In the first of its kind, a novel lightweight, high toughness auxetic structure made from carbon fiber reinforced polymer composite laminates has been developed by a team of researchers, led by Changfang Zhao, from Nanjing University of Science and Technology.

The auxetic structure can be used for constructing primary structures in the transport industry, where the drive to implement electrification in vehicles, as well as aircraft and marine vessels, demands that these different modes of transportation be made of lightweight materials to enhance the power efficiency.

Normal structures or materials exhibit the usual phenomenon when stretched (compressed), i.e. they become thinner (thicker) in the direction perpendicular to the applied load. Auxetics refers to structures or materials that exhibit an unusual behavior when mechanical loaded.

When stretched (compressed), they become thicker (thinner) perpendicular to the applied force. They can exhibit this behavior because of their particular (engineered) internal structure. In the form of material, we say that the auxetic exhibits negative Poisson’s Ratio. More importantly, the auxetic exhibits robust shock-absorbing properties, and this lend to wide applicability, such as armour, vehicle bumper, aircraft body, the hull of a marine vessel, and in packaging.

However, making the auxetic lightweight, tough with sufficiently stiffened components could be even more attractive as it endows the auxetic with multi-functional applications, for instance as a primary structure for electric vehicles, including aircrafts and marine vessels, demands that these be made using lightweight materials for high power efficiency, that can also deliver maximum protection to the vehicle and passengers in the event of a collision.

Changfang Zhao and his team designed and fabricated ‘cell structures’, i.e. small-scale versions of the auxetic structure, using laminated carbon fiber reinforced plastics (CFRP), which is the same material used in the Boeing 787 aircraft fuselage.

The cell structures were subjected to compressive loading in three orthogonal directions to evaluate the effectiveness of energy absorption in the respective direction. Using a state-of-the-art digital correlation imaging system to observe the deformation of the cell structure, they concluded that the cell structures were capable of auxetic behavior in two orthogonal directions and possesses high energy-absorbing capacity in the third direction. Future design of CFRP auxetic structures as lightweight primary structures will be able to leverage the directional dependent properties for both structural support and impact resistance.

The members of his team of international researchers are hailed from Nanjing University of Science and Technology (comprising Jianlin Zhong, Kebin Zhang, Zhendong Zhang, Jie Ren, Guigao Le), National University of Singapore (Heow Pueh Lee) and Newcastle University (Kheng Lim Goh).