Hybrid natural fiber composites in a helicopter cabin door – Mechanical properties and ecological efficiency

The overall goal of this work is the application of bio-based materials in an aerospace structure, while maintaining the structural-mechanical performance in accordance with its certification standards. This goal was pursued through the use of hybrid composites made from a combination of conventional (carbon) and bio-based fiber composites (flax). The cockpit door of an ultralight helicopter was chosen to prove the applicability of this hybrid composite. A reference door, built from carbon-fiber-reinforced polymers, was considered a benchmark to the requirements in terms of mass, stiffness, damping, ecological efficiency and costs. First, the benchmark door was built and characterized. Then the geometry was redesigned for the application of flax fiber composites, leading to an increase of the areal moment of inertia. The new geometry was then analyzed using multiple gravity loads. Highly loaded areas were locally reinforced with carbon prepregs. Tensile tests and sub-component cantilever beam tests were iteratively analyzed for the development and advancement of the finite element analysis (FEA) model. A significant material nonlinearity in form of a reduction of elasticity with increasing strains is inherent in flax-fiber composites, this phenomenon was interpreted as a yield point and included in the failure analysis. The resulting hybrid design showed promising results, as the stiffness matched the reference doors and a mass reduction of 6% with an increase in the bio-based material mass from 0 to 43% was achieved. But, due to manufacturing issues, additional epoxy layers had to be used in the flax weave, which significantly reduced the fiber volume content. As a result, the built hybrid door consisted of a bio-based mass of only 30%. The ecological efficiency, in terms of material primary production, of the hybrid door is significantly better than the reference doors. By calculation, the hybrid door’s materials consumed 221.7 MJ - eq less energy, while emitting 11.0 kg CO₂ - eq less carbon. However, drawbacks in the operational life due to an increased mass are inherent and break even on the ecological efficiency. This weight-drawback needs to be prevented, which could be achieved through a denser flax-fiber weave. In order to point out the potential of natural fibers, the designed hybrid version is also compared to the reference. Furthermore, benefits in terms of costs, vibration behavior, and mechanical damping are itemized in this work.