Fiber Architecture Governs Collapse Mode, Load Transfer, and Fatigue Resistance of Ultrasonically Welded CF / PC Tubular Composite Rivets

ABSTRACT

To address the limitations of conventional joining methods in lightweight composite structures, three carbon fiber/polycarbonate (CF/PC) composite rivets with distinct fiber architectures: a woven fabric tube (Fabric tube), a unidirectional fiber tube (UD tube), and a unidirectional solid rod (UD rod)—were fabricated and joined to open‐hole CF/PC laminates via ultrasonic welding. The UD tube and UD rod formed regular disk‐shaped rivet heads, whereas the Fabric tube produced irregular joint geometries due to the impeded radial flow caused by transverse fiber bundles. Three configurations exhibited a two‐stage response: the Stage 1 was governed by interfacial interlocking and adhesion, while the Stage 2 was dominated by crushing of the rivet. The UD tube achieved a maximum load of 5728 N and a specific load of 6288 N/g, which were 217.2% and 24.0% higher than those of the UD rod, respectively, with an interfacial adhesion efficiency of 11 N/mm ² . This advantage stems from the hollow tubular cavity facilitating uniform melt flow, whereas the solid structure suffered from restricted flow. A curved tube wall finite element model was developed for the UD tube, reproducing experimental failure morphology with deviations within 5%. Fatigue comparison showed the UD tube's fatigue limit at 10 ⁶  cycles was 2873 N (52.1% higher than RAHJ) and its specific fatigue load of 7858 N/g was 9.37 times that of RAHJ. Welding creates a compliant layer from outward fibers, easing stress and shifting load to the rivet, localizing damage. Fatigue then only affects the rivet, sparing parent material for easy repair.