoroughly validated against passive regenerator cases with an analytical solution. Compared to the fully implicit scheme, the proposed scheme achieves more accurate results, prevents numerical errors and requires less computational effort. In AMR simulations the new scheme can reduce the computational time by 89%.     

The effect of fibre dispersion on initial failure strain and cluster development in unidirectional carbon/glass hybrid composites

By adding glass fibres to carbon fibre composites, the apparent failure strain of the carbon fibres can be increased. A strength model for unidirectional hybrid composites was developed under very local load sharing assumptions to study this hybrid effect. Firstly, it was shown that adding more glass fibres leads to higher hybrid effects. The hybrid effect was up to 32% for a hybrid composite with a 10/90 ratio of carbon/glass fibres. The development of clusters of broken fibres helped to explain differences in the performance of these hybrid composites. For 50/50 carbon/glass hybrids, a fine bundle-by-bundle dispersion led to a slightly smaller hybrid effect than for randomly dispersed hybrids. The highest hybrid effect for a 50/50 ratio, however, was 16% and was achieved in a composite with alternating single fibre layers. The results demonstrate that thin ply hybrids may have more potential for improved mechanical properties than comingled hybrids.     

Eigenstrain formulation of boundary integral equations for modeling particle-reinforced composites

A novel computational model is presented using the eigenstrain formulation of the boundary integral equations for modeling the particle-reinforced composites. The model and the solution procedure are both resulted intimately from the concepts of the equivalent inclusion of Eshelby with eigenstrains to be determined in an iterative way for each inhomogeneity embedded in the matrix. The eigenstrains of inhomogeneity are determined with the aid of the Eshelby tensors, which can be readily obtained beforehand through either analytical or numerical means. The solution scale of the inhomogeneity problem with the present model is greatly reduced since the unknowns appear only on the boundary of the solution domain. The overall elastic properties are solved using the newly developed boundary point method for particle-reinforced inhomogeneous materials over a representative volume element with the present model. The effects of a variety of factors related to inhomogeneities on the overall properties of composites as well as on the convergence behaviors of the algorithm are studied numerically including the properties and shapes and orientations and distributions and the total number of particles, showing the validity and the effectiveness of the proposed computational model.     

Mathematical modeling of flows from canals

In the hydrodynamic formulation, two-dimensional steady filtration in homogeneous isotropic ground from canals through a soil layer to the underlying highly permeable pressure water-bearing stratum is considered in the presence of the ground capillarity and evaporation from the free surface. To study filtration, a combined multiparametric boundary-value problem of the theory of analytical functions is formulated, which is solved using the P. Ya. Polubarinova-Kochina method and procedures of conformal mapping of regions of a special kind that are characteristic of the problems of subsurface hydromechanics. On the basis of this model an algorithm of calculating the capillary water spreading and the filtration discharge is developed for the situations where in water filtration from canals provision is made for the ground capillarity, evaporation from the free-surface of groundwater, and the additional pressure from the side of water of the underlying wellpermeable bed. With the aid of the obtained accurate analytical relations and numerical calculations a hydrodynamic analysis is made of the structure and character of specific features of the modeled process as well as of the effect of all physical parameters of the scheme on the filtration characteristics.     

RTM processing and electrical performance of carbon nanotube modified epoxy/fibre composites

This investigation focuses on nanoparticle filtration in the processing of multiscale carbon and glass fibre composites via resin transfer moulding. Surface modified and unmodified carbon nanotubes (CNTs) were incorporated into a commercial epoxy resin. The dispersion quality was evaluated using electrical measurements of the liquid suspensions. The manufacturing process was adapted to the challenges posed by the modified rheological behaviour of the CNT loaded resin. Nanoparticle filtration was observed; with some of the unmodified systems following so called ‘cake filtration’ behaviour. This resulted in nonlinear flow behaviour that deviated from the ideal response observed in RTM filling in conventional composites. The electrical conductivity of relatively high fibre volume fraction multiscale carbon and glass laminates increased by less than an order of magnitude with the addition of the nanotubes.     

A fast multipole boundary element method for 2D viscoelastic problems

A fast multipole formulation for 2D linear viscoelastic problems is presented in this paper by incorporating the elastic–viscoelastic correspondence principle. Systems of multipole expansion equations are formed and solved analytically in Laplace transform domain. Three commonly used viscoelastic models are introduced to characterize the time-dependent behavior of the materials. Since the transformed multipole formulations are identical to those for the 2D elastic problems, it is quite easy to implement the 2D viscoelastic fast multipole boundary element method. Besides, all the integrals are evaluated analytically, leading to highly accurate results and fast convergence of the numerical scheme. Several numerical examples, including planar viscoelastic composites with single inclusion or randomly distributed multi-inclusions, as well as the problem of a crack in a pressured viscoelastic plane, are presented. The results are verified by comparison with the developed analytical solutions to illustrate the accuracy and efficiency of the approach.     

Electrical resistance-based strain sensing in carbon nanotube/polymer composites under tension: Analytical modeling and experiments

This paper presents an analytical and experimental study on the strain sensing behavior of carbon nanotube (CNT)-based polymer composites. Tensile tests were conducted on CNT/polycarbonate composites and the responses in the electrical resistance were measured during the tests. An analytical model incorporating the electrical tunneling effect due to the matrix material between CNTs was also developed to describe the electrical resistance change as a result of mechanical deformation. The model deals with the inter-nanotube matrix deformation at the micro/nanoscale due to the macroscale deformation of the nanocomposites. A comparison of the analytical predictions and the experimental data showed that the proposed model captures the sensing behavior. In addition, the effect of the micro/nanoscale structures on the strain induced resistance change was discussed to provide useful information for designing CNT-based polymer composites with high strain sensing capability.