Human Body Models: BOSS-Models - Preventive Biomechanics - page 194

Fig. 5.22 FE models: a fat model, and b muscle/bone model with prescribed nodes

(highlighted), c compound model including fat, muscle, bone and indenter

boundary surface of all deformation steps and the position of the indentation axis
(see Fig. 5.22 a).

FE-Compound Model: The reconstructed surface data was meshed using the
H YPERMESH (A LTAIR , Michigan/USA) pre-processor. Both tissue components, skin/
fat and muscle, were modelled with second-order tetrahedral continuum elements,
Fig. 5.22 a and b. The nodes in the fat-muscle-interface as well the muscle-bone-
interface were rigidly connected. Thus, relative motion between the components
was not assumed since shear stress was intended to be minimized by orienting the
indentation axis accordingly, as described in Sect. 5.2.2.1 . The bone structure as well
as the indenter head were modelled as rigid bodies with the bone structure fixed in
space and the indenter fully constrained, except in the indentation direction.

Skin/fat Model and Muscle Model: To optimize the material parameters for
skin/fat and muscle, the compound model was split into one model containing the
skin/fat layer and another model containing the muscle layer with pelvic bone.
To simulate both models separately, adequate boundary conditions were applied to
both models. This was implemented by prescribing the nodes of the fat-muscle-
boundary (Fig. 5.21 b). That means that the top surface nodes in the muscle-model
(Fig. 5.22 b) and the bottom surface nodes of the fat-model (Fig. 5.22 a) meet all
reconstructed fat-muscle-boundary layers for all deformation steps. The indenter
head in the skin/fat model was moved accordingly. The assumption made hereby,
is that the node prescription between the single deformed layers ise normal to the
particular deformed surface. Boundary conditions for the fat-muscle-boundary, i.e.
the node displacements of the fat-muscle-boundary nodes were derived for the
global directions via a coded routine.

5.2.3.2 Parameter Identification

Constitutive Equation: According to previous outlines regarding mechanical fat
and muscle tissue property evaluation, both tissues exhibit distinct viscoelastic
material behaviour, cf. Fig. 5.15 b and d. The following section is concerned with
the determination of the mechanical tissue properties of the steady state or equi-
librium elasticity, i.e. long-term behaviour after relaxation. Hereby, for the con-
tinuum mechanical description of the tissue material, the phenomenological