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

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 ( σ 2 , τ 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 ( σ 2 , τ 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. • Complex failure behaviour of natural fibre-reinforced polymers (NFRP) under biaxial loading: Natural fibre-reinforced polymers (NFRP), such as flax (FFRP) or hemp (HFRP) fibre-reinforced composites, exhibit strongly non-linear and orthotropic mechanical behaviour — particularly under multiaxial stress conditions. This non-linearity significantly influences inter-fibre failure (IFF) in multidirectional laminates, posing challenges for both structural design and failure prediction. The present study emphasises the necessity of accounting for such non-linear effects and critically evaluating the applicability of established failure models, such as the Puck criterion, to FFRP under biaxial loading. • Development and validation of a novel biaxial test setup for FFRP: The new biaxial test rig based on the Iosipescu shear test enables efficient failure analysis of FFRP under combined loading. This approach allows reproducible testing with flat specimens rather than tubes, ensuring controlled failure initiation. As a result, the new experimental setup enabled a detailed characterisation of FFRP behaviour, confirming the suitability of the Puck criterion while highlighting the limitations of LaRC04 for FFPR. • Implications for predicting failure in multiaxial FFRP laminates: The study demonstrates that accurate failure prediction in FFRP laminates critically depends on accounting for their pronounced material non-linearity under multiaxial loading. Neglecting this non-linearity — especially in inter-fibre failure mode B — leads to significant errors in strength estimation. The findings underscore the necessity of using well-parametrised failure criteria tailored to the unique mechanical behaviour of FFRP to ensure reliable structural assessment.