Thermal shock and fatigue behavior of ceramic tubes

A narrow circumferential helium quench was used to thermally shock and fatigue internally heated alumina, reaction bonded, and sintered alpha silicon carbide tubes at 500°C and 1000°C. During these tests, transient temperature measurements required for thermal and stress-profile calculations were obtained through the use of microthermocouples positioned along the internal surface of the tubes. Acoustic emissions were also employed for in situ monitoring of crack initiation and propagation of the resident flaw populations during the single and repeated (up to 5) thermal shocks. Post-quench inspections and destructive burst tests were used to correlate the existence, extent, and statistical (Weibull) nature of the damage induced by the cycling. Results indicated progressive strength degradation in alumina tubes with repeated thermal cycles. In contrast, the thermally-cycled silicon carbide samples either showed no damage at all, or suffered minimal progressive strength degradation after the first cycle. In any case, the complex stress distributions computed from an FEA-based inverse heat transfer analysis were required to understand the observed damage (crack paths) and apparent fatigue behavior.