The figure shows compressive stress-strain curve for an elastomeric foam, an elastic-plastic foam and one which is elastic-brittle. They have broadly similar shapes, but for different reasons. All of them show a linear-elastic regime followed by a plateau of roughly constant stress, leading to a final regime of steeply rising stress. Each regime is associated with a mechanism of deformation. On the first loading, the cell walls bend, giving linear elasticity (provided, of course, that the cell wall material is itself linear-elastic). However, when critical stress is reached the cells begin to collapse: in elastometric materials collapse is caused by the elastic buckling of the cell walls and therefore, it is recoverable. In materials with a plastic yield point, it is caused by the formation of plastic hinges at the section of maximum moment in the bent members. Finally, in brittle materials it is caused by brittle fracture of the cell walls. The last two cases, of course, are not recoverable. Eventually, at high strains, the cells collapse sufficiently enough to allow opposing cell walls touch (or their broken fragments packs together) and further deformation compresses the cell wall material itself. This fact generates the final, steeply rising portion of the stress-strain curve labelled densification.