Biaxial testing and failure criterion validation for flax fibre-reinforced plastics using a novel test method

Abstract

Natural fibre-reinforced polymers (NFRP), particularly those based on flax or hemp fibres, exhibit orthotropic and non-linear material behaviour. This makes accurate strength prediction using physically based failure criteria — such as the models proposed by Puck or Cuntze — especially challenging. In particular, the interaction of normal and shear stresses under combined $({\sigma }_2,{\tau }_{21})$ loading may lead to stress redistributions and, consequently, to an unpredictable shift in the inter-fibre failure (IFF) mode. Due to the experimental complexity involved, the investigation of such complex failure behaviour and the validation of physically-based failure criteria for NFRP have rarely been addressed in the scientific literature. In light of these challenges, a novel biaxial test rig was developed in this study, extending the established Iosipescu shear test according to ASTM D5379. Experimental results on 90 ° Iosipescu specimens made from unidirectional flax fibre-reinforced polymers (FFRP) laminates reveal non-linear stress–strain curves under combined $({\sigma }_2,{\tau }_{21})$ loading. These non-linearities further complicate strength predictions of multi-axial laminates based on physically-based failure criteria. The novel test method offers a reproducible, practical alternative to tubular testing and provides a foundation for improved failure modelling of multidirectional NFRPs under realistic multiaxial loading.