The initial stage of fatigue failure has not been thoroughly clarified for carbon fiber reinforced plastics (CFRPs). Although the initiation of fatigue cracks has been regarded to be interfacial debonding between the carbon fiber and polymer matrix, their detection among numerous carbon fibers, whose diameter is only 7 µm, is extremely difficult. In this study, a single carbon fiber was transversely embedded in a dumbbell-shaped epoxy sample to focus on the interfacial debonding and was observed using synchrotron radiation (SR) X-ray computed tomography (CT). A tabletop fatigue testing machine driven by a piezoelectric actuator was developed to apply static and cyclic loads along the beamline. 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). The carbon fiber was entirely captured by micro-CT and then magnified at both ends on the free surfaces. Nano-CT clearly visualized the interfacial debonding under 30 MPa static tensile load and the implication of the coalescence of nano-voids along the interface under 50 MPa. Under cyclic loads, the interfacial debonding gradually progressed under a 8–40 MPa sinusoidal stress after 10,000 cycles, whereas it did not propagate under a stress below 30 MPa.

The direct observation ofthe interfacial debonding between carbon fibers and an epoxy matrix was achieved through in-situ tensile and fatigue testing on the beamline of a synchrotron radiation facility,SPring-8.Preliminarytestswereconducted using miniatureepoxyspecimenswith multiple embedded carbon fibers under a high magnification digital microscope.Thestaticloadingunloading tests revealed clear interfacial debonding between the carbon fibers and epoxy matrix under a nominal stress of 50 MPa and indicated debonding initiation at 30 MPa. Synchrotron radiation X-ray computed tomography ( CT) was employed to observe the internal shape of the debonded interface.Although it washardlyvisibleundertheconventionalX-rayabsorption contrast CT (micro-CT),the3-dimensionalshapeofthedebonded interfacewasclearlyobservableunderthe high-resolution phase-contrast X-ray CT ( nano-CT), even under a nominal stress of 30 M Pa. The debonding between thecarbon fibersand epoxymatrix initiated at theinterfacewheretheneighboring fibers were closely spaced under static loading. Debonding did not propagate under the cyclic loading; however, further propagation was confirmed along the exterior interface of the fiber bundles.