Understanding Failure Mechanisms of Overwrapped 3D-Printed Polymeric T-joints Joined To CFRP Laminates

This study developed four different T-joint configurations: simple hybrid T-joints combining 3D printed polylactic acid (PLA) or carbon-fiber-reinforced PA12 (PA12 CF) with carbon fiber reinforced polymer (CFRP) laminates, as well as two additional T-joint configurations reinforced with carbon fiber overwrap layers. The four T-joint types were systematically evaluated through quasi-static monotonic tensile and shear tests, coupled with finite element simulations using the continuum damage mechanics and cohesive zone modeling. Experimental results show that the carbon-fiber-overwrapped 3D-printed PLA T-joint (PLA-O-T-joint) achieved maximum tensile and shear loads of 3548.9 N and 5625 N, respectively, whereas the carbon-fiber-overwrapped 3D-printed PA12 CF T-joint (PA-O-T-joint) exhibited the highest tensile and shear stiffness values of 799.2 N/mm and 1308 N/mm, respectively. Although PA12 CF itself outperformed PLA in terms of strength, stiffness, and ductility, when joined with CFRP laminate, PLA exhibited better interfacial bonding with the laminates. Failure mode analysis indicated that the carbon fibre overwrapped (O-type) joint method effectively enhanced load transfer, shifting the failure mode from interface debonding to fracture of the 3D printed web, thereby improving damage tolerance. Numerical simulations were consistent with experimental findings, with multiple Hashin failure parameters in the PA-O-T-joint reaching critical values, revealing a coupled failure mechanism involving fibre breakage and matrix cracking. This study highlights the excellent adhesion behaviour between PLA and epoxy resin, as well as the broad application potential of PA12 CF-based hybrid structures in engineering. Adhesion performance can be further improved through interface-enhancing co-curing design concepts and optimisation of overwrap carbon fibre layers, to meet the requirements of lightweight and high-strength composite structures.