This thesis presents a new method for determining mode II interlaminar fracture toughness in fiber reinforced polymers (FRP) using the essential work of fracture (EWF) method. Fracture tests were performed on a tabbed double edge notched shear (DENS) specimen, made from a unidirectional glass/epoxy laminate, in an Iosipescu fixture. The EWF in mode II was found to agree with the GIIc for fracture initiation value from an ENF test.

A finite element model was used to determine mechanisms involved in the DENS specimen. Interlaminar fracture was simulated using cohesive elements and the unstable crack growth path was simulated using the Riks arc length method. The numerical results closely match the empirical results. The model reveals that even though the EWF could be found using the conventional linear extrapolation, elastic strain energy, rather than the usual plastic energy, is the dominant geometry dependent property.