Multiscale Composite Materials Research Articles

High grafting strength from chemically bonded 2D layered material onto carbon microfibres for reinforced composites and ultra-long flexible cable electronic devices

Surface modification of carbon microfibres (CF) represents a promising and challenging alternative for creating multiscale and multifunctional hierarchical lightweight high-performance composite materials. In this study, a low-temperature chemical method is developed to graft layered graphene oxide (GO) directly onto CF through covalent bonding, and the grafting is characterised by SEM, FTIR, Raman and XPS spectroscopy. The GO failure stress (FS) measured using in situ nanomechanical pull-out tests is 36.2% higher than the literature values. Since GO fracture is the only breaking mechanism observed, there exists a strong carbon-carbon covalent bonding at the GO-CF interface indicating that the actual grafting strength (GS) is greater than the FS obtained. This elevated GS can substantially increase the interfacial and impact properties necessary in high performance composites. CF and GO-CF are fabricated to current-collector free ultra-long flexible cable-supercapacitors and their electrochemical properties in gel electrolyte are systematically investigated. GO-CF supercapacitor leads to an electrochemical capacitance of 7.8 F cm−3 and an energy density of 6.93 × 10−4 Wh cm−3 at 0.02 A cm−3. Thus, these grafted materials along with advanced composites have substantial promise in ultra-long flexible cable electronics with superior strengths and performances.

Influence of specific mechanical energy during compounding in a co-kneader on electrical and rheological properties of multiscale composite materials

Abstract This paper focuses on the effect of different scales of fillers, from nano to macrometer, on the rheological and electrical behaviour of thermoplastic-based composites. Composites based on poly (etherimide) matrix (PEI) with multiscale reinforcement are processed by melting in a co-kneader to disperse homogeneously carbon nanotubes and glass fibers. We clearly demonstrate the strong link between the liquid-solid transition and the electrical conductivity. We show that the specific mechanical energy is a key parameter. Glass fibers allow to increase the electrical conductivity of blends based on carbon nanotubes and to obtain a lower decreasing of conductivity at the liquid transition which takes place in the same time as rheological and electrical parameters.

Editorial: Multiscale Lattices and Composite Materials: Optimal Design, Modeling and Characterization

The Research Topic “Multiscale lattices and composite materials:” (MLCM) is focused on the optimal design, modeling, and characterization of novel lattices, composite materials, and structures at different scales, through the control of the internal architecture of the system. A fundamental goal of this article collection is the study of mechanical metamaterials that are able to form next-generation-generation cellular solids; lattice materials, multiscale composites; and structural-scale systems. The collection took inspiration from the peculiar behaviors exhibited by structured materials at multiple scales (Bosia et al., 2018). The latter include, for example, high stiffness, strength, and toughness at extremely low densities (Meza et al., 2014), phononic band-gaps (Lu et al., 2009), sound control ability (Cummer et al., 2016); negative effective mass density (Liu et al., 2000); localized confined waves (Theocharis et al., 2013), to name but a few examples. The research reported devoted special attention to the creation of complex mechanical systems with properties derived mainly from their geometric design rather than their chemical composition (Cummer et al., 2016; Bertoldi et al., 2017). Also investigated was the use of multiscale lattices to optimally design reinforcing elements for novel composite materials (Fleck et al., 2010; Li et al., 2014). The chosen modeling and experimental approaches were able to predict and characterize the intrinsically complex mechanical behavior of the analyzed systems through multiscale techniques.

Effect of testing procedure on the in-plane shear properties of CNF/glass/epoxy composites

Many investigations have demonstrated that the addition of nanoscale particles could affect in-plane shear properties of the laminated composites. Besides, a variety of testing procedures were introduced to evaluate the in-plane shear properties of the multiscale composite materials. In the current research, Iosipescu shear, double V-notched rail, and off-axis tensile testing methods were used to measure in-plane shear modulus and strength of the glass/epoxy and carbon nanofiber (CNF) as 0.25 wt% CNF/glass/epoxy laminated composites. In-plane shear properties of the CNF/glass/epoxy specimens were increased in comparison with the neat glass/epoxy specimens using all three testing procedures. However, the improvements were not identical for all the testing methods. The maximum improvements in the in-plane shear modulus and strength recorded using off-axis tensile test method were as 11% and 15.6%, respectively. In the off-axis tensile test method, all in-plane stress components are activated in the fracture plane parallel to the fiber orientation which are responsible for the failure initiation and propagation. Consequently, enhancing the resin’s mechanical property and interface bonding quality using CNF could remarkably enhance the in-plane shear property of the CNF/glass/epoxy specimens. On the other hand, the special fiber orientation of the specimens in Iosipescu shear and V-notched rail methods prevents the reinforcing effects of the CNF particles to be effectively revealed.

Application of a Laser-Based Time Reversal Algorithm for Impact Localization in a Stiffened Aluminum Plate

Non-destructive testing and structural health monitoring techniques using elastic guided waves are often limited by material inhomogeneity or geometrical irregularities of the tested parts. This is a severe restriction in many fields of engineering such as aerospace or aeronautics, where typically one needs to monitor composite structures with varying mechanical properties and complex ge- ometries. This is particularly true in the case of multiscale composite materials, where anisotropy and material gradients may be present. Here, we provide an impact localization algorithm based on time reversal and laser vibrometry to cope with this type of complexity. The proposed approach is shown to be insensitive to local elastic wave velocity or geometrical features. The technique is based on the correlation of the measured impact response and a set of measured test data acquired at various grid points along the specimen surface, allowing high resolution in the determination of the impact point. We present both numerical finite element simulations and experimental measure- ments to support the proposed procedure, showing successful implementation on an eccentrically stiffened aluminium plate. The technique holds promise for advanced structural health monitoring, potentially in real time, of geometrically complex composite structures.

Reiterated homogenization of a laminate with imperfect contact: gain-enhancement of effective properties

A family of one-dimensional (1D) elliptic boundary-value problems with periodic and rapidly-oscillating piecewise-smooth coefficients is considered. The coefficients depend on the local or fast variables corresponding to two different structural scales. A finite number of imperfect contact conditions are analyzed at each of the scales. The reiterated homogenization method (RHM) is used to construct a formal asymptotic solution. The homogenized problem, the local problems, and the corresponding effective coefficients are obtained. A variational formulation is derived to obtain an estimate to prove the proximity between the solutions of the original problem and the homogenized problem. Numerical computations are used to illustrate both the convergence of the solutions and the gain of the effective properties of a three-scale heterogeneous 1D laminate with respect to their two-scale counterparts. The theoretical and practical ideas exposed here could be used to mathematically model multidimensional problems involving multiscale composite materials with imperfect contact at the interfaces.

High sensitive damage sensors based on the use of functionalized graphene nanoplatelets coated fabrics as reinforcement in multiscale composite materials

Functionalized graphene nanoplatelets networks created through glass fiber fabrics were used to detect and locate damage in multiscale composite materials. The electrical behavior of multiscale composite materials was strongly influenced by microstructural features. Coated fabrics presented high sensitivity to breakage of fibers due to the preferential orientation of f-GNPs through fibers. This sensitivity was higher when damage was induced perpendicular to the fiber direction and the region where damage could be detected was bigger in the case of locating the measuring electrical channels perpendicular to fiber direction. These phenomena are related again to the morphology of the electrical network through the coating. Due to the insulating character through thickness of composites, detection and location was limited to layers of fabric of the composite. Nevertheless, self-sensors had the capacity of detecting and locating damage with high sensitivity by means of abrupt increases in electrical resistance induced by breakage.

Multimodel approach for accurate determination of industry-driven properties for Polymer Nanocomposite Materials

The need for effective and efficient design and production of sophisticated materials with advanced performances on a competitive time scale is strongly driving the integration of material modeling and simulation techniques into material selection decision processes. Specifically, for complex structural materials such as polymer-based nanocomposites (PNCs), there is a strong industrial demand for chemistry/physics-based models and modeling workflows able to predict relevant material properties in an accurate and reliable way. Under this perspective, in this work we describe the application of multiscale molecular modeling techniques for the choice of PNC materials for aerospace applications. The results are obtained in the framework of the European project Multi-scale Composite Material Selection Platform with a Seamless Integration of Materials Models and Multidisciplinary Design Framework (COMPOSELECTOR), funded by the European Commission within the H2020 call Advancing the integration of materials modeling in business processes to enhance effective industrial decision making and increase competitiveness.

Mechanical and Thermal Properties of Multi-scale Carbon Nanotubes–Carbon Fiber–Epoxy Composite

Multi-scale composite material was developed by combining carbon fabric and multi-wall carbon nanotubes (MWCNTs) in epoxy matrix. Varying concentrations of MWCNTs were ultrasonically dispersed in epoxy resin and applied on carbon fabric followed by curing. Tensile and flexural tests were conducted to observe the influence of MWCNTs on the mechanical properties of carbon fiber-reinforced plastic (CFRP). Coefficient of thermal expansion (CTE), thermal stability and glass transition temperature were determined through dilatometry, thermo-gravimetric analysis and differential scanning calorimetry, respectively. The MWCNTs dispersion in the epoxy was characterized through optical microscopy and scanning electron microscopy. Fractured surfaces of tensile samples were examined through high-resolution scanning electron microscope to investigate the failure mechanism. Tensile and flexural strengths were improved by 60 and 54%, respectively, on addition of 0.25 wt% MWCNTs in CFRP composite. This enhancement was attributed to the micro-crack bridging effect of the MWCNTs. Nanotubes acted as nano-stitches by holding the matrix together, thereby increasing the load bearing capability. Glass transition temperature and CTE of the CFRP was reduced by addition of MWCNTs whereas MWCNT–CFRP was found thermally stable up to 350 $${^{\circ }}$$ C. Microscopy results showed that the nanotubes were well dispersed in the epoxy matrix. However, higher weight fraction of MWCNTs in the epoxy had promoted agglomeration.

Sensitivity, influence of the strain rate and reversibility of GNPs based multiscale composite materials for high sensitive strain sensors

The addition of functionalized graphene nanoplatelets (f-GNPs) into the epoxy resin of glass fiber multiscale composite materials creates an electrically conductive network. This electrical network is highly sensitive to strain induced in the material. For this reason, it is a competitive material to be used in structural health monitoring (SHM). Sensitivity of multiscale composite materials is ∼50 under tensile loads and the behavior under compression loads differs, making possible the detection of different load conditions. Independency of the strain rate is related to the enhanced interface between f-GNPs and the epoxy matrix due to functionalization. Additionally, the electrical behavior is reversible without the loss of efficiency in sensorial properties.

Strain Sensing Based on Multiscale Composite Materials Reinforced with Graphene Nanoplatelets.

The electrical response of NH2-functionalized graphene nanoplatelets composite materials under strain was studied. Two different manufacturing methods are proposed to create the electrical network in this work: (a) the incorporation of the nanoplatelets into the epoxy matrix and (b) the coating of the glass fabric with a sizing filled with the same nanoplatelets. Both types of multiscale composite materials, with an in-plane electrical conductivity of ~10-3 S/m, showed an exponential growth of the electrical resistance as the strain increases due to distancing between adjacent functionalized graphene nanoplatelets and contact loss between overlying ones. The sensitivity of the materials analyzed during this research, using the described procedures, has been shown to be higher than commercially available strain gauges. The proposed procedures for self-sensing of the structural composite material would facilitate the structural health monitoring of components in difficult to access emplacements such as offshore wind power farms. Although the sensitivity of the multiscale composite materials was considerably higher than the sensitivity of metallic foils used as strain gauges, the value reached with NH2 functionalized graphene nanoplatelets coated fabrics was nearly an order of magnitude superior. This result elucidated their potential to be used as smart fabrics to monitor human movements such as bending of fingers or knees. By using the proposed method, the smart fabric could immediately detect the bending and recover instantly. This fact permits precise monitoring of the time of bending as well as the degree of bending.

Strain Sensing Based on Multiscale Composite Materials Reinforced with Graphene Nanoplatelets

Strain Sensing Based on Multiscale Composite Materials Reinforced with Graphene Nanoplatelets

Finite Element-Based Mechanical Assessment of Bone Quality on the Basis of In Vivo Images

Beyond bone mineral density (BMD), bone quality designates the mechanical integrity of bone tissue. In vivo images based on X-ray attenuation, such as CT reconstructions, provide size, shape, and local BMD distribution and may be exploited as input for finite element analysis (FEA) to assess bone fragility. Further key input parameters of FEA are the material properties of bone tissue. This review discusses the main determinants of bone mechanical properties and emphasizes the added value, as well as the important assumptions underlying finite element analysis. Bone tissue is a sophisticated, multiscale composite material that undergoes remodeling but exhibits a rather narrow band of tissue mineralization. Mechanically, bone tissue behaves elastically under physiologic loads and yields by cracking beyond critical strain levels. Through adequate cell-orchestrated modeling, trabecular bone tunes its mechanical properties by volume fraction and fabric. With proper calibration, these mechanical properties may be incorporated in quantitative CT-based finite element analysis that has been validated extensively with ex vivo experiments and has been applied increasingly in clinical trials to assess treatment efficacy against osteoporosis.

Electrically conductive functionalized-GNP/epoxy based composites: From nanocomposite to multiscale glass fibre composite material

Abstract The electrical behaviour and mechanical properties of functionalized graphene nanoplatelets (GNPs) reinforced epoxy-based composites were evaluated. These properties were analysed scaling up from the nanocomposite to the multiscale composite material. The electrical conductivity of 12 wt % GNPs reinforced nanocomposites was in the order of 10 −4 S/m, in contrast to the in-plane conductivity of multiscale composite materials with the same GNPs content, which was found to be ∼10 −3 S/m. This was attributed to microstructural features that also provoke the electrical conductivity through the thickness diminish down to ∼10 −4 S/m in multiscale composites. Additionally, mechanical properties were enhanced by the dispersion of GNP in nanocomposites but experienced a reduction in glass fibre composites because of the formation of a weak interface between the GNPs filled matrix and fibres.

3D nanomechanical evaluations of dermal structures in skin

Skin is a multilayered multiscale composite material with a range of mechanical and biochemical functions. The mechanical properties of dermis are important to understand in order to improve and compare on-going in vitro experiments to physiological conditions, especially as the mechanical properties of the dermis can play a crucial role in determining cell behaviour. Spatial and isotropy variations in dermal mechanics are thus critical in such understanding of complex skin structures. Atomic force microscopy (AFM) based indentation was used in this study to quantify the three dimensional mechanical properties of skin at nanoscale resolution over micrometre length scales. A range of preparation methods was examined and a mechanically non-evasive freeze sectioning followed by thawing method was found to be suitable for the AFM studies. Subsequent mechanical evaluations established macroscale isotropy of the dermis with the ground substance of the dermis dominating the mechanical response. Mechanical analysis was extended to show significant variation in the elastic modulus of the dermis between anatomical locations that suggest changes in the physiological environment influence local mechanical properties. Our results highlight dependence between an isotropic mechanical response of the dermal microenvironment at the nanoscale and anatomical location that define the variable mechanical behaviour of the dermis.

An update on the constitutive relation of ligament tissues with the effects of collagen types

The musculoskeletal ligament is a kind of multiscale composite material with collagen fibers embedded in a ground matrix. As the major constituent in ligaments to bear external loads, collagens are composed mainly of two collagen contents with different mechanical properties, i.e., types I and III collagen. The constitutive relation of ligaments plays a critical role in the stability and normal function of human joints. However, collagen types have not been distinguished in the previous constitutive relations. In this paper a constitutive relation for ligament tissues was modified based on the previous constitutive relation by considering the effects of collagen types. Both the collagen contents and the mechanical properties of sixteen ligament specimens from four cadaveric human knee joints were measured for determining their material coefficients in the constitutive relation. The mechanical behaviors of ligaments were obtained from both the uniaxial tensile and simple shear tests. A linear regression between joint kinematic results from in vitro and in silico experiments was made to validate the accuracy of this constitutive relation. The high correlation coefficient (R2=0.93) and significance (P<0.0001) of the regression equation revealed that this modified constitutive relation of ligaments was accurate to be used in studying joint biomechanics. Another finite element analysis with collagen contents changing demonstrated that the effect of variations in collagen ratios on both joint kinematics and ligament biomechanics could be simulated by this constitutive relation.

Continuous-Microflow Synthesis and Morphological Characterization of Multiscale Composite Materials Based on Polymer Microparticles and Inorganic Nanoparticles

This paper presents a new route to the synthesis of uniform and size-controlled inorganic/organic composite microparticles by means of microreaction technology. Au-nanoparticles in the range of 3 to 14 nm are synthesized by reduction of tetrachloroauric acid, while ZnO-nanoparticles (200–2000 nm) are synthesized in a continuous-flow two-step process using microtube arrangements for microsegmented flow. Both inorganic nanoparticles have a well-controlled size and narrow size distribution. Upon surface modification, the nanoparticles are then mixed on one hand with an acrylate-based monomer and, on the other hand, with an aqueous solution of acrylamide. Both solutions were then emulsified into uniform core-shell droplets by means of a capillary-based microfluidic device. Droplet's shell was hardened through UV-induced polymerization, whereas the core led to a hydrogel upon thermal-induced polymerization. Core-shell polymer microparticles (200–300 µm) with inorganic nanoparticles selectively incorporated int...

Effective Transport Properties of Porous Electrochemical Materials — A Homogenization Approach

This study concerns the determination of effective transport coefficients for multiscale composite materials used in various electrochemical systems. The effective transport coefficients are indispensable in macroscale modeling of such systems. We propose an integrated approach which for a given two-phase or three-phase microstructure allows us to systematically determine the exact values of different effective transport coefficients such as diffusivity of a species or electric conductivity. In addition to electron microscopy, this approach combines state-of-the-art techniques of mathematical homogenization, image processing and numerical computation. When only partial information about the microstructure is available, rigorous upper and lower bounds are available on the effective transport coefficients and we demonstrate that the commonly used Bruggeman’s formula may in fact violate the lower bound in some regimes. These upper bounds also allow one to quantify how much the transport properties of the material with a given composition could be improved. The proposed approach is illustrated by analyzing a three-phase electrode material of an actual Li-ion battery. We also quantify the uncertainty of the effective transport coefficients resulting from possibly imprecise information about the material properties of the individual phases and address the question concerning the importance of resolving all three phases.

Cement lines and interlamellar areas in compact bone as strain amplifiers – Contributors to elasticity, fracture toughness and mechanotransduction

Bone is multi-scale hierarchical composite material making the prediction of fragility, as well as pinning it to a certain cause, complicated. For proper mechanical simulation and reflection of bone properties in models, microscopic structural features of bone tissue need to be included. This study sets out to gain a mechanistic insight into the role of various microstructural features of bone tissue in particular cement lines and interlamellar areas. Further the hypothesis that compliant interlamellar areas and cement lines within osteonal bone act as strain amplifiers was explored. To this end, a series of experimentally-based micromechanical finite element models of bovine osteonal bone were developed. Different levels of detail for the bone microstructure were considered and combined with the results of physical three-point bending tests and an analytical composite model of a single osteon. The objective was to examine local and global effects of interface structures. The geometrical and microstructural characteristics of the bone samples were derived from microscopy imaging. Parametric finite element studies were conducted to determine optimal values of the elastic modulus of interstitial bone and interlamellar areas. The average isotropic elastic modulus of interfaces suggested in this study is 88.5MPa. Based on the modelling results, it is shown that interfaces are areas of accumulated strain in bone and are likely to act as potential paths for crack propagation. The strain amplification capability of interface structures in the order of 10 predicted by the models suggests a new explanation for the levels of strain required in bone homoeostasis for maintenance and adaptation.

Electrical, rheological and mechanical characterization of multiscale composite materials based on poly(etherimide)/short glass fibers/multiwalled carbon nanotubes

Composites based on poly(etherimide) matrix (PEI) with nanoscale reinforcements (multiwalled carbon nanotubes (MWCNTs)) and multiscale reinforcements (MWCNTs and Glass Fibers (GFs)) are processed by melt blending. For nanocomposite materials, we demonstrate that addition of MWCNTs (i) significantly improve electrical conductivity of PEI matrix, (ii) do not significantly improve the mechanical properties (young and flexural modulus) but strongly degrades the strain at break. In the case of multiscale composite materials PEI_GF_MWCNT, strain at break increases with the nanofiller content, and the electrical properties are the same than that in the case of nanocomposites. TEM observations are performed at the PEI/GF interfaces and clearly show a preferential location and orientation of MWCNTs along and perpendicularly to the GFs which explain the evolution of the strain at break for the multiscale composite materials.