Loading rate dependency of Mode I interlaminar fracture toughness for unidirectional composite laminates

This study was conducted to investigate the effect of the loading rate on the Mode I interlaminar fracture toughness of unidirectional carbon/epoxy laminates. Double cantilever beam (DCB) test geometry was employed for both quasi-static and dynamic fracture tests. A novel dual electromagnetic Hopkinson bar was employed to load the DCB specimens dynamically and symmetrically with velocities in the range of 10-30 m/s. A hybrid experimental-numerical method was used to determine the interlaminar fracture toughness for both quasi-static and dynamic loading conditions using the virtual crack closure technique (VCCT). The results indicate the presence of a critical loading rate for interlaminar fracture, below which the fracture toughness remains constant and beyond which the fracture toughness increases rapidly. Fractography results suggest that the failure mechanism transitions from a fiber/matrix interface failure under quasi-static loading to a brittle cleavage fracture of the matrix material with microbranching under dynamic loading.