Fracture toughness evolution of a carbon/carbon composite after low-cycle fatigue

High fracture resistance of carbon/carbon (C/C) composites as a material for aircraft braking systems is essential for their safe service. This paper presents a study on fatigue behavior and residual mechanical properties after cyclic load of discretely-reinforced C/C composite. The influence of cyclic loads on the fracture toughness of frictional C/C composite was examined. Results of fracture toughness tests after compression fatigue load were determined using the ASTM single-notch three point bend test and correlated with fracture surface using scanning electron microscopy (SEM). It is revealed that C/C composite fracture toughness strongly depends on the fiber-matrix interfacial bonding strength. Cyclic loads below fatigue limit improve residual fracture toughness of the material up to 25% by the mechanism of interfaces weakening. Cyclic loads above fatigue limit decrease residual fracture toughness of the material by the fracture of carbon fibers. Observed results may be useful in developing friction C/C composites to enhance fracture resistance in aircraft braking systems, increasing safety and operating lifetime.