Ceramics are brittle and fail catastrophically when subjected to steady state or non-steady state mechanical or thermomechanical stresses. In many applications ceramics are exposed to thermomechanical stresses because of the thermal shock/transient thermal conditions (a sudden change in temperature), for example, in a high temperature furnace or in a gas turbine engine. Under these thermal transient conditions ceramics are susceptible to catastrophic failure because of their brittleness, relatively low thermal conductivity, and high Young's modulus of elasticity. In the past four decades, extensive studies have been devoted to the understanding and improvement of the thermal shock behaviour of ceramics, but those studies were mainly concerned with monolithic ceramics. More recently, continuous fiber-reinforced ceramic composites have been developed because of their enhanced fracture toughness. However, our knowledge on the behaviour of fiber- reinforced ceramic composites under thermal transients is rather limited. Therefore, the objective of this study was to study the thermal shock behaviour of continuous fiber-reinforced ceramic composites (CFCC's) subjected to thermal transients. This objective was accomplished by performing thermal shock studies using water quench test on CFCC's and characterizing thermal shock damage using nondestructive and destructive techniques. These tests were performed on 2-D woven Nicalon fiber CVI SiC matrix composites. The thermal shock damage was characterized by destructive techniques using mechanical tests and optical microscopy. Nondestructive techniques of Resonant Ultrasound and Grindosonic were used to characterize thermal shock damage and related to microscopic evidence of the nature of damage obtained by detailed optical microscopy. It is shown that the composites possessed superior resistance to thermalshock damage than the monolithic ceramics. Catastrophic failure due to severe thermal stresses is prevented in composites and a significant portion of their original strength is retained at a quenching temperature difference upto 1000 °C. It is also shown that nondestructive techniques of Resonant Ultrasound and Grindosonic can be used to characterize the thermal shock damage in CFCC's. These results along with an analysis of the thermal shock damage based on the destructive and nondestructive tests will be presented. An attempt will also be made to identify the mechanism of thermal shock damage in CFCC's.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 2, pages 725 - 728 Publisher:Ashgate Publishing Company
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