Open-cell flexible foams are industrially important materials with widespread use in household furniture and car seating. The cushioning application of these materials depends critically on how they deform under a compressive load.

Using 3-D X-ray computed microtomography on the ID19 beamline it has been possible to study in fine detail the exact mechanisms by which open-cell flexible foams deform in compression. The characterisation of the deformation process at the microscopic length scale in 3-D, in the bulk, represents a key step towards a better understanding of the relationship between the structure of foams and their mechanical properties.

In order to observe the deformation mechanisms in these relatively low-density materials two criteria need to be satisfied. Firstly, a spatial resolution better than 10 micrometres is required to clearly resolve the fine struts making up the foam structure. Secondly, it is preferable to perform "local tomography" on a small region inside a larger sample so that edge effects during the compression are minimised. The actual experiment involved compressing a 2 cm-diameter cylinder of foam in a custom-built compression rig mounted onto the rotation stage of the tomography setup. The imaged volume was a 7 mm x 7 mm x 7 mm cube in the centre of the foam cylinder. Tomographic scans were taken at successively increasing levels of compressive strain. A standard filtered back-projection algorithm was used to reconstruct the volume from the local projection data. By setting one edge of the imaged volume coincident with the stationary plunger, it has been possible to correlate the structure seen at one level of compression with the structure observed at each other level of compression.

Figure 106 shows 3-D reconstructions of the sample at three different strain levels. The face of the stationary plunger can be seen at the bottom of each image. The 3-D renditions represent volumes of 7 mm x 7 mm x 1.4 mm.

A comparison of the struts around the cells marked C and D between Figure 106a and Figure 106b shows clearly that the initial phase of the compression occurs by a process in which struts bend. This early deformation is accompanied by a roughly linear elastic response. By comparing Figure 106b and Figure 106c it can be seen that a whole band of the structure comprising cells labelled A to E actually collapses. A plateau in the stress/strain curve accompanies this second mode of deformation. Although these processes have been understood for some time, this experiment allows them to be visualised in the bulk in unprecedented detail and will allow further clarification of the mechanisms of foam deformation.

Software applications have been developed at the ESRF to extract the locations of the nodes in the reconstructed foam volumes and also to determine the connectivity of the structures. The data will be used to construct finite element models and a comparison of the simulated deformation and the actual observed deformation will be performed. The advantage of using the modelling approach in tandem with the experimental characterisation is that insight into the actual stresses inside the foam may be deduced from the calculations.

Authors
A.H. Windle (a), J. Elliott (a), R.J. Oldman (b), J.R. Hobdell (c), G. Eeckhaut (c), J. Baruchel (d), S. Bouchet (d,e), W. Ludwig (d), P. Cloetens (d), E. Boller (d).

(a) Department of Materials Science and Metallurgy, Cambridge University (UK)
(b) ICI Chemicals and Polymers Ltd., Runcorn (UK)
(c) Huntsman Corporation, Everberg (Belgium)
(d) ESRF
(e) CMM, ENS des Mines de Paris (France)