Investigating Nonlinear Behavior and Stiffening Phenomenon in Flax Fiber Laminates During Quasi-Static Monotonic and Cyclic Testing: Analysis of Influential Parameters

Abstract

Abstract The nonlinear response and stiffening behavior of flax fiber laminates under monotonic and cyclic loading are investigated, with a focus on the effects of fiber orientation, resin type, loading rate, and temperature. Unidirectional flax fabrics (0 deg, 30 deg, 45 deg, 60 deg, and 90 deg) and mat reinforcements combined with either unsaturated polyester or Elium® resin were used to fabricate laminates by vacuum infusion. The mechanical tests were done at temperatures of 20 °C, 50 °C, and 80 °C, with varying strain rates (1, 5, 25, and 50 mm/min). Fiber orientation plays a crucial role: materials with fibers aligned in a single direction (0 deg) are more resistant to damage than oblique or random orientation. The stiffening effect was more pronounced in unsaturated polyester resin than Elium® resin due to its rigid thermoset structure, which limits fiber mobility. This stiffening effect became stronger at higher strain rates (25 and 50 mm/min). Mechanical properties were significantly affected by temperature: at 50 °C, the intermediate level of matrix flexibility enhanced the load transfer; however, at 80 °C, matrix softening weakened the stiffness and enhanced the propagation of damage. These results show that material properties and thermomechanical loading conditions are vital in controlling the nonlinear behavior and stiffening of flax-based composites.