A comparative study of in-plane shear behavior of flax/PP composite laminates with different reinforcement structures at elevated temperatures

Thermoforming of natural fiber-reinforced thermoplastic composites has emerged as a promising fabrication method due to its rapid processing capabilities, recyclability, and sustainability. Among various deformation mechanisms during this process, in-plane shear deformation plays a critical role, especially in achieving defect-free forming of complex geometries. While prior studies have primarily focused on woven natural fiber composites, recent interest has shifted toward unidirectional (UD) reinforcements for their high mechanical performance and compatibility with automated manufacturing. Spread-tow woven composites have also gained attention as an alternative architecture that may combine desirable features of both UD and woven structures. However, the in-plane shear behavior of both UD and spread-tow woven flax/polypropylene composites under realistic thermoforming conditions remains largely unexplored. To address this, bias extension tests were conducted at various temperatures and displacement rates, with in-plane shear deformation captured using 3D digital image correlation (DIC). The results provide comparative data on the shear performance of these two architectures, offering insights that will support future simulation-based process optimization and help material selection for thermoformed natural fiber composites.