Mechanics of Composite Materials最新文献

Within the framework of the nonlocal strain gradient and the Euler–Bernoulli beam theories, a model of electrostatically actuated functionally graded (FG) microbeams is developed taking into account the intermolecular interaction forces in it. In addition to the electrostatic forces resulting from the applied DC voltage between a fixed substrate and the FG microbeam, the Casimir and van der Waals forces were also considered. The governing equation of motion of FG microbeams was derived by employing the Hamilton principle. Utilizing the Galerkin and the equivalent linearization methods, an analytical solution was obtained for the nonlinear vibration problem with zero initial conditions. To validate the accuracy of the results obtained, our solution was compared with the numerical and analytic solutions published in the literature.

A promising mathematical model is considered for studying the aeroelastic stability of sandwich shells supported by an internal hollow elastic cylinder and annular ribs. Using the relations derived, the influence of thickness of the inner hollow cylinder, the length of the shell, and the shear modulus of core on the critical air flow speed leading to the occurrence of flutter was studied. Based on the calculations performed, recommendations were developed for choosing parameters of the sandwich composite structure considered. The mathematical model presented makes it possible to expand the range of topical scientific and applied problems that can be solved in the field of aeroelastic stability of sandwich shells.

The problem of forced bending vibrations of a plane rod with a finite-length fastening section under the action of an external transverse force at its free end was solved. The classical Kirchhoff–Love model in the classical geometrically nonlinear approximation was used to describe the deformation process of the free part of the rod. The deformation of its fixed part was described by the Timoshenko refined shear model that takes into account transverse strains. The conditions of kinematic conjugation of the free and fixed parts of the rod were formulated. The equations of motion, the corresponding boundary conditions, and the force conditions of conjugation of the rod parts were obtained using the Hamilton–Ostrogradsky variational principle. An exact analytical solution of the problem of forced vibrations of a rod under the action of a harmonic transverse force at the free end of the unfastened part of the rod was deduced. Numerical experiments were carried out to study the resonant vibrations of rods made of unidirectional fiber composite. The effect of a noticeable increase of the amplitudes of transverse vibrations of the ends of the cantilever parts of the rods studied due to transverse contraction of the fixed section was revealed. Taking into account the transverse contraction caused an almost twofold reduction of the maximum transverse shear stresses in the fixed part of the duralumin rod.

The main, well-known scientific results were obtained by outstanding scientist Yu. N. Rabotnov in the field of creep theory, hereditary elasticity, damage mechanics, technical theory of shells. However, in the last years of his life, the academician Yu. N. Rabotnov was keen on some problems in the field of mechanics of fiber-reinforced composite materials. He proposed new, original strength criteria, simplified methods for ply-by-ply calculation of composite laminates. One of Yu. N. Rabotnov’s scientific passions were connected with description of specific fracture modes of layered fibrous composites having the types of delamination and splitting. On the base of energy fracture criterion, it’s possible to estimate the scale effect of strength – the dependence of critical stresses on the absolute sizes of composite members. Such models of specific fracture modes make it possible to estimate the danger of such technological defects as thin polymeric film between composite layers.

The aim of the present study is to use beech wood flour (WF) and expanded ethylene vinyl acetate (EVA) copolymer industrial waste to develop a sustainable composite and its production method for further engineering use. Polyamide (PA) powder waste obtained after multiple selective laser sintering (SLS) thermal cycles was used to increase the strength and adhesion between the waste composite components. The morphological, mechanical, and thermal properties of the EVA/WF composites were characterised along with their interfacial wetting and water absorption properties. Optical and electron microscopy investigations revealed that the composites prepared have homogeneous dispersion and good interfacial adhesion between EVA and wood. The addition of SLS waste PA powder increases the strength and stiffness of the composite developed. The composite with 40 wt% WF exhibited the best water absorption, mechanical properties, and processability among the various compositions. The sustainable composite proposed can replace commercially available materials, which helps to save resources and reduce waste.

A new analytical study for nonlinear buckling and postbuckling behavior of porous functionally graded material (FGM) circular plates and shallow spherical caps resting on nonlinear elastic foundation was carried put using the nonlinear Reddy’s higher-order shear deformation theory (HSDT). The spherical caps/circular plates under thermo-mechanical loadings were considered, and the nonlinear elastic foundation was used to model the behavior of hardening and softening foundations. The total potential energy expressions of caps/plates were established, and the Ritz energy method was applied. The analytical expressions of load-deflection relationships were obtained. The critical buckling loads and postbuckling behavior of shells/plates were determined and analyzed. The remarkable influences of geometrical parameters, material parameters, and nonlinear foundation stiffnesses on the nonlinear static stability behavior of caps and circular plates were noted.

Hybrid composite materials reinforced with nanoscale additives provide better mechanical properties than traditional composites. Carbon-fiber-reinforced polyether ether ketone with various layups modified by 0.05-0.36 wt% of single-wall carbon nanotubes was experimentally studied. The influence of the nanotubes on the mechanical properties of the composites, found in tensile and bending tests, and on their electrical conductivity was studied and discussed. Adding 0.15 wt% of SWCNTs between layers of prepregs led to an increase in the tensile strength by 9.9% and flexural strength by 5.5%. The electrical conductivity of the unidirectional composite has not been changed significantly after the incorporation of SWCNTs, while for orthotropic layup it increases by 65%.

A systematic analytical study of the mathematical properties of the previously constructed nonlinear model for shear flow of thixotropic viscoelastic-plastic media, which takes into account the mutual influence of the deformation process and structure evolution, is continued. A set of two nonlinear differential equations describing the processes of shear at a constant rate and stress relaxation is obtained. Equation set describing creep is derived; a general solution of the Cauchy problem for the set is constructed in an explicit form (the equations of the families of creep, and structuredness curves are derived). For arbitrary six material parameters and (increasing) material function that govern the model, basic properties of the families stress-strain curves at constant strain rates, stress relaxation curves and creep curves generated by the model, and the features of structuredness evolution under these types of loading are analytically studied. The dependences of these curves on time, shear rate, stress level, initial strain, and initial structuredness of the material, as well as on the material parameters and function of the model, are studied. Several indicators of the applicability of the model are found which are convenient to check with experimental data. It was examined what effects typical for viscoelastic-plastic media can be described by the model and what unusual effects (unusual properties) are generated by a change in structuredness in comparison with typical stress-strain curves, relaxation curves, and creep curves of structurally stable materials. In particular, it is proved that creep curves always increase in time and have oblique asymptote, and structuredness under constant stress is always monotonous (unlike other loading modes), but can decrease or increase depending on the relation between the stress level and initial structuredness. The same condition controls creep curves to be convex up or down: at a certain (calculated) critical load creep curves change from convexity up (under smaller loads) to convexity down, and the structuredness becomes ascending instead of descending. The analysis proved the ability of the model to describe behavior of not only liquid-like viscoelastoplastic media, but also solid-like (thickening, hardening, hardened) media: creep, relaxation, recovery, a number of typical properties of experimental relaxation curves, creep and stress-strain curves, strain rate and strain hardening, flow under constant stress and so on.

This study aims to analyze the in-plane free vibrations of arches comprising the laterally symmetric functionally graded materials. Emphasis is placed on the circular arch whose material properties vary laterally symmetrically about the centroidal axis by a power-law function. The differential equations governing the mode shape of the arch were derived under the boundary conditions and were numerically solved to calculate the natural frequencies using the Runge–Kutta and Regula–Falsi methods. Calculation results of this study for natural frequencies compare well with those of the finite element method. The effects of various arch parameters on natural frequencies are highlighted and discussed in detail.

It is known that the initial fiber waviness affects the stiffness and strength of the polymer composite material. The influence of degree of waviness on stiffness characteristics under uniaxial tension and compression of a polymer composite material was investigated by using numerical modeling. A computational approach based on a special periodicity cell with different fiber waviness was developed. The hypothesis regarding the impact of manufacturing stresses, appearing during a curing process on waviness growth, was tested. The results obtained explain the mechanism that causes difference in the stiffness observed in fiber composites in the longitudinal direction under uniaxial tension and compression.