Researchers from the University of Sunderland have shown that adding graphene to glass-fibre composites could improve their crash performance.
There’s no doubt that the automotive industry is in a state of transition. Hybrid and electric vehicles are growing in popularity, and driverless cars no longer feel quite so futuristic. But there’s another trend that’s all about improving fuel efficiency. Lightweighting – moving away from bulk steel, and towards fibre-reinforced composite materials. The energy absorption capacity per unit weight of composites is significantly higher than metals, and a new paper, published in Applied Materials Today [DOI: 10.1016/j.apmt.2016.11.003], has shown that the addition of graphene may improve it even further.
The work focused on modelling the mechanical performance of a small section of car bumper (known as a crash box), in order to assess its macroscopic crashworthiness. The material of choice was a 3-phase composite, consisting of short glass fibres, mixed with disc-shaped platelets of graphene, all embedded in a polymer matrix. Measurements of the tensile and compressive strength, fracture toughness and impact performance of the composite’s individual components were carried out using standard techniques. By varying the volume fraction of the graphene platelets, it was possible to design a series of composites, and apply a multiscale modelling approach (both finite element techniques and mean-field homogenisation) to each, in order to derive the overall response of each crash box.
To identify the optimal composition, a crushing load of 800 kg was mathematically applied to the upper side of each crash box. The impact strength, broadly characterised by its ability to withstand a ‘sudden’ shock, was found to increase as the volume fraction of graphene platelets increased. In addition, the graphene seemed to act as a stiffening agent – the softening behaviour seen in tensile strength testing of glass-fibre-only composites was greatly reduced by the addition of graphene.
Most notably, graphene was shown to improve the material’s specific energy absorption (SEA). In fact, with 2% graphene, 60% glass fibres and 38% polymer, the materials’ SEA was higher than that of steel. However, in ‘peak force’ crush tests, steel still outperformed the glass-graphene-polymer composites. Regardless, these results certainly demonstrate the potential for using graphene in energy-absorbing applications. The next stage of the work will be to investigate the role of other, non-mechanical properties of these composites, beginning with their electrical conductivity.
A. Elmarakbi, W. Azoti, M Serry, “Multiscale modelling of hybrid glass fibres reinforced graphene platelets polyamide PA6 matrix composites for crashworthiness applications”. Applied Materials Today, 6, 2017, 1–8. DOI: 10.1016/j.apmt.2016.11.003