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Mechanics of Composite Materials (v.36, #5)

Solution of Dynamic Deformation Problems for Prestressed Laminated Plates Based on the Three-Dimensional Theory of Elasticity by A.V. Marchuk; V. G. Piskunov (pp. 345-354).
A three-dimensional analytical solution describing forced harmonic vibrations of prestressed laminated plates is found for the case of a hinged support. The solution is based on the analytical separation of variables. It is assumed that the prestressed state is homogeneous, subcritical, linear, and momentless and that the vibration amplitudes are small. A solution based on a model with a polynomial approximation of the required displacement functions across the plate thickness is also considered. These functions are found on the front surfaces of the structure. This allows us to solve the problem both in the continuous and discrete structural approaches. In the continuous structural approach, the order of the resolving system of equations is independent of the number of layers. In the discrete structural approach, for rigid contact of layers with similar boundary conditions at the plate end face, an algorithm can be introduced which reduces significantly the number of operations required for realization of the model proposed. In the numerical examples presented, both rigid and sliding contacts of layers and various prestressed conditions are considered. Both approaches give results that agree well.

Keywords: prestressed laminated plates; dynamic strain; three-dimensional elastic solution; applied theory

Stress State of Multilayer Structural Elements Subjected to Static Loading and Optimization of Laminated Beams and Bars by J. Bareisis; D. Garuckas (pp. 355-364).
The results of FEM investigation of the triaxial stress state in multilayer structural elements subjected to axial and bending loads are presented. The distribution regularities of the stiffness and stresses or strains depending on the geometric and mechanical characteristics of layers and their position in the cross section of beams and bars are examined. The optimization of these elements is carried out using the dependences of the Bareisis—Paulauskas method and the “Optim-98” computer program created by the present authors. As the optimization criteria, the strength, stiffness, mass, and cost of the structural elements are considered.

Keywords: stress state; composite materials; bar; beam; static loading; optimization

Structural Changes in the Surface Friction Layer of a Polymeric Endoprosthesis Cup by E. A. Tsvetkova; Zh. V. Kadolich; V. A. Goldade; L. S. Pinchuk (pp. 365-372).
A concept of creation of a polymeric insert for hip joint endoprostheses with the physiological and biomechanical properties typical of natural cartilage is proposed. The spherical friction surface of the insert is coated with a microporous layer imitating cartilage. This layer carries an electret charge, which improves the lubrication of the endoprosthesis with synovia and serves as a carrier of drugs, thus ensuring their prolonged discharge into the operation wound. Tribotechnical characteristics of an endoprosthesis with such an insert are investigated. It is shown that a drop in the friction coefficient of such a pair is accompanied by a change in the microrelief of the friction surface and in the degree of crystallinity of the material of the porous layer.

Keywords: polymeric insert; structure; friction coefficient; porous layer; hip joint endoprosthesis; cartilage; synovia

Numerical Simulation of the Influence of Rough Bone-Callus Interface on the Healing of Fractured Bone by V. Beilin; G. N. Pande; K. Ito (pp. 373-378).
The process of healing of fractured bone is known to be influenced by the mechanical environment and the loads exerted by physical activity of the patient or otherwise. We compute mechanical fields in the soft connective tissue of the healing fracture using Biot's poroelasticity model and a finite element (FE) method for low-frequency loading. A two-scale FE framework is used to model effects of the rough bone-callus contact surface. We look at the difference the interface roughness makes with respect to different possible mechanostimulating agents.

Keywords: finite element method; poroelasticity; cellular stimulation; fracture healing

The Effect of Physical Activity on the Properties of Bone Tissue by I. Pontaga; A. Paeglitis (pp. 379-384).
The effect of sports training or lack of physical activity on the properties of bone tissue is investigated. People with normal physical activity, sportsmen, and hypodynamic persons are tested. Ultrasound as a safe and noninvasive method is used. In normal activity and after short-time sports training or bed rest, the maximum ultrasound velocity is observed in the middle part of the tibia diaphysis because this region has the highest bending moment. Long-time sports training (more than seven years) in basketball and freestyle skiing alters the distribution of ultrasound velocity along the tibia: the maximum velocity is found in the distal diaphysis of the tibia due to the existence of high mechanical stresses in this region caused by the intense external impact loads and the calf muscle contraction. The maximum velocity for rowers is in the middle diaphysis or in the region between the proximal and middle diaphysis, since high impact loads are not characteristic of their foot joints. Long-time hypodynamia weakens the distal diaphysis of the tibia due to the lack of naturally occuring high stresses in this part of the bone.

Keywords: bone tissue; ultrasound velocity; sports training; hypodynamia

Utilization of Composite Materials in Saddle-Shaped Cable Roofs by D. Serdjuks; K. Rocens; L. Pakrastinsh (pp. 385-388).
A saddle-shaped cable roof formed by two orthogonal groups of cables joined with a compliant support contour is considered. The kinematic invariability of the roof obtained by prestabilizing the cables makes it possible to avoid heavy slabs of reinforced concrete in the construction of roofs and to use light composite materials. The main geometric characteristics for a cable roof with CFRP load-carrying and contour cables and steel stabilizing cables are determined by a numerical experiment.

Keywords: cable net; carbon-fiber-reinforced composite cables; saddle-shaped roof

Cracking Analysis of FRP-Reinforced Concrete Flexural Members by M. A. Aiello; L. Ombres (pp. 389-394).
The paper is dedicated to the cracking analysis of FRP (Fiber-Reinforced Polymer)-reinforced concrete elements. A general nonlinear calculation procedure, based on the slip and bond stresses, is described and adopted for the prediction of the crack width and crack spacing in FRP-reinforced concrete beams. An analytical expression of the bond-slip law is estimated using the corresponding experimental results available in the literature. A numerical investigation is carried out and the influence of the mechanical and geometrical parameters of the material (bond-slip law, reinforcement ratio, concrete strength, diameter of rebars, etc.) on the crack formation is investigated. Referring to glass-FRP-reinforced concrete beams, a comparison between the theoretical predictions and experimental results is made. The results obtained are presented and discussed.

Keywords: reinforced concrete; fiber-reinforced polymer rebars; bond; slip; bending; cracks; prediction

Environmental Effects on the Mechanical Properties of Glass-FRP and Aramid-FRP Rebars by M. A. Aiello; L. Ombres (pp. 395-398).
In the paper, the experimental results on the effect of temperature and moisture on the mechanical properties of FRP (Fiber-Reinforced Polymer) reinforcements are presented. FRP rebars made from glass and aramid fibers were subjected to cyclic thermal actions at temperatures ranging between 20 and 70°C, typical of natural hot-climate environments. Tensile tests were also carried out on FRP rebars. The effect of moisture was investigated by cyclic wetting and drying the FRP rebars under laboratory conditions before their testing in tension. Finally, the elastic modulus and tensile strength of the FRP rebars exposed to these cyclic actions were compared with those obtained for unexposed ones, in order to evaluate the mechanical damage caused by environmental conditions.

Keywords: composite rebars; thermal cycling; cyclic wetting-drying; tension tests; mechanical properties

Technological Anisotropy of Artificial Stone and Practical Use of This Phenomenon by V. A. Lapsa (pp. 399-402).
The technology of building stones from concrete and lime-sand mixes includes the process of unidirectional pressure molding. As stated at the Laboratory of Concrete Mechanics of Riga Technical University, this leads to anisotropy of mechanical properties of the material obtained, e.g., the acoustic anisotropy, found by measuring the velocity of ultrasonic pulse waves, is about 1.1-1.25. This is due to a large number of microdefects appearing in the horizontal planes after very quick unloading of the raw mix compacted in the mould. This harmful phenomenon has found practical use in elaborating a new and simple splitting technology for production of more lightweight facing tiles. These new tiles are essentially lower in cost than the analogous traditional materials - the split facing brick or sedimentary natural stone.

Keywords: concrete; strength anisotropy; nondestructive tests; technology

Characteristics of Biodegradable Poly(Ester-Urethanes) with Side Chains by U. Stirna; V. Yakushin; A. Dzene; V. Tupureina; I. Shits (pp. 403-408).
Two series of segmented poly(ester-urethanes) (SPEU) have been studied. The flexible segment of SPEU was formed from polycaprolactonediols (PCL diols) with a molecular mass of 600 to 10000 and the rigid one — from a blend of 2.4 and 2.6-toluene diisocyanates (TDI) and a chain extender. The first series of SPEU contained no side branches, whereas in the second series, side branches in the form of long chains of aliphatic structure were present at the rigid segment. The tensile strength of SPEU decreased when the molecular mass of the flexible segment increased from 600 to 2000; in this case, the specimens were of amorphous structure. An increase in the molecular mass of the flexible segment from 2000 to 10000 led to an increase in its degree of crystallinity and in the melting point, fusion enthalpy, tensile strength, yield stress in tension, and packing coefficient of SPEU. The side chains at the rigid segment affected the degree of phase separation insignificantly, but decreased the order of the structure, the glass transition temperature, and strength properties of SPEU, whereas the side chains at the flexible segment reduced its crystallinity.

Keywords: segmented poly(ester-urethanes); caprolactonediols; tensile properties; thermal properties; side chains

Effect of the Work Expended on Mixing and Extrusion on the Properties of Thermoplastic Elastomer Compounds by I. V. Volkov; V. I. Kimel'blat; S. I. Vol'fson; I. N. Musin (pp. 409-416).
The effect of the processing parameters of thermoplastic elastomer compounds on their properties is studied. The compounds consist of ethylene-propylene rubber and high- or low-density polyethylene (HDPE or LDPE) mixed with extenders in equal amounts. The variations in the degree of crystallinity under different mixing modes are examined by means of differential scanning calorimetry. The variations in the molecular weight distribution (MWD) of polymeric constituents of the compounds are estimated based on the results of rheological tests. An increase in the mixing and extrusion rates affects, up to certain limits, the mechanical properties positively. The optimum preparation modes for HDPE- and LDPE-based compounds are different. A noticeable mechanical destruction of HDPE-based compounds, estimated from the MWD parameters, occurs when the work of mixing exceeds 3.4 kJ/g and is accompanied by cross-linking. Up to 7.1 kJ/g, the work of mixing improves the properties of LDPE-based compounds. The extrusion effects do not smooth out the mixing effects — both these effects add up.

Keywords: thermoplastic elastomer compounds; polyolefin; relaxation; properties

Properties of Bitumens Modified by Thermoplastic Elastomers by Y. N. Khakimullin; V. I. Kimel'blat; I. G. Chebotareva; E. V. Muruzina; A. V. Murafa; V. G. Khozin; S. I. Vol'fson (pp. 417-422).
The possibilities of modification of bitumens by blend thermoplastic elastomers (TPEs) with predominantly maximum long main chains are studied. The evaluation of their properties shows that the blend TPEs display a stronger effect in low-viscosity bitumens, which is associated with improved compatibility of TPEs in bitumens, as evidenced by convergence of the solubility parameters of TPEs and of the aliphatic part of the malthene fraction of bitumens. The relaxation properties of the modified bitumens allow us to judge their stability under operating conditions, to evaluate this effect quantitatively, and to determine the activation energy of the relaxation process.

Keywords: bitumens; thermoplastics; modification; relaxation properties

Investigation of the Structure and Characteristics of Curing Bitumen-Thiokol Compositions by Yu. N. Khakimullin; A. V. Murafa; Z. O. Sungatova; E. I. Nagumanova; V. G. Khozin (pp. 423-428).
Thiokol-modified bitumens are investigated. Based on the measurements of time-dependent viscosity parameters (melting point, penetration, and spin-spin relaxation time T 2), it is established that MnO2 considerably decreases but ZnO increases the cure time of thiokols in the presence of bitumens. At a content of thiokol more than 25%, formation of a continuous phase is possible, which shows up as a sudden change in the bitumen properties and is associated with the phase inversion. The method of bitumen modification suggested can be recommended for obtaining roofing materials.

Keywords: bitumen; thiokol; modification; rheology

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