AN INVESTIGATION OF THE EFFECTS OF MICROSTRUCTURE ON FATIGUE DAMAGE IN A SYMMETRICAL [0 90](2S) SILICON-CARBIDE (SCS6) FIBER-REINFORCED TITANIUM MATRIX COMPOSITE/

The results of a systematic study of the effects of microstructure on the mechanisms of Fatigue damage in a symmetric eight ply [0/90](2s) T i-15Al-3Cr-3Al-3Sn/SiC (SCS6) composite are presented. Damage mechanis ms are elucidated using optical/scanning electron microscopy and acous tic emission techniques. Damage initiation under cyclic loading is sho wn to occur early in life, and is dominated by longitudinal and transv erse interfacial cracking. Subsequent damage occurs by matrix and fibe r cracking, slip band formation and crack coalescence prior to the ons et of catastrophic failure. However. the sequence of the damage is sen sitive to changes in the metastable beta matrix and interfacial micros tructure. Based on the experimental evidence. a micromechanics model i s developed for the prediction of fatigue life. This model involves th e use of crack-tip shielding concepts in the assessment of crack bridg ing phenomena during fatigue crack growth.