Design and Optimization of Motorsport Safety Structures using Flax Fiber and Hybrid Composites

Over the past decades, composite materials have gained prominence across several industrial sectors, from civil engineering and biomedical applications to transportation, as alternatives to traditional metals, driven by their outstanding specific mechanical properties that significantly surpass those of conventional materials. The application sector of interest in this study is the automotive industry, with a focus on Formula 4 racing vehicles. In particular, the analysis presented here focuses on the design and performance assessment of crash-box structures, which are key structural components that provide passive safety by protecting the driver and critical vehicle components during impact events. Indeed, the automotive industry is increasingly relying on lightweight, crashworthy composite structures to improve performance, safety, and sustainability. By combining multiple layers of specific materials, laminates offer advantages such as lightweight, high strength, corrosion resistance, ease of manufacture, and tailored properties, making them well-suited for high-performance end-use applications. This work investigates carbon- and flax-fiber reinforced polymer laminate composites, highlighting material hybridization as a promising compromise between mechanical efficiency and environmental impact. The experimental activity starts with the mechanical characterization of carbon/epoxy and flax/epoxy laminated structures. Then, elementary structures, i.e., coupons, are tested under impact with different loading conditions to assess their crushing resistance, energy-absorption capability, and the influence of geometry on crushing efficiency. Square plate coupons are tested under Low-Velocity Impact (LVI) and subjected to varying impact energies until perforation. A finite element (FE) modeling strategy is developed using an explicit solver in LS-DYNA, aiming to reproduce the experimentally observed impact behavior of composite plate laminates. In this context, the material cards for carbon and flax composites are calibrated and optimized to develop a robust material model that accurately represents the crushing behavior of composite structures and the associated damage response, enabling reliable predictions for larger and more complex geometries, such as crash-boxes. Subsequently, to assess the crushing load-carrying capability of the composite laminates of interest and the influence of geometry, dynamic in-plane crashworthiness tests are first conducted on flat rectangular coupons and corrugated (S-shaped) coupons. Tubular components are then analyzed, as thin-walled structures are widely employed under impact loading due to their ability to efficiently absorb energy through controlled deformation. Accordingly, circular and square tubes, as simple reference configurations for structural energy absorbers, are experimentally tested under axial crushing in both quasi-static and dynamic loading conditions. The effects of different cross-sectional geometries are evaluated, showing that the resulting crushing efficiency is higher than that of flat geometries. At the same time, a FE model is employed as a reliable predictive tool to reproduce the experimentally observed responses. In this context, a comparison between macro-scale (single-shell) and meso-scale (multi-shell) approaches is conducted to identify the optimal balance between accuracy and computational cost. Finally, the analysis is extended to the large-scale component, namely the rear crash-box, under the dynamic impact homologation test. In this regard, a macro-scale modeling approach is adopted to avoid excessive computational burden, and the obtained results achieve the desired accuracy. Experimental results from coupon-level to structural-level tests indicate that components reinforced solely with flax fibers are not suitable for crashworthiness applications that meet standard automotive requirements. In contrast, the carbon crash-box satisfies the homologation criteria. Moreover, among the investigated hybrid sample configurations, solutions with outer carbon plies exhibit crushing efficiency and specific energy absorption (SEA) values comparable to those of fully carbon-reinforced structures, with a reduction of only about 15% relative to the pure carbon case. These encouraging results suggest that hybrid solutions can also be potentially implemented for crash-box applications to balance environmental benefits and mechanical efficiency. For this reason, computationally efficient numerical simulations are essential for implementing an optimization procedure to identify the optimal carbon–flax hybrid crash-box configuration. Although the adopted macro-scale modeling approach provides adequate accuracy, the overall simulation time required for each analysis remains too high to enable the implementation of a formal and fully automated optimization procedure. A comparison with a single-shell model implemented in LS-DYNA and Abaqus, using the CZone add-on, is presented. The latter approach uses additional input parameters beyond the traditional material parameters, namely the crush stress data. These values are not intrinsic composite properties but depend on several variables, such as material type, fiber orientation, and geometry, including curvature. Curvature-dependent crush stress data, which are obtained from experimental in-plane crashworthiness tests on flat rectangular specimens and axial crushing tests on circular tubes, are used to model the different flat and curved regions of the crash-box FE structure. This strategy proves effective, yielding a considerable reduction in computational time and improved accuracy. Based on the improved FE model, to identify the optimal flax–carbon hybrid configuration, an optimization procedure using a generational integer-coded genetic algorithm is implemented to ensure crashworthiness certification while maximizing the carbon footprint reduction. This underscores the potential of hybrid materials for use as impact energy absorbers, offering a promising alternative to purely carbon fiber-based structures. The results suggest that the use of natural fibers is feasible in structural applications, not only in paneling, provided that appropriate design adjustments, such as material hybridization, are implemented. This work lays the foundation for research on the application of natural fiber composites (NFCs) in the automotive sector. It advances development goals toward continuous performance improvement and sustainability in the motorsport sector. Finally, following the investigation of monolithic composite laminates, the study is extended to sandwich configurations with two carbon-reinforced epoxy skin layers and an aluminum honeycomb core, which more closely represent antiintrusion panels. Such panels are included in the survival cell of racing cars to protect the pilots from external penetration. The LVI study highlights the impact behavior and induced damage mechanisms in the upper and lower skins. In contrast, quasi-static indentation aims to characterize skin–core interactions, core crushing, and the onset of delamination under highly concentrated loading. An effective FE model is developed using the standard Abaqus software, providing a macro-scale representation of the honeycomb core. The agreement observed between numerical simulations and standard experimental results provides a starting point for further enhancing the prediction of crack propagation by adopting a meso-scale technique. Future work may involve explicitly representing the topology of the core; this approach could more accurately capture the indentation response and its interaction with the composite skins, albeit at the expense of increased computational cost.