Influence of Textile Reinforcement Architecture on Delamination Initiation and Post-Initiation Behaviour of Flax/Epoxy Curved Laminates Under Four-Point Bending

Delamination in curved flax/epoxy laminates is governed not only by the stress required to initiate interlaminar separation, but also by the ability of the reinforcement architecture to maintain load transfer once damage has begun. This study investigates these aspects by comparing five six-layer flax/epoxy configurations: technical embroidery with fixation spacings of 2, 4 and 8 mm; a parallel-roving non-interlaced unidirectional reference (UD-R); and a woven fabric architecture. The curved L-shaped specimens were manufactured by vacuum-assisted resin infusion and tested under four-point bending following the ASTM D6415 framework. The results show a strong architecture-dependent response: reduced embroidery spacing increases resistance to delamination initiation and limits subsequent crack development, while the woven and unidirectional architectures exhibit distinct combinations of initiation resistance and post-initiation load transfer. The interlaminar tensile strength (ILTS) average is 16.18, 9.59, 2.46, 4.79 and 24.90 MPa for spacings of 2, 4, 8 mm, UD-R and woven, respectively. The ILTS values at 4 and 8 mm are 40.7% and 84.8% lower than those for the 2 mm configuration. Taken together, the results show that curved flax/epoxy laminates should be evaluated through three complementary aspects: the overall four-point bending response, the corrected curved-section state at delamination initiation, and the structural reserve retained after initiation. These aspects do not necessarily evolve together and cannot be represented adequately by a single peak load, Curved-Beam Strength (CBS) or strength parameter.