Auxetic structures for energy absorption applications

Abstract

Auxetic materials are new class of materials exhibiting negative Poissons ratio. This unusual behavior results in improvement of mechanical properties such as energy absorption capability. This research focuses on design of auxetic materials in order to enhance and control mechanical properties. Mechanical design of auxetic structures has been developed for both high and low stiffness applications. For high stiffness applications, auxetic structures were designed to be used for making auxetic materials. Among several auxetic structures, re-entrant structures have been selected due to their potential of modeling auxetic materials. The basic mechanical properties and impact characteristics have been determined using analytical and numerical methods. The analytical formulation has been validated by finite element analysis whereas the numerical results have been corroborated against experimental results. For validation, the basic mechanical properties and energy absorption capacity have been compared accordingly to subsequently carry out further analyses. As additional results, dynamic analysis of viscoelastic structures under impact loading was also demonstrated to examine the amount of impact resistance. For low stiffness applications, negative Poissons ratio polyurethane foam was precisely fabricated through a modified fabrication process which later involved experimental works to measure and control the mechanical properties. The effects of fabrication parameters namely hydraulic pressure, heating temperature and time on auxeticity of specimens have also been investigated. More importantly, a new method based on image processing technique has been proposed for measuring Poissons ratio of foam. In addition to this, energy absorption capability of auxetic foam was measured by using a high speed camera and falling weight system. Overall, the results highlight the pronounced effect of unit cell cross section and unit cell angle on the auxeticity and energy absorption characteristics. The primary outcome of this thesis is development of auxetic structure design for high stiffness application and modification of fabrication process of auxetic foam. Furthermore, the results demonstrated the importance of analyzing auxetic foam-filled thin-walled tubes as part of an energy absorbing system.