*3.4. Effect of CNF Addition on the Mechanical Properties of BF/CNF Fibrous Preforms*

The fibrous preform's mechanical properties are closely related to the final FRP product manufactured as a reinforcing agent. Figure 7 summarizes the tensile properties of the fabricated BF/CNF nonwoven preforms. The tensile test results could not be obtained for BF nonwoven samples without CNFs because the fibers loosened as soon as the tension load was applied, and the nonwoven structure was damaged. Alternatively, a tensile test was possible for BF/CNF5. As a result, it was determined that the tensile strength was 8.1 MPa, and Young's modulus was 570 MPa. CNFs begin to act as coatings and binders as they penetrate the BF nonwoven structure. The mechanical reinforcing effect of CNFs increased as the CNF quantity rose. BF/CNF15 demonstrated a tensile strength of 13.8 MPa, and Young's modulus of 770 MPa. In addition, as the number of CNFs increased, the tensile elongation of the BF/CNF preform also grew. As CNFs are input, the breakage of BF/CNFs is converted from BF-oriented brittle fracture behavior to elastic and progressive failure due to the energy dissipation effect of the CNF bundle. Vilchez prepared chicken hair fiber/CNF preforms and confirmed that as the quantity of CNFs enlarged, progressive failure appeared, increasing tensile elongation [18]. This progressive failure behavior of the BF/CNFs by CNFs consequently contributed to a substantial surge in the BF/CNF preform toughness.

**Figure 7.** Mechanical properties of BF/CNF fibrous preforms. (**a**) Representative tensile stress–strain curve, (**b**) tensile stress and Young's modulus, (**c**) toughness, and (**d**) tensile strain.
