  This work investigates the mode I and II interlaminar fracturing behavior of
laminated composites and the related size effects. Fracture tests on
geometrically scaled Double Cantilever Beam (DCB) and End Notch Flexure (ENF)
specimens were conducted to understand the nonlinear effects of the cohesive
stresses in the Fracture Process Zone (FPZ). The results show a significant
difference between the mode I and mode II fracturing behaviors. It is shown
that, while the strength of the DCB specimens scales according to the Linear
Elastic Fracture Mechanics (LEFM), this is not the case for the ENF specimens.
Small specimens exhibit a pronounced pseudo-ductility with limited size effect
and a significant deviation from LEFM, whereas larger specimens behave in a
more brittle way, with the size effect on nominal strength closer to that
predicted by LEFM. This behavior, due to the significant size of the Fracture
Process Zone (FPZ) compared to the specimen size, needs to be taken into
serious consideration. It is shown that, for the specimen sizes investigated in
this work, neglecting the non-linear effects of the FPZ can lead to an
underestimation of the fracture energy by as much as 55%, with an error
decreasing for increasing specimen sizes. Both the mode I and II test data can
be captured very accurately by Ba\v{z}ant's type II Size Effect Law (SEL).
