Impacts of prior ablative damage and fiber breakage influence on flexural strength of 2D C/C composites with and without secondary high temperature treatments

For reliable use of 2D Carbon–Carbon (C/C) composites in aerostructural applications, the sensitivity of the strength properties to prior damage (ablative and fiber breakage) needs to be assessed. A 0°/90° carbon fiber preform underwent a 6-cycle polymer infiltration and pyrolysis (PIP) schedule (standard process (SP)) using phenolic resin. A subset of samples was subjected to a high temperature treatment (HTT) at 2400 °C followed by another two PIP cycles. The HTT specimens exhibited a statistically significant increase in flexural strength compared to the SP specimens, due to an increased ordering of the matrix atomic structure, higher bulk density, and lower porosity. The HTT C/C presented a mixed mode failure type in flexural testing, with fiber-matrix debonding and fiber pullout leading to interlaminar shear at the neutral axis of the sample. The SP C/C failed from a crack emanating at the tensile free surface. The HTT C/C was more resistant to ablation than the SP C/C, with the HTT samples not exhibiting a statistically significant debit (6.6 %) in flexural strength due to prior ablation. Conversely, the SP specimens exhibited a 57.9 % reduction in strength after prior ablation. For the SP C/C, the presence of a small notch resulted in broken fibers and load redistribution, which significantly debited the flexural strength, although the notch had minimal effect on the flexural strength of previously ablated samples. This work quantifies the residual strength in the case of prior damage, thus identifying the processing-structure-properties relationships and mechanisms that regulate the durability of C/C composites. • For standard processed (SP) 2D C/C composite, a high temperature treatment (HTT) increased strength. • Failure mechanism shifted from tensile cracking (SP C/C) to delamination (HTT C/C). • HTT led to increased degree of matrix atomic ordering, density, thermal conductivity. • Prior ablation for SP decreased strength and had insignificant effect on HTT C/C. • A notch in SP C/C resulted in decreased or minimal changes in strength for cases without or with prior ablation, respectively.