Increasing the damping properties of carbon laminates by flax fiber hybridization for rotorcraft applications

Abstract

Vibrations play a crucial role in the design of helicopter structures and are in many cases the cause for structural changes in the late development phase. Often, dynamic interactions and excitations can only be determined in flight tests while structural dynamic simulations can only make inaccurate predictions. This is even more true for unconventional designs such as eVTOLS, for which practical experience is limited. One possible solution to minimize risks in the design is to increase the global damping in the helicopter structure, for which natural fibers offer a promising approach. This paper lays the groundwork for further investigations using flax fibers in hybridization with carbon fibers for selective damping enhancement of helicopter structures. For this purpose, the damping of flax, carbon and flax/carbon hybrid laminates is investigated at laminate level. High-performing flax and carbon prepregs were used to manufacture test specimen in a vacuum/autoclave process. Experimental modal analyses, using a contactless measurement method and impulse technique, were performed on hybrid and non-hybrid laminate test specimen with different stacking sequences and flax contents to determine their dynamic behavior. The damping is calculated using the Half-Power Bandwidth Method and set in relation to their mechanical properties, quantifying the damping potential for different load cases. The superior damping potential of flax was confirmed, especially in fiber direction. The distance to the neutral axis was shown to be the main influence between the oppositely behaving stiffness and damping in the laminates. However, the use of flax fibers in the hybrid laminates made it possible to achieve an advantageous ratio in favor of the damping. Flax fiber orientation in the direction of loading (UD0) is particularly suitable for this purpose.