AN INVESTIGATION OF THE EFFECTS OF INTERFACIAL MICROSTRUCTURE ON THE FATIGUE BEHAVIOR OF A 4-PLY [75](4) CONTINUOUS SILICON-CARBIDE (SCS-6)FIBER-REINFORCED TITANIUM MATRIX COMPOSITE

The effects of interfacial microstructure/thickness on the strength an d fatigue behavior of a model four-ply [75](4) Ti-15V-3Al-3Cr-3Sn/SiC (SCS-6) composite are examined in this article. Interfacial microstruc ture was controlled by annealing at 815 degrees C for 10, 50, or 100 h ours. The reaction layer and coating thickness were observed to increa se with increasing annealing duration. Damage initiation/propagation m echanisms were examined in as-received material and composites anneale d at 815 degrees C for 10 and 100 hours. Fatigue behavior was observed to be dependent upon the stress amplitude. At high stress amplitudes, the failure was dominated by overload phenomena. However, at all stre ss levels, fatigue crack initiation occurred by early debonding and ma trix deformation by stress-induced precipitation. This was followed by matrix crack growth and fiber fracture prior to the onset of catastro phic failure. Matrix shear failure modes were also observed on the fra cture surfaces in addition to fatigue striations in the matrix. Correl ations were also established between the observed damage modes and aco ustic emission signals that were detected under monotonic and cyclic l oading conditions.