The direct observation of the 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. Preliminary tests were conducted using miniature epoxy specimens with multiple embedded carbon fibers under a high magnification digital microscope. The static loading-unloading 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 was hardly visible under the conventional X-ray absorption contrast CT (micro-CT), the 3-dimensional shape of the debonded interface was clearly observable under the high-resolution phase-contrast X-ray CT (nano-CT), even under a nominal stress of 30 MPa. The debonding between the carbon fibers and epoxy matrix initiated at the interface where the neighboring 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.
