Damage tolerance and notch sensitivity of bio-inspired thin-ply Bouligand structures

Different bio-inspired Bouligand thin-ply Carbon-Fibre-Reinforced Plastic (CFRP) laminates with a pitch angle as low as 2.07∘ are realised, which is the smallest pitch angle realised in literature. The angle is therefore close angles found in biological microstructures. Low-Velocity Impact (LVI) and residual compressive strength tests determined the damage tolerance of the structures. Investigated were two different interlaminar fracture toughnesses and two different metal-Bouligand-CFRP-layups. The low pitch angle results in significantly higher residual strengths than 45∘ quasi-isotropic (QI) layups, despite the significantly lower proportion of 0∘ fibres. Higher fracture toughness and hybridisation with steel layers lead to reduced matrix damage without increasing residual compressive strength. In-plane plane tension properties are determined with a pitch angle of 2.59∘. The results reveal, that the unnotched tensile strength is significantly lower. However, only helicoidal, sub-critical matrix cracking and no delaminations occur before final failure. The sub-critical matrix cracking leads to almost no notch sensitivity and a similar open-hole-tensile strength to 45∘-QI layups despite the low number of 0∘-fibres