Multiscale nano-integration in the scarf-bonded patches for enhancing the performance of the repaired secondary load-bearing aircraft composite structures

This study investigates a novel approach of improving the mechanical performance of scarf-repaired carbon fiber reinforced composites by integrating nanomaterials in the patch constituents. Two distinct types of carbon-based nanomaterials, thermally exfoliated graphene oxide grade-2 (TEGO) and Epocyl T 128-06 (MWCNT) are integrated into the patch resin matrix and fiber/matrix interface in an upscaled manner. Another group of composites are repaired with pristine patches in accordance with the existing and prevalent composite repair method. Compared to the reference panel, 109.9%, 99.7% and 99.3% stiffness recoveries are achieved for the patches with CNT and TEGO-incorporated resin matrices and TEGO-electrosprayed fibers, respectively. Location-wise analyses of the test data show that the stiffness, strength, Poisson's ratio, and strain values depend on the number of patch plies in each specimen. Fractographic inspections show that the failure sites shift towards the outer areas of the scarf region demonstrating an enhanced stress redistribution due to the nanomaterials. SEM observations show that nanoparticles affect toughening mechanisms based on the type, location, and alignment of the nano-reinforcement, which in turn limits the shear-dominated failures (SDF) in the baseline and pristine patch repair system. In CNT- and TEGO-reinforced resin patches, efficient crack bridging and fracture plane tilting/twisting or crack bifurcations are observed whereas the electrosprayed TEGO particles positioned at the perimeter of fibers operate as a shield for the fibers to prevent SDFs. These findings demonstrate that failure behavior of repair systems and therefore their mechanical performance are governed by the type of nanoreinforcement and its integration process.