THE MICROMECHANICS OF AMBIENT-TEMPERATURE CYCLIC FATIGUE LOADING IN ACOMPOSITE OF CAS GLASS-CERAMIC REINFORCED WITH NICALON FIBERS

The behavior of a Nicalon fiber reinforced glass ceramic composite cyc licly loaded has been evaluated at ambient temperature using high-reso lution micromechanical test methods. On this basis, the events leading to fracture have been found to be similar to those accompanying fract ure in unidirectional tension tests. Matrix strains were determined lo cally at the point of matrix fracture. Crack opening displacements (CO Ds) were measured as a function of loading cycles, and fiber strains w ere determined, in some cases. It is concluded that debonding of fiber s begins at the point of matrix cracking and rapidly increases. Most o f the cyclic lifetime of the material is spent with fibers debonded ov er large distances (fractions of a millimeter); these fibers are pulle d out of the matrix on each loading cycle. Final debond length, as det ermined by fractography, is a function of the number of cycles lo frac ture, and of the applied stress level.