Design of adhesion/cohesion fracture resistant FRP composite repair system for V-notched Al-6061-T6 Aircraft skin

Designing a highly fracture-resistant FRP composite repair system by considering crucial bonded region (adhesion/cohesion)-type joint fractures in resin infilled V-notched Al-6061-T6 aircraft skin material has been reported. Three-dimensional LEFM-based VCCT integrated FE modelling with parametrized APDL codes for V-notch region joint fractures such as; VNS-AF (V-Notch-Surface-Adhesion-Fracture), VNE-AF (V-Notch-Edges-Adhesion-Fracture), and VNT-CF (V-Notch-Tip-Cohesive-Fracture) have been established for analyzing: (i) effect of patch size variation on critical fracture modes and (ii) suggesting optimum patch size that gives maximum joint fracture resistance. SERR (Strain Energy Release Rate)-based fracture modes with corresponding critical locations on the fracture front are extracted through varying patch sizes along length/width directions. Numerical analyses reveal predominant fracture mode for different joint fractures, i.e. (VNT-CF→Mode-I), (VNE-AF→Mode-III), and (VNE-AF→Mode-III) remains invariant for different patch sizes. Both edge regions of the fracture front for VNS-AF and VNT-CF are critical throughout the patch length/width, but VNE-AF, which propagates unsymmetrically and critical at the central region of the fracture front. In addition, SERR-reduction rates conclude that patch length variation is more critical than patch width for escalating joint fractures, with VNS-AF being the critical fracture responsible for poor design against bond line resin metal interface failure at all patch sizes. SERR-based approach with implemented failure criteria suggests two optimum patch sizes, (40x42) mm² and (40x52) mm^2, that arrest joint region fractures and are further to be verified through corelating with experimental results. Experimental analyses are carried out to (i) validate (resemblance in experimental failure load between the maximum and optimum patch cases) the FE model and (ii) support the numerically achieved optimized patch size by examining failure mode with post-fracture damaged surfaces. A larger proportion of leftover damaged bond line resin on both interface surfaces, highly damaged pattern of resin infilled materials, and the extent of debonding at the FRP patch's side edges support the numerically computed optimum patch size (40x42) mm².