In-situ Observation of Interfacial Debonding between a Single Carbon Fiber and Epoxy Matrix by Synchrotron Radiation X-ray Nano-Imaging

Although the initial stage of fatigue failure accounts for failure in mechanical structures, this has not been thoroughly clarified for Carbon Fiber Reinforced Plastics (CFRPs), thereby prompting the need to develop innovative fatigue tests. In this study, interfacial debonding between a single carbon fiber and the epoxy matrix was observed using synchrotron radiation (SR) X-ray computed tomography (CT). A single carbon fiber was embedded perpendicular to the loading direction in a dumbbell-shaped epoxy sample. A tabletop fatigue testing machine driven by a piezoelectric actuator was developed to apply static and cyclic loads along the beamline for the in situ observation. The SR X-ray multiscale CT imaging was conducted by switching between an absorption-contrast projection method (micro-CT) and a phase-contrast imaging type X-ray microscopic CT (nano-CT). Micro-CT had insufficient resolution in detecting interfacial debonding, whereas nano-CT clearly visualized it under a static tensile load of 30 MPa. A gradual increase in the debonding length was detected until a static load of 50 MPa with nanovoid formation along the interface. Under cyclic loading, the debonding length increase along the interface was captured under a sinusoidal stress of 8–40 MPa after 10,000 cycles, whereas it did not propagate under a stress below 30 MPa.