Narrow Results

Classical percolation theory is concerned with the onset of geometrical connectivity and the accompanied onset of electrical connectivity in disordered systems. It was found, however, that in many systems, such as various composites, the geometrical and electrical onsets of the connectivity are not simultaneous and the correlation between them depends on physical processes such as tunneling. The difference between the above two types of systems and the consequences for the electrical transport properties of the latter composites have been largely ignored in the past. The application of scanning local probe microscopies and some recent theoretical developments have enabled a better understanding of the latter systems and their sometimes "strange" behavior as bona fide percolation systems. In this review we consider the above issues and their manifestation in three types of systems: Carbon Black–Polymer composites, metal–insulator cermets and hydrogenated microcrystalline silicon.

Peridynamics theory is a nonlocal meshless method that replaces differential equations with spatial integral equations, and has shown good applicability and reliability in the analysis of discontinuities. Further, with characteristics of clear physical meaning and simple and reliable numerical calculation, the bond-based peridynamics method has been widely applied in the field. However, this method describes the interaction between material points simply using a single elastic “spring”, and thus leads to a fixed Poisson’s ratio, relatively low computational efficiency and other inherent problems. As such, the goal of this review paper is to provide a summary of the various methods of bond-based peridynamics modeling, particularly those that have overcome the limitations of the Poisson’s ratio, considered the shear deformation and modeling of two-dimensional thin plates for bending and three-dimensional anisotropic composites, as well as explored coupling with finite element methods. This review will determine the advantages and disadvantages of such methods and serve as a starting point for researchers in the development of peridynamics theory.

The properties of fiber-matrix interface pay an important effect on the mechanical properties of carbon fiber-reinforced composites. To improve the interfacial properties in carbon fiber/phenolic resin (CF/PF) composites, CF adhered CNTs (CNTs-CF) were prepared by CVD method. The surface of treated CF and the distribution of CNTs in the interfacial region of CF were detected by scanning electron microscopy (SEM). SEM images identified 700${}^{\circ }$C being the optimum growth temperature to obtain CNTs-CF, in which CNTs randomly dispersed surrounding the individual fiber surfaces. Compared to crude CF/PF composites, the tensile strength, bending strength, shear strength, impact strength, thermal conductivity at 25${}^{\circ }$C of CNTs-CF/PF composites increase by 144.5%, 59.2%, 129.0%, 75.9% and 41.7%. The improvement of mechanical properties is due to that the homogenously dispersed CNTs serve as a supplementary reinforcement to the interface and further reduce inter-laminar stress concentration. This represents an important step toward CNT-reinforced polymers with high mechanical properties.

In this paper, various research works have been performed on synthesis and functionalization after the invention of graphene (Gr) and its spectral properties. The reinforcing of Gr family members into the matrix leads to the development of novel composites. Its characterization in terms of morphological and physicochemical properties is required for the discovery of wide-ranging applications. This paper reviews the various emerging features and future scope for reduced graphene oxide (rGO). The possibilities of composites development through the different matrix are proposed to evaluate application potential, various fabrication techniques for Gr, and its role in improving composite properties. The work initially examines multiple ways to synthesize rGO. Its addition to different materials includes metals, metal oxides, ceramics, polymers, and organic compounds through various composite preparation techniques. Finally, this paper gives an overview of the enhancement in properties obtained due to the addition of rGO, which opens the door for a broad set of applications. This paper concludes with a comparative study that defines a suitable preparation technique according to the required material composites, desired properties, and specific applications. An attempt has been proposed to examine the exceptional feature of rGO/composite for cost-effective functions in manufacturing sectors.

Modern fiber placement machines allow laminates with spatially varying stiffness properties to be manufactured. In earlier research, the authors optimized variable stiffness plates for maximum buckling load, demonstrating significant improvements in load-carrying capacity. In aerospace applications, panel structures are often permitted to enter the postbuckling regime during service. It is, therefore, not only important to understand their postbuckling behavior, but also to develop fast analysis methods that can subsequently be used in a design optimization framework. The aim of the present research is to study the postbuckling behavior of the optimized plates using a perturbation method that has been developed earlier within a general-purpose finite element environment. The perturbation approach is used to compute postbuckling coefficients, which are used to make a quick estimate of the postbuckling stiffness of the panel and to establish a reduced-order model. In the present work, the postbuckling analysis of variable stiffness plates is carried out using the reduced-order model, and the potential of the approach for incorporation within the optimization process is demonstrated.

The embedded atom method (EAM) is used to construct an interatomic potential for modelling interfaces in Cu–Nb nanocomposites. Implementation of the Ziegler–Biersack–Littmark (ZBL) model for short-range interatomic interactions enables studies of response to ion bombardment. Collision cascades are modelled in fcc Cu, bcc Nb, and in Cu–Nb layered composites in the experimentally-observed Kurdjumov–Sachs (KS) orientation relation. The interfaces in these composites reduce the number of vacancies and interstitials created per keV of the primary knock-on atom (PKA) by 50–70% compared to fcc Cu or bcc Nb.

This work presents the results of study of the electrophysical properties of composite polymer ceramics (1$-x)$[KNN-LTSN]–$x$PVDF at $x$ = 25 vol.% and $x$= 50 vol.% in the temperature range of $T$ = 20–160${}^{\circ }$C and frequency range of $f$ = 2 × 10¹–2 × 10⁶ Hz. The concentration dependence of piezomodules of the studied materials has been analyzed as a function of temperature. X-ray measurements have also been carried out. A model of description of revealed dielectric parameters dispersion in the material is presented. The nonclassical modified Havriliak–Negami model written for complex electrical conductivity has been used to describe the temperature–frequency properties. It is shown that the dielectric spectra of the studied composites include three relaxation processes in the temperature ranges of 40–80${}^{\circ }$C, 80–120${}^{\circ }$C and 120–150 ${}^{\circ }$C, which were confirmed by the dynamics of changes in the dependences of ${\gamma {}^{\prime }}^{(}f)$, tg$\delta (f)$, ${M{}^{\prime }}^{(}f)$, $M^{\mathrm{\prime \prime }}$($f$) and $M\mathrm{\prime \prime }$($M\prime )$. All three processes are almost exactly described by this model and well correlated with the studies by other researchers of the composites based on PVDF. The results of this work show that the use of such experimental model is suitable for describing the complex dielectric spectra of any nonlinear dielectrics including composite materials.

In this paper, closed-form approximate solutions for the geometrically nonlinear behaviour of rectangular laminated plates with flexural orthotropy under longitudinal compression are presented. Based on the governing Marguerre-type differential equations postulated for imperfect plates, two plate configurations are discussed in detail, representing important application cases in practical engineering work. The first configuration is a laminated plate that is simply supported at all four edges (the so-called SSSS plate), while for the second configuration clamped unloaded longitudinal edges are considered (denoted as the SSCC plate). For both plate configurations, rather simple closed-form approximations in the form of trigonometric shape functions are employed for the description of the out-of-plane postbuckling plate deflections. Based on the chosen shape functions, the compatibility condition with respect to the in-plane strains is fulfilled exactly, while the out-of-plane equilibrium condition for a deflected plate element is not, but is solved using a Galerkin-type formulation instead. Eventually, very simple closed-form solutions for all postbuckling state variables (deflections, in-plane edge displacements, and effective widths) are derived that can be used very conveniently in engineering practice. The high accuracy of the presented analysis methods is established by comparison with the results of other authors.

In this work, Ni-doped ZnO/Al composites were prepared by a facile chemical co-precipitation method. The morphology and structure of the as-prepared composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), respectively. It was found that the flake-like Al powders were successfully coated by Ni-doped ZnO nanoparticles with slight aggregation and Ni${}^{2+}$ was successfully doped into the crystal lattice of ZnO. Moreover, the effects of ZnO concentration and doped Ni concentration on the infrared emissivity of ZnO/Al composites at the waveband range of 8–14$\mu$m were studied. The results showed that the ZnO/Al composites exhibited the lowest infrared emissivity of 0.34 with 50wt.% ZnO concentration. Meanwhile, the electromagnetic parameters and microwave absorbing properties of Ni-doped ZnO/Al composites in the frequency range of 2–18GHz were explored. Significantly, 12mol.% Ni-doped ZnO/Al composites presented the lowest infrared emissivity of 0.37 and the maximum reflection loss reached $-$32.5dB at 13.6GHz with a thickness of 4.5mm. The excellent microwave absorbing properties could be attributed to the good impedance match, crystal lattice defects and interfacial polarization. It was believed that the Ni-doped ZnO/Al composites could be used as potential infrared-microwave compatible stealth materials.

A novel category of polyphenylene oxide/high-impact polystyrene (PPO/HIPS) alloy was used as the polymer matrix (abbreviated as mPPO) and loaded with different volume fractions (0, 10, 20, 30, 40, 50 vol.%) of MgTiO₃–Ca${}_{0.7}$La${}_{0.2}$TiO₃ (abbreviated as MTCLT) ceramics to prepare composites by injection molding. Its micromorphology, density, dielectric, thermal and mechanical properties were analyzed in detail. The experimental results show that the composites possess a compact microstructure because HIPS increases the fluidity of PPO. Due to the excellent dielectric properties of both mPPO and MTCLT, the composites have an extremely low dielectric loss. The realization of the high ceramic filler fraction greatly limits the thermal expansion of the polymer chain by introducing the interphase, so that the coefficient of thermal expansion of the composite material could be as low as 21.8 ppm/${}^{\circ }$C. At the same time, the presence of ceramic particles could reinforce the mechanical property of the composites. When the ceramic filler fraction is higher than 20 vol.%, the bending strength of the composite material is around 110 MPa. When the ceramic filler fraction is 40 vol.%, the composite possesses the best comprehensive performance. The dielectric constant is 6.81, the dielectric loss is 0.00104, the thermal expansion coefficient is as low as 25.3 ppm/${}^{\circ }$C, and the bending strength is 110.4 MPa. Due to its excellent properties, this material can be a good candidate in the field of microwave communication.

In this paper, a comparative experimental analysis of die-sinking electric discharge machining (EDM) to two most exhaustively used aluminum metal matrix composites (AMMCs) has been performed using Copper and Tungsten as tool electrodes. AMMCs containing silicon carbide (SiC) and alumina (Al₂O${}_3)$ as reinforcement (10wt%) were fabricated by stir casting method. The Box–Behnken Design (BBD) approach of response surface methodology was used to develop experimental models for material removal rate (MRR) and surface roughness (SR). Effect of input parameters such as current ($I=4$–12A), gap voltage ($V_g=40-60$V), pulse-on time ($T_{\mathrm{\text{on}}}=100-200\mu$s), and duty factor ($\tau =4$–6) on the output responses has been investigated with response surface plots. Effectiveness of design of experiment (DoE) and evolutionary algorithm-based multi-objective optimization (MOO) technique have been compared to find the best feasible optimal solution. ANOVA analysis reveals that for alumina reinforced AMMC interaction between $I\times V$ has significant effect on both MRR and SR using Cu electrode. But for tungsten, electrode interaction between $I\times T_{\mathrm{\text{on}}}$, $I\times \tau$, and $V\times \tau$ have major role on MRR whereas SR is mostly influenced by interaction between $I\times V_g$ and $I\times T_{\mathrm{\text{on}}}$. The parametric analysis reveals that an increase of current from 4A to 12A at a higher pulse-ontime increases the MRR more significantly, and higher MRR occurs in cases of alumina-reinforced AMMC. Increase of pulse-ontime at low current (4A) reduces the MRR in AMMC/Al₂O₃. Good surface finish can be obtained by combining high voltage (60V) with either small current (4A) or small duty factor (4) for both AMMCs. Both DoE and metaheuristic-based MOO technique reveals that copper electrode should be preferred for die-sinking EDM of AMMC/SiC. Metaheuristic approach should be preferred for optimization of die-sinking EDM of AMMCs using different electrodes because it requires low current for effective machining of different AMMCs.

Advanced composites are increasingly being used as a structural material because of their balanced properties, higher impact resistance, and easier handling and fabrication compared with unidirectional composites. However, complex architecture of these composites leads to difficulties in predicting the mechanical response necessary for product design. Different methods for micromechanical analysis for the evaluation of effective mechanical properties of advanced composites are compared. Difficulties in modeling are highlighted and recommendations are given for micromechanical analysis using the finite element method.

This study reports the optimal frequencies and damping factor of the honeycomb sandwich composite plates. The sandwich panel face sheets have been considered as layered composite and honeycomb core. The higher-order shear deformation theory has been adopted to formulate the structural model and solve the governing equations of motion of sandwich structures to compute the frequencies. An optimal layout of the honeycomb composite laminated sandwich structure is being utilized to improvise both the fundamental natural frequencies and damping factors using a teaching–learning-centered artificial bee colony (TLABC). An experimental investigation is performed to demonstrate the effectiveness of the current TLABC algorithm to identify the optimal values by comparing them with numerically obtained results. Additionally, for the optimal layer sequences and the fiber orientations of the composite laminated plates, several optimization problems are developed with the objective functions of frequency maximization and modal damping factors (MDF). The TLABC algorithm integrated with finite element method has been utilized to evaluate the said responses. Hence, it is concluded that the efficient design layout of a honeycomb sandwich composite plate configurations would provide the guidelines for the designer to control the vibration effectively.

This paper presents a computational approach for composite materials subjected to ballistic impact. The composite materials are composed of layers of fabric embedded in a matrix material. The fabric is represented by bar elements and the matrix material is represented by solid elements. The geometric characteristics of these composite materials are three dimensional, which in turn requires a three dimensional analysis. The strength and failure characteristics of the fabric are determined directly from the actual fiber characteristics. For the matrix material it is necessary to modify the solid elements, because much of the volume of the solid elements is occupied by the volume of the fabric. The resulting modifications include the determination of effective density, stiffness, strength and failure characteristics for the solid elements. This approach has been shown to provide good agreement with experimental ballistics data. Because this approach treats each of the component materials individually (instead of blending them into a homogeneous anisotropic material), it is possible to examine the effects of individual material properties for both the fabric and the matrix material. This includes the capability to allow for failure of the fabric material and/or the matrix material. Parametric computational results are included to illustrate some of these effects.

Laser cleaning of artworks and antiquities is reaching a mature enough stage both in terms of the procedures employed and the understanding of the interaction processes involved. To this effect, a series of studies have been pursued aiming to process optimization. Appropriate model systems have been studied for elucidating the interaction mechanisms both in polymerized phases as well as inorganic encrustation. An overview of these issues presented and their implications for future directions outlined.

The majority of ultrasonic characterizations are done on thermoplastics, with just a few articles available on the characterization of thermoset resin characteristics. A non-destructive methodology for monitoring fatigue and static deformation induced by mechanical loading on a fiber-reinforced plastic is presented. However, these materials’ dynamics of elastic waves are considerably more complicated. A large part was devoted to the calculation of dispersion curves of guided waves in composites. Therefore, this study presents a thorough description of the Glass/Epoxy system by comparing ultrasonic and mechanical data. Ultrasonic wave propagation at high frequencies, functioning as a dynamic mechanical deformation, may be utilized to calculate longitudinal and shear moduli during static and dynamic loading. The evolution of attenuation and velocity during loading is linked to the significant changes that occur during the aging process. The experimental transfer function is determined by the Fourier transform of all the obtained ultrasonic echoes.

Owing to the unique microstructure and the excellent dielectric properties, carbon nanotubes (CNTs) were decorated with CoFe₂O₄ nanoparticles to synthesize the CoFe₂O₄/CNTs nanocomposites by the solvothermal method. The phase structure, morphology, magnetic properties and microwave absorption performance of the as-prepared CoFe₂O₄/CNTs were characterized and discussed by X-ray diffraction (XRD), thermal gravity analysis (TGA), transmission electron microscope (TEM), vibrating sample magnetometer (VSM) and vector network analyzer (VNA). All results indicated that the diameter of CoFe₂O₄ nanoparticles decorating on the surface of CNTs increased with the solvothermal temperature. CoFe₂O₄/CNTs prepared at 180°C, 200°C and 220°C exhibited superparamagnetism, while the other samples presented ferromagnetism at room temperature. And with the increasing solvothermal temperature, the saturation magnetization and coercivity increased up to 72 emu/g and 2000 Oe for the sample prepared at 260°C (S-26). And the reflection loss of CoFe₂O₄/CNTs nanocomposites increased with the solvothermal temperature up to -15.7 dB for S-26 with the bandwidth of 2.5 GHz.

A novel approach to determine the translaminar crack resistance curve of composite laminates by means of a machine learning model is presented in this paper. The main objective of the proposed method is to extract hidden information of crack resistance from strength values of center-cracked laminates. Compared to traditional measurements, the notable advantage is that only tensile strength values are required which can be obtained by a rather simpler experimental procedure. This is achieved by the incorporation of the finite fracture mechanics, which links crack resistance with strength values. In order to get training dataset, a semi-analytical method using both finite element method and finite fracture mechanics is employed to generate strength values of center-cracked specimens with different random R-curves, which serve as inputs for our artificial neural network. Regarding the outputs, principal component analysis is performed to reduce dimensionality and find suitable descriptors for crack resistance curves. After successfully training machine learning model, experimental studies on basalt fiber reinforced laminates are conducted as validation. Results have proven the effectiveness of the proposed strategy for predicting crack resistance curves, as well as the feasibility of using machine learning-based framework to find out more information about composites from simple experimental data.

Natural fibers are extracted from natural resources such as stems of plants. In contrast to synthetic fibers (e.g., carbon fibers), natural fibers are from renewable resources and are eco-friendlier. Plant fibers are important members of natural fibers. Review papers discussing the microstructures, performances and applications of natural plant fiber composites are available in the literature. However, there are relatively fewer review reports focusing on the modeling of the mechanical properties of plant fiber composites. The microstructures and mechanical behavior of plant fiber composites are briefly introduced by highlighting their characteristics that need to be considered prior to modeling. Numerical works that have already been carried out are discussed and summarized. Unlike synthetic fibers, natural plant fiber composites have not received sufficient attention in terms of numerical simulations. Existing technical challenges in this subject are summarized to provide potential opportunities for future research.

Interlaminar delamination and brittle fracture of matrix have been a dilemma that fiber-reinforced composites have been faced with. Herein, the polyimide (PI) nanofiber-toughened glass fiber fiber-reinforced epoxy composites were prepared by electrospinning method and subsequent vacuum assistant resin transfer molding. The effect of spinning parameters including PI concentration, applied voltage, collector distance, jet speed and ambient humidity on the resultant fiber diameter and its distribution was systematically evaluated. The surface properties of obtained PI nanofibers were characterized by FT-IR, TG-DSC and water contact angle. The effect of PI concentration on tensile strength of PI membranes was also studied. The mode I (G${}_{\mathrm{\text{Ic}}})$ and mode II (G${}_{\mathrm{\text{IIc}}})$ interlaminar fracture toughness were measured. The results indicated that G_Ic and G_IIc increased by 127.69% and 85.33%. The improvement of interlaminar fracture toughness may be attributed to the bridging effect of PI nanofibers.