Basic modeling of the visco elastic behavior of flax fiber composites

In the middle of the 20th century, fiber reinforced composites became mainstream, with carbon and glass fibers at the forefront of this developing industry. However, they have certain disadvantages which have driven the research towards the discovery of novel materials. Today's demands go towards cheap and renewable materials. It has been proven that flax fiber possesses some of the best mechanical properties of bio-reinforcements, close to the ones of the glass fibers. These advantages, together with a lower density than glass and the possibility of recycling have made the flax fiber reinforcement a candidate for the next generation of composites. Along with these strong points, comes a behavior, which, for the composites mentioned earlier, can often be neglected: the time dependent behavior. The present work aims at studying this property and to propose a basic model for its evolution. The analyzed composite is an epoxy resin reinforced by long flax fibers, oriented at +/- 45 degrees, which show the shear behavior of the material. The experimental procedure consists of a tensile test to determine the mechanical properties of the composite on this direction and to establish the stress levels for the time dependent tests. Secondly, four levels of creep recovery tests are conducted with a creep of one hour and recovery of five hours. In the last part of the paper, linear modeling of this phenomenon is presented, based on the Zener and Burger models.