Auxetic Materials and Structures for Defense Applications

Poisson’s ratio is the fundamental mechanical property of a material which is usually positive for conventional materials. However, it is negative for the special class of materials known as auxetic. These materials expand laterally when stretched longitudinally, and this characteristic is quite common in materials having cellular geometry like honeycomb (re-entrant type). The auxetic behavior is dependent upon the cell geometry however it is scale-independent; hence, it can be achieved at various structural levels ranging from molecular to macroscopic levels. The auxetic behavior can be generated artificially in the structures by peculiar tessellation of unit cells. This auxetic behavior in a structure depends upon the shape, size, and deformation behavior of each unit cell connected to each other. It is reported that the auxetic effect enhances the overall mechanical properties such as indentation resistance, fracture toughness, shear strength, etc., making them suitable for lightweight applications in automobiles, sports, defense, sensors, aerospace, and space. Military vehicles and soldiers are prone to ballistic threats, so lightweight armor that is able to absorb energy locally and able to dissipate it efficiently and swiftly is in demand. Auxetics are quite suitable for such applications. Apart from this, synclastic behavior of auxetics makes them potential candidates for making seamless helmets or knee pads. The synclastic curvature is useful in aerospace applications such as nose cones and wing panels. In this paper, auxetics are reviewed extensively in terms of the geometry of the structure, namely re-entrant, rotating polygon, chiral, and other models. Further, the auxetic properties, mechanics, and applications are explained concisely in addition to the challenges and drawbacks of these structures in engineering applications.