EFFECT OF MISSION CYCLING ON THE FATIGUE PERFORMANCE OF SIC-COATED CARBON-CARBON COMPOSITES

The response of quasi-isotropic laminates of SiC-coated carbon-carbon (C-C) composites under flexural fatigue was investigated at room tempe rature. Virgin as well as mission-cycled specimens were tested to stud y the effects of thermal and pressure cycling on the fatigue performan ce of C-C. Tests were conducted in three-point bending with a stress r atio of 0.2 and frequency of 1 Hz. The fatigue strength of C-C has bee n found to be quite high: approximately above 90% of the ultimate flex ural strength. The fatigue strength appears to be decreasing with the mission cycling of the specimens. This lower strength with the mission -cycled specimens is attributed to the increase in interfacial bond st rength due to thermal and pressure cycling of the material. C-C is als o found to be highly sensitive to the applied stress level during cycl ic loading, and this sensitivity is observed to increase with the incr ease in the amount of mission cycling. Weibull characterization of the fatigue data has been performed, and a wide scatter in the Weibull di stribution is discussed. Fractured as well as untested specimens were C-scanned, and the progressive damage growth during fatigue is present ed. Optical microscopy including scanning electron microscopy for the failed specimens was also performed, to analyse the failure behaviour. Degradation and separation of the porous matrix structure, localized damage and filament splitting within the yarns, complete failure of th e fibre bundle, and delamination near the loading zone was observed.