Crashworthiness performance of optimized auxetic foamfilled tubes under axial loadings

Abstract

Filling thin-walled tubes with foam cores is a typical method to promote a desirable energy absorption performance and stabilize the crushing responses of thinwalled tubes under impact loading. Auxetic foams as new class of cellular materials have recently gained popularity within the research community due to their enhanced mechanical properties. However, the energy absorption performance of auxetic foamfilled tubes design information is very limited. The aim of this study is to evaluate the crush response, the energy absorption capacity and the deformation behavior of auxetic foam-filled square and circular tubes under quasi-static and dynamic axial loadings. For comparison, energy absorption performance of empty and conventional foamfilled square and circular tubes was also experimentally and numerically examined with respect to deformation modes and load-displacement responses. All tube specimens were crushed at a constant loading rate of 3 mm/min for quasi-static loading and an initial impact velocity of 5 m/s was adopted for dynamic loading. In order to investigate the influence of tube effective parameters such as wall thickness, diameter, width and height, a series of parametric studies were conducted using validated finite element (FE) models. The initial finding reveals that both auxetic foam-filled square and circular tubes are superior to empty and conventional foam-filled tubes in terms of energy absorption capacity without a significant increase in the initial peak load. From the initial finding and due to the great potential of auxetic foam as cores, a new fabrication technique called Quasi Tri-axial Compression Method (QTCM) was developed to fabricate the auxetic foam with the maximum achievable negative Poisson’s ratio. The fabricated auxetic foam with optimal re-entrancy was then introduced as the core for the tubes. Moreover, energy absorption capacity of auxetic foam-filled tubes was experimentally quantified with the foam Poisson’s ratio ranging from -0.13 to -0.32. The results show that the energy absorbed by auxetic foam-filled square and circular tubes loaded dynamically are approximately 34.7% and 22% greater than that of conventional foam-filled square and circular tubes respectively. This is practically beneficial when higher kinetic energy needs to be absorbed in order to reduce the impact force transmitted to the occupant’s compartment. Furthermore, it is evident that an increase in the auxeticity level of foam filler enhances crashworthiness performance of filled tubes under both quasi-static and dynamic loading conditions. Above all, the primary outcome of this thesis is a design guideline for the use of an auxetic foam as a core for energy absorbing devices where axial impact loading is anticipated.