  This work presents a comprehensive investigation of the elastic and fracture
behavior of ply-to-ply angle interlock three-dimensional woven composites. The
research investigated novel splitting and wedge-driven out-of-plane fracture
tests to shed light on the tensile fracture behavior in the thickness direction
and to provide estimates of the out-of-plane tensile strength and fracture
energy. In addition, size effect tests on geometrically-scaled Single Edge
Notch Tension (SENT) specimens were performed to fully characterize the
intra-laminar fracture energy of the material and to study the scaling of
structural strength in this type of three-dimensional composites. The results
confirmed that size effect in the structural strength of these materials is
significant. In fact, even if the range of sizes investigated was broader than
in any previous size effect study on traditional laminated composites and
two-dimensional textile composites, all the experimental data fell in the
transition zone between quasi-ductile and brittle behavior. This implies strong
damage tolerance of the investigated three-dimensional composites. The analysis
of the data via Bazant's Type II Size Effect Law (SEL) enabled the objective
characterization of the intra-laminar fracture energy of three-dimensional
composites for the first time. Finally, Arcan rig tests combined with X-ray
micro-computed tomography allowed unprecedented insights on the different
damage mechanisms under multi-axial nominal loading conditions, particularly
tension-dominated and shear-dominated conditions.
