Delayed Failure at Intermediate Temperatures (600°–700°C) in Air in Silicon Carbide Multifilament Tows

Unexpected failure of Nicalon/SiC composites is observed under low stresses at intermediate temperatures (600°–800°C) in air. This effect has been designated as a nuisance by a few authors. The lifetime of SiC (Nicalon) multifilament tows is investigated in static fatigue in air at temperatures in the range of 600°–700°C. The various features that have been identified suggest that delayed failure under low stresses results from a slow-crack-growth mechanism. Analysis of acoustic emission data allows the number of failures during the tests to be determined. Scanning electron microscopy examination of tow fracture surfaces allows typical crack patterns to be identified. A model of delayed failure is proposed. The model is based on the Paris law and on the well-known model of subcritical crack growth devised by Davidge et al. for ceramics. The model takes into account the statistics of fiber failure. The slow-crack-growth constants have been determined from stress-rupture data. Finally, the model allows satisfactory description of the lifetime of tows and prediction of the lifetime of Nicalon/SiC minicomposites.