Editing Plasticity (physics) (section)

=== Cellular materials ===
These materials plastically deform when the bending moment exceeds the fully plastic moment. This applies to open cell foams where the bending moment is exerted on the cell walls. The foams can be made of any material with a plastic yield point which includes rigid polymers and metals. This method of modeling the foam as beams is only valid if the ratio of the density of the foam to the density of the matter is less than 0.3. This is because beams yield axially instead of bending.  In closed cell foams, the yield strength is increased if the material is under tension because of the membrane that spans the face of the cells.