  Design of large composite structures requires understanding the scaling of
their mechanical properties, an aspect often overlooked in the literature on
composites. This contribution analyzes, experimentally and numerically, the
intra-laminar size effect of textile composite structures. Test results of
geometrically similar Single Edge Notched specimens made of 8 layers of 0
degree epoxy/carbon twill 2 by 2 laminates are reported. Results show that the
nominal strength decreases with increasing specimen size and that the
experimental data can be fitted well by Bazant's size effect law, allowing an
accurate identification of the intra-laminar fracture energy of the material.
The importance of an accurate estimation of Gf in situations where
intra-laminar fracturing is the main energy dissipation mechanism is clarified
by studying numerically its effect on crashworthiness of composite tubes.
Simulations demonstrate that, for the analyzed geometry, a decrease of the
fracture energy to 50% of the measured value corresponds to an almost 42%
decrease in plateau crushing load. Further, assuming a vertical stress drop
after the peak, a typical assumption of strength-based constitutive laws
implemented in most commercial Finite Element codes, results in an strength
underestimation of the order of 70%. The main conclusion of this study is that
measuring accurately fracture energy and modeling correctly the fracturing
behavior of textile composites, including their quasi-brittleness, is key. This
can be accomplished neither by strength- or strain-based approaches, which
neglect size effect, nor by LEFM which does not account for the finiteness of
the Fracture Process Zone.
