Biomedical Engineering Reference
In-Depth Information
Fig. 4.5 Plate compression test scheme employing a zwicki Z-5.0 testing machine: front view
without test assembly and side view with mounted transducer, plane indenter plate and foam
specimen
open-cell PU-foam is depicted in Fig.
4.4
(for clarity in Fig.
4.4
the compressive
force axis is mirrored). In the densifying process, three stages can be distinguished:
(I) a linear elastic region at small strain (\4-5%) due to cell wall bending, (II) a
plateau of deformation at moderate force increase (the plateau is distinctive at open-
cell PU foams), caused by elastic buckling of the cell ligaments (cf. Fig.
4.3
a II), and
(III) a densifying region where cell edges touch, thus increasingly resistant to further
densification. During this state material blocking occurs, resulting in a rapid increase
of loading force with minor displacement (see
Sect. 3.2.2.1
in ''Behaviour of human
soft tissue and polymer soft foams''). A detailed overview about polymer physics
can be found in (Treloar 2005), and information on structure and micromechanics in
(Gibson and Ashby 1997).
4.2.1.2 Experiments I: Elasticity
Experimental Set Up: To mechanically describe foam materials and characterize
long-term foam material parameters, the following deformation modes were dis-
placement-driven to create more than one state of strain:
1. quasi-static compression test using a plate
2. quasi-static compression test using a hemispherical indenter (Ø100 mm)
3. quasi-static quad lap shear test
A zwicki Z-5.0 testing machine by Z
WICK
/R
OELL
, as depicted schematically in
Fig.
4.5
for the plate and the indenter compression test was used to produce the
