Although carbon-fiber-reinforced polymers (CFRPs) have excellent mechanical properties and are considered sustainable, an improved understanding of the fatigue behavior of these materials is crucial for expanding their application scope. The aim of this study is to clarify the fatigue mechanism of CFRPs using in situ and offline mapping by small-angle/wide-angle X-ray scattering (SAXS/WAXS) measurements. The resulting findings provide information about the interface between the carbon fibers (CFs) and epoxy matrix, as well as changes in epoxy chain structure and internal structure of the CFs. For example, the in situ SAXS profiles revealed an increase in intensity with cycling, which may be related to void and crack formation. Furthermore, the in situ WAXS profiles indicated that the 0° fibers became less ordered and that some destruction of the turbostratic structure occurred. Offline SAXS/WAXS mapping after fatigue tests with low and high loads revealed information about changes in the epoxy resin and carbon fibers, respectively, as well as the shapes of cracks. In particular, offline mapping confirmed that epoxy chain relaxation occurred rapidly after failure.
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