Decreasing propagation rate of interfacial debonding between a single carbon fiber and epoxy matrix under cyclic loading

The interfacial debonding of a single carbon fiber transversely embedded in a dumbbell-shaped epoxy sample was generated under cyclic loading, and images were captured using synchrotron radiation X-ray computed tomography. A fatigue testing machine driven by a piezoelectric actuator placed along the beamline for in situ observation was developed for precise alignment. Interfacial debonding was initially observed under a static tensile load and was confirmed to be almost of the same length at both ends of the carbon fiber, implying negligible bending deformation due to inclination. Cyclic loads were then applied to the sample to capture the progressive debonding. The propagation rate of the interfacial debonding decreased as the number of cycles increased. Another sample with a single carbon fiber aligned parallel to the loading direction was prepared following a single-fiber fragmentation test. Interfacial debonding was clearly observed around the fiber breakage. Cyclic loads were also applied to this sample; however, no progression of the interfacial debonding was evident. Degradation of the interfacial strength between the carbon fiber and epoxy matrix was not confirmed under cyclic loading within the elastic deformation range.