Optimization Potential of the Method - Preventive Biomechanics - page 320

Fig. 7.41 Absolute direct (modulus) and shear tissue stress at the ischium subject to various
foam material stiffness (original stiffness of foam RD-59 scaled by factor 0.25, 2.0 and 5.0)
plotted against seat cushion thickness
reported by (Ragan et al. 2002). Furthermore, it can be observed from Fig. 7.40
that for the investigated foam material RD-59, both stress quantities remain nearly
constant at a cushion thickness of approx. s = 100 mm, and direct stress has its
maximum where shear stress exhibits its lowest values. The thicker the cushion the
more body surface achieves contact, supporting and redistributing the body load,
whereby tissue loading directly underneath the ischium is lowered.
In addition, the widening contact area caused mounting involvement of the
lateral buttock tissue which was increasingly displaced in a vertical direction,
affecting tissue shear stress magnitude. When comparing different foams with
varying stiffness to judge the influence of long-term foam stiffness on tissue stress,
direct/shear stress relation becomes more complicated. In this context, long-term
stiffness of commercially employed soft foam material RD-59 was scaled by
factors of 0.25, 2.0 and 5.0 and appropriate material parameters were optimized to
obtain four different foam materials (cf. Sects. 7.1 , 7.1.1.3 , Fig. 7.4 ). Comparison
of tissue stress based on the resulting generic foam materials is shown in Fig. 7.41 .
In Fig. 7.42 , direct stress is plotted against shear stress values as given in
Fig. 7.41 where the cushion thickness was eliminated. Interestingly, the direct
vs. shear stress relations are nearly linear over the investigated cushion thickness
range for the reference foam material (RD-59), as well for the scaled generic
materials (excluding the initial horizontal slope of foam stiffness scaled with factor
0.25), Fig. 7.42 . In addition, the gradients of the single curves are nearly equal.
This indicates that, irrespective of the particular material, the ratio of the change of
direct stress and (due to that change) the resulting change in shear stress may be
almost constant.