Optimization Potential of the Method - Preventive Biomechanics - page 289

Fig. 7.1 FE-Model of the base system

shorter simulation times, the model of the bedding system—just like the human
body model (B OSS -Model)—was generated as a half model. Furthermore, existing
curvatures at the T-section heads and the foam cover were modelled as right-angles
to achieve less elements and larger element edges (due to time-step considerations
in explicit integration). The foam cover at the sides and on the floor of the T-section
guiding were neglected since they do not provide any supporting function. The
foam filling between the T-sections was also neglected to avoid additional resis-
tance of foam and T-beams, which would also not contribute to system judgement.
The exterior T-beams were not modelled (they do not contribute to body support) to
further simplify the system. Under the assumption of insignificant deformations
compared to the soft foam and human tissue materials, the T-beams and their
guiding have been modelled as rigid material. Modelling of the air-mattress and the
air-filling was done using membrane and fluid elements, cf. Fig. 7.1 .

7.1.1.2 Model Variations of the Base System

To compare the effects of different systems on tissue, two alternative bedding
systems were generated: a three chamber system (3CS) and a profiled system (PS),
Fig. 7.2 . In contrast to the base system, the three chamber system consisted of
three unconnected identical chambers in which the air fillings did not ''commu-
nicate'', Fig. 7.2 a. The profiled system had wider T-section heads and additional
notches between the single T-sections and the foam material, Fig. 7.2 b.

7.1.1.3 Data for Membrane and Foam Material

The material parameters for the membrane material were derived via cyclic
(tensile) loading and unloading experiments with holding periods (1 h), following
the procedure outlined in Sect. 4.2.1.2 . Material samples with the dimensions
100 9 20 mm 2 were employed. Figure 7.3 a depicts the force-displacement data
obtained through tensile testing and the simulated model curve based on the O GDEN
model ( 3.272 ) . The parameters employed for the air filling of the mattress were
adopted from the literature (ambient pressure: 1.01325 bar, density: 1.2 kg/m 3 ).