Performance Simulation of Vulcanized Natural Rubber based Auxetic Cellular Structures

Abstract

Polymeric auxetic foams are formed from conventional polymeric foams when subjected to a compression and heating process which results in the foam re-entrant structure characteristic of auxetic foams. The re-entrant structure imparts enhanced mechanical properties to the material, along with a negative Poisson's ratio, which can be changed by controlling processing parameters. Although the mechanical behavior of these materials is counter-intuitive in nature due to the negative Poisson's ratio, its application in important areas like impact resistance, biological applications, homeland security, sports, etc., makes the creative utilization of its unique properties all the more important. The two-step post-foaming auxetication procedure is applicable to thermoplastic foams, whose base material have a definite softening temperature. However, for thermoset polymers and vulcanized elastomers, a relatively unexplored one step procedure would be required for generation of auxetic cellular structure, in which the auxetic structure would have to be formed prior to final curing of the cellular structure. The purpose of this research was to develop carbon black nanoparticle reinforced natural rubber vulcanizates, and to investigate the effects of carbon black reinforcement on the mechanical and viscoelastic behavior of the auxetic foam by using modeling and numerical simulation techniques. This has been achieved by mechanically characterizing vulcanized natural rubber of various nanoparticle loadings, and then using the test data to perform finite element simulations on the foam computer generated models. Viscoelastic as well as hyperelastic material models have been used to simulate time-dependent behavior, and large compressive deformation, respectively, by fitting experimental data to theoretical material models in the simulation software. An approximated idealized foam model has been developed based on material properties of the base rubber, and large deformation analysis of this foam structure has been done to determine structural re-organizations, stress distributions, reaction forces, and negative Poisson ratio effect in the foam V structure. The aim was to characterize the auxetic foams of varying carbon black loading for moderate impact applications. The idealized model developed to study the auxetic structure provides information for further research in rubber based cellular auxetic materials.