Internal friction behavior of unidirectional carbon/carbon composites after different fatigue cycles

Internal friction behavior was utilized as an indirect metric to study structural change in carbon/carbon composites after fatigue tests. In this work, two kinds of unidirectional carbon/carbon composites with different densities were prepared by isothermal chemical vapor infiltration (ICVI), and loaded under stress level of their fatigue limit. The internal friction behavior of the composites after different fatigue cycles was studied. After the initial 10⁴ fatigue cycles, since the matrix began to break and shed, frictional damping that happened between the fiber and matrix interfaces increased and the bulk internal friction increased rapidly. Between 10⁴ and 5×10⁵ fatigue cycles, holes formed at interfaces because of continued shedding of the matrix. Since the holes reduced contact area between different phases, frictional damping decreased, as along with the internal friction increment ratio. Between 5×10⁵ and 10⁶ fatigue cycles, interfacial bonding between carbon fiber and matrix was in a steady state, so structure and internal friction of the composites did not change. The results revealed that internal friction was an effective method to characterize structural change of carbon/carbon composites after fatigue tests.