Enhancing Lap Shear Strength of Hybrid Polymer‐Composite Single Lap Joint Fabricated Through Injection Molding Process Using Multiscale Z‐Pin and Carbon Nanofiber

ABSTRACT In this paper, the effect of multi‐scale synergistic reinforcement of carbon nanofiber (CNF)/Z‐pin on the lap shear properties of polymer‐thermoplastic composites single lap joint (SLJ) fabricated through the injection molding process was studied. The lap shear performance of unreinforced, CNF reinforced, Z‐pin reinforced, and multi‐scale CNF/Z‐pin synergistically reinforced injection joints were studied. The effects of Z‐pin parameters (volume fraction, implantation depth, and material type) on the shear performance of injection‐molded SLJ were systematically examined. Injection‐molded SLJs were analyzed for load–displacement behavior, ultimate shear strength, and failure morphology. Joints reinforced with a 0.9 volume fraction (vol%) metal Z‐pins exhibited a 106.22% shear strength increase compared to unreinforced joints, and this enhancement surpassed carbon fiber Z‐pinned joints by 22%. Metal Z‐pinned joints with 1.0 wt% CNF additive and 0.9 vol% implantation density exhibited 118.4% greater interlaminar shear strength versus unreinforced joints, while showing 12.2% strength augmentation over metal Z‐pinned injection‐molded joints without CNF reinforcement. CNF/Z‐pin multi‐scale reinforced joints showed a 118.4% shear strength increase, outperforming CNF‐only (8.7%) and Z‐pin‐only (106.2%) joints. This synergy effect originates from CNFs enhancing interfacial friction by surface roughening and amplifying Z‐pin crack‐bridging through multiscale stress coupling.