Quantification of hygrothermal aging-induced interfacial debonding of carbon fiber/epoxy composites at nano-to-micrometer length scales

This work investigated the phenomenon of interfacial debonding due to hygrothermal aging (HA), which was quantified using Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM). Several micrographs of carbon fiber/epoxy cross-ply laminate coupons were analyzed, including unaged (HA/0) samples, as well as those aged for 1000 h (HA/1000) and 2000 h (HA/2000). To quantify the extent of debonding, an image thresholding tool was employed. For HA/0, HA/1000, and HA/2000 coupons, the average interfacial debonding arc length of inner plies was longer than that of the respective outer plies by 14%, 17%, and 21%, respectively. Residual stress variation across inner plies is more pronounced compared to outer plies due to the latter's freedom of expansion. The debonding that occurred without external loading was due to the combined effects of matrix and interface plasticization, chemical reaction, viscoelastic changes, and swelling. The AFM 3-D surface topography revealed that HA/0, HA/1000, and HA/2000 specimens had progressively lower peak-valley distances of 135 nm, 113 nm, and 82 nm, respectively, due to matrix swelling.