Interface Pressures and Deformations Calculated From FE Analyses

In each of the analyses described in the last chapter, the interface pressures and deformations when the weight of the body was supported through the seat were calculated. For the models with the ischial tuberosity, it was assumed that half the weight transferred through the buttocks during sitting, was transferred through the seats in the FE analyses (166-9 N). For the analyses with the sacrum, the weight transferred through the seat sections was estimated from interface pressure measurements on a planar seat (0 38 N).

Approximate pressure distributions between the body and the seat surfaces in the FE models were calculated from the position and forces on the nodes at the interface. The normal forces on nodes at the interface were assumed to be associated with half of the surface area of each of the two adjacent elements. In the axi-symmetric analyses, the forces on the central axis were calculated to be zero. The formulation of STIF84 elements was such that the forces through the

mid-side nodes were less than those through the corner nodes. Therefore the pressures associated with the nodes at the mid-side and corners of the elements were calculated and averaged in order to obtain smoothly varying distributions. A Stineman interpolation was made between the non-zero pressure results in analyses 1 to 5 (Cricket Graph Macintosh Software, Reference Manual). The data was not extrapolated to the central axis, due to the limited load information in this region. The calculated pressure distributions are shown in Figure 36.

300 , ANALYSES 1 to 6 50 -, ANALYSES 6 to 10 Model 1 Model 2 Model 3 Model 4 Model 5 Model 6 i 150 9 20 - Model 6 Model 7 Model 8 ™ 100 Model 9 Model 10 100 120

Distance from axis of revolution (mm) Distance from axis of revolution (mm)

150

ANALYSES 1 ,7 ,1 1 ,1 2 & 13

Model 1 (planar seat)

Model 7 (seat depth =105 mm) Model 11 (seat depth=80 mm) Model 12 (seat depth =60 mm) Model 13 (seat depth=40 mm)

20 40 120

FIGURE 36

Interface pressure distributions when a vertical load of 187 N was supported

through seat in FE analyses In Sections 7.2 and 7.3 Distance from axis of revolution (mm)

In analyses 1 to 5 the maximum pressures were under the ischial tuberosity and they then decreased with distance from the axis of revolution. The peak pressure was very much smaller

in the analyses with contoured seat shapes, compared to the analysis with a planar seat. In analyses 6 to 10 the pressure distributions were very similar. The interface pressures increased with distance from the central axis. The distributions went from zero on the axis to near-constant values of about 40 mmHg (5 3 KPa) over most of the interface. The pressures further increased by about 10 mmHg before the interface boundary was reached. Thus the ring of peak pressure was around the boundary of the interface. In analyses 6 to 9, the trough in the pressure distribution under the ischial tuberosity was wider, when the seat was locally deepened along the axis of revolution.

Compared to the results with shallow seats, the interface pressure distributions were more evenly distributed over the interface and the peak pressures were lower in the analyses with deeply shaped seats. The maximum pressure values ranged from under 50 mmHg (6 7 KPa) when the seat depth was 105 mm (analysis 7) to over 80 mmHg (10 7 KPa) when the seat depth was 40 mm (analysis 13).

In the plane strain analyses of the central section through the buttock, the maximum interface pressures were between 36 mmHg (analysis B) and 54 mmHg (analysis E). The pressure peak was at or close to the tip of the sacrum in analyses A, 0, and F. The interface pressure peak was anterior to the tip of the sacrum in analyses B and D. In analysis E there were two pressure peaks, one close to the sacrum and one located at a more anterior position along the interface. The calculated pressure distributions are shown in Figure 37.

The soft tissue surface deformations from each analysis were compared. The outlines were graphically aligned such that the bony interfaces were superimposed. In general the body surfaces moved inward in the areas where there was contact between the body and the seat, and moved outward in the areas outside the boundary of contact. When contact was locally lost due to a depression in the seat, the soft tissue surface moved either inwards or outwards by a small amount in these regions. The soft tissue surface deformations from analyses 1 to 13 are shown in Appendix 11. The soft tissue surface deformations were largest along the axis of revolution in analyses 1 to 5. They ranged from over 48 mm in analysis 1 to less than 22 mm in analysis 5.

When the seats were shaped similarly to the soft tissue surface (6 to 10), the body deformations were a lot smaller than in analyses 1 to 5. For analysis 6 the maximum displacement of a node at the surface of the body (relative to the bone-soft tissue interface) was 13 5 mm. For analyses 6 to 10, the maximum inward displacement was located lateral to the central axis. There were lower levels of deformation at the tissue surface under the ischial tuberosity, when the depth of the seat along the axis of revolution was increased (analyses 6 to 9). The deformations were smaller and more evenly distributed when the seats were deeply contoured. For example, the maximum surface deformation in the area of contact was only 15 mm in model 7.

Analysis A

2 40- 330-

cJistanpe along Interface (mm|

'V,

Max. deformation of node In contact with seat =11.2 mm

Analysis B

Total vertical load = 0.40 N

Analysis D o> 50- E 40- S 30- Q. 20- Ô 50 100 distance alorig interface

Q .

I

djstance along interface (mm)

Max. deformation of node in contact with seat = 8.0 mm Total vertical load = 0.38 N

Max. deformation of node in contact with seat = 9.8 mm Total vertical load = 0.38N

Analysis E 60 - O) 50 - 40 - 30 - 20 - 10 - 50 100 , 150 Î along interface (mm) ince

Max. deformation of node in

Analysis C 60-

!

i

I

Max. deformation of node in contact with seat = 14.4 mm Total vertical load = 0.38 N

Analysis F o> 50-1 40- 30- 20- 10- Oj 50 100 distance; jalong interf^cç (mm)

Max. deformation of node in contact with seat = 8.2 mm

Total vertical load = 0.35 N

contact with seat =10.1 mm Total vertical load = 0.37 N

FIGURE 37

Interface pressures and surface deformations on sagittal section through sacrum from FE anaiyses in Section 7.4

From the analyses of the central section through the buttock, the maximum displacement of nodes in contact with the seat was between 8 0 mm (analysis B) and 14 4 mm (analysis C). For most of the analyses, the maximum displacements were located near the mid-point of the body- seat interface. The soft tissue surface deformations from each of the plane strain analyses are shown In Figure 37.

In the next section, the interface quantities which have been described in this section were related to the stress state in the soft material, to see whether it might be possible to gain information on the stress distribution in the soft tissues from outside the body.

8.3 Correlation in FE Analyses between Interface Data and the Stress State