MECHANICAL-BEHAVIOR OF SIC(F) SIC COMPOSITES AND CORRELATION TO IN-SITU FIBER STRENGTH AT ROOM AND ELEVATED-TEMPERATURES/

Mechanical properties of Nicalon-fiber-reinforced silicon carbide matr ix composites were evaluated, in flexure, at various temperatures rang ing from ambient to 1300 degrees C. First matrix cracking stress range d from 250 to 280 MPa and was relatively insensitive to test temperatu re, The measured ultimate strength showed a small increase from a room -temperature value of 370 to 460 MPa at 800 degrees C. Beyond 800 degr ees C, however, strength dropped to as low as 280 MPa at 1300 degrees C. This decrease in ultimate strength at elevated temperatures is beli eved to be partly due to degradation of in situ Nicalon fiber strength , Scanning electron microscopy was employed to evaluate the in situ Ni calon fiber strengths via fracture mirror size measurements. Degradati on of Nicalon fiber strength is attributed to thermal damage and to st ructural changes to the fiber at elevated temperatures, Measured value s of ultimate strength of the composites were compared with prediction s made on the basis of in situ fiber strength characteristics and an a vailable analytical model.