Stiffness Of Composites Research Articles

Vibration Analysis of a Magneto Thermo Electrical Nano Fiber Reinforced with Graphene Oxide Powder Under Refined Beam Model

The present article express the magneto thermo electric deformation of composite nano fiber reinforced by graphene oxide powder (GOP). To reach the governing equation of the problem a higher-order trigonometric refined beam model is utilized according to Hamilton’s principle. The effect of a nonuniform magnetic and thermo piezo electric field is applied to the governing equations by combining the field relations with the displacement field equations. Then, obtained equations are solved by using Galerkin’s method to consider the influence of different boundary conditions on the vibrational responses of the fiber. The accuracy and efficiency of the presented model is verified by comparing the results with that of published researches. Further, the effects of different variant on the dimensionless frequency of GOP reinforced magneto piezo thermo elastic composite fibers are highlighted through tables and dispersion curves. The weight fraction of GOP and the magneto thermo electro effects have significant influence in the stiffness of the nano composites.

Mechanical effects of circular liquid inclusions inside soft matrix: role of internal pressure change and surface tension

The mechanical effects of dilute liquid inclusions on the solid-liquid composite are explored, based on an analytical circular inclusion model incorporating the internal pressure change of the liquid and the surface tension of the interface. Several simple explicit dependences of the stress field and effective stiffness on the bulk modulus and the size of the liquid, the surface tension, and Poisson’s ratio of the matrix are derived. The results show that the stresses in the matrix are reduced, and the stiffness of the solid-liquid composite is enhanced with the consideration of either the surface tension or the internal pressure change. Particularly, the effective Young’s modulus predicted by the present model for either soft or stiff matrices agrees well with the known experimental data. In addition, according to the theoretical results, it is possible to stiffen a soft solid by pressured gas with the presence of the surface tension of the solid-gas interface.

Adaptive surrogate-based harmony search algorithm for design optimization of variable stiffness composite materials

Having the advancement in automated fiber placement (AFP) and three-dimensional (3D) printing, fabricating novel composites reinforced with curvilinear fibers, namely variable stiffness composites (VSCs), have gradually become feasible. Contrasted to the traditional straight fiber-reinforced composites, VSCs possess significantly enhanced mechanical properties and increased mechanical performance. However, the optimization of a VSC panel is still a challenging problem, which can be attributed to the high non-convexity of the feasible design space and highly involved computation resources. To accelerate the design and optimization of VSC structures, in this study, we propose an effective and efficient adaptive surrogate-based harmony search algorithm (ASBHSA). The VSC is modeled using the fiber path methodology through which the fiber trajectory is generated by a mathematical formulation. In addition, the manufacturability and quality of the product are guaranteed by considering the curvature constraint. For the ASBHSA, a distance-based infill criterion in combination with a radial basis function (RBF) is proposed to guide the adaptive sampling. The global best harmony search (GHS) with a good ability to find the global minimum is integrated into the developed optimization framework. After solving some popular benchmark optimization problems as well as making comparisons of the optimal designs of VSCs with those in published works, the proposed optimization framework is demonstrated being able to effectively and efficiently find the global minimum. Then, a series of designs and optimizations of laminated VSC plates under various boundary conditions and different loading conditions are carried out systematically. The reported interesting findings may provide some insights and guidance in the design of laminated VSC composites. Moreover, our proposed optimization model provides a useful tool for solving various kinds of optimization problems.

Stiffness and Energy Dissipation of Polymer Matrix Composites Containing Embedded Metallic Negative Stiffness Structures

Conventional composites used in damping applications exhibit an undesirable tradeoff between stiffness and energy dissipation. Recent research demonstrates that it is possible to simultaneously achieve increased stiffness and energy dissipation for a configuration of a viscoelastic polymer matrix placed in parallel with a negative stiffness structure (NSS). This configuration resulted in energy dissipation equal to the sum of its components but is difficult to implement in practice. In this paper, an alternative configuration is investigated in which the NSS is embedded simultaneously in series and parallel with the matrix. The main objectives are to examine the tradeoff between the stiffness and energy dissipation of the composite and to identify the mechanisms for enhanced energy dissipation. To achieve this, FEA models were used to match the stiffness of a polymer matrix with that of a metallic NSS. Multiple specimens were manufactured and tested under quasi-static compressive loads to determine the force versus displacement curves and calculate the energy dissipation and stiffness. These tests demonstrate that the total energy dissipation of the composite can be greater than the sum of its components, while maintaining the benefit of increasing the stiffness and damping capacity simultaneously. The results also demonstrate that the applied strain rate plays a critical role in activating the NSS, which is essential to achieve the desired increase in energy dissipation. The results indicate that localized strain and strain rate at the interface between the NSS and polymer matrix are the main contributors to achieving energy dissipation beyond the sum of its components. Furthermore, it was demonstrated that the strain rate affects the activation of the NSS and therefore composites containing mechanically activated NSS must be designed for the strain rate of interest.

A simulation-driven design approach to the manufacturing of stiff composites with high viscoelastic damping

Materials with enhanced vibration damping properties are of ever-growing technological interest. The ability of a material to limit mechanical vibrations, indeed, not only may extend its service life, but also reduces its susceptibility to mechanical noise generation. The selection and design of damping structural materials usually aims to maximize the loss coefficient, while sacrificing the stiffness, to achieve the proper vibration damping capability. In this work, we overcome this typical stiffness tradeoff by geometrically confining thin viscoelastic layers between stiff thicker ones in a layered composite, thus enhancing its viscoelastic shear response. The preparation of the composite is guided by a simple design methodology based on the effective properties of its homogenized counterpart. An optimal design window, defined for the viscoelastic volume fraction, directs the manufacturing of the composite. This approach allows to manufacture composites that exhibit enhanced damping properties, with over three orders of magnitude increase of loss coefficient and less than one order of magnitude decrease of stiffness compared to the constituent stiff material. Such materials might find applications in fields where high stiffness is required but a damping component is necessary to avoid the build-up of dangerous vibrational modes: from the aerospace industry to the design of “silent” infrastructures.

Interface Strength and Fiber Content Influence on Corn Stover Fibers Reinforced Bio-Polyethylene Composites Stiffness.

Stiffness of material is a key parameter that allows the use of material for structural or semi-structural purposes. Besides, lightweight materials are increasingly calling the attention of the industry. Environmental impact is also increasing in its importance. Bio-based materials produced from renewable sources can be good candidates for structural purposes combining lightweight and low environmental impact. Nonetheless, similar mechanical properties of commodities have to be reached with such materials. In this work, composite materials from corn stover fibers as a bio-polyethylene reinforcement were produced and tested. The effect of coupling agents to improve the fiber–matrix interface has been evaluated. It has been found that coupling agent content influenced the stiffness of the materials, increasing the Young’s modulus and the material processability. The best performance was achieved for a 6% of coupling agent, corresponding to 4.61 GPa for 50 w/w% of corn stover fibers. Micromechanics showed the impact of the semi-random orientation of the fibers and the lesser impact of its morphology. It was possible to determine a triangular packing of the composites as a hypothesis for future research.

Carbon fiber-reinforced PEEK in implant dentistry: A scoping review on the finite element method

Objective : The aim of the present study was to perform an integrative systematic review on the stress distribution assessed by finite element analysis on dental implants or abutments composed of carbon fiber-reinforced PEEK composites. Method: An electronic search was performed on PUBMED and ScienceDirect using a combination of the following search terms: PEEK, Polyetheretherketone, FEA, FEM, Finite element, Stress, Dental implant and Dental abutment. Results: The findings reported mechanical properties and the stress distribution through implant and abutment composed of PEEK and its fiber-reinforced composites. Unfilled PEEK revealed low values of elastic modulus and strength that negatively affected the stress distribution through the abutment and implant towards to the bone tisues. The incorporation of 30% carbon fibers increased the elastic modulus and strength of the PEEK-matrix composites although some studies reported no statistic differences in stress magnitude when compared to unfilled PEEK. However, an increase in short carbon fibers up to 60% revealed an enhancement on the stress distribution through abutment and implants towards to the bone tissues. PEEK veneering onto titanium core structures can also be a strategy to control the stress distribution at the implant-to-bone interface. Conclusions: The stiffness and strength of PEEK-matrix composites can be increased by the improvement of the carbon fibers’ network. Thus, the content, shape, dimensions, and chemical composition of fibers are key factors to improve the stress distribution through abutment and implants composed of PEEK-matrix composites.

An empirical FRP-concrete bond-slip model for externally-bonded reinforcement on grooves

The externally-bonded reinforcement on grooves (EBROG) has been recently developed as a substitute for the conventional externally-bonded reinforcement (EBR) technique used to retrofit reinforced concrete (RC) structural members with fiber-reinforced polymer (FRP) composites. While the presently available bond-slip models predict the bond-slip behavior of FRP composites adhered to the concrete surface via the conventional EBR method, there is a lack of reports in the literature on rigorous bond-slip model for FRP composites attached to concrete surfaces via the EBROG technique. Using a nonlinear regression on experimental data, the current research attempts to introduce an empirical model for such bond-slips. For this purpose, FRP composite strips were bonded to 68 concrete prism specimens retrofitted through the EBR and EBROG technique, before subjecting them to the single lap-shear experiment (108 tests in all). The test parameters included compressive strength of concrete, grooves dimensions (width and depth), composite strip width, and composite stiffness. A bilinear bond-slip model was proposed. Maximum shear stress, fracture energy, and associated slip were determined by analyzing on-groove and out-of-groove stresses and strains via the image processing method of particle image velocimetry (PIV). Finally, analysis of variance and different statistical tools were employed to validate the results obtained from the bond-slip model developed.

Thermo-physical and mechanical characteristics of composites based on high-density polyethylene (HDPE) e spent coffee grounds (SCG)

In this study we attempt to obtain ecological composites, produced from high density polyethylene (HDPE) and spent coffee grounds (SCG), 10–30% w/w, through extrusion and injection molding. The influence of the SCG concentration on the mechanical properties, microstructure, thermal and physical stability of the composites was determined. The incorporation of SCG fibers provided an increase of 49% (modulus of elasticity), and 108% (modulus of flexion) in the stiffness of composites. There was also an increase of approximately 13% in the elasticity of composites with 10% w/w SCG. In general, SCG particles up to 20% w/w were homogeneously dispersed in the HDPE matrix, showing good interaction and compatibility, without the need of a compatibilizer. Another important result was the maintenance of the thermal profile of the HDPE matrix with the incorporation of SCG, without causing processing modifications. Overall, the HDPE/SCG-based approach to obtaining natural fiber reinforced composite (NFRC) was promising, enabling the production of green composites using an underutilized resource, without negatively affecting its properties.

Minimum weight design of curvilinearly grid-stiffened variable-stiffness composite fuselage panels considering buckling and manufacturing constraints

In this paper, optimum designs of a tow-steered composite fuselage skin stiffened by a grid of curvilinear composite stiffeners are obtained for minimum weight subject to buckling and manufacturing constraints. A simply supported cylindrically curved panel loaded by either uniaxial compression or combined uniaxial compression and shear is addressed. Using Python-Abaqus interface for modelling the panels and genetic algorithm implemented in MATLAB as an optimization platform, an integrated design framework is developed for obtaining the designs. The design variables include the number of stiffeners, the stiffener profile and the stiffener orientations as well as the fiber orientation angles of the variable-stiffness skin. Two different fiber paths with linear and circular arc variations are adopted to tailor the stiffeners and skin. For comparison with traditional grid-stiffened composite fuselage panels, minimum weight designs are also obtained for orthogrid and anglegrid stiffened quasi-isotropic skins. The goal is to identify the lightest weight of the three competitive designs. It is shown that the curvilinearly grid-stiffened variable-stiffness panel yields the most weight efficient design. Such a design provides for a weight reduction of up to 30% and 18% compared to orthogrid and anglegrid designs, respectively. As opposed to the stiffener trajectories, it is observed that the fiber trajectories of the skin have sharp curvatures. It is also observed that the shallow fuselage panels designed using the circular arc path satisfy the manufacturing constraints represented by the minimum turning radius of both the fiber and stiffener paths and have favourable buckling patterns to aircraft structures.

Multiscale modeling framework to predict the effective stiffness of a crystalline-matrix nanocomposite

Recently, multiscale modeling frameworks combining micromechanics-based homogenization methods and atomistic simulations have been widely applied to predict the effective stiffness of particulate-reinforced composites. Although most studies demonstrated that theoretical predictions incorporating interfacial damage are necessary to explain atomistic simulation results, the microscopic origin of the interfacial damage has not been systematically analyzed in terms of interatomic potential and interfacial structure. In this study, first, we conduct a series of particle simulations of two fictitious model crystalline composites with coherent interfaces: one has a two-dimensional triangular structure described by a bead-spring model and the other has a face-centered cubic structure described by the artificial Lennard-Jones potential. By comparing the simulation results with micromechanics theory, we obtain the interfacial bonding (damage) parameter used in the homogenization method in terms of parameters at the atomistic level. Second, we study the effects of the interfacial structures (coherent/incoherent) because of lattice or crystallographic orientation mismatch on the effective properties of composites. We obtain the elastic stiffness of Si(matrix)-Ge(nanoparticle) nanocomposites with different interfacial structures (coherent/incoherent structures) using atomistic simulations and observe that nanoparticle-size-dependency occurs only for the composite with incoherent interfaces. We propose a homogenization scheme considering the pre-stress (or residual stress) and interfacial imperfection, and explain the results from Si-Ge nanocomposite simulations.

Surface-Directed Mineralization of Fibrous Collagen Scaffolds in Simulated Body Fluid for Tissue Engineering Applications.

The use of polymer additives that stabilize fluidic amorphous calcium phosphate is key to obtaining intrafibrillar mineralization of collagen in vitro. On the other hand, this biomimetic approach inhibits the nucleation of mineral crystals in unconfined extrafibrillar spaces, that is, extrafibrillar mineralization. The extrafibrillar mineral content is a significant feature to replicate from hard connective tissues such as bone and dentin as it contributes to the final microarchitecture and mechanical stiffness of the biomineral composite. Herein, we report a straightforward route to produce densely mineralized collagenous composites via a surface-directed process devoid of the aid of polymer additives. Simulated body fluid (1×) is employed as a biomimetic crystallizing medium, following a preloading procedure on the collagen surface to quickly generate the amorphous precursor species required to initiate matrix mineralization. This approach consistently leads to the formation of extrafibrillar bioactive minerals in bulk collagen scaffolds, which may offer an advantage in the production of osteoconductive collagen-apatite materials for tissue engineering and repair purposes.

Bird strike resistance of a CFRP morphing leading edge

The implementation of morphing technologies into the leading edge a regional aircraft wing promises some benefits in terms of reduction of perceived noise on the ground as well as reduction of fuel consumption. In the design of a morphing leading edge made from fiber reinforced composites, two competing requirements need to be considered: compliance requirements allowing morphing of the composite skin and stiffness requirements ensuring bird strike resistance of the morphing leading edge. This article presents finite element simulations of the impact behavior of a 2 kg bird onto a CFRP morphing leading edge. It is shown, that the composite skin is always perforated. Therefore, an internal bird strike protection system was designed capable of deflecting the bird to ensure that the fuel tank is not impacted by the bird.

Gut microbiota composition and arterial stiffness measured by pulse wave velocity: case–control study protocol (MIVAS study)

IntroductionIntestinal microbiota is arising as a new element in the physiopathology of cardiovascular diseases. A healthy microbiota includes a balanced representation of bacteria with health promotion functions (symbiotes). The aim...

Coupled flexural torsional analysis and buckling optimization of variable stiffness thin-walled composite beams

Present work examines flexural–torsional coupling for static and stability analysis of thin-walled composite beams having variable stiffness. Variable stiffness composites (VSCs), manufactured with curvilinear fibers, are considered in this work. Compared to straight fiber laminates, variable angle tow (VAT) sections may significantly improve structural response as they redistribute the in-plane stresses. A generalized beam kinematic model which considers the effect of flexural, torsional and, warping deformation is adopted and using the principle of minimum potential energy, governing equations are derived. The variable fiber angles can be defined using a general Lagrangian polynomial and as a particular case, the solution is presented for linearly varying fiber angles. Stability analysis considers geometric nonlinearity. The present model neglects transverse shear deformations, and governing equations are numerically solved using a set of C 1 continuous finite elements. Important observations are made on the coupled response of thin-walled VSCs for static and stability analysis, which indicates the necessity of optimization study. An efficient ‘Teaching Learning Based Optimization’ (TLBO) algorithm is applied to obtain the most efficient curvilinear fiber path. The results show that lateral torsional buckling load capacity can be increased by approximately 15% in optimized VSCs compared to unidirectional fiber composites. The present kinematic model and numerical scheme can be easily extended to the optimization studies considering manufacturing constraints, which may be further implemented in commercial designs.

Nano-silica reinforced epoxy resin/nano-rubber composite material with a balance of stiffness and toughness

In the electronics and aerospace industries, epoxy resins are generally regarded as economical and efficient adhesives and have a high status. However, epoxy resins are highly crosslinked polymers and are very brittle adhesives where they are prone to fast crack propagation under dynamic loads. Therefore, it is very necessary to enhance the toughness of epoxy resin adhesives. Nano-rubber has been proved to be an important toughening agent for epoxy resin, which can significantly improve the fracture toughness of epoxy resin. However, increasing the toughness of epoxy resin by adding nanomaterials is often accompanied by decreasing the strength and stiffness of resin. Therefore, in this work, rigid nano-silica particles were added to improve the rigidity and tensile strength reduction caused by the addition of rubber particles. And further increase the toughness of the epoxy resin to obtain an epoxy adhesive with balanced stiffness-toughness. As a result, it can be found that the addition of silica particles can significantly improve the decrease in stiffness caused by the addition of rubber particles. For example, Young’s modulus and tensile strength are increased by 28%, and 23%, respectively, with 4% silica is added based on rubber particles. Through the single lap shear experiment, it is found that the shear strength of the epoxy/RnP/silica composite adhesive has increased, which further proves that the addition of nano-silica particles can increase the stiffness of the epoxy composite. The dynamic mechanical analysis experiment found that after adding nano-silica particles, the storage modulus of epoxy composites increased, which also shows that adding nano-silica particles can improve the stiffness of epoxy composites. Scanning electron microscopy analysis was performed to study the reinforcement mechanism of epoxy/RnP/silica composite materials. The thermal stability of epoxy composites was characterized by Dynamic mechanical analysis and thermogravimetric analysis.

The Effect of Functionally‐Graded Voids and GNPs on the Damage Tolerance of Polyurethane Foam Core

AbstractThe weight of sandwich composite structures with foam cores mainly depends on the volume fraction and the distribution of air voids in its porous structure. A considerable reduction of the weight can be achieved at the expense of mechanical and strength properties by increasing the fraction of air voids in the sandwich composite's core. Therefore, high stiffness nano‐fillers such as graphene nano‐platelets (GNPs) are frequently used to improve the specific stiffness and mechanical strength of sandwich composites without sacrificing from the lightweight requirements. In this contribution, the effect of the square‐ and circle‐shaped distributions of air‐voids and GNPs on damage tolerance properties of thermoplastic polyurethane (TPU) resin is investigated. TPU foams with randomly oriented air voids and GNPs are used as a baseline to evaluate the improvement of the properties mentioned above. For that purpose, finite element based micro‐mechanical models for representative volume elements (RVEs) are used to homogenize the properties of multi‐scale models of TPU foam. The analysis results show that the air pores increase the ductility of brittle TPU resin significantly when the pores are non‐linearly distributed in circular functionally graded (FG) shape. Besides, it is concluded that Young's modulus degradation associated with the use of linearly distributed air‐voids composed of the square‐shaped FG foam can be compensated by using GNPs. The results of the study suggest that the functional grading design of TPU foam has excellent potential for applications, which require meeting the challenging weight, strength, stiffness and toughness requirements at the same time.

A inclusive review: Fatigue behaviour and its failure mechanisms of aluminium - carbon nanotubes (CNT) composites: Part 3

Carbon nanotubes (CNT) are an outstanding development in the field of nanostructure. Unique characteristics of Carbon Nanotubes (CNT) make them a suitable reinforcing agent in Aluminium (Al) Metal Matrix composites. These composites are being anticipated in aviation for their high explicit quality just as utilitarian materials for their energizing mechanical attributes. Most researches have focused on their strength and stiffness of Al-CNT composites for more than a decade. There is no specific review paper for the mechanism of fatigue failure of CNT-Al composites. Thus the objective of this review paper to addresses the fatigue failure mechanism of Al - CNT composites.

Mechanical Properties, Microstructure and Surface Quality of Polypropylene Green Composites as a Function of Sunflower Husk Waste Filler Particle Size and Content

Agricultural waste is a still untapped source of materials that can, in case of proper utilization, significantly improve the sustainability of polymers and their composites. In this work, polymer composites based on isotactic polypropylene were produced incorporating ground sunflower husk in the amount of 10 wt% and 20 wt%. The work’s main objective is to evaluate how preliminary fractioning of this agricultural waste filler affects the thermomechanical properties, microstructure and surface topology of polypropylene-based injection molded composites. The composites were analyzed for mechanical properties (tensile, impact strength and hardness), thermomechanical properties (Vicat softening point VST, heat deflection temperature HDT, and dynamic thermomechanical analysis DMTA) with reference to morphological changes evaluated using scanning electron microscopy (SEM). The quality of the produced composites was assessed on the basis of the analysis of the surface topology of the injected composites. It has been shown that the larger particle size of used filler has a direct impact on increasing composite stiffness in the room and elevated temperature. Moreover, a relationship was demonstrated between the size of the filler and the deterioration of the tensile strength in the case of composites with a higher content of filler. The results show that the addition of sunflower husk as a particle-shaped waste filler is an effective method to increase sustainability of polypropylene-based green composites with beneficial thermomechanical properties and to reduce the residue of sunflower husk from industrial oil production.

Thermomechanical Properties of Carbon Fabric Reinforced Epoxy Laminates with h-BN and MoS2 Fillers

Abstract This work endeavors to investigate thermomechanical performance of carbon fabric reinforced epoxy composite (CEC) with fillers – hexagonal boron nitride (h-BN) and molybdenum disulfide (MoS2). The filler of 2, 4, 6, and 8 Wt.% was dispersed precisely in the epoxy resin through magnetic stirring and ultrasonication, prepared the filler loaded CEC using vacuum bag technique and studied the composite behavior at an elevated temperature by dynamic mechanical analysis. The 6 Wt.% MoS2-CEC showed 70% enhancement of storage modulus since the filler in the epoxy networks increased the composite stiffness. The increase in G-band intensity of Raman spectra in the filler loaded composites confirmed the improved matrix-fiber stress transfer. The 4 Wt.% BN-CEC revealed the highest glass-transition temperature 92°C. The thermogravimetric analysis of the composite exhibited a two-step thermal decomposition: epoxy matrix (nearby 260°C) and carbon fibers (beyond 420°C). The 4 Wt.% MoS2-CEC showed the maximum degree of crosslinking, twice the neat CEC, the MoS2 restrained the mobility of the epoxy chains and decreased the thermal decomposition. Both the filler loaded composites have comparable thermal stability and are significantly improved than the neat CEC. Thus, the composite containing solid lubricant filler up to 6 Wt.% shall be used for high-temperature applications.