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Mechanics of Composite Materials (v.41, #5)

Mode II Delamination of a Unidirectional Carbon Fiber/Epoxy Composite in Four-Point Bend End-Notched Flexure Tests by E. Zile; V. Tamuzs (pp. 383-390).
Results from an experimental study on the delamination of a unidirectional carbon fiber/epoxy composite by using the four-point bend end-notched flexure (4ENF) test are presented. It was found that the compliance data obtained in load-unload-reload and continuous loading tests were very similar. The R-curves for specimens of different thickness were also found experimentally. These curves showed an appreciable toughening with crack advance, which can be explained by the presence of fiber bridging. The finite-element method with cohesive elements allowing us to model the progressive delamination was used to analyze the 4ENF test.

Keywords: four-point bend end-notched flexure; delamination; Mode II; cohesive elements

Interaction of Plane Stress Waves in a Three-Layer Structure. 1. Degenerate Solutions in Terms of Characteristics for a Homogeneous Structure by V. A. Polyakov; R. P. Shlitsa; V. V. Khitrov; V. I. Zhigun (pp. 391-406).
Exact expressions in terms of characteristics for calculating the normal-stress waves propagating across the layers of different materials are deduced. A one-dimensional boundary-value problem is considered for a three-layer structure of sandwich type. The faces of the layered structure are free from loads or one of them is rigidly fixed (variant 1), or one face is rigidly fixed and the other is subjected to an impact of a mass M with a speedV0 (variant 2). For the boundary conditions of variant 1, relationships are obtained which allow one to reduce the analytical continuation of a solution in time to a periodic procedure if solely the initial disturbances of the strain field in the layers are given. It is shown that, in this case, the Cauchy problem with the initial strain field is reduced to graphoanalytically constructing the superposition patterns of the forward and backward waves. The fundamental features of the construction are demonstrated for a uniform bar with a piecewise constant distribution of strains along its length. To solve the problem of impact loading in variant 2, analytical results for a uniform plate are used, which allows us to account for the direction of mass forces in collision. In the latter case, the possibility of mass recoil is revealed in the first and second time cycles. The analytical constructions presented are focused on an exact calculation of stresses upon response of a layered plate to initial disturbances within its layers, as well as to an external dynamic action.

Keywords: layered structure; wave problem; graphoanalytical method in terms of characteristics; transverse normal stress

Predicting the Deformability of Expanded Polystyrene in Long-Term Compression by I. J. Gnip; V. I. Kersulis; S. I. Vaitkus (pp. 407-414).
The interval prediction of creep strain on the basis of 15 years is carried out for slabs of expanded polystyrene (EPS) subjected to a compressive load. The expansion of the confidence interval caused by the discounted prediction information is allowed for by an additional factor. The creep compliance $$ar J$$ c (t = 15) of the EPS is determined based on empirically estimating the long-term creep of this material subjected to a compressive stress σ c = 0.3σ10% for 15 years. A relationship between $$ar J$$ c (t = 15) and EPS density in the slabs is established.

Keywords: interval prediction; creep; creep compliance; long-term compression; slabs of expanded polystyrene

Large Elastic Strains of Plastic Foams by D. A. Chernous; S. V. Shil'ko (pp. 415-424).
The elastic deformation of plastic foams with a low (< 6%) volume fraction of solid phase is described based on a 4-rod equivalent element. A criterion is proposed which allows one to determine the parameter of structure of this element. Based on an analysis of the equivalent element, a procedure is developed for constructing the compression diagram of plastic foams in the region of large (> 70%) strains. The calculation results are compared with data found in the literature and experimental results for polyurethane foams obtained by the present authors.

Keywords: foams; implants; volume fraction of solid phase; structural element; effective Young's modulus; nonlinear elastic deformation; loss of stability

Comparative Studies on the Mechanical Properties of a Thermoset Polymer in Tension and Compression by R. D. Maksimov; E. Z. Plume; J. O. Jansons (pp. 425-436).
Results of an experimental investigation into the mechanical properties of a polyester resin in tension and compression are reported. Features of the stress-strain curves obtained are discussed. Data on the elastic modulus, Poisson ratio, and volume strains are obtained. The results of creep behavior of the material in tension and compression are also presented. It is found that the time-dependent creep obeys a power law, but the nonlinear stress dependence can be described by using the hyperbolic sine function. The effect of load type (tension or compression) on the nonlinearity of the creep is analyzed.

Keywords: polyester resin; tension; compression; strength; elastic modulus; Poisson ratio; creep

Stability of Composite Cylindrical Shells with Noncoincident Directions of Layer Reinforcement and Coordinate Lines by N. P. Semenyuk; V. M. Trach (pp. 437-444).
The stability problem is solved for cylindrical shells made of a laminated composite whose directions of layer reinforcement are not aligned with coordinate axes of the shell midsurface. Each layer of the composite is modeled by an anisotropic material with one plane of symmetry. The resolving functions of the mixed variant of shell theory are approximated by trigonometric series satisfying boundary conditions. The stability of the shells under axial compression, external pressure, and torsion is investigated. A comparison with calculation data obtained within the framework of an orthotropic body model is carried out. It is shown that this model leads to considerably erroneous critical loads for some structures of the composites.

Keywords: composite cylindrical shells; stability; axial compression; external pressure; torsion; one plane of symmetry

Analysis of Thick Laminated Composite Plates on an Elastic Foundation with the Use of Various Plate Theories by S. S. Akavci (pp. 445-460).
In this study, various theories of composite laminated plates are extended to rectangular composite laminates resting on an elastic foundation. First, an analysis based on the classical theory of laminated plates is employed. Then the first-order Reissner-Mindlin theory is used for analyzing the laminates. At last, the Reddy shear deformation theory, which allows for the transverse shear strains, is applied to the bending analysis of the laminates. In the analysis, the two-parameter Pasternak and Winkler foundations are considered. The accuracy of the present analysis is demonstrated by solving problems numerical results for which are available in the literature. Some numerical examples are presented to compare the three methods and to illustrate the effects of parameters of the elastic foundations on the bending of shear-deformable laminated plates.

Keywords: laminate; composite; plate; shear; Winkler; Pasternak

Multicriteria Optimal Design of a Rectangular Composite Plate Subjected to Biaxial and Thermal Loading by G. Teters (pp. 461-466).
Multicriteria optimization of the structure and geometry of a laminated anisotropic composite plate subjected to the thermal and biaxial action is considered. From known properties of the monolayer and the given values of variable structural parameters, the thermoelastic properties of the layered composite are determined. The criteria to be optimized—the transverse critical load and the longitudinal thermal stresses—depend on two variable design parameters of composite properties and temperature. In the space of the optimization criteria, the domain of allowable solutions and the Pareto-optimal subdomain are found.

Keywords: multicriteria optimization; composite plate; thermal action; biaxial loading

The Method of Three-Point Bending in Testing Thin High-Strength Reinforced Plastics at Large Deflections by A. K. Arnautov (pp. 467-476).
A method is presented for determining the flexural strength of unidirectional composites from three-point bending tests at large deflections. An analytical model is proposed for calculating the flexural stress in testing thin bars in the case of large deflections. The model takes into account the changes in the support reactions at bar ends and in the span of the bar caused by its deflection. In the model offered, the influence of transverse shear and the friction at supports are neglected. The problem is solved in elliptic integrals of the first and second kind. The results obtained are compared with experimental tension data. The method elaborated for calculating flexural stresses has an obvious advantage over the conventional engineering procedure, because the calculation accuracy of the stresses increases considerably in the case of large deflections.

Keywords: advanced composites; test methods; thin bar; analytical model; three-point bending; large deflections; tension; strength

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