Fatigue strengthening of carbon/carbon composites modified with carbon nanotubes and silicon carbide nanowires

Fatigue strengthening of composites refers to the increase of residual strength after a specific number of loading cycles. This eccentric behavior widely exists in carbon/carbon composites and some ceramic matrix composites while few reasonable mechanisms have been proposed. In the present work, a comparative study of two different types of C/C composites (CNT-C/C with enhanced fiber-matrix interface and SiCNW-C/C composites with strengthened matrix) is performed to probe into the inherent mechanism responsible for fatigue strengthening. Damage evolution during the fatigue tests is systemically characterized by a combination of monitoring the hysteresis loop, internal friction, residual thermal stress and fiber bundle push-in strength. Our results indicate that the enhanced residual strength of the composites is realized by the shielding of main crack tip and deflection of the main crack due to the appearance of subcritical cracks. Further analysis reveals that these subcritical cracks are closely related to the anisotropy of the matrix and result from the debonding of turbostratic stacked pyrocarbon grains under cyclic loads.