Properties Of Composite Structures Research Articles

The effects of delamination deficiencies on compressive properties of composite grid-stiffened structures

ABSTRACTThis article provides a systematic approach to accurately predict the effects of delamination deficiencies on compressive properties of composite grid-stiffened structures. The deformation of the stiffener and the skin is investigated during a mechanical test. Cohesive elements in the finite element method were employed to calculate the ultimate load-carrying capacity. The delamination deficiencies in the interface between the skin and the stiffener have a significant effect upon its ultimate load-carrying capability compared with the delamination deficiencies in the skin and in the stiffener.

Dynamic properties of hybrid composite structures based multiwalled carbon nanotubes

The present paper investigates an experimental approach concerning the determination of dynamic behavior and damage kinetics of composite materials based on multiwalled carbon nanotubes (MWCNTs), embedded in electrospun reactive nanofibers in the Taylor impact test. Different impact energies have been considered namely; 21J and 39J to investigate the composite response. Projectiles are manufactured from a commercial steel 2071 with a nominal diameter of 50 mm and 1600 g of weight. The projectile was fired against a composite specimen initially hooked on a cell effort by a compressed gas gun within the velocity of 5 m/s and 7 m/s. Three types of specimens are considered: (1) MAT1 (carbon fiber reinforced epoxy polymer composite), MAT2 (consists of MAT1 and electrospun Polybenzmideazole-Bismaleimide (PBI-BMI) nanofibermats between carbon fiber layers) and MAT3 (consists of MAT2, where PBI-BMI nanofibermats are reinforced with multiwalled carbon nanotubes (MWCNTs)). The effect of the MWCNTs on the dynamic properties of the composite structures was studied. Microscope observations reveal damage progressive, buckling and crush-front propagation during tests. Application of the PBI-BMI reactive nanofibermats reinforced with MWCNTs leads to damage prevention, reducing damage area in composite samples.

Impact of hydration on the micromechanical properties of the polymer composite structure of wood investigated with atomistic simulations

A model of the secondary layer of wood cell wall consisting of crystalline cellulose, hemicellulose, and lignin is constructed and investigated with molecular dynamics simulations in the full range of hydration: from dry to saturated state. The model is considered a composite with the cellulose fibrils embedded in hemicellulose and lignin, forming a soft amorphous matrix. Its complex structure leads to nonlinear and anisotropic swelling and mechanical weakening. The water diffusivity through the pores is affected by an interplay between stiff cellulose fibers and weakening amorphous polymers. The formation and breaking of hydrogen bonds within the polymers and at the interfaces is found to be the underlying mechanism of adsorption-induced mechanical softening. The model is tested for adsorption isotherm, mechanical moduli, hydrogen bonds, and water diffusivity that all undergo a substantial change as the hydration increases. The determined physical and mechanical properties, changing with hydration, agree qualitatively with experimental measurements.

Effect of sintering parameters using the central composite design method, electronic structure and physical properties of yttria-partially stabilized ZrO2 commercial ceramics

Abstract In this work, the effect of sintering parameters on electronic structure and physical properties of yttria-partially stabilized ZrO2 (YPSZ) commercial ceramics has been studied using the central composite design (CCD) method. The CCD method allows using empirical modelling with better fitting, by considering the interaction between both factors. Different temperature ranges and sintering times for processing of YPSZ ceramics have been used in order to evaluate the grain growth, hardness and volumetric shrinkage by the CCD method. X-ray diffraction patterns and Rietveld refinement data indicate that non-sintered YPSZ ceramics exhibits two phases related to tetragonal and monoclinic structures, while the sintered YPSZ ceramics exhibits a single phase related to a tetragonal structure. Moreover, the monoclinic structure presents zirconium (Zr) atoms coordinated to seven oxygen (O) atoms, while in the tetragonal structure Zr atoms are coordinated to eight O atoms. Field emission scanning electron microscopy images were employed to monitor the sintering and growth process. In addition, the response surfaces obtained from calculations presented the effect of thermal and kinetic variables on the physical properties such as average grain size, volumetric shrinkage and hardness of YPSZ ceramics.

Pressure-Induced Densification of Oxide Glasses at the Glass Transition

Densification of oxide glasses at the glass transition offers a novel route to develop bulk glasses with tailored properties for emerging applications. Such densification can be achieved in the technologically relevant pressure regime of up to ~ 1GPa. However, the present understanding of the composition–structure–property relationships governing these glasses is limited, with key questions, e.g., related to densification mechanism, remaining largely unanswered. Recent advances in structural characterization tools and high-pressure apparatuses have prompted new research efforts. Here, we review this recent progress and the insights gained in the understanding of the influence of isostatic compression at elevated temperature (so-called hot compression) on the composition–structure–property relationships of oxide glasses. We focus on compression at temperatures at or around the glass transition temperature (Tg), with relevant comparisons made to glasses prepared by pressure quenching and cold compression. We show that permanent densification at 1 GPa sets-in at temperatures above 0.7Tg and the degree of densification increases with increasing compression temperature and time, until attaining an approximately constant value for temperatures above Tg. For glasses compressed at the same temperature/pressure conditions, we demonstrate direct relations between the degree of volume densification and the pressure-induced change in micro-mechanical properties such as hardness, elastic moduli, and extent of the indentation size effect across a variety of glass families. Furthermore, we summarize the results on relaxation behavior of hot compressed glasses. All the pressure-induced changes in the structure and properties exhibit strong composition dependence. The experimental results highlight new opportunities for future investigation and identify research challenges that need to be overcome to advance the field.

Multi-scale simulations of apatite–collagen composites: from molecules to materials

We review scale-bridging simulation studies for the exploration of atomicto-meso scale processes that account for the unique structure and mechanic properties of apatite-protein composites. As the atomic structure and composition of such complex biocomposites only partially is known, the first part (i) of our modelling studies is dedicated to realistic crystal nucleation scenarios of inorganic-organic composites. Starting from the association of single ions, recent insights range from the mechanisms of motif formation, ripening reactions and the self-organization of nanocrystals, including their interplay with growth-controlling molecular moieties. On this basis, (ii) reliable building rules for unprejudiced scale-up models can be derived to model bulk materials. This is exemplified for (enamel-like) apatite-protein composites, encompassing up to 106 atom models to provide a realistic account of the 10 nm length scale, whilst model coarsening is used to reach μm length scales. On this basis, a series of deformation and fracture simulation studies were performed and helped to rationalize biocomposite hardness, plasticity, toughness, self-healing and fracture mechanisms. Complementing experimental work, these modelling studies provide particularly detailed insights into the relation of hierarchical composite structure and favorable mechanical properties.

Development of a ceramic-based composite for direct bonded copper substrate

In the present paper, a computational approach is presented to design alumina-based composite with tailored properties that could replace commercial alumina used in Direct Bonded Copper (DBC) substrates for applications in power electronic modules. A mean-field homogenization and effective medium approximation (EMA) using an in-house code is used for predicting potential optimum thermal and structural properties for DBC substrates by considering the effect of filler type, volume, and size in the alumina matrix. The primary goal for designing such alumina-based composites is to have enhanced thermal conductivity for effective heat dissipation and spreading capabilities together with a coefficient of thermal expansion (CTE) value that is close to the silicon chips in electronic circuits in order to avoid interface layers. At the same time, other functional properties like elastic modulus and electrical conductivity have to be maintained. Our strategy incorporates thermal and structural properties of composites as a constraint on the design process. Among various metallic and carbon-based fillers, chromium, silicon carbide and diamond fillers were found suitable candidates that could enhance the thermal and structural performance of the alumina-based substrates. As a validation, we developed alumina-silicon carbide (Al2O3-SiC) composites in line with the designed range of filler size and volume fraction using Spark Plasma Sintering (SPS) process. Thermal and structural properties including thermal conductivity, CTE, and elastic modulus are measured to complement the computational design. It is found that the developed computational design tool is accurate enough in predicting the desired properties of composite materials for DBC substrate applications.

Interplay of composition, structure, and electron density of states in W-Os cathode materials and relationship with thermionic emission

The presence and composition of W-Os alloys have been found to significantly affect the thermionic emission properties of Os-coated tungsten dispenser cathodes. However, the comprehensive understanding of structure–property relationships needed to design improved tungsten cathodes with larger thermionic emission is still lacking. In this study, composition–structure–property relationships governing thermionic emission from W-Os alloys were investigated using quantum mechanical calculations. Low-energy W-Os atomic configurations at various compositions were determined from first-principles calculations based on density functional theory in combination with cluster expansion calculations. Electronic properties were investigated in terms of the electron density of states. The relative position of the Fermi level with respect to peaks and pseudogaps in the density of states for different W-Os structures can be used to explain, at least in part, observed variations in thermionic emission from Os-coated tungsten dispenser cathodes.

Scattering Analysis of Nonperiodic Composite Metallic and Dielectric Structures Using Synthetic Functions

A synthetic basis functions method (SBFM) is used to analyze scattering properties of nonperiodic composite metallic and dielectric structures. Specifically, the model of targets hidden in jungles is analyzed here. We first propose a new approach on synthetic functions’ construction. It constructs synthetic functions for PEC bodies and dielectric bodies independently in a mixed problem. Then, we theoretically explain that the new constructing method can decrease computational complexity and memory cost of synthetic functions’ expansion coefficients matrix compared to traditional ways. Moreover, the approach can be used to analyze nonperiodic mixed structures in an efficient way. Finally, effectiveness and accuracy of the proposed approach are validated by both simulating and measured results.

Role of electrochemically in-house synthesized and functionalized graphene nanofillers in the structural performance of epoxy matrix composites.

The present study focuses on the intriguing enhancement in the mechanical properties of an epoxy-based composite structure resulting from the incorporation of in-house synthesized functionalized graphene nanosheets (f-GNSs) as nanofillers. The f-GNSs were obtained by anionic electrochemical intercalation and exfoliation with 2 M H2SO4, HClO4, and HNO3 protic electrolytes. The structural properties of the as-synthesized GNSs were analyzed by XRD and Raman spectroscopy. The (002) and (001) lattice planes of graphene and graphene oxide are observed at around 24.5° and 11° (2θ), respectively, in the XRD spectra. The characteristic peaks at around 1345, 1590, and 2700 cm-1 correspond to the D, G, and 2D bands of the GNSs in the Raman spectra. Quantification of the functional groups and sp2 contents in the GNSs were further analyzed by XPS. Morphological characterization of the f-GNSs reveals that the exfoliated carbon sheets consist of 2-8 layers. The composites are then fabricated by addition of these f-GNSs nanofillers, and the effect of the wt% of the nanofillers on the mechanical properties of the composites is analyzed with the three-point bend test and fractography analysis through interfacial morphological analysis. The addition of 0.1 wt% of nitric-acid-exfoliated f-GNSs nanofiller results in maximum increases of 42.6% and 28.2% in the flexural strengths of neat epoxy resin and glass fiber/epoxy polymer composite structures, respectively. Similarly, the moduli increase by 33.5% and 57.7% in the neat epoxy resin and glass fiber/epoxy polymer composite structures, respectively. The effect of epoxy/f-GNSs interfacial bonding in the composite structure was studied by DSC analysis.

Mechanical Properties of Lab Joint Composite Structure of Glass Fiber Reinforced Polymers

Glass fiber composite laminates have competitive properties than monotonic material for their superior mechanical strength. Lab joints in composite structure are of great importance in aerospace and aircraft industry. Therefore, lab joints’ strength and failure of composite laminates structure are experimentally investigated. Composites laminates of four different stacking sequences and layup are manufactured using hand layup technique and curing at room temperature. Specimens of unidirectional laminates of [0]8 stacking sequence are used to test lamina mechanical properties while [0/90]2s, [0/60/90]s and woven are used to test the mechanical properties of lab joints. Lab joints of single row and double row are produced and tested in bearing using simple mode I test (tension test). The results illustrate that bearing strength of quasi-brittle laminates of [0/60/90]s has more stability and strength than that of woven glass fiber, then the cross ply laminates of [0/90]2s; this can be attributed to increase of anisotropy of cross ply laminates than other composite laminate structure.

A Numerical-Experimental Method for Drop Impact Analysis of Composite Landing Gear

The special material performance, manufacturing process, machining behavior, and operating condition of composite materials cause uncertainties to inevitably appear in the mechanical properties of composite structures. Therefore, variability in mechanical properties must be considered in a mechanical response analysis of composite structures. A method is proposed in this paper to predict the dynamic performance of composite landing gear with uncertainties using experimental modal analysis data and nonlinear static test data. In this method, the nonlinear dynamic model of the composite landing gear is divided into two parts, the linear and the nonlinear parts. An experimental modal analysis is employed to predict the linear parameters with a frequency response function, and the nonlinear parameters caused by large deflection are identified by a nonlinear static test with the nonlinear least squares method. To check the accuracy and practicability of the method, it is applied to drop impact simulations and tests of composite landing gear. The results of the simulations are in good agreement with the test results, which shows that the proposed method is perfectly suited for the dynamic analysis of composite landing gear.

Analysis of noise-protective properties of sheet material composite structures

The object of this research is sound insulation properties of sheet material composite structures. The basis of the working hypothesis is the fact that particular importance has the minimum gap between the layers for the theoretical insulation calculations of the double sheet material structure. Dependence observation of the air gap size between the two layers of interior walls suggests that for maximum sound insulation ability the air gap size should be no less than five times greater than the maximum thickness of one of them. Comparison of the estimated sound insulation characteristics R N with the measured frequency response of air sound insulation R shows that the actual sound insulation of two sheet structure is significantly lower than estimated. This difference is between 3 to 5 dB over the entire frequency range. It also confirms the low acoustic efficiency of modern plastic windows. The results of this work will allow as a promising direction for further research to identify the program to adjust the sound insulation properties of any translucent walling (including window fillings), also consisting of sheet materials.

Design and Optimization of Novel ZnS–Metal Core-Shell Random Structures for Light Harvesting—a Computational Study

We present a systematic computational study on the optical properties of a new composite structure based on ZnS–metal core-shell particles, which can find applications in light trapping. Three metals Au, Ag and Cu whose surface plasmon resonance (SPR) fall in visible region are used as shell materials. Effect of shell material, thickness and core radius on the SPR values is studied using Mie theory. Optimization of aspect ratios for each composite is done for maximum extinction in visible region. Uniform random distribution of selected particles with optimized aspect ratios is modelled using Lumerical FDTD software. Transmission of wavelengths 400, 450, 500, 550, 600 and 650 nm through random media is investigated using finite difference time domain simulations. Shell material is found to have a large influence on light trapping ability of the composite. Depending on the shell material, thickness and core radius, wavelengths at different regions of visible spectrum can be trapped, thus offering scope for designing novel light harvesting systems tunable over specific wavelength regions.

Investigation into the material properties of wooden composite structures with in-situ fibre reinforcement using additive manufacturing

In contrast to subtractive manufacturing techniques, additive manufacturing processes are known for their high efficiency in regards to utilisation of feedstock. However the various polymer, metallic and composite feedstocks used within additive manufacturing are mainly derived from energy consuming, inefficient methods, often originating from non-sustainable sources. This work explores the mechanical properties of additively manufactured composite structures fabricated from recycled sustainable wood waste with the aim of enhancing mechanical properties through glass fibre reinforcement.In the first instance, samples were formed by pouring formulation of wood waste (wood flour) and thermosetting binder (urea formaldehyde), with and without glass fibres, into a mould. The same formulations were used to additively manufacture samples via a layered deposition technique. Samples manufactured using each technique were cured and subsequently tested for their mechanical properties. Additively manufactured samples had superior mechanical properties, with up to 73% increase in tensile strength compared to moulded composites due to a densification of feedstock/paste and fibre in-situ directional alignment.

Coupling of experimentally validated electroelastic dynamics and mixing rules formulation for macro-fiber composite piezoelectric structures

Piezoelectric structures have been used in a variety of applications ranging from vibration control and sensing to morphing and energy harvesting. In order to employ the effective 33-mode of piezoelectricity, interdigitated electrodes have been used in the design of macro-fiber composites which employ piezoelectric fibers with rectangular cross section. In this article, we present an investigation of the two-way electroelastic coupling (in the sense of direct and converse piezoelectric effects) in bimorph cantilevers that employ interdigitated electrodes for 33-mode operation. A distributed-parameter electroelastic modeling framework is developed for the elastodynamic scenarios of piezoelectric power generation and dynamic actuation. Mixing rules (i.e. rule of mixtures) formulation is employed to evaluate the equivalent and homogenized properties of macro-fiber composite structures. The electroelastic and dielectric properties of a representative volume element (piezoelectric fiber and epoxy matrix) between two neighboring interdigitated electrodes are then coupled with the global electro-elastodynamics based on the Euler–Bernoulli kinematics accounting for two-way electromechanical coupling. Various macro-fiber composite bimorph cantilevers with different widths are tested for resonant dynamic actuation and power generation with resistive shunt damping. Excellent agreement is reported between the measured electroelastic frequency response and predictions of the analytical framework that bridges the continuum electro-elastodynamics and mixing rules formulation.

Modeling of gradient composite structures for shielding of microwaves

ABSTRACTThe simulation of shielding properties of gradient composite structures (g-CMs) nanocarbon-polymer in the frequency range of electromagnetic radiation (EMR) (25.5 – 55.5 GHz) has been implemented in C++ code. The simulation has shown a significant decrease of EMR reflection index (R) and sufficient increase of EMR absorption (A) and shielding efficiency in gradient composite structures with filler concentration growth in the direction of EMR propagation as compared with composite with uniform distribution of nanocarbon filler. The simulated results coincide with the experimental data for the set of the epoxy composites with gradient distribution of 5 wt.% of graphite nanoplatelets.

Mechanical and damping properties of CNT-reinforced cementitious composites

The mechanical and damping properties of CNT-reinforced cementitious composite structures were experimentally examined. In the experiments, an aromatic modified polyethylene glycol ether named TNWDIS and polyvinylpyrrolidone (PVP) were used to disperse CNTs, which were ultra-effective and compatible with cement hydrates. The growth of cement hydrates on the CNT surface was observed by Scanning Electron Microscopy (SEM) and identified by Energy Dispersive Spectrometry (EDS). X-ray Powder Diffraction (XRD) analysis suggested that the mechanism by which the CNTs and cement hydrates were combined was a physical process. The compressive and flexural strengths of CNT/cement composites were improved by 17.3 and 16.3%, respectively, through the addition of 0.1wt.% CNTs dispersed by PVP, while the addition of CNTs dispersed by TNWDIS led to a very limited improvement in strength. In addition, the loss factor of CNT/cement matrix was measured, and 0.1wt.% CNTs dispersed by TNWDIS improved the loss factor by 25.9%, which is nearly twice greater than the improvement caused by 0.1wt.% CNTs dispersed by PVP.

A Monte Carlo Study on the Effect of Energy Barriers on the Thermoelectric Properties of Si

Abstract Energy filtering by energy barriers has been proposed to interpret observations on large thermoelectric power factor (TPF) enhancement in highly doped nanocrystalline Si (nc-Si). Previous Boltzmann transport equation (BTE) modeling indicated that high TPFs could be explained as the result of the presence of energy barriers at the grain boundaries, the high Fermi energy due to the high doping level, and the formation of a low thermal conductivity second phase. To test the assumptions of the BTE modeling and provide more realistic simulations, we have performed Monte Carlo (MC) simulations on the transport properties of composite nc-Si structures. Here, we report on (i) the effect of an energy barrier, and (ii) the effect of multiple barriers on the conductivity and the Seebeck coefficient. In short structures, a TPF enhancement was found and it has been attributed to energy filtering by the energy barrier. The MC indicated that the TE performance can be improved by multiple barriers in close separation. It has been shown that TPF enhancement is possible even when the condition for thermal conductivity non-uniformity across the composite structure is not-fulfilled.

Damage monitoring of aircraft structures made of composite materials using wavelet transforms

The present article is dedicated to the study of the acoustic properties of composite materials and the application of non-destructive testing methods to aircraft components. A mathematical model of a wavelet transformed signal is presented. The main acoustic (vibration) properties of different composite material structures were researched. Multiple vibration parameter dependencies on the noise reduction factor were derived. The main steps of a research procedure and new method algorithm are presented. The data obtained was compared with the data from a three dimensional laser-Doppler scanning vibrometer, to validate the results. The new technique was tested in the laboratory and on civil aircraft at a training airfield.