The SFIR Test: An Innovative Hydrostatic Compression Test to Characterize the Volumetric Behavior of Polymeric Foams

Background

Polymeric foam materials can show a strongly non linear compressible elastic response. For certain applications, it is necessary to know the volumetric behavior of the material under hydrostatic compression. Existing devices for hydrostatic compression testing use a multiaxial testing machine or a fluid to transmit pressure to the foam. They are either complex to set up, or do not allow for hydrostatic pressures of several MPa to be applied or for volume variations of several tens of percent to be achieved. Besides, when pressure is applied to the sample via a fluid, it is difficult to prevent penetration of the fluid into the foam, particularly when it is open-cell.

Objective

This paper presents a hydrostatic compression test for polymeric foams that does not present these limitations.

Methods

A cylinder of a nearly incompressible material (silicone) is molded around a spherical sample of the polymeric foam of interest. The whole set is subjected to confined compression in a rigid chamber. Post-processing is developed, based on finite element analysis, to determine the hydrostatic stress in the foam and its volume ratio from the axial load and displacement data.

Results

Finite element simulations show that the foam sample is subjected to a state close to hydrostatic compression. The test was applied to several samples of elastomeric microcellular polyurethane foams of different densities. The results are in line with expectations, with limited scattering.

Conclusions

The Sphere Foam In Rubber (SFIR) test allows to reach volume reductions of several tens of percents and hydrostatic stress levels of several MPa, on any kind of polymeric foams, provided that its bulk modulus is at least 100 times lower than that of the surrounding nearly incompressible material used. It can be easily implemented with very standard equipment.