Strengthening of composite T-joints using Ø 0.11 mm Z-pins via an ultrasound-guided insertion process

•The novel ultrasound-guided fine Z-pin insertion units are developed and arranged in an array of 2 × 5 into an industrial robot.•The composite T-joints are reinforced with Ø 0.11 mm carbon or steel pins by the ultrasound-guided fine Z-pin insertion system.•Thinner pins can increase the peak load and total energy absorption capacity of composite T-joints more effectively than conventional size Z-pins. This paper investigates the effect of Ø 0.11 mm Z-pinning on the structural properties of the composite T-joints via the ultrasound-guided insertion process. The structural properties of the joints are determined by the stiffener web pull-off testing. The experimental results show that carbon Z-pins exhibit a better improvement effect than steel Z-pins due to the interface between carbon Z-pins and laminate owning higher pullout strength. When the skin-flange area of the joints is inserted with 0.2 vol% of Ø 0.11 mm carbon Z-pins, the peak load and total energy absorption capacity can be improved by 72.4% and 436.5%, respectively. With the Ø 0.11 mm Z-pinning, the tensile strength decreased by only 2% when the volume content reached 0.2%. The improvements to the T-joint structural properties are explained by the enhancement mechanism of Ø 0.11 mm Z-pinning on mixed mode I/II interlaminar fracture toughness of laminates. Finally, the Ø 0.11 mm Z-pinned T-joint model is established to verify the experimental results.