Biomedical Engineering Reference

In-Depth Information

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
.

model parameter N = 2 is performed via the optimization routine described in

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

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