Biomedical Engineering Reference
In-Depth Information
Fig. 4.3 PU-foam cells (a-I) undeformed configuration, (a-II) compressed along the rise direction
and (b-I) undeformed configuration, (b-II) compressed perpendicular to the rise direction
Fig. 4.4 Representative
force-displacement curve
obtained from stepwise
uniaxial loading of low-
density open-cell PU-foam.
Characteristic regions of
elastomeric foam behavior
are labeled: (I) linear elastic
at small strain, (II) elastic
buckling, and (III)
densification
The difference in material response is due to a possible elongation of foam cells
in the foaming (rise) direction. The cell structure and, as a result, the macro-
structure, is therefore geometrically (mechanically) anisotropic. Consequently, the
mechanical response of polymeric foams depends on the intrinsic properties of the
employed polymers, the cell architecture and the material of the cell walls. The
cell architecture is determined by the cell ligament dimensions and dimension
distribution of the cells making up the framework.
Mechanical Behaviour: In geometrically anisotropic cell structures, compression
loading perpendicular to the rise direction results in a nearly monotonic force increase,
whereas loading along the rise direction results in a distinct plateau in the load-
displacement curve. As noted in (Mills 2000), loading forces at 50% strain are about 3-
fold higher when loading perpendicular to the cell rise direction, compared to loading
along the rise direction. International standard regulations (ISO 3386, ASTM D-3574)
prescribe the compression force deflection value (CFD) for soft urethane foams to be
determined by compression perpendicular to the foam rise direction.
When loaded in compression, open-cell foam exhibits characteristic behaviour. A
representative material response from stepwise uniaxial compression loading of
