Supports - Preventive Biomechanics - page 163

Table 4.3 Long-term
material parameters of
VE-foam material

VE-Foam
j

l j [MPa] a j [-] m j [-]

1 5.280146E - 03 2.208547E ? 01 1.587915E - 04

2 1.042675E - 03 2.669817E ? 01 2.855843E - 03

and VE-foam material, Fig. 4.13 , is shown based on optimized long-term material
parameters provided in tables Table 4.2 and 4.3 .
Identification of long-term parameters a k , l k and b k of equation ( 3.274 ) using
model parameter N = 2 is performed via the optimization routine described in
Sect. 3.4 . In the quality functional ( 3.364 ) , f i M (h i , p) is the model function with the
indenter force-displacement relation generated on the basis of ( 3.274 ), f i E and h i
are the measured force and displacement data of the indenter and the coordinates
of parameter vector p with the material parameters a k , l k and b k . According to
( 3.364 ), using the norm parameter m = 2, the following material parameters are
derived, meeting the requirements deduced in Sects. 3.4 and 3.4.8 such as
restrictions based on classical linear theory, the B AKER -E RICKSEN inequalities and
polyconvexity, Tables 4.2 and 4.3 (correlation between m i and b i see ( 3.275 ) ).
In the parameter sets listed above, stable (nominal) strain ranges according to
the D RUCKER stability criterion for the deformation modes of uniaxial compression
and uniaxial tension as well as simple shear are shown:

HR-Foam % VE-Foam %

Uniaxial compression Stable for all strains Stable for all strains

Uniaxial tension \340 \180

Simple shear \340 \170

4.2.1.4 Experiments II: Viscoelasticity (Creep)

Experimental Set Up: To mechanically characterize the time domain viscoelas-
ticity of the foam materials HR and VE (see ''Employed Elastomeric Materials'' in
Sect. 4.2.1.2 , Table 4.1 ), the following deformation modes at constant climatic
conditions (20C; 50% humidity) were performed under constant indenter weights
and gravity loading:

1. plate compression creep tests with a plate mass of 17.0 kg
2. hemispherical punch creep tests (punch: Ø48 mm, indenter mass: HR-foam:
27.0 kg, VE-foam: 3.3 kg

Test (1) encorporated foam specimen loading with a rigid plane plate with
variable weight. Plate displacement over time was recorded. Test (2) was primarily
used to verify the derived parameters under a multiaxial strain state, cf. Fig. 4.14 .
In

case of deviation from the experimental results of the hemispherical