Laser ablation of CFRP surfaces for improving the strength of bonded scarf composite joints

Repairing damaged composite parts using scarf technique requires a careful selection of treatment methods for composite surface. Laser treatment is one of the emerging techniques to treat the milled composite surface by unlocking various levels of morphological changes and, thus, optimizing joint strength. However, laser parameters, e.g., energy density (fluence), should be carefully determined to ensure the acceptable structural recovery. Here, the influence of CO₂ laser with relatively high fluence (ablation effect) on the surface characteristics (roughness, morphology, wettability) and scarf joint strength with associated failure modes of unidirectional (UD) and quasi-isotropic (QI) carbon fiber-reinforced plastic (CFRP) laminates is studied. We found that the ablation effect using CO₂ laser at 3.6 J/m² was considered safe for UD laminates as their joint strength was comparable with that treated by manual sanding. The ablation at higher fluence (8.4 J/m²) reduced the joint strength in UD laminates due to severe damage occurred in 0^∘ fibers that triggered adhesive failure. In QI laminates, 3.6 J/m² laser fluence could improve joint strength since the cohesive failure was activated in off-axis plies (90^∘, +45^∘, −45^∘).