Control of multi-scale cracking for improvement of the reliability of carbon/carbon composites via design of interlaminar stress

Controlled cracking at micro/mesoscale is crucial for decreasing the strength scatter and improving the reliability of carbon/carbon composites (C/Cs). In this work, the controlled cracking is realized by a designed interlaminar residual thermal stress (RTS), which is introduced inside the C/Cs by the alternate stacking of two types of C/C plies with different coefficients of thermal expansion. Firstly, multiscale finite element (FE) modeling is performed to capture the interlaminar RTS induced by the alternate stacking and evaluate the risk of delamination due to the free edge effect. Secondly, mechanisms responsible for the controlled cracking are elaborated with the aid of the FE modeling and experimental characterizations, and the controlled deflections and bifurcations of the crack strongly shield the influence of multiscale flaws. Finally, by adopting this strategy, the flexural strength of the C/Cs increases by 66% while the statistical Weibull modulus also increases from 4.9 to 8.3, verifying the simultaneously improved strength and reliability of the C/Cs.