Mode‐I Fracture Behavior of Carbon Fiber/Epoxy Composites Toughened Using <scp>3D</scp> ‐Printed Polypropylene Grid Structures With Chopped Glass and Carbon Fibers

ABSTRACT This study examines mode‐I fracture behavior of carbon fiber/epoxy composite laminates toughened with 3D printed grids made from pure polypropylene (PP), polypropylene with 15 wt% chopped glass fiber (PP‐GF15), and carbon fiber (PP‐CF15). Laminates with/without grid interleaves are manufactured with vacuum‐assisted resin infusion followed by out‐of‐autoclave curing. Mode‐I fracture energies of the laminates are evaluated using double cantilever beam (DCB) specimens. PP‐CF15 laminates demonstrate the highest improvement, increasing the initiation mode‐I fracture energy ( G IC ) by ~38% compared to the base laminates. PP and PP‐GF15 exhibit increases of 28% and 9% in G IC , respectively. The propagation fracture energies ( G IR ) of PP and PP‐CF15 laminates are comparable to the base laminates, whereas the laminates with PP‐GF15 grids exhibit a 40% reduction. Rising R‐curves in PP and PP‐CF15 toughened specimens confirm extrinsic toughening (i.e., bead bridging), where thermoplastic beads span cracks and resist delamination through plastic deformation and pull‐out. The linear R‐curve of PP‐GF15 indicates limited toughening. Fractography shows resin‐rich zones around grids, with interfacial debonding and plastic deformation as key crack resistance mechanisms. These results highlight the potential of additively manufactured thermoplastic grids to enhance fracture toughness, with filler type critically influencing toughening efficiency.

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