Laser-induced synergistic modifications to enhance mode-I fracture toughness for adhesively bonded thermoset CFRP joints

Abstract

Adhesive bonding has emerged as a critical joining technology for the carbon fiber reinforced polymer (CFRP), however, residual release agents from CFRP demoulding process significantly impair interfacial compatibility with structural adhesives, leading to high risk of interface debonding. This study adopts three treatment methods, including mechanical sanding, plasma and laser treatments, to induce characteristic modifications on CFRP material surface for enhancing adhesive bonding performance of CFRP materials, and mode-I fracture toughness (G_IC) was served as the evaluation metric. Sand and plasma treatments maximally achieve 173 % and 365 % increase in G_IC value compared to untreated joints, respectively. Notably, laser treatment yields an unprecedented 1030 % increase in G_IC value, and exclusively shifts the fracture mode from interface debonding to cohesive fracture. Mechanical sanding causes non-selective material removal and serious damages to matrix fibers, while plasma treatment primarily grafts functional groups of -NH₂ and C-O/C=O. In contrast, laser treatment creates porous protrusion microstructures on CFRP surface, and simultaneously facilitates effective elimination of F-containing contaminants and concentration increasing of O-containing components. Laser-induced synergistic combination of mechanical interlocking and chemical activation underpins the enhancement of cohesive fracture transition and 1030 % G_IC increase. Importantly, laser-treated CFRP surface exhibits quite stable and no apparent decline is found for mode-I G_IC after 72h exposure to atmospheric environment. Our investigation pioneers a comparative analysis of surface modification methods targeting stringent manufacturing requirements of CFRP, and establishes laser treatment as the most industrially viable solution.