R-curve behavior of adhesively bonded composite joints with highly toughened epoxy adhesive under mixed mode conditions

abstract The fracture toughness of an adhesively bonded carbon fiber reinforced plastic joint under mixed mode (I and II) conditions was determined by means of the Fernlund–Spelt type double cantilever beam (DCB) test, where a rubber-modified epoxy adhesive was used. The R curves were determined by calculating the energy release rates, G I and GII, based on the beam theory, along with a finite element analysis. The total energy release rate GT (= GI + GII) strongly depended on the I–II mixed mode. This means that it remarkably increased with increasing mode-II component; furthermore, it varied with crack growth from G T C (at the onset of cracking) to G T S (at the steady crack-propagation region). The difference between G T C and G T S increased with the increase in the mode II component. Such R-curve characteristics were discussed based on observation of the fracture surfaces with a scanning electron microscope and the corresponding crack propagation-path behavior.