Cell wall stress-strain relations

Because no coupling between the neighboring cell walls is assumed, only the main diagonal of the material matrix has nonzero values and, due to the nonlinear elastic-plastic struchual behavior with damage of the cell walls, their components are functions of the corresponding strain components. As mentioned above, the overall stress-strain relation of the foam is obtained by adding both contributions of the skeleton and the diffuse continuum ... 

Cellular materials can collapse by another mechanism. If the cell-wall material is plastic (as many polymers are) then the foam as a whole shows plastic behaviour. The stress-strain curve still looks like Fig. 25.9, but now the plateau is caused by plastic collapse. Plastic collapse occurs when the moment exerted on the cell walls exceeds its fully plastic moment, creating plastic hinges as shown in Fig. 25.12. Then the collapse stress (7 1 of the foam is related to the yield strength Gy of the wall by... 

The different main polymers in wood appear to have structural roles that are related to this microarchitecture. Cellulose, an intrinsically rigid crystalline material, provides the reinforcing framework of the cell walls. Theoretical treatments of stress-strain relationships in wood under load indicate that the stiflhess of the material is imparted primarily by cellulose fibrils 10). The main function of hemicellulose and lignin is to buttress the fibrils. Degradation of any of these wood constituents results in a decrease in the strength of the material. 

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