FAILURE MECHANISMS IN MISSION-CYCLED CARBON-CARBON COMPOSITES UNDER FLEXURAL LOAD AT ROOM AND ELEVATED-TEMPERATURES

The response of quasi-isotropic laminates of SiC coated carbon/carbon (C/C) composites under flexural load was studied. Mission-cycled as we ll as virgin specimens were tested to compare the thermal- and pressur e-cycling effects. Variation of flexural strength and stiffness with t emperature was investigated to study the load-deflection behaviour and the thermal stability of C/C composites up to 1371-degrees-C. Increas e in flexural strength and stiffness were observed with the rise in te mperature. A distinct shift in failure modes from compressive to tensi le was found with the mission-cycled specimens with the increase in te st temperatures, while the failure mode for virgin material was found always on the tensile side. Change in the load-deflection behaviour wa s examined and increase in non-linearity of the stress-strain behaviou r with the mission cycling was observed. Although the number of test s pecimens was few, Weibull characterization on the flexure data was per formed to study the variation of Weibull modulii and the characteristi c lives. Failed, as well as untested, specimens were C-scanned to iden tify the location and the extent of the damaged zone. Post-failure ana lyses through optical microscopy and scanning electron microscopy were performed to study the damage growth and failure mechanisms, Degradat ion and separation of the porous matrix structure, localized damage of the reinforcing fibres in the transverse direction, complete fibre bu ndle failure in the mission-cycled specimens, and delamination near th e loading zone were observed.