Biomedical Engineering Reference
In-Depth Information
especially within the first 150 s of relaxation, and most distinctively at indenter
displacements C30 mm
Corresponding to the specific ramp displacement, force relaxation values at cut-
off times were 2.1, 6.7, 17.6 and 49.3 N. Oscillations in the experimental force-
relaxation curves were due to chest motion caused by breathing. The actual tissue
response is characterized by the lowest points corresponding to each load peak
(filtered line in Fig.
5.30
). The filtered data at the 40 mm ramp displacement was
used as a target function in the parameter optimization process.
Following guidelines by (Oyen et al. 2005), it can be shown that the normalized
(by the time at peak force t Pmax and the peak force P
max
) filtered experimental
responses, t
=
t Pmax and P
=
P
max
of Fig.
5.30
a have identical shapes (cf. Fig.
5.30
c).
This indicates that the relaxation behaviour of the examined tissue (compound)
material is strain independent and thus can be approached with a quasi-linear
viscoelasticity (QLV) theory (cf.
Sect. 5.2.5
).
As a result of the tissue compound behaviour, it can be concluded that the
relaxation behaviour of the single components, i.e. gluteal fat and (transversally
loaded) passive muscle, is also strain independent. Further indication of the
applicability of the QLV theory for use in soft (muscle) tissue mechanics is
provided in (Van Loocke et al. 2008).
5.2.5 Material Identification II: Viscoelasticity
FE-Buttock-Models: The viscoelastic characterization of gluteal fat and muscle
tissue and the identification of creep parameters is based on the models depicted in
Figs.
5.21
and
5.22
from volunteer M1. Since experimental creep and relaxation
approaches were established independently, generation of relaxation parameters were
based on more recent anatomical data of the same subject. The anatomy, i.e. fat and
muscle thickness as well as the physical condition of the subject changed between
both experiments. Fat and muscle thickness at the time of the creep experiment were
27 and 56 mm, respectively, (cf. Fig.
5.27
) and at the time of the relaxation experi-
ment 23 and 67 mm, respectively (cf. Fig.
5.29
). Conduction of material parameter
identification based on two different FE-models is thus indispensible, since, due to
different anatomies and physical condition, different material parameters are expected
(cf. guidelines given in the ''Critical Review'' part of
Sect. 5.2.3.2
).
Constitutive Equation for Viscoelastic Behavior: Linear viscoelastic material
behavior at finite strains was assumed for tissue creep and relaxation. The continuum
mechanical description is based on the model proposed by (Simo 1987) with the
and
3.2.6.5
).
