Characterisation and description of compressible rubbery materials for component modelling

Rubbery foams and similar highly compressible rubbery solids form an important class of materials for technology. In order for components made from such materials to be modelled, in Finite Element Analysis (FEA) for example, appropriate constitutive laws are required.The review of the literature revealed that rubbery foams lacked the array of comprehensive data sets that have been published for near-incompressible rubbery materials for example. Moreover there were important contradictions between accounts of how rubbery foams behaved at finite strains. The review of the literature also raised some doubts on what the value of Poisson’s ratio (ν) should be for such materials in simple extension – let alone in compression.Most finite strain FEA of rubbery foams seems to use in the constitutive law a finite strain version of ν based on logarithmic strains: here called the Poisson index, υ. Some of the implications of such approaches have been explored via theory and experiments. Some doubt has thereby been cast on such approaches.In this project experiments performed in simple compression on a normal natural rubber latex foam have confirmed that the apparent Poisson ratio can fall to very low values even at rather small strains; the apparent Young modulus can drop to low values even at very small strains. The experiments have also indicated that: ν, and therefore υ at small strain simple extension, can be well below the value of 1/3 usually assumed; the value of υ can rise as extension progresses and can exceed ½