Cellular foams with centralized density for improving resistance to indentation and impact

Abstract

Cellular foams are highly porous structures with interesting combinations of physical, mechanical, thermal, electrical and acoustic properties. They are commonly used as materials for lightweight space-filler, energy absorption, and thermal managements.

A great deal of research and techniques concerning the geometrical design, property characterization and industrial production of cellular foams have been reported. However, there is a lack of good understanding of the mechanical response and the energy absorption capacity of cellular foams which is vital to foam designs and their industrial applications.

Although the mechanical performance of cellular foams under quasi-static and dynamic compression tests have been extensively studied with experiments and computer simulations, few studies of the mechanical properties of cellular foams under indentation and impact loading tests have been reported. However, the indentation and impact properties of cellular foams are essential for foam design of bumpers in vehicle collisions or of protective packaging in impact resistance applications.

This thesis proposes a centralized density cellular foam model which is mainly designed for sustaining indentations and impacts. Its indentation and impact properties are studied in detail and compared with homogeneous density cellular models using finite element analysis method. Cellular modelling technique with Voronoi tessellation is adopted to construct numerical models, for exploring the macro mechanical properties of foams ad observation of the inner cell collapse and tearing process.

Both quasi-static and dynamic indentation tests are used to measure the mechanical behaviors and energy absorption abilities of centralized density foams. The influences of cell regularity, punch velocity, relative density, punch size and density difference are firstly studied compared with homogeneous density cellular foams with the same relative density. It is concluded that the centralized density cellular are preferable to homogenous density cellular foams in mechanical behaviors and energy absorption abilities under indentation tests. Then, the cell tearing effects in the centralized density cellular foams are estimated under quasi-static indentation tests. Finally, the influence of the strain-sensitivity effects and inertia effects are studied in dynamic indentation tests.

Low velocity impact loading tests are also used to exploit the mechanical properties and energy absorption abilities of centralized density cellular foams. Firstly, the effects of relative density, punch velocity, punch size and density difference on mechanical behaviors of the cellular foams are investigated. Furthermore, the centralized density part of centralized density model is optimized using surrogate optimization method based on the highly nonlinear performance of cellular foams under the impact tests.

By investigating the indention and impact properties of centralized density cellular foams, this thesis may contributes to offering a design guide for cellular foams used in impact-resistance and energy absorption applications.