One of the main drivers for the development of thermoplastic composites has been the increasing need for materials with high specific stiffness and strength without compromising recyclability. In recent years, the development of composite materials took advantage of their inherent heterogeneity and anisotropy to combine the well-known load-bearing functions of these materials with novel functionalities in the form of embedded elements. This way, the resulting “smart” composite material enables gains in efficiency by integrating extra functions into the structure, which may act during processing, during part manufacturing and assembly, or during service to adapt to the service conditions.
In SALIENT, CIDAUT is working on the integration of Nitinol (Ni-Ti) Shape Memory Alloy (SMA) wires as functional elements into a carbon fibre-reinforced polyamide 6 composites with the purpose of enhancing the mechanical performance of the resulting structure. The main goal of embedding SMA wires into the composite is to actively change the composite properties using electrical stimulation, thanks to the phase change induced in the SMA wires, from austenite to martensite phase. Specifically, greatly stiff composites such as the one employed in SALIENT can suffer from extremely brittle failure due to a lack of plastic deformation, leading to non-optimal impact properties because of the premature appearance of barely visible impact damage.
The activities executed in SALIENT allowed defining an optimal strategy for integrating SMAs into CFRP composite structures, covering aspects such as the selection of the manufacturing method and processing steps; the definition of the triggering strategy (Joule heating) and its effect on the overall structure; and material optimisation through mechanical testing on coupons with different SMA configurations.
The conclusions obtained at coupon level were employed to build a crash box prototype. This component was then tested in crushing mode at CIDAUT using a double impactor machine. The results showed that the introduction of SMA wires dampens the elastic waves that propagate through the material, decreasing the oscillations of the force signal. Consequently, the evolution of the crushing becomes smoother, with less pronounced peaks, especially when the wires are activated electrically.
The research leading to these results has received funding from the Horizon EU Programme under grant agreement No. 101069600 and UKRI grant agreements No. 10047227 and No.10047305 (SALIENT Project).