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 well as virgin specimens were tested to compare the thermal- and pressure-cycling effects. Variation of flexural strength and stiffness with temperature was investigated to study the load-deflection behaviour and the thermal stability of C/C composites up to 1371 °C. Increase in flexural strength and stiffness were observed with the rise in temperature. A distinct shift in failure modes from compressive to tensile was found with the mission-cycled specimens with the increase in test temperatures, while the failure mode for virgin material was found always on the tensile side. Change in the load-deflection behaviour was examined and increase in non-linearity of the stress-strain behaviour with the mission cycling was observed. Although the number of test specimens was few, Weibull characterization on the flexure data was performed to study the variation of Weibull modulii and the characteristic lives. Failed, as well as untested, specimens were C-scanned to identify the location and the extent of the damaged zone. Post-failure analyses through optical microscopy and scanning electron microscopy were performed to study the damage growth and failure mechanisms. Degradation and separation of the porous matrix structure, localized damage of the reinforcing fibres in the transverse direction, complete fibre bundle failure in the mission-cycled specimens, and delamination near the loading zone were observed.