Asymmetrical Auxetic Structures for Impact Force Mitigation

Abstract

Abstract This research is aimed at proposing an enhanced re-entrant hexagonal structure and examining its auxetic behavior in compressive or tensile load conditions. Auxetic materials, also known as materials with negative Poisson’s ratio (NPR), have been designed and fabricated for diverse applications that utilized normal materials which follow Hooke’s law but still show the NPR properties. One of the major applications is body protection pads that are comfortable to wear and effective in protecting body parts by reducing impact force and preventing injuries in high-risk individuals such as elderly people, industry workers, law enforcement and military personnel, and sports players. Another application is soft robotics for which the auxetic structures are used as sensors and actuators while protecting the wearer in the event of falling or collision. It is important to develop a new garment that best combines each individual’s requirements for wearing comfort, ease of fitting, ensured protection, and cost-effectiveness. The garment would be made from multilayer materials and adaptive structures to achieve unique multifunctional properties such as high hardness, impact toughness, adjustable stiffness, lightweight, and excellent shock absorption. This paper reports an integrated theoretical, computational (finite element analysis, FEA), and experimental investigation conducted for newly designed asymmetrical auxetic structures that exhibit the NPR effect and good impact force absorption or diversion capability. When an impact force is applied to a protective pad, especially right at the most vulnerable location of possible injury, absorption, and diversion of the impact force are the critical factor for the protection of the human body. For use in soft robotics, different patterns of deformations are also desirable for different motions of body parts and joints. Asymmetry along a longitudinal axis and lateral axis and also repetitive patterns in different directions were studied. Various combinations of auxetic cell patterns were modeled and investigated. 3D CAD models of auxetic structures were developed and their structural characteristics were evaluated through static analyses of FEA models. Through FEA, force and stress distribution on the bottom of the auxetic pads were examined while impact forces were applied on the top surface. The 3D-printed prototypes were then tested, and the results were compared with the computational prediction. Different profiles of stress and force distribution were observed for different patterns. The mitigation of impact forces was also observed. The simulation and test results showed that the asymmetrical auxetic material has high NPR and can be effective for absorbing or diverting impact forces.