Biomedical Engineering Reference
Fig. 7.6 a Stress path 1 and
b stress path 2 indicated by
region of maximum shear stress S 23 at the tailbone, ending at the coccyx, Fig. 7.6 a.
Path 2 begins at skin level and progresses through the tissue to the region of
maximum normal stress S 33 at the tailbone, Fig. 7.6 b. The stress components were
chosen based on the fact that both S 23 and S 33 (pointing in gravitational direction),
were at least one order of magnitude larger than other normal and shear stress
In Fig. 7.7 the tissue stress component S 33 is shown along both paths with
factorized foam material F0.25 at varying membrane pressure. In Fig. 7.8 the
tissue stress component S 23 is shown along path 1 for materials F0.25 and F0.5 at
varying membrane pressure. Along path 1 maximum stress (S 33 ) occurred at the
fat/muscle interface (pressure stress) and at the tailbone (tensile stress). Further-
more, S 33 increases with increasing air mattress internal pressure.
Stress path 2 reflects moderate S 33 -values at skin level, and increasing stress
towards the (tail) bone surface. Shear stress S 23 was maximum at the fat/bone
interface and decreased towards the path end. Similar to normal stress, S 23
increased with increasing air mattress internal pressure. Stress maxima occurred
for both stress components at the fat/bone interface. These stress characteristics
were apparent with all the previously introduced (factorized) foam materials.
Analysis shows that an air mattress internal pressure of 2.8 mbar represents a
minimum mattress pressure value where the T-section bars deeply intrude into the
membrane surface. Membrane material failure is thus likely to occur.
The simulations involving bedding system PS showed unrealistically large
deformation in the upper profiled region, which would likely lead to material
failure. Hence, not all possible material combinations were simulated, only those
with realistic outcome.