The effect of modified tannic acid (TA) eco-epoxy adhesives on mode I fracture toughness of bonded joints

Abstract Two synthesized eco-epoxy components based on TA: (A) glycidyl ether and (B) glycidyl phosphate ester, are used, as a replacement for the Bisphenol A (BPA) based epoxy component, for bonding aluminum (Al) and carbon fiber reinforced polymer (CFRP). Their effect on the mode I fracture toughness ( G I ) is evaluated by Double Cantilever Beam (DCB) testing while using Digital Image Correlation (DIC) for in-situ crack tip monitoring. Compared to the reference adhesive, an improvement of ( G I ) of Al (43%) and CFRP (100%) is obtained when using adhesive B. Moreover, regardless of the adherend material, a stick-slip pattern of crack growth is observed. Weak adhesion of the reference adhesive leads to an adhesive failure vs. a cohesive-adhesive failure in the case of adhesive B. On the contrary, the modification of adhesive A has an adverse effect on the G I of Al (-33%) and CFRP (-78%) as opposed to their reference counterparts.