A Thermoplastic Polymer Coating for Improved Impact Resistance of Railways CFRP Laminates

Due to the superior specific mechanical properties and low density of composite materials, their demand has risen prolifically within several industrial fields over the last decade including railway industry. The latter considers composite materials as a much more attractive alternative to standard metallic solutions. However, while composite materials have already been used in the manufacturing of parts of rolling stock (overhead structures, cab fronts, seats, doors), there is currently no procedure to certify a rail vehicle built entirely - or in large part - from non-metallic materials. In this context, safety based technological improvements criteria are mandatory for any transportation system. One of the safety features for a train is the missile protection, which indicates that the vehicle shell must not permit any flying objects to penetrate the coach/vehicle. Hence, the analysis of the characteristics of composite impact damage is mandatory to apply mitigation actions against structural integrity detriment, also considering that even though regulation requirements for composite structures exist, they are not comprehensive enough to improve the overall structural safety. The aim of this paper was design and characterisation of Carbon Fibres Reinforced Polymer (CFRP) laminates hybridised with thermoplastic polyurethane (TPU), to enhance impact energy absorption and satisfy light weight requirements. In order to simulate flying ballast, a Low Velocity Impact (LVI) tests campaign was carried out at different impact energy levels (2 J and 3 J) by mean of a drop tower impact test on 150 x 100 mm CFRP specimens with and without TPU. Impacted samples were subjected to a non-destructive analysis campaign using Phased array to evaluate the extent of internal damaged areas and the results showed a significant benefit towards impact damage tolerance when a thin polyurethane layer is applied on CFRP components. Results from the experimental campaign show significant benefit against impact damage tolerance due to the thermoplastic material damping properties which are able to modify the energy absorption mechanism, reducing the extent of the internal delamination.