Thermal Expansion and Shrinkage of Unidirectional Composites at Elevated Temperatures
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THERMAL EXPANSION AND SHRINKAGE OF UNIDIRECTIONAL COMPOSITES AT ELEVATED TEMPERATURES Ì. K. Kucher,1 O. S. Yakovleva,
UDC 539.4
and O. O. Chyzhyk The model of multiphase media analyzes the effectiveness of describing thermal expansion and shrinkage of unidirectional polymer composites at their uniform heating by linear law. It is shown that the nature of change in thermal deformation as a function of the composite temperature is similar for an unreinforced matrix, and the maximum absolute value of chemical shrinkage is slightly lower than that of a matrix. The calculated deformations describing thermal expansion are compared with experimental deformations obtained in the process of thermal-oxidative destruction. Keywords: unidirectional composite, ablation of material, thermal expansion and shrinkage of composite, elevated temperatures. Introduction. It is known that complex irreversible physical and chemical transformations known as “ablation” [1–4] take place in polymer composites under the influence of high temperatures. This term implies a process of substance removal from the surface of a solid body under the influence of irradiation or gas flow [5]. When the physical and chemical structures of a composite change, its mechanical and thermophysical parameters change significantly. This refers to significantly reduced density of composite material, nonmonotonically changing coefficient of thermal conductivity, and significantly increased gas permeability of the medium. Since elastic and strength characteristics of the material change non-uniformly in different directions, the material shrinkage occurs at certain temperatures, inducing thermal stresses in composites even in the absence of temperature gradient. The dependence of state equations on temperature and heating rate is characteristic of ablation composite materials. Prediction of the thermomechanical behavior of such materials is carried out with the help of multiphase media mechanics models [1–3]. They can be used in engineering calculations to determine the effective characteristics of elasticity, strains, and stresses, as well as strength parameters of composites described in [3, 6, 7]. It is known that changes in the density of the unreinforced ablation matrix, carbon fiber bundle, and unidirectional polymer composites at elevated temperatures are well described by the models of multiphase media based on the modified integral representative function [3, 6, 7]. At the same time, for example, epoxy un-reinforced matrix and carbon-fiber plastic made on its basis lose more than 97 and 100% of initial density in the process of thermal-oxidative destruction [6]. Some features of deformation of ablation composites with different types of fibers and matrixes are given in [3, 8–11]. It should be noted that thermal destruction is understood as the process of oxidation and decomposition of materials at elevated temperatures in an inert medium, thermal-oxidative destruction (TOD) – oxidation and decomposition in an oxidizing medium (for example, in the air). P
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