Potential of natural fiber composites for automotive crashworthiness applications

In the next future, industries and companies need to be compliant with the Agenda 2030 (and 2050) requirements on sustainable development and climate change mitigation: some of the seventeen goals concern the minimum threshold of bio-based raw materials in the product manufacturing, and the limitation of gas emissions into the atmosphere. To achieve long-term effects leading to more sustainable components, the use of natural fibers as a reinforcement for composite materials answers well to the raised environmental issues, combining interesting damping properties, lightweight, and short supply-chain. The research activity, in particular, focused on the mechanical design of automotive components for energy absorption, such as crash boxes or bumpers. A suitable redesign of these energy absorbers could be achieved by replacing the nature of reinforcing fibers: from synthetic to natural, with fabric layers embedded into epoxy resin, even stacked in a hybrid solution. To do this, an in-depth analysis of the natural fibers composites (NFCs) was first required, where both mechanical characterization and low-velocity impact (LVI) tests were performed to understand their behavior, as well as their limitations and critical issues. In particular, this first investigation involved the following reinforcing fabrics: flax, unbalanced hemp, balanced hemp, and sisal. Then, relying on the reported performance, the study focused on flax fiber reinforcement along with carbon fiber using a toughened epoxy resin, in an increasingly specific and targeted research. Hybrid configurations – i.e., carbon-flax-carbon (CFC), and flax-carbon-flax (FCF) – were also tested to explore the performance under impact. Moreover, two different stacking sequences were analyzed, with the layers placed at 0° and in a quasi-isotropic (ISO) orientation. Lastly, the research activity progressed to more complex geometries, through the study of tubular samples subjected to axial crushing, with the same raw materials as in previous tests. Numerical finite element (FE) models of the performed tests were designed and solved using LS-DYNA software, to first reproduce the experimental curves, and, then, have a validated predictive tool for future analysis. In particular, the models of the mechanical characterization tests allowed for the parameters calibration in the material cards, subsequently adopted in the LVI models. The results obtained showed an agreement between experimental and numerical trends, highlighting the possibility of designing and manufacturing structural components in composite material reinforced with natural fibers. Finally, a practical case study involved the numerical modeling of a carbon crash box, with the aim of replacing synthetic layers with natural fibers ones, as mentioned earlier. Future developments will concern this automotive component for energy absorption, both in hybrid and flax configuration as regards materials, with optimized stacking sequence to achieve the best performance.