Evaluation of Surface Treatment on the Fracture Performance of <scp>CFRP</scp> / <scp>TC4</scp> Adhesively Bonded Joints: Effect of Physical Methods

ABSTRACT With the application of carbon fiber reinforced polymer (CFRP)/metal adhesively bonded structures, selecting appropriate metal surface treatments is crucial for structural reliability. This study investigates the effect of physical surface treatment (sand‐blasting, unidirectional brushed finish, laser treatment, and polishing) of TC4 adherend on the fracture performance of CFRP/TC4 adhesively bonded joints. The results indicate that the surface wettability of the TC4 adherend is closely associated with the dominant failure mode and consequently the fracture toughness. Strong linear correlations exist between surface free energy (SFE) and fracture toughness, which can be interpreted within the framework of the thermodynamic work of adhesion: higher surface energy correlates with stronger interfacial bonding, a factor that may promote cohesive failure and engage the adhesive's energy‐dissipation mechanisms. As a result, the sand‐blasted TC4 surface exhibits the highest wettability (contact angle of 31.87° and SFE of 63.44 mN/m), showing cohesive failure and the highest fracture toughness (Mode I and Mode II fracture toughness of 480.84 and 2970.19 J/m 2 ). While the brushed TC4 surface exhibits the poorest wettability (contact angle of 84.12° and SFE of 28.9 mN/m), showing adhesive failure and the lowest toughness (Mode I and Mode II fracture toughness of 182.16 and 476.57 J/m 2 ). The laser‐treated and polished surfaces exhibit intermediate fracture performance, consistent with their respective surface free energies. Therefore, sand‐blasting is recommended as the preferred physical surface treatment method for TC4 adherend.