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Abstract
We report results of theoretical and experimental research aimed at establishing the mechanisms for the formation of a defective layer at the machined surfaces made from carbon composite materials, specifically those from carbon-carbon and carbon-polymeric groups. Possessing a set of unique physical and mechanical properties, the latter are increasingly applied in aviation and space technologies. However, since the properties of a material are predetermined not only by the components applied but also by the processes to obtain products (laying of reinforcing fibers, orientation of threads), conducting mechanical tests of samples-witnesses is a compulsory stage in the operations performed. Based on the generalization of statistical and theoretical-analytical information, we have developed a model of the emergence and propagation of cracks in a quasi-fragile material, particularly the carbon-carbon and carbon-polymeric composites, caused by the action of a cutting wedge. It is shown that the stresses that occur in a surface layer predetermine the intensity of crack growth while a direction of microcracks propagation is due to the applied force load. Therefore, control over the direction of force action, as well as the application of certain technical means, including a hydroabrasive jet, could enable the localization of microcracks in small quantities at the surface of the formed edge. The established regularities in the formation of a defective layer at machining (including the hydroabrasive cutting) have made it possible to identify ways to improve the quality of a sample and to reduce the layer thickness to 0.05 mm. The derived dependences of the destruction zone parameters on the stresses that occur at cutting allowed us to obtain the rational sequence of machining transitions, at which the defective surface layer is the smallest. The results obtained provide a possibility to significantly increase the accuracy of mechanical tests of carbon composite materials, thereby reducing the variance in the measurements of controlled parameters by 30‒40 %. The results have been actually implemented industrailly, and are of interest for the further research aimed at the hybridization of processes, as well as the development of technologies based on a functional-oriented approach.
Highlights
Wide use of composite materials based on carbon-polymer fibers is predetermined by the high durability of goods, their light weight, the capability to perceive different kinds of force loads
Insufficient knowledge about the issues of machining of carbon-carbon composite materials became a prerequisite for undertaking a comprehensive study, with its relevance emphasized by the prospects and uniqueness of a given type of the material
As well as parameters of the applied tool, directly define the machining efficiency, and the thickness of a defective surface layer, which must be taken into consideration when machining parts made from carbon-carbon composites
Summary
Wide use of composite materials based on carbon-polymer fibers is predetermined by the high durability of goods, their light weight, the capability to perceive different kinds of force loads. Carbon-carbon materials possess in addition the capability to maintain mechanical properties at high temperatures. The workpieces for products made from carbon-carbon composites are obtained by laying the wire strands, threads or fabrics onto the appropriate models followed by the saturation with pyrocarbon under conditions of high temperatures [1]. Parts made of the carbon-polymeric materials are obtained by using prepregs with laying onto the model and subsequent impregnation with components of polyester resins. This makes it possible at the stage of forming a material to assign the main axes of stiffness and strength, thereby achieving the desired level of anisotropy of properties of the structural elements and the finished product in general. Established regularities could form a basis for developing an engineering procedure for predicting the level of quality of edges to be obtained and for determining appropriate modes of conducting the machining, as well as a sequence of technological transitions
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