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Mechanics of Composite Materials (v.40, #1)

Multifractal Parametrization of the Structure of Deformed Carbon Fibers by V. U. Novikov; L. P. Kobets; I. S. Deev (pp. 1-16).
Deformed carbon fibers are investigated, and their failure model is proposed based on the Sierpinski set and the hypothesis of two — brittle and viscous — fracture modes, whose existence is confirmed by examples of a correlation between the mechanical strength and elastic modulus of the fibers. For the first time, a multifractal diagram is obtained, which allows one to justify the classification of carbon fibers into brittle and inelastic ones.

Keywords: microstructure; multifractal formalism; carbon fiber; parameter; deformation; mechanical properties; model; dimension; diagram

Longitudinal Flexure as a Method for Determining the Flexural Strength of Composite Materials by A. K. Arnautov; Yu. M. Tarnopol'skii (pp. 17-28).
A method is presented for determining the flexural strength of composites from tests on pinned specimens in axial compression. This scheme is close to the so-called Euler buckling model, but the load is applied eccentrically, and the bar is bent practically from the beginning of loading. An analytical model is proposed for calculating the flexural stress in longitudinal flexure of a thin bar in the case of large displacements. The problem is solved in elliptic integrals of the first and second kinds. The analytical model shows an excellent agreement with experiments. The results obtained are compared with experimental tension data. A device is also designed for testing composite bars in longitudinal flexure. The method offered has an obvious advantage over the conventional three- or four-point bending — the gage zone in this scheme is far away from the loading zone of the test specimen.

Keywords: advanced composites; test methods; thin bar; analytical model; flexural strength; longitudinal flexure; tension; buckling

A Study of the Relation between the Mechanical Properties and the Adhesion Level in a Laminated Packaging Material by S. Kao-Walter; J. Dahlström; T. Karlsson; A. Magnusson (pp. 29-36).
The mechanical properties of a laminate consisting of aluminum-foil, adhesive, and polymer layers were studied in relation to the adhesion level. A special application for liquid-food packaging materials was considered. In experiments, laminates with and without adhesive layers were tested. Tensile tests were first run for every layer of the laminate, and the data obtained were then used in analyzing the results of tensile tests on the entire laminate, as well as in theoretical and finite-element calculations. Relations between different mechanical properties (such as Young's modulus, the peak stress, and the strain at the peak stress) and the adhesion level were analyzed. It was found that the tensile strength and the strain at the peak stress increased with adhesion level. Only slight differences in Young's modulus were observed at different adhesion levels.

Keywords: laminate; packaging material; mechanical properties

Limit Regimes of Braking of Disc-Type Composite Flywheels Made by Filament Winding by G. Portnov; I. Cruz; R. P. Fiffe; F. Arias (pp. 37-44).
A set of invariants describing the limit regimes of braking disk-type wound composite flywheels is suggested. The limit regime is determined by using a linear failure criterion. It is shown that, for generating short-time impulses of very high power, a set of small identical flywheels is more effective than one large flywheel with an equivalent mass. Particular examples illustrating the theory considered are discussed.

Keywords: flywheel; disk; composite; power; energy; braking; limit regime

Calculation of the Ultimate State of Reinforced Plastics and Unoriented Polymers in Asymmetric High-Cycle Tension–Compression by V. P. Golub; V. I. Krizhanovskii; A. D. Pogrebniak (pp. 45-56).
The fatigue failure of reinforced plastics and unoriented polymers under a joint action of static and high-cycle loadings is considered. The fatigue failure strength is estimated from stress range diagrams with a static tensile component. The models of ultimate state are constructed based on the hypothesis of existence of a unified ultimate diagram invariant with respect to the number of cycles to failure. The unified diagram is given by a transcendental power function whose exponent characterizes the sensitivity of the material to the stress-cycle asymmetry. The models of ultimate state make it possible to span practically all forms of ultimate diagrams of composite and polymer materials, including concave, rectilinear, S-shaped, and convex ones.

Keywords: reinforced plastics; unoriented polymers; high-cycle loading; asymmetric cycle; fatigue failure; stress range diagrams; fatigue life

Effects of Moisture and Stresses on the Structure and Properties of Polyester Resin by E. A. Faitel'son; V. P. Korkhov; A. N. Aniskevich; O. A. Starkova (pp. 57-66).
The results of a complex study of structural changes in a cured Norpol 440 polyester resin under the action of damp environment and mechanical loading are presented. A considerable effect of absorbed moisture on the structure and some characteristics of the material is revealed by using thermophysical methods and X-ray diffractometry. The joint effect of moisture and mechanical stress is estimated by investigating the creep in stationary and nonstationary moisture conditions. The anisotropy of the material structure formed during creep is evaluated from the results of dilatometric measurements. It is found that the degree of anisotropy of the material after creep accompanied by moisture sorption is higher than that after creep in the conditions of moisture equilibrium with atmosphere. It is established that the aftercure and relaxation of the residual creep deformation come to an end at heating to 80-85°C. At a further rise in temperature and repeated heating, changes in the material structure are not observed.

Keywords: polyester resin; moisture; creep; dilatometry; X-ray diffractometry; thermophysical characteristics

Evaluation of Deformation and Strength Characteristics of Composites Based on Low-Density Polyethylene and Linen Yarn Production Waste by the Methods of Mathematical Statistics by J. E. Lejnieks; J. A. Kajaks; S. A. Reihmane (pp. 67-74).
The possibilities of using various methods of mathematical statistics for processing and analyzing the results of deformation and strength tests on composites made from a low-density polyethylene and linen yarn production waste are evaluated. The hypothesis that the experimental strength data agree with the Gaussian distribution is examined by the Shapiro–Wilk test (W-test.) It is shown that the Gaussian distribution, both for systems unmodified and modified with diphenylmethane diisocyanate (DIC), is valid only for two parameters: the maximum tensile strength σmax and the elastic modulus E t. For the other parameters (the relative elongation εmax corresponding to σmax, the specific total work of failure A b), and the specific work of failure to the tensile strength A max), a non-Gaussian distribution is observed. An analysis of measurements for different specimens by the Bartlett test shows that the E t data have equal variances for both systems (with and without DIC), but for the system containing DIC, the σmax data have different variances. A two-factor ANOVA analysis reveals that DIC considerably affects the tensile strength and modulus of composites, but the influence of test conditions is a statistically significant factor only for the modulus. The coefficient of variation is considerably lower for σmax than for E t and can be used as a quantitative measure for the degree of heterogeneity of the composites investigated.

Keywords: low-density polyethylene; linen waste; composites; deformation and strength characteristics; data analysis; mathematical statistics

Linear Viscoelasticity of Liquid-Crystalline Polymers by E. E. Jakobson; L. A. Faitel'son (pp. 75-86).
A linear (small-amplitude) periodic shear deformation of anisotropic viscoelastic liquids obeying the Akay–Leslie rheological model is considered. The frequency dependences of the real and imaginary components of the complex shear modulus and complex normal-stress coefficient are determined. A comparison between calculation results and test data on the shear flow of poly(γ-benzylglutamate) in m-cresol is carried out. It is stated that, if the material is characterized by some initial orientation, both components of the complex shear modulus contain a multiplier which depends on the degree of the initial orientation and increases the values of the components compared with those for an initially isotropic material. The model predicts that, in a periodic shear flow, the components of shear and normal stresses are constant and, like the components of shear modulus, are independent of deformation frequency. If the parameter d 0 of the Akay–Leslie model is equal to zero, the values of its other parameters can be determined from experimental results on periodic shear flow.

Keywords: anisotropic viscoelastic liquid; linear viscoelasticity of liquid-crystalline polymer; complex normal-stress coefficient

Notes for Contributors (pp. 87-89).
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