MODERNIZATION OF AN EXPERIMENTAL INSTALLATION AND A PROCEDURE FOR INVESTIGATING THE ANISOTROPIC VISCOELASTIC PROPERTIES OF COMPOSITE MATERIALS AT ELEVATED TEMPERATURES

Authors

V. G. Martynenko, National Technical University “Kharkiv Polytechnic Institute”, (2, Kyrpychova St., Kharkiv, 61002, Ukraine), e-mail:  martynenko.volodymyr@gmail.com, ORCID: 0000-0002-9471-0905

Yu. N. Ulianov, National Technical University “Kharkiv Polytechnic Institute”, (2, Kyrpychova St., Kharkiv, 61002, Ukraine), e-mail:  gradedegree@gmail.com

Abstract

 The paper describes the process of modernizing the existing installation designed for performing long-term tests of steel and aluminum cylindrical specimens for high-temperature creep with the purpose of conducting the experimental studies of the anisotropic strength and viscoelastic characteristics of planar composite specimens at elevated temperatures. In view of the differences in the approaches to finding the mechanical properties of metals and composite materials, the modernization required that special methods be developed for its implementation. In order to achieve the objective set, a scheme for reconstructing the specimen holders in the experimental installation was proposed, as well as the method of fixing them, implementing uniaxial stress-stain state and enabling  one to avoid stress concentration where the grippers are used. The specimens for the experiment were cut out in accordance with their optimal shape from one sheet of orthogonally reinforced composite material at different angles to the reinforcement direction, which allowed obtaining their anisotropic mechanical properties. The preparation of the specimens for conducting the experimental study was performed in accordance with international standards, which ensured the accuracy of obtaining the desired mechanical quantities. The developed, designed and built automatic temperature control block for the electric furnace allowed maintaining elevated temperature with a sufficiently small error during its long use, which was necessary for studying the mechanical properties of composite specimens, as well as regulating the heating temperature in a given range. When performing a series of experiments, an optimal temperature was chosen that was higher than the glass transition temperature of the composite material and lower than its phase transition temperature. Its observance made it possible to measure the viscoelastic properties of the composite with a high accuracy when the relaxation time reached half of the measuring period and guarantee a complete construction of creep curves. Conducting the experimental study of the instantaneous and long-term mechanical properties demonstrated the effectiveness of the improvements made for the experimental installation, as applied to the realization of such experiments. The developed procedure can be used for finding the anisotropic vicoelastic properties of the composite materials dependent on time and temperature, as well as setting the level of anisotropy of such properties for its subsequent consideration in the mathematical models of the mechanical behaviour of structural and installation elements made of composite materials.

Keywords: anisotropic viscoelasticity, composite material, experimental investigation, elevated temperature, relaxation curve

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Received 17 January 2018

Published 30 June 2018