Damage tolerance of carbon-carbon composites in aerospace application

We investigate fatigue-cracking behavior of unidirectionally reinforced carbon-carbon composites with different fiber orientations aimed for aerospace applications. Through digital image correlation (DIC), full field displacements are recorded in-situ, which capture the evolution of strain localizations during cyclic loading. DIC displacement fields are further utilized to determine crack driving forces via a regression analysis of orthotropic constitutive relations. Microscopic computerized tomography (micro-CT) scans disclose the competing nature of damage micromechanism e.g. pore coalescence, fiber bridging etc. for an advancing crack. Electron microscopy of fractured surfaces reveals widespread fiber/matrix interface debonding and fiber pullout, which chiefly contribute to cyclic cracking resistance. Upon sufficient progression, cyclic crack growth is observed to be self-arresting in nature unless applied loads are further increased. The origin of such behavior is attributed to: (a) reduction of driving forces due to continually degrading composite elastic modulus and (b) enhanced damage impedance originating from resistive tractions due to pervasive fiber bridging and pullout in the wake. [Display omitted]