  This work is focused on investigating the impact of out-of-plane stitches on
enhancing mode-II interlaminar fracture toughness (or energy) and
characterizing damage progression and crack arrestment in stitched
resin-infused composites. For the experimental work, End-Notched Flexure (ENF)
quasi-isotropic specimens were manufactured using +/-45 non-crimp carbon-fiber
fabrics through a resin-infusion process. Both stitched and unstitched specimen
sets were designed for comparison. For a size effect study, the ENF specimens
were geometrically scaled with three scaling levels. Based on the
load-displacement data (i.e., global analysis), the fracture energy of the
specimen material was analyzed using the compliance calibration method and a
size effect theory. The fracture energy values were compared between the
stitched and unstitched cases to characterize the enhanced fracture toughness
of stitched composites. For local analysis, two types of digital image
correlation (DIC) systems were employed: microscopic and macroscopic (i.e.,
coupon-scale) DIC systems. By analyzing in-plane displacement through the
thickness, separation development was characterized along predicted fracture
process zones. The impact of out-of-plane stitches on separation propagation
along fracture process zones was discussed based on the DIC analysis. This work
will contribute to developing a high-fidelity damage model for stitched
resin-infused composites in the form of a traction-separation for high-speed
aircraft applications.
