Over the years, the industries of structural, mechanical, aerospace engineering, materials science, medicine, prosthetics, and many more have called for stronger, lightweight, low maintenance materials to withhold the stresses and loads that come with everyday usage of materials. Material scientists and engineers have studied and manufactured composite materials to cater to the needs of these various industries; however, composites have limitations as well. Many composites need repairs after damage from high impact velocities (typically in aerospace applications), and that can induce additional costs. [1] Auxetic materials are a relatively new class of materials that may be used in place of typical materials while being structurally sound and
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Auxetic structures typically conform to three main geometries. Re-entrant structures are the most common, and are characterized by bendable ribs that allow tensile and compressive deformation. Rotating structures consist of uniform shapes connected at vertices that expand by rotating about the vertices. A third structure is the nodule and fibril structure, where nodules move away from the fibrils when they are pulled in tension, causing an auxetic effect. Knowing the geometry of auxetic structures allow for the manufacture of synthetic auxetics from typically non-auxetic materials. The most common auxetic unit shape is the re-entrant honeycomb; materials have already been designed with the re-entrant honeycomb lattice on the microscopic scale, creating potential for filters at that scale [3]. Auxetic polymers such as polytetrafluoroethylene (PTFE), ultra-high molecular weight polyethylene (UHMWPE) and polypropylene (PP) have also been researched and synthesized as auxetic polymers [5]. The microstructure for PTFE is found to be auxetic, where PTFE has a nodule and fibril structure on the microscopic scale, making it useful in small spaces [3]. Foam is one of the most common forms of auxetic materials; auxetic foams also display fracture toughness and crack resistance in addition to the other properties of auxetics mentioned earlier [10]. Non-auxetic foams are manufactured to be auxetic through a process of compression on all axes and thermal deformation [3] as well as soaking the foams in electromagnetic fluids to alter their physical properties
Auxetic structures typically conform to three main geometries. Re-entrant structures are the most common, and are characterized by bendable ribs that allow tensile and compressive deformation. Rotating structures consist of uniform shapes connected at vertices that expand by rotating about the vertices. A third structure is the nodule and fibril structure, where nodules move away from the fibrils when they are pulled in tension, causing an auxetic effect. Knowing the geometry of auxetic structures allow for the manufacture of synthetic auxetics from typically non-auxetic materials. The most common auxetic unit shape is the re-entrant honeycomb; materials have already been designed with the re-entrant honeycomb lattice on the microscopic scale, creating potential for filters at that scale [3]. Auxetic polymers such as polytetrafluoroethylene (PTFE), ultra-high molecular weight polyethylene (UHMWPE) and polypropylene (PP) have also been researched and synthesized as auxetic polymers [5]. The microstructure for PTFE is found to be auxetic, where PTFE has a nodule and fibril structure on the microscopic scale, making it useful in small spaces [3]. Foam is one of the most common forms of auxetic materials; auxetic foams also display fracture toughness and crack resistance in addition to the other properties of auxetics mentioned earlier [10]. Non-auxetic foams are manufactured to be auxetic through a process of compression on all axes and thermal deformation [3] as well as soaking the foams in electromagnetic fluids to alter their physical properties