  This work is focused on the experimental characterization of
traction-separation laws for cohesive modeling of mode-II interlaminar fracture
in composites. For the experimental investigation, damage progression in
end-notched flexure specimens under three-point bending was captured using
microscopic and macroscopic Digital Image Correlation (DIC) techniques. The
specimens were geometrically scaled with three scaling levels for a size effect
study. The fracture energy of the material was analyzed based on both linear
elastic fracture mechanics and Bazant's type-II size effect law for comparison.
For damage modeling, traction-separation laws were built from the experimental
data, and the fracture process zones of the specimens were modeled using
cohesive interactions. Difficulties in characterizing traction-separation laws
merely relying on load-displacement curves (i.e., global behaviors) were
demonstrated and discussed through extensive simulations. To address this
issue, through-thickness deformation analysis and progressive DIC methods were
proposed and implemented in this work. The observations and proposed methods
herein will contribute to characterizing a single traction-separation law for a
given composite material by capturing global and local fracture behaviors on
any geometric scale.
