Thermal shock studies on carbon-carbon composites: Experimentation and analysis.

摘要

The oxidation behavior of C/C composites under thermal shock conditions in air is understood and predicted experimentally and by computational efforts. In Chapter. 1, both compressive properties and oxidation behavior of pristine and thermal shock exposed 2D C/C composite specimens were examined. Pristine test specimens were exposed to thermal shock conditions with temperatures ranging from 400°C to 1000°C in an oxidizing environment, followed by compression tests on pristine and thermal shock exposed specimens to obtain their compressive responses.;Similarly, in Chapter. 2, the influence of thermal shock conditions on both, the extent of carbon materials decomposition and the through-thickness compressive mechanical response of 2D woven C/C composites is investigated by computational efforts using ABAQUS. First, C/C composite specimens and carbon fibers are exposed to thermogravimetric (TG) experiments under isothermal conditions at 400°C, 600°C and 800°C. The weigth loss with time is recorded and TG curves are obtained for the C/C composite specimens and carbon fibers, which are utilized to predict the degree of decomposition of the carbon matrix by using rule of mixtures. Finally, carbon fiber tows and carbon matrix TG curves are related to the Arrhenius equation to determine the corresponding kinetic parameters, which are required as inputs to the proposed computational scheme. A novel computational framework consisting of two main steps is proposed. In the first step, a finite element radiation heat transfer analysis is developed on a meso-scale representative model of a C/C composite specimen exposed to thermal shock conditions, with peak temperatures in the range of 400°C to 800°C. The radiation heat transfer analysis is coupled to a HETVAL subroutine to determine the total amount of heat flux at every integration point and time within the meso-scale model. In the second step, a UMAT subroutine is added to the computational framework to account for the carbon materials degradation with temperature and time. Following this, a static analysis is performed by applying a through-thickness compressive load on the meso-scale model to determine its compressive stiffness. Finally, the predicted compressive responses of the meso-scale model under various thermal shock conditions are compared with the experimental results obtained in Chapter. 1, resulting in good agreement. In conclusion, the proposed computational framework can be used by material engineers to aid in the complex design and expensive manufacturing of parts made of C/C composites. This computational model can also be extended to other composites by changing individual material properties, fiber architecture, weave pattern and/or fiber volume fraction.