Upscaling of viscoelastic properties of highly-filled composites: Investigation of matrix–inclusion-type morphologies with power-law viscoelastic material response

In this paper, homogenization schemes for upscaling of elastic properties in the framework of continuum micromechanics are extended towards upscaling of viscoelastic material properties. Hereby, the Laplace–Carson transform method is applied to the Mori–Tanaka scheme, the self-consistent scheme, and the generalized self-consistent scheme and solved numerically by the Gaver–Stehfest algorithm. The performance of the so-obtained upscaling schemes is: (i) illustrated for an academic example (a 2-phase composite with Maxwellian-type creep response of the phases) and (ii) assessed considering a polyester matrix/marble dust filler composite with respective experimental data taken from the literature. Hereby, for the investigated range of volume fractions of inclusions, ranging from 29 vol.% to 55 vol.%, and a matrix/inclusion-type morphology, the transformed generalized self-consistent scheme emerged as the most suitable scheme for determination of the effective viscoelastic properties of this highly-filled composite material, resulting in a sound representation of the experimental data.     

Multi-Inclusion modeling of multiphase piezoelectric composites

Recent work on multifunctional materials has shown that a functionally graded interface between the fiber and matrix of a composite material can lead to improved strength and stiffness while simultaneously affording piezoelectric properties to the composite. However the modeling of this functional gradient is difficult through micromechanics models without discretizing the gradient into numerous layers of varying properties. In order to facilitate the design of these multiphase piezoelectric composites, accurate models are required. In this work, Multi-Inclusion models are extended to predict the effective electroelastic properties of multiphase piezoelectric composites. To evaluate the micromechanics modeling results, a three dimensional finite element model of a four-phase piezoelectric composite was created in the commercial finite element software ABAQUS with different volume fractions and aspect ratios. The simulations showed excellent agreement for multiphase piezoelectric composites, and thus the modeling approach has been applied to study the overall electroelastic properties of a composite with zinc oxide nanowires grown on carbon fibers embedded in the polymer. The results of this case study demonstrate the importance of the approach and show the system cannot be accurately modeled with a homogenized interphase.     

Effective moduli of multi-scale composites

The effective moduli of a multi-scale composite are evaluated by a bottom-up (hierarchical) modeling approach. We focus on a two-scale structure in which the small scale includes a platelet array inside a matrix, and the large scale contains fibers inside a composite matrix. We demonstrate that the principal moduli of the multi-scale composite can be fine-tuned by the platelet arrangement and orientation. As a case study, we consider the phenomenon of fiber micro-buckling within the multi-scale composite. It is found that the compressive micro-buckling strength can be considerably increased for specific platelet orientations. The multi-scale design approach presented here can be used to generate novel families of composite materials with tunable mechanical properties.     

Pseudo-percolation: Critical volume fractions and mechanical percolation in polymer nanocomposites

Polymer nanocomposites offer a basis for the design and manufacture composite materials with greatly enhanced properties at relatively low volume fractions of the included phase. One underlying mechanism, thought to contribute to these properties is the presence of an interfacial region, ∼15 nm thick, between the polymer matrix and included particles. The size of the interface makes relatively little contribution to the effective properties of composites with micro-sized particles but, because its thickness is comparable to the size of the nanoscaled included phase, its potential impact within nanocomposites is much greater. In particular, percolated nano-microstructures may result at volume fractions below theoretical thresholds, due to connectivity achieved through rod-interface-rod, or ‘pseudo-percolation’, contact. In this work the influence of the interface layer is incorporated into estimates of critical volume fraction through an excluded volume model. Results show a significant reduction in the range of critical volume fractions. These values are incorporated into a mean-field micromechanics model to illustrate mechanical percolation through changes in predicted effective elastic composite properties.     

Unified microporomechanical approach for mechanical behavior and permeability of misaligned unidirectional fiber reinforcement

The microporomechanical approach (via homogenization schemes) has been used and combined with triaxial tests to verify the Biot theory for the perfectly straight unidirectional fiber assembly in a previous paper [Tran T, Binetruy C, Comas-Cardona S, Abriak NE. Microporomechanical behaviour of perfectly straight unidirectional fiber material: theoretical and experimental. Compos Sci Technol 2009;69:199–206.]. The comparison of theoretical and experimental results is in good agreement, i.e. the Biot coefficients are clearly lower than one for densely packed fiber array. This result will be developed in this article in the case where the fibers are not perfectly straight but in misalignment (unidirectional fiber assembly in localized contact). Furthermore, within the same theoretical framework, the transverse compression modulus and the hydraulic permeability will be also estimated for the fiber reinforcement of double-scale porosity. The homogenization schemes used in this article are the self-consistent and the one proposed by Mori–Tanaka. The estimated and, when possible, bibliographical results for different types of fibrous materials (carbon, kevlar and glass fibers) are compared and show good agreement.     

A micromechanical characterization of angular bidirectional fibrous composites

A micromechanical numerical algorithm to efficiently determine the homogenized elastic properties of bidirectional fibrous composites is presented. A repeating unit cell (RUC) based on a pre-determined bidirectional fiber packing is assumed to represent the microstructure of the composite. For angular bidirectional fiber distribution, the symmetry lines define a parallelepiped unit cell, representing the periodic microstructure of an angular bidirectional fiber composite. The lines of symmetry extrude a volume to capture a three dimensional unit cell. Finite element analysis of this unit cell under six possible independent loading conditions is carried out to study and quantify the homogenized mechanical property of the cell. A volume averaging scheme is implemented to determine the average response as a function of loading in terms of stresses and strains. The individual elastic properties of the constituents’ materials, as well as, the composite can be assumed to be completely isotropic to completely anisotropic. The output of the analysis can determine this degree. The logic behind the selection of the unit cell and the implementation of the periodic boundary conditions as well as the constraints are presented. To verify this micromechanics algorithm, the results for four composites are presented. The results in this paper are mainly focused on the impact of the fiber cross angles on the stiffness properties of the composites chosen. The accuracy of the results from this micromechanics modeling procedure has been compared with the stiffness/compliance solutions from lamination theory. The methodology is to be accurate and efficient to the extent that periodicity of the composite material is maintained. In addition, the results will show the impact of fiber volume fraction on the material properties of the composite. This micromechanics tool could make a powerful viable algorithm for determination of many linear as well as nonlinear properties in continuum mechanics material characterization and analysis.     

Structural health monitoring of glass fiber reinforced composites using embedded carbon nanotube (CNT) fibers

In the present work, carbon nanotube (CNT) fibers had been embedded to glass fiber reinforced polymers (GFRP) for the structural health monitoring of the composite material. The addition of the conductive CNT fiber to the non-conductive GFRP material aims to enhance its multi-function ability; the test specimen’s response to mechanical load and the insitu CNT fiber’s electrical resistance measurements were correlated for sensing and damage monitoring purposes. It is the first time this fiber is used in composite materials for sensing purposes; CNT fiber is easy to be embedded and does not downgrade the material’s mechanical properties. Various incremental loading–unloading steps had been applied to the manufactured specimens in tension as well as in three-point bending tests. The CNT fiber worked as a sensor in both, tensile and compression loadings. A direct correlation between the mechanical loading and the electrical resistance change had been established for the investigated specimens. For high stress (or strain) level loadings, residual resistance measurements of the CNT fiber were observed after unloading. Accumulating damage to the composite material had been calculated and was correlated to the electrical resistance readings. The established correlation between these parameters changed according to the material’s loading history.     

Strength prediction of multi-layer plain weave textile composites using the direct micromechanics method

Previously developed micromechanical methods for stiffness and strength prediction are adapted for analysis of multi-layer plain weave textile composites. Utilizing the direct micromechanics method (DMM) via finite element modeling, three methods are presented: (a) direct simulation of a multi-layer plain weave textile composite; (b) micromechanical analysis of a single layer of interest from the force and moment resultants acting on that layer; and (c) application of the previously developed quadratic stress-gradient failure theory to the layer of interest. In comparison to direct modeling, the other two techniques show only 5% difference over a number of random test cases. Several practical design examples of strength prediction are included to illustrate the importance and accuracy of method implementation.     

A composite material used as a membrane for ophthalmology applications

New biomaterials for intracorneal ophthalmologic implants were designed, manufactured and characterized. A composite material in the form of a membrane was manufactured in a two-stage process. The first stage of the process depended on preparation of multidimensional (MD-type) fibrous polymer composite. A stable terpolymer polytetraflouroethylene-co-polyvinylene fluoride-co-polypropylene (PTFE–PVDF–PP) was used as a composite matrix, and sodium alginate-based biopolymer (NA) in the form of short fibres and/or powder were used as porogenic constituents. The composite materials were subjected to physicochemical treatment in order to remove a water soluble biopolymer. The treatment led to about 50% of open porosity within the polymer matrix. Depending on the membrane type the mean pore size determined with SEM microphotographs was 15–25 μm. Permeability and durability of the membranes in simulated eye fluid (culture medium enriched with albumin) was tested. The size and shape of the pores before and after the permeability test were compared (SEM), and they depend on the porogen form. Mechanical parameters of the composite materials such as; tensile strength, Young’s modulus, and strain to failure were measured. A membrane derived from fibres and particles showed better mechanical properties than a membrane derived from porogen particles. Microstructure and mechanical properties make the membranes a good candidate for ophthalmological implants.     

Positive size and scale effects of all-cellulose composite laminates

Negative size effects are commonly reported for advanced composite materials where the strength of the material decreases with increasing volume of the test specimen. In this work, the effect of increasing specimen volume on the mechanical properties of all-cellulose composites is examined by varying the laminate thickness. A positive size effect is observed in all-cellulose composite laminates as demonstrated by a 32.8% increase in tensile strength as the laminate thickness is increased by 7 times. The damage evolution in all-cellulose composite laminates was examined as a function of the tensile strain. Enhanced damage tolerance concomitant with increasing specimen volume is associated with damage accumulation due to transverse cracking and strain delocalisation. A transition from low-strain failure to tough and high-strain failure is observed as the laminate thickness is increased. Simultaneously, scale effects lead to an increase in the void content and cellulose crystallinity at the core, with increasing laminate thickness.     

Effect of nanotube geometry on the elastic properties of nanocomposites

Many analytical models replace carbon nanotubes with “effective fibers” to bridge the gap between the nano and micro-scales and allow for the calculation of the elastic properties of nanocomposites using micromechanics. Although curvature of nanotubes can have a direct impact on these properties, it is typically ignored. In this work, the nanotube geometry in 3D is included in the calculation of the elastic properties of a modified effective fiber. The strain energy of the nanotube and the effective fiber are calculated using Castligiano’s theorem and constraints imposed by the matrix on the deformation are taken into consideration. Model results are compared to results from archived literature, and a reasonable agreement is observed. Results show that the effect of nanotube curvature on reducing the modulus of the effective fiber is not limited to in-plane curvature but also to curvature in 3D. The impact of the nanotube curvature on the elastic properties of nanocomposites is studied utilizing the modified fiber model and the approach developed by Mori–Tanaka. Analytical results show that for a low weight fraction of nanotubes the effect of curvature seems to be minor and as the weight fraction increases, the effect of nanotube curvature becomes critical.     

Meso-scale modeling of hypervelocity impact damage in composite laminates

ObjectivesThis paper presents an approach to numerical modeling of hypervelocity impact on carbon fiber reinforced plastics (CFRP).MethodsThe approach is based on the detailed meso-scale representation of a composite laminate. Material models suitable for explicit modeling of laminate structure, including fiber-reinforced layers and resin-rich regions, are described. Two numerical impact tests with significantly different impact energies were conducted on thermoplastic AS4/PEEK materials with quasi-isotropic layups. Simulations employed both SPH and Finite element methods.ResultsResults of simulations are verified against experimental data available from the literature and demonstrate good correlation with the experiments.ConclusionsDeveloped modeling approach can be used in simulations where post-impact damage progression in composite material is of the main focus.     

Effect of short carbon fibers and MWCNTs on microwave absorbing properties of polyester composites containing nickel-coated carbon fibers

Multiphase composite materials filled with multiwall carbon nanotubes (MWCNTs), short nickel-coated carbon fibers and millimeter-long carbon fibers with various weight fractions and compositions are developed and used for the design of wide-band thin radar-absorbing screens. The effective complex permittivity of several composite samples is measured in the frequency range from 8 GHz to 18 GHz. The obtained results show that the addition of the MWCNTs into the mixture allows tuning the EM properties of the composite filled with the short nickel-coated fibers. Numerical simulations are also performed in order to design new radar-absorbing shields. Single-layer and bi-layer thin dielectric Salisbury screens are designed to exhibit minimum reflection coefficient at 10 GHz and at 15 GHz, and maximum bandwidth at −10 dB. It results that the total thickness of the screen can be reduced below 2 mm by using a lossy sheet made with the composite filled with MWCNTs and nickel-coated carbon fibers, whereas the bandwidth at −10 dB can exceed 6 GHz in a bi-layer structure.     

Methods to measure interlaminar tensile modulus of composites

This work expands a recently developed short-beam method coupled with the Digital Image Correlation full-field surface deformation measurement technique to enable assessment of the interlaminar tensile stress–strain constitutive properties of polymer–matrix composite materials. This work also expands the American Society for Testing and Materials Standard D 6415 curved-beam method as another means for measurement of the interlaminar tensile stress–strain constitutive behavior. The interlaminar tensile modulus values resulting from both methods are compared for Hexcel IM7/8552 carbon/epoxy tape composite material system.     

Effect of the stochastic nature of the constituents parameters on the predictability of the elastic properties of fibrous nano-composites

The stochastic nature and the variability of the constituents of nano-composites materials affect the predictability of their properties. The few studies that dealt with the probabilistic nature of the micromechanics of fibrous nano-composites, focused on the effect of statistical variation of individual parameters. This study presents a systematic analysis of the influence of parameter randomness on the theoretical predictions of the elastic properties of nano-composites. To this end, Monte-Carlo simulations are performed using a modified version of the Mori–Tanaka Mean-Field theory under different combinations of parameter randomness. The results indicate that the randomness in interface imperfection, fibre orientation and length, and fibre stiffness have a significant influence on the variability of the composite properties. The analysis provided an insight into the sensitivity of the predictions of the elastic tensor to the probabilistic variations of the aforementioned parameters. A probabilistic model for the effective properties is called for in place of deterministic models.     

Processing-structure–property relationships of SWNT–epoxy composites prepared using ionic liquids

Experimentally achieved mechanical properties of nanotube–epoxy composites fail to match theoretical expectations; shortcomings are mainly attributed to poor dispersion. The elastic modulus of a well-dispersed single walled carbon nanotube (SWNT)-ionic liquid-epoxy composite was evaluated in tension and compared to predictions by a micromechanics homogenization model. The model takes into account the mechanical properties of the constituent phases in addition to SWNT aspect ratio, spatial distribution, dispersion, and agglomeration. These parameters were evaluated using information obtained via scanning and transmission electron microscopy. The Young’s modulus of the composite shows excellent agreement with the model at low concentrations, while discrepancies at high SWNT concentrations are possibly due to composite processing limitations. At high concentrations the uncured composite mixture is above the rheological percolation threshold. As the polymer network reaches its maximum capacity for well-dispersed SWNTs, increasing volume fraction does not result in further significant reinforcing effects.     

Load and health monitoring in glass fibre reinforced composites with an electrically conductive nanocomposite epoxy matrix

Fibre reinforced polymers (FRPs) are an important group of materials in lightweight constructions. Most of the parts produced from FRPs, like aircraft wings or wind turbine rotor blades are designed for high load levels and a lifetime of 30 years or more, leading to an extremely high number of load cycles to sustain. Consequently, the fatigue life and the degradation of the mechanical properties are aspects to be considered. Therefore, in the last years condition monitoring of FRP-structures has gained importance and different types of sensors for load and damage sensing have been developed.

In this work a new approach for condition monitoring was investigated, which, unlike other attempts, does not require additional sensors, but instead is performed directly by the measurement of a material property of the FRP. An epoxy resin was modified with two different types of carbon nanotubes and with carbon black, in order to achieve an electrical conductivity. Glass fibre reinforced composites (GFRP) were produced with these modified epoxies by resin transfer moulding (RTM). Specimens were cut from the produced materials and tested by incremental tensile tests and fatigue tests and the interlaminar shear strength (ILSS) was measured. During the mechanical tests the electrical conductivity of all specimens was monitored simultaneously, to assess the potential for stress/strain and damage monitoring.

The results presented in this work, show a high potential for both, damage and load detection of FRP structures via electrical conductivity methods, involving a nanocomposite matrix.     

Overall behavior and microstructural deformation of R-CNT/polymer composites

Carbon nanotubes (CNTs) are an excellent candidate for the reinforcement of composite materials owing to their distinctive mechanical and electrical properties. Reticulate carbon nanotubes (R-CNTs) with a 2D or 3D configuration have been manufactured in which nonwoven connected CNTs are homogeneously distributed and connected with each other. A composite reinforced by R-CNTs can be fabricated by infiltrating a polymer into the R-CNT structure, which overcomes the inherent disadvantages of the lack of weaving of the CNTs and the low strength of the interface between CNTs and the polymer. In this paper, a 2D plane strain model of a R-CNT composite is presented to investigate its micro-deformation and effective stiffness. Using the two-scale expansion method, the effective stiffness coefficients and Young’s modulus are determined. The influences of microstructural parameters on the micro-deformation and effective stiffness of the R-CNT composite are studied to aid the design of new composites with optimal properties. It is shown that R-CNT composites have a strong microstructure-dependence and better effective mechanical properties than other CNT composites.     

Estimating mechanical properties of 2D triaxially braided textile composites based on microstructure properties

Textile composites manufactured using Resin Transfer Modeling (RTM) can offer advantages in some automotive applications including reduction in weight, while being relatively simpler to fabricate than standard laminated composites used for aerospace applications. However, one of the challenges that arise with these textile composite materials is that the mechanical properties are inherently dependent on the local and final (in-situ) architecture of the textile itself as a result of the molding and curing processes. While this provides additional latitude in the composite design process it also necessitates the development of analytical models that can estimate the mechanical properties of a textile composite based on the textile architecture and the properties of the manufactured component.In this paper, an analytical model is developed and its estimations are compared against experimental in-plane engineering properties for composites with various textile architectures. Results from the model are also compared against finite element (FE) based computational results. The microstructures of the 2D triaxially braided composite (2DTBC) studied were extensively characterized. The microstructure properties thus measured were used in the analytical model to estimate the mechanical properties. Uniaxial tension and V-notched rail shear tests were conducted on 2DTBC with different textile architectures. Good agreement between the analytical, computational, and experimental results were observed and are reported here. Furthermore, computational estimations of matrix mechanical properties are limited to the linear elastic range of a representative material volume (unit cell) and coupon data. Full mechanical response of larger 2DTBC structures, albeit of prime interest, is beyond the scope of this work and could be the focus of follow up studies.     

Novel wear-resistant materials - Carbon fiber reinforced low-carbon Stellite alloy composites

This paper reports the design and development of a class of new composite materials, which are low-carbon Stellite alloy matrices reinforced with carbon fibers. The focus of the research is to compare the different effects of carbon fibers versus carbides on Stellite alloys. Stellite 25 was selected as the matrix because of its very low carbon content (0.1 wt.%), thus minimal carbide volume fraction. The composite specimens are fabricated using the hot isostatic pressing and sintering techniques. The microstructures of the specimens are examined with optical microscopy in order to identify the possible carbide formation from the carbon fibers. The material characterization of the specimens is achieved through hardness test, sliding wear test and corrosion test. These novel materials exhibit superior properties compared to existing Stellite alloys and are expected to spawn a new generation of materials used for high temperature, severe corrosion, and wear resistant applications in various industries.