Enhancing indentation and impact resistance in auxetic composite materials

Auxetic materials exhibiting a negative Poisson's ratio are of great research interest due to their unusual mechanical responses and a wide range of potential deployment. However, due to the cellular structure and the bending or rotation deformation nature of the elements in auxetic materials, they usually have relative low stiffness, limiting their applications where high stiffness, strength, hardness, and energy absorption are simultaneously desired. To overcome this limitation, we apply the auxetic lattice structures as the reinforcements combining with the nearly incompressible soft materials as the matrix to create a class of high-performance composites. This coupled geometry and material design concept is enabled by the state-of-the-art additive manufacturing technique. Guided by static and dynamic experimental testing, we systematically study the indentation behavior of the 3D printed auxetics reinforced composites and achieve a significant enhancement of their indentation stiffness and impact resistance compared with the non-auxetic reinforced composites. By digital image correlation processing of experimental tests and numerical simulation, this improved mechanical performance is found due to two deformation mechanisms. The first mechanism is the negative Poisson's ratio effect of the auxetic reinforcements, which makes the matrix in a state of biaxial compression during indentation and impact and hence provides additional support. The second mechanism is the negative Poisson's ratio effect of the overall composites, which makes the auxetic reinforced composites denser at the site of the indentation and therefore are more resistant to indentation. The results show that auxetic structures can lead to design stiffer, harder and tougher composite materials. The material design strategy provides insights into the development of classes of novel auxetic composites with a wide range of mechanical and structural applications.