The lower value of modulus E, compared to the initial modulus Ei, is mainly attributed to the non-linear behavior of the flax fibers. This behavior is caused partly by fiber defects (bands and kinks that reduce the load transfer) and partly by the realignment of the microfibrils of the fiber [24,26]. The location of the knee formed by the transition from Ei to E was evaluated by calculating the intersection of the two slopes as shown in Fig. 6. The knees of the flax/PLA and flax-paper/PLA composites are located at 0.158% ± 0.006% and 0.149% ± 0.015%, respectively.