In theory, a negative Poisson’s ratio (i.e. auxetic) material has improved hardness, impact resistance, fracture toughness, and shear modulus over one with a positive Poisson’s ratio and comparable stiffness. These enhanced properties make them attractive candidates for a wide range of applications, including ones in the biomedical industry. Over the past two decades there has been increasing interest in auxetic materials leading to the discovery and development of new auxetic materials at the micro- and macro-scales. It has been reported in the literature that some human trabecular bone is auxetic; however verification of this claim and measurement of the Poisson’s ratio of biological materials remains a challenge. This research need is the motivation for the current work. The central research objective of this project is to develop an approach to gain fundamental insight into the geometric characteristics of auxetic open cell materials by studying the underpinning mechanics behind the transition from positive to negative Poisson’s ratio that takes place when open-cell foam is compressed and heat treated. The approach involves detailed image analysis and finite element modeling of the microstructure of polyurethane foam, which is commonly used as a test material to model the structure of human cancellous bone. By studying both conventional and auxetic foam, and the process by which conventional foams are transformed to auxetic, we seek to identify the critical features of auxetic open-cell structures. The results will lead to a better understanding of auxetic cellular materials in general, and will be used to develop a framework for use in determining the mechanical properties and potential auxeticity of human trabecular bone and to aid in the design of synthetic auxetic biomaterials. In this paper we report on preliminary results of our modeling efforts.
