Assessing fracture toughness performance of adhesively bonded carbon fiber/epoxy composite joints accompanied by acoustic emission inspection: Effect of surface treatment methods

This study investigates the impact of mechanical abrasion (MA), atmospheric pressure plasma activation (APA), and peel-ply (PP) treatments on the fracture toughness, damage mechanisms and damage progression of adhesively bonded carbon-fiber (CF)/epoxy composite joints. The chemical and physical properties of treatment applied adherend surfaces are examined through various methods. Double cantilever beam (DCB) and end-notched flexure (ENF) tests are conducted to evaluate the fracture toughness of joints. The acoustic emission (AE) method is employed during DCB and ENF tests to evaluate damage mechanisms and damage progression within specimens. The results demonstrate that MA treatment provides the highest fracture toughness, with mode-I (GIC) and mode-II (GIIC) toughness values increasing by 59% and 43%, respectively, compared to untreated specimens. APA-treated specimens show improved GIC and GIIC values by 27% and 30%, respectively, which is attributed to enhanced surface energy and chemical functionality. PP treatment contributes to a 20% increase in GIC and a 14% rise in GIIC due to improved surface roughness and surface energy. The findings highlight that the mechanical interlocking effect induced by MA treatment significantly strengthens the bond, while surface chemistry modifications achieved through APA treatment benefit bonding in applications where material integrity is critical. AE analysis reveals distinct damage mechanisms associated with each surface treatment.