Showing papers in "Mechanics of Composite Materials in 2012"
TL;DR: In this article, an analytical investigation into the nonlinear response of thick functionally graded double-curved shallow panels resting on elastic foundations and subjected to thermal and thermomechanical loads is presented.
Abstract: An analytical investigation into the nonlinear response of thick functionally graded double-curved shallow panels resting on elastic foundations and subjected to thermal and thermomechanical loads is presented. Young’s modulus and Poisson’s ratio are both graded in the thickness direction according to a simple power-law distribution in terms of volume fractions of constituents. All formulations are based on the classical shell theory with account of geometrical nonlinearity and initial geometrical imperfection in the cases of Pasternak-type elastic foundations. By applying the Galerkin method, explicit relations for the thermal load–deflection curves of simply supported curved panels are found. The effects of material and geometrical properties and foundation stiffness on the buckling and postbuckling load-carrying capacity of the panels in thermal environments are analyzed and discussed.
51 citations
TL;DR: In this paper, the design of laminates with a minimum number of layers for obtaining the given elastic properties is addressed in the general case, where no simplifying hypotheses are made about the type of their stacking sequence.
Abstract: The design of laminates with a minimum number of layers for obtaining the given elastic properties is addressed in the paper. The problem is treated and solved in the general case — no simplifying hypotheses are made about the type of their stacking sequence. The problem is stated as a nonlinear programming problem, where a unique objective function includes all the requirements to be satisfied by the solutions. The optimum solutions are found within the framework of the polar-genetic approach, since the objective function is written in terms of polar parameters of the laminates, while a nonclassical genetic algorithm is used as the optimization scheme. The optimization variables include the number of layers, layer orientations and layer thicknesses. In order to include the number of plies among the design variables, certain modifications of the genetic algorithm have been done, and new genetic operators have been developed. Some examples and numerical results concerning the design of laminates with a minimum number of layers for obtaining some prescribed elastic symmetries, bending-extension uncoupling, and quasi-homogeneity are shown in the paper.
43 citations
TL;DR: In this paper, the effect of hollow glass microspheres with a density of 125 kg/m3 on the properties of low-density rigid polyurethane foams is investigated.
Abstract: The effect of hollow glass microspheres with a density of 125 kg/m3 on the properties of low-density (54-90 kg/m3) rigid polyurethane foams is investigated. The thermal expansion coefficient of the foams and their properties in tension and compression in relation to the content of the microspheres (0.5-5 wt.%) are determined. An increase in the characteristics of the material in compression in the foam rise direction with increasing content of filler is revealed. The limiting content of the microspheres above which the mechanical characteristics of the filled foams begin to decrease is found. The distribution of the microspheres in elements of the cellular structure of the polyurethane foams is examined.
24 citations
TL;DR: The results of an experimental investigation of round concrete columns confined by a wound basalt filament yarn are presented in this paper, where it is shown that the basalt FRP confinement provides a considerable strengthening effect.
Abstract: The results of an experimental investigation of round concrete columns confined by a wound basalt filament yarn are presented. Basalt is an attractive material for strengthening purposes due to its low cost coupled with a good mechanical performance, especially at high temperatures. It is shown that the basalt FRP confinement provides a considerable strengthening effect. The winding equipment employed in this study has the ability to set a desired pretension force of the yarn and thereby to produce a prestressed confinement. It is found that the prestressed confinement notably delays the onset of intense internal cracking of concrete.
23 citations
TL;DR: In this article, various composites were prepared from recycled waste printed circuit boards and an epoxy resin, and some of their mechanical properties (hardness, elongation at break, tensile and flexural strengths and moduli, and impact energy) were determined experimentally.
Abstract: Various composites were prepared from recycled waste printed circuit boards and an epoxy resin, and some of their mechanical properties (hardness, elongation at break, tensile and flexural strengths and moduli, and impact energy) were determined experimentally. The results obtained are presented in the form of diagrams.
17 citations
TL;DR: In this article, an efficient method of solving 3D elasticity problems for thick and thin laminated composite plates is presented, based on a new concept of reference surfaces inside the plate, and in each nth layer, in arbitrary reference surfaces parallel to the midsurface are introduced, and the displacement vectors of these surfaces are chosen as unknown functions.
Abstract: An efficient method of solving 3D elasticity problems for thick and thin laminated composite plates is presented. It is based on a new concept of reference surfaces inside the plate. According to this concept, into each nth layer, In arbitrary reference surfaces parallel to the midsurface are introduced, and the displacement vectors of these surfaces are chosen as unknown functions. Such a choice allows one to represent the governing equations of the high-order theory of plates proposed in a very compact form and to derive strain–displacement relationships correctly describing all rigid-body motions of laminated plates.
17 citations
TL;DR: In this article, a physical-mechanical description of the mechanical characteristics of a three-dimensional cross-linked plasticized elastomer filled with solid particles is presented, where the basic structural parameters of the composition and the temperature-rate equivalence in the uniaxial tension of specimens are taken into account.
Abstract: A physical-mechanical description of the mechanical characteristics of a three-dimensional cross-linked plasticized elastomer filled with solid particles is presented. The basic structural parameters of the composition and the temperature-rate equivalence in the uniaxial tension of specimens are taken into account. Examples of solution of direct and inverse prediction problems are given.
16 citations
TL;DR: In this paper, a three-dimensional analytical model for a unitcell representative volume element (RVE) consisting of a monofiber, a matrix, and an interphase region is developed.
Abstract: Based on the energy method and the principle of minimum total complementary potential energy, a new three-dimensional analytical model is developed. A three-dimensional analysis is performed for a unit-cell representative volume element (RVE) consisting of a monofiber, a matrix, and an interphase region. The model is evaluated using the finite-element technique. In order to study the effects of neighboring fibers on the residual stresses, several microstructural finite-element models are considered. It is proved that the RVE used in the analytical model is suitable for predicting the thermal microresidual stresses in polymer composites. In comparison with the analytical model, the finite-element analysis presents a little higher residual stresses. According to the results of the analytical model, the interfacial shear stress reaches a maximum in the vicinity of fiber ends, while the finite-element analysis gives a maximum shear stress at composite ends. This is due to the edge singularity, which is not considered in the finite-element model. Although the interphase region affects the distribution of interfacial radial stresses only slightly, it has a significant effect on the maximum interfacial radial stresses at fiber ends.
15 citations
TL;DR: Using the Rayleigh-Ritz method, the natural frequencies of layered cylindrical shells with delaminations are derived in this paper, and the results are compared with FE computations and with data available in the literature.
Abstract: Using the Rayleigh–Ritz method, the natural frequencies of layered cylindrical shells with delaminations are derived. The results are compared with FE computations and with data available in the literature. It is demonstrated that the transverse shear effects can play a significant role in a correct and accurate evaluation of the natural frequencies of such shells. Numerical investigations into the problem of radial excitations of cylinders with delaminations have also been performed. From the results obtained, it follows that their optimal excitation frequency is related to the natural frequency of a local region containing the delamination.
15 citations
TL;DR: In this paper, the authors compare the numerical and experimental results with a closed-form solution for the dynamic responses of composite sandwich panels subjected to a low-velocity impact, using an equivalent two-degree-of-freedom system to predict the response at different boundary conditions.
Abstract: The objective of this research is to compare the numerical and experimental results with a closed-form solution for the dynamic responses of composite sandwich panels subjected to a low-velocity impact. An equivalent twodegree-of-freedom system is used to predict the response at different boundary conditions. The experimental results were obtained from drop-weight impact tests. Numerical calculations were performed using the LSDYNA software to verify the analytical results. The numerical simulation showed a good agreement with experimental data. Meanwhile, with increasing impact velocity, the analytical model showed some disagreement with experiment. This can be explained by the static analysis and minimization of the potential energy used in the model.
14 citations
TL;DR: In this paper, the dynamic stability of foam-filled sandwich plates with stiff composite laminated faces subjected to an arbitrary periodic load is studied and the effects of load parameters and core layer thickness on the excitation frequency, dynamic stability region and dynamic instability index are discussed.
Abstract: In this paper, the dynamic stability of foam-filled sandwich plates with stiff composite laminated faces subjected to an arbitrary periodic load is studied. The load is taken to be a combination of periodic bending and normal stresses. The governing equations are established by using the Galerkin method with a reduced eigenfunctions transformation. The equations of motion of Mathieu type are derived and used to determine the regions of dynamic instability based on Bolotin’s method. A numerical approach is developed to determine the dynamic stability of foam-filled sandwich plates. The effects of load parameters and core layer thickness on the excitation frequency, dynamic stability region and dynamic instability index are discussed.
TL;DR: In this paper, the effect of surface modification of carbon nanotubes (CNTs) on the mechanical properties of polyimide (PI) composites with and without MoS2 was investigated.
Abstract: The aim of this work was to investigate the effect of surface modification of carbon nanotubes (CNTs) on the mechanical properties of polyimide (PI) composites with and without MoS2. A three-phase system of CNT/PI/ MoS2 laminates were fabricated with an extrusion-grade PI to produce CNT-reinforced laminates with MoS2 volume fractions of 1–5%. The tensile and impact properties of CNT/PI composites and CNT/PI/MoS2 laminates were also measured and compared. Results showed that the introduction of CNTs as reinforcing additives improved the tensile properties of the composites, but worsened their impact properties. Furthermore, the addition of MoS2 increased the impact strength of the CNT/PI composites greatly. The optimum contents of CNT and MoS2 have been found.
TL;DR: In this paper, the elastic properties of a nanocomposite based on a rigid polyurethane and organically modified montmorillonite are analyzed, and data on the influence of volume fraction of the filler and of anisometry (aspect ratio) and orientation of the plate-like filler particles on the elastic constants of the nanocompositionite are reported.
Abstract: A variant for determining the elastic characteristics of composites containing platelike filler particles is considered. The elastic properties of a nanocomposite based on a rigid polyurethane and organically modified montmorillonite are analyzed. Data on the influence of volume fraction of the filler and of anisometry (aspect ratio) and orientation of the montmorillonite particles on the elastic constants of the nanocomposite are reported.
TL;DR: In this article, the tension curves of short-flax-fiber-reinforced polypropylene were used to evaluate the interfacial shear strength and the fiber orientation factor of the composite.
Abstract: An important factor affecting the mechanical characteristics of composite materials is the adhesion of constituents which can be characterized by the interfacial shear strength (IFSS). Most of the experimental methods for the evaluation of IFSS involve model single-fiber composites. By contrast, the method proposed by Bowyer and Bader is based on an analysis of the stress–strain curve of a short-fiber-reinforced composite in tension. In the present study, the method is applied to the experimental tension curves of short-flax-fiberreinforced polypropylene to evaluate the IFSS and the fiber orientation factor of the composite. The IFSS was found to depend on the volume fraction of fibers and the treatment to improve adhesion.
TL;DR: In this paper, the cumulative distribution function (cdf) of the fatigue life of every individual microvolume (WMV) is calculated using the Markov model of fatigue, where the authors consider the presence of several weak sites where fatigue damage can accumulate and a loading with an arbitrary (but positive) stress ratio.
Abstract: As a development of the model where only one weak microvolume (WMV) and only a pulsating cyclic loading are considered, in the current version of the model, we take into account the presence of several weak sites where fatigue damage can accumulate and a loading with an arbitrary (but positive) stress ratio. The Poisson process of initiation of WMVs is considered, whose rate depends on the size of a specimen. The cumulative distribution function (cdf) of the fatigue life of every individual WMV is calculated using the Markov model of fatigue. For the case where this function is approximated by a lognormal distribution, a formula for calculating the cdf of fatigue life of the specimen (modeled as a chain of WMVs) is obtained. Only a pulsating cyclic loading was considered in the previous version of the model. Now, using the modified energy method, a loading cycle with an arbitrary stress ratio is “transformed” into an equivalent cycle with some other stress ratio. In such a way, the entire probabilistic fatigue diagram for any stress ratio with a positive cycle stress can be obtained. Numerical examples are presented.
TL;DR: In this article, the authors derived nonlinear vibration equations of motion based on the Mindlin plate theory for a hybrid composite plate with an initial stress on elastic foundations, and the results revealed that the foundation stiffness and initial stresses lead to a drastic change in the non-linear vibration behavior of the plate.
Abstract: Nonlinear vibration equations of motion based on the Mindlin plate theory are derived for a hybrid composite plate with an initial stress on elastic foundations. Using the governing equations derived, the nonlinear vibration behavior of an initially stressed hybrid composite plate on Pasternak and Winkler elastic foundations is studied. The initial stress is taken to be a combination of a pure bending stress and a tensile stress in the plane of the plate. The Galerkin method is employed to reduce the governing nonlinear partial differential equations to ordinary nonlinear differential equations, and the Runge–Kutta method is used to obtain the nonlinear frequencies. The linear frequency can be calculated by neglecting the nonlinear terms in the ordinary nonlinear differential equations. The results obtained reveal that the foundation stiffness and initial stresses lead to a drastic change in the nonlinear vibration behavior of the plate. The effects of various parameters on the nonlinear vibration of hybrid composite plates are investigated and discussed.
TL;DR: In this paper, the results of an experimental investigation and analytical description of creep of a nonlinear viscoelasticplastic material (a Upilex-S polyimide film) in tension are presented.
Abstract: The results of an experimental investigation and analytical description of creep of a nonlinear viscoelasticplastic material (a Upilex-S polyimide film) in tension are presented. The elastic modulus, yield stress, and ultimate strength were found from quasi-static tests with a constant deformation rate. The total strain of the material was assumed to be the sum of elastic, viscoelastic, and irreversible strains. The reversible elastic and viscoelastic and the irreversible deformation of the material were investigated in experiments on creep with the subsequent recovery of deformations (creep recovery). The nonlinearity of its viscoelastic behavior was taken into account using the principle of stress-time superposition. The results obtained allow one to describe the creep behavior of a material at various stages of loading and unloading.
TL;DR: In this article, two types of layered composite structures based on an aramid fabric covered with alumina microparticles and on polycarbonate were considered and the minimum distance from a first puncture where the stubbing protection is safe was found by using the digital image correlation method.
Abstract: Two types of layered composite structures — based on an aramid fabric covered with alumina microparticles and on polycarbonate — have been considered. Experimental investigation of the mechanical properties and puncture resistance of the first material was carried out. The results of quasi-static, dynamic, and firearm tests of the composite plates are presented. The minimum distance from a first puncture where the stubbing protection is safe was found by using the digital image correlation method. A FEM analysis of bending and tension of composite plates with a defect of complex geometry was performed. The analytical results obtained are compared with test data.
TL;DR: In this paper, the properties of polyethylene (PE) with small additions of multiwall carbon nanotubes (CNTs) are reported, and a theoretical analysis of elastic properties of the PE/CNT composite is carried out by using the Mori-Tanaka theory of an equivalent medium.
Abstract: Results of an investigation into the properties of polyethylene (PE) with small, no more than 5 wt.%, additions of multiwall carbon nanotubes (CNTs) are reported. Specimens of the composite were prepared using a concentrate containing 31.6 wt.% of nanotubes in the polyethylene matrix. The concentrate was fabricated by a patent in situ polymerization method. Experimental data on the influence of CNT additions on the thermograms of differential scanning calorimetry, the crystallinity of the polyethylene matrix, and the indices of mechanical properties (yield stress, strength, elastic modulus, ultimate elongation, and long-term creep) of PE/CNT composite are obtained. A theoretical analysis of elastic properties of the PE/CNT composite was carried out by using the Mori–Tanaka theory of an equivalent medium. The calculation results are compared with experimental data.
TL;DR: In this article, the effect of initial stress concentrations around two cylindrical horizontal holes in an orthotropic rectangular simply supported plate is investigated, where the plate is loaded by uniformly distributed normal tensile or compressive forces on its two opposite end faces parallel to the longitudinal axes of the holes.
Abstract: The effect of initial stress concentrations around two neighboring cylindrical horizontal holes in an orthotropic rectangular simply supported plate is investigated. The plate is loaded by uniformly distributed normal tensile or compressive forces on its two opposite end faces parallel to the longitudinal axes of the holes and by uniformly distributed normal forces on its upper face. The corresponding problem is formulated within the framework of the 3D linearized theory of elasticity and is solved numerically by using the 3D finite-element method. The influence of material and geometrical parameters, interaction between the holes, and initial stresses on the concentration of stresses around the holes is shown and discussed.
TL;DR: In this article, a procedure for calculating the deformation of foldcores of any geometry and configuration made of various composite materials with an arbitrary stacking sequence of layers has been created for the cases of transverse compression and shear by using the ANSYS\LS-DYNA software.
Abstract: A procedure for calculating the deformation of foldcores of any geometry and configuration made of various composite materials with an arbitrary stacking sequence of layers has been created for the cases of transverse compression and shear by using the ANSYS\LS-DYNA software. The results obtained for carbon and aramid foldcores in transverse compression are compared with experimental data. The influence of size of the finite-element mesh and of the mass scaling technique on calculation results is investigated. The possibility of using the cell of periodicity instead of the entire three-layer panel in the calculations is shown. Transverse compression and shear simulations for three various foldcore configurations — Z-, M-, and V-crimps — are carried out, with the use of two models of composite materials with failure. The failure criteria used in this research are analyzed in detail. An APDL program allowing one to prescribe a geometry and material properties parametrically has been created for Z-, M-, and V-crimps, with a possibility of addition of new types of crimps.
TL;DR: In this paper, the elastic modulus of polymeric materials from deformation diagrams of thin-walled circular cylindrical shells in compression and tension in the region of geometrical nonlinearity has been elaborated.
Abstract: A method for determining the elastic modulus of polymeric materials from deformation diagrams of thin-walled circular cylindrical shells in compression and tension in the region of geometrical nonlinearity has been elaborated. A numerical solution is found by the finite-element method (ANSYS.) The existence of a unified deformation diagram in generalized coordinates is established, from which the elastic modulus is determined. To validate the method, the eigenfrequencies of cylindrical specimens were found experimentally. The results obtained are compared with FEM calculations.
TL;DR: In this paper, the damage behavior of a type-3 hydrogen storage vessel is modeled based on a meso-macro approach and takes into account the damage behaviour of the composite and the elastoplastic deformation of the liner.
Abstract: The damage behavior of a type-3 hydrogen storage vessel is modeled. The vessel consists of a metal envelop, called liner, coated with a filament winding. The model proposed allows simulating the mechanical response of the structure to a quasi-static loading. The model is based on a meso-macro approach and takes into account the damage behavior of the composite and the elastoplastic deformation of the liner. The results obtained are compared with experimental data. Finally, the effect of stacking sequence of filament layers on the damage level in the composite is investigated.
TL;DR: In this paper, the behavior of organic and carbon-fiber-reinforced plastics based on three different binders was investigated under three test modes: stress relaxation, strain relaxation, and creep.
Abstract: The behavior of organic- and carbon-fiber-reinforced plastics based on three different binders was investigated under three test modes: stress relaxation, strain relaxation, and creep. In the theory used, the linearized generalized Maxwell–Gurevitch equation is employed. From stress relaxation diagrams, the relaxation constants of the equation are determined, which are utilized for calculating creep diagrams. The elastic and strength characteristics of the composites are also investigated.
TL;DR: In this article, a model for calculating the linear elastic constants of high-porosity cellular plastics by orientationally averaging the rigidity tensor of a structural element consisting of an air sheath and a loadcarrying element in the form of a straight strut with a piecewise constant cross section is presented.
Abstract: A model is presented for calculating the linear elastic constants of high-porosity cellular plastics by orientationally averaging the rigidity tensor of a structural element consisting of an air sheath and a loadcarrying element in the form of a straight strut with a piecewise constant cross section. The load-carrying element can resist the axial and shear loads and bending moments applied to its ends. The nonuniform orientational distribution of the elements is also taken into account. The calculation results obtained are compared with some literature data.
TL;DR: In this paper, the experimental investigation of reinforced concrete (RC) slabs, repaired and reinforced with an alumina/polymer (AP) mortar and a prestressing (PS) strand in the tension zone, under static and fatigue loadings was conducted.
Abstract: While the extent of repair and rehabilitation of existing old concrete structures is rapidly increasing, a vast number of repaired and rehabilitated structures do not function properly during their remaining service life. Especially in the case of using heterogeneous repair materials, it is very important to maintain the bonding performance between materials and to prevent the interface failure under static and fatigue loads. This paper focuses on the experimental investigation of reinforced concrete (RC) slabs, repaired and reinforced with an alumina/polymer (AP) mortar and a prestressing (PS) strand in the tension zone, under static and fatigue loadings. The variables in this experimental study were the space of strengthening, the number of strands, and AP mortar thickness. Attention is concentrated on the overall load-carrying capacity, deflection, strains of reinforcing bars, and the efficiency of repaired and reinforced RC slabs. Test results showed that the deflection of the repaired and reinforced RC slabs was approximately 40% lower than that of control RC slabs. The initial and horizontal cracking loads of a RC slab with an AP mortar of thickness 30 mm in the static test were approximately the same as those of a RC slab with a 20-mm-thick one. In the fatigue test, the deflection, the strain of reinforcing bars at the midspan, and the maximum shear stress of the repaired and strengthened RC slab were about 40~70% lower than those of the control RC slab. Therefore, it can be concluded that RC slabs with an AP mortar and PS strands have a good strengthening efficiency under both static and fatigue loadings, thanks to the high bonding capacity of the AP mortar.
TL;DR: In this paper, a polyoxymethylene/carbon fiber (POM/CF) composites containing nano-SiO2 were prepared, and their mechanical properties were investigated.
Abstract: Polyoxymethylene/carbon fiber(POM/CF)composites containing nano-SiO2 were prepared, and their mechanical properties were investigated. At a content of 1-5 vol.%, the nano-SiO2 exerted an obvious reinforcing effect on POM, leading to an increase in the elastic modulus and stiffness of the composites.
TL;DR: In this article, the average elastic and strength characteristics of a honeycomb core of hexahedral structure made of a NOMEX polymeric paper were determined in compression and shear in two mutually perpendicular planes.
Abstract: To determine the average elastic and strength characteristics of a honeycomb core of hexahedral structure made of a “NOMEX” polymeric paper, tests of special specimens in compression and shear in two mutually perpendicular planes were conducted. Proceeding from the experimental and computational-experimental data found, the dimensionless correcting parameters entering into solutions derived previously in the form of analytical formulas and meant to determine the honeycomb core characteristics mentioned are identified. It is established that, in compression, the average elastic and strength characteristics of the filler considered are practically formed only at the expense of two walls glued together, but in shear in the plane of the walls — at the expense of all four walls of the cell of periodicity.
TL;DR: In this paper, a failure analysis and shape control of smart composite laminates under coupled thermal (hygro), electric, and mechanical stimuli is studied. But the authors focus on the failure analysis only.
Abstract: The present study focuses on the failure analysis and shape control of smart composite laminates under coupled thermal (hygro), electric, and mechanical stimuli. A linear thermo(hygro)electroelastic constitutive model for transversely isotropic materials is used for each ply in the composite laminate and for the piezoelectric materials that are integrated with laminates of the composite. Piezoelectric materials, such as lead zirconate titanate, and piezoelectric fiber composites, such as an active fiber composite or a microfiber composite, are considered as actuators for controlling unwanted bending deformations to avoid failure in such composite laminates. Due to the high stress concentrations at the interfaces between an active layer and the host structure, which may cause debonding, embedded actuators in which the active material is placed as part of the plies to form geometrically continuous plies are considered in order to minimize the stress concentration while improving the actuation capability. The first-ply failure and the ultimate laminate failure criteria of composite laminates are used to predict the failure stress and mode of the smart composite laminates, where commonly known macroscopic failure criteria, such as the Tsai–Hill, Tsai–Wu, and maximum stress criteria, are employed for each lamina. Piezoelectric materials can be used to prevent the failure from hygrothermal and mechanical loadings by applying an electric voltage in order to counteract laminate deformations. Based on the deformation and failure analyzes of smart composite laminates having various stacking sequences, fiber and matrix constituents, and piezoelectric materials, we could estimate the overall properties and failure envelopes of the laminates, which is useful in the preliminary design of smart composite structures.
TL;DR: In this article, the effects of the amount of nanoclay and surface treatment were studied by a capillary rheometer, which was inspected by using the scanning electron microscopy.
Abstract: The purpose of this research was to investigate the rheological behavior and extrudate swell of polylactic acid (PLA) filled with a nanoclay. The effects of the amount of nanoclay and surface treatment were studied by a capillary rheometer. The dispersion of the nanoclay was inspected by using the scanning electron microscopy. Generally, PLA/nanoclay composites exhibited a pseudoplastic rheological behavior as the shear stress and die swell increased with increasing shear rate. However, the shear viscosity decreased. The die swell also increased with increasing shear stress. The elongation viscosity decreased with increasing elongation rate. The incorporation of nanoclay decreased the shear stress and shear viscosity to a lesser degree than the elongation viscosity.