Investigations of seat pressure distributions at the human-seat
contact interface by analyses with a FE buttocks model
Citation for published version (APA):
Verver, M. M., Oomens, C. W. J., & Hoof, van, J. (2005). Investigations of seat pressure distributions at the human-seat contact interface by analyses with a FE buttocks model. In J. Middleton, & N. G. Shrive (Eds.), Computer Methods in Biomechanics & Biomedical Engineering - 5 [5S] FIRST Numerics Ltd.
Document status and date: Published: 01/01/2005
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INVESTIGATION OF SEAT PRESSURE DISTRIBUTIONS AT TE
HUMAN-SEAT CONTACT INTERFACE BY ANALYSES WITH A FE
BUTTOCKS MODEL
M.M. Verver1, C.W.J. Oomens2, J. van Hoof1
ABSTRACT
This paper describes a finite element model of the human buttocks suitable for analyses of seat pressure distributions. The model comprises a realistic three dimensional geometric description of the bony structures and the soft tissues. The bony structures are modelled rigid. The soft tissues have been lumped together. The skin has been modelled separately. The model is validated for seat pressure distributions based on volunteer tests on a rigid and a soft cushion. The model response agrees reasonably well with the volunteer responses in both conditions. A sensitivity study shows that the FE buttocks model is able to predict the influence of variations in geometrical and mechanical seat cushion properties on human-seat contact interaction.
1.
INTRODUCTION
The contact interaction between human and seat has been studied in literature for several purposes. For example, there is the clinical problem of pressure ulcer development with disabled people caused by the prolonged seating and inability to change posture. On the other hand, for the automotive industry comfort is an important issue and the seat pressure distribution at the human-seat contact interface is an important factor for this. For both areas the current standard of investigation is mainly based on experimental testing. This is time consuming and costly and restricted by technical and ethical limitations. Numerical models could act as a complementary tool for the current standard. In an early stage of the design process of a new seat, computer simulations will provide information about the seat pressure distributions at the contact interface between human and seat. Moreover, in simulations also the internal stress distribution in the soft tissues due to this contact interaction and the shear stresses at the contact interface can be analysed.
Accurate prediction of the maximum pressure in a pressure distribution, occurring under the ischial tuberosities, requires an accurate geometric description of the bony structures. Main limitations of models published in literature are the simplified geometry of the bony structures (4;10), the coarseness of the mesh (5;6) and the need to combine of several sources to create a complete model (13).
This paper presents a finite element model of the human buttocks suitable for prediction of seat pressure distributions. The geometry of the bony structures has been modelled in detail to be able to predict realistic maximum stresses in the contact interaction between human and seat. Geometry of bones, soft tissues and skin have been obtained from one Keywords: buttocks model, seat pressure distributions, finite element techniques
1TNO Automotive, PO box 6033, 2600 JA Delft, The Netherlands
source. The main focus of the present study is to investigate the sensitivity of the model to various human soft tissues and seat parameters.
2.
BUTTOCKS MODEL
A numerical model was developed using MADYMO v6.0, a simulation program that combines finite element and multi-body techniques (7). Fig. 1 shows the finite element buttocks model. The 3D shape of the bony structures and the skin of the model is based on a 78 year old male subject with a weight of 80 kg and standing and sitting height of 1.73 m and 0.92 m, respectively. The geometry of the iliac wings, sacrum, coccyx, femora and skin was used to define a mesh for the presented model. The skin and bony structures have been modelled with triangular shells. The lumped soft tissues have been modelled by tetrahedron elements (11).
The model consists of a set of rigid bodies for the pelvis-sacrum, the upper legs, lower legs and the upper body to which the finite elements parts and joints (hip, knee and upper body) have been connected. The FE parts of the iliac wings, the sacrum and the coccyx have been attached to the sacrum-pelvis body. The FE-parts of the femora have been attached to the bodies of the upper legs. The multi-body parts of the pelvis-sacrum and the upper legs have negligible mass, such that the mass of this body part is only determined by the properties of the finite element parts. The masses of the upper body and lower legs have been based on values of an average male (9).
In the model it has been assumed that the bony structures are rigid. The skin has been modelled by a linear elastic isotropic material model. Its stiffness has been based on the average of values reported in literature, E = 1.5 MPa, ρ = 1100 kg/m3, ν = 0.33 (5;6;10).
In the present study, the human soft tissues properties have been included by a hyperelastic isotropic Mooney Rivlin material model. The strain energy function is
defined by: 2 3 4 2 3 3 3 2 1 1(J 3) A (J 3) A(J 1) A(J 1) A W = − + − + − − + −
with J1, J2, J3 the invariants of the right Green strain tensor. The right
Cauchy-Green tensor is defined by: C=FT ⋅F, with F the deformation tensor. J1, J2 and J3 have
been defined as: J1 =trace(C), J
(
trace (C) trace(C ))
2 2 2 2 1 − = , J3 =det(C)
The second Piola-Kirchhof stress tensor is obtained by differentiating the strain energy function with respect to the right Cauchy-Green strain tensor:
C W S ∂ ∂ = 2 . The material
Fig. 1: The finite element model of the pelvis and thighs. Left: the bony structures. Middle: the bony structures and the human soft tissues. Right: the complete model -bony structures, human soft tissues and skin.
parameters A3 and A4 are dependent of A1 and A2: 2 1 3 2 1A A A = + , ) ( ) ( A ) ( A A
ν
ν
ν
2 1 2 5 11 2 5 2 1 4 − − + − =The values for A1, A2 and
ν
have been based on averages reported in literature: A1 =3300, A2 = 6700, ρ = 1000, ν = 0.49 (1;3;4;5;6;10).
3.
VALIDATION STUDY
For the validation study, volunteer experiments have been used on a rigid and soft cushion. The cushion has been represented by 8-node hexahedron (brick) elements. The material properties of the soft cushion used in the volunteer experiments have been implemented. Under all conditions, the FE buttocks model has been positioned just above the seat and allowed to settle into the seat cushion due to an applied gravitational field finding an equilibrium with the seat cushion. The average and maximum pressure, as well as the size of the contact area have been used as evaluation parameters in the present study.
Fig. 3a depicts the pressure distribution predicted by the buttocks model with the pressure distribution of the volunteer for the rigid surface condition. The FE buttocks model predicts, similar as recorded in the experiments, the maximum pressure located under the bony structures. Fig. 4 compares the values of the maximum and average pressure and contact area predicted by the FE buttocks model with the volunteer data. The average pressure approaches the experimental value well, while the contact area in the simulation is a little smaller than in the experiment (Fig. 4).
FE buttocks model FE buttocks model
Volunteer Volunteer
(a) Rigid surface (b) Soft surface
Fig. 3: The pressure distribution (Pa) predicted by the FE pelvis model and a human male for a rigid (a) and soft (b) cushion condition.
0 10 20 30 40 50 60 70 Rigid Soft kP a
Model - max. pressure Exp - max. pressure Model - ave. pressure Exp - ave. pressure
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 Rigid Soft C on ta ct a re a [m 2] Model Experiment
Fig. 4: Overview of the validation results: the values for maximum and average pressure (left) and the contact area (right) predicted by the FE buttocks model with the volunteer results.
In Fig. 3b the pressure distribution predicted by the buttocks model and the volunteer pressure distribution are compared for the soft cushion condition. The values for the maximum pressure and the average pressure predicted by the FE buttocks model are smaller than the values measured in the experiments but in the same order of magnitude (Fig. 4). The contact area agrees well with the contact area measured in the experiments. Generally, it can be summarised that the FE buttocks model shows reasonable correlation with the volunteer response for both the rigid and the soft cushion condition.
4.
SENSITIVITY STUDY
This section describes a sensitivity study with the FE buttocks model. The study investigates whether the output of the FE buttocks model is sensitive for variations in cushion parameters, that are key parameters for seat manufacturers in early stages of the design process. Car and seat manufacturers all over the world have been contacted and asked for their key parameters with respect to seat pressure distributions in the design of a new more comfortable car seat. Variations in cushion geometry and cushion stiffness were mentioned most and for that reason used in the sensitivity study.
The geometry of the cushion of a standard car seat has been used. The seat model was discretized, using 8 node hexahedron elements. The reference seat comprises six layers of these elements with an element size of 10 mm. Realistic material properties have been added. The study on seat pressure distributions considers the following variations:
• increase and decrease of the width and depth of the seat cushion with 50 mm • increase and decrease of the thickness with 20 mm
• increase and decrease of the foam stiffness and friction with a factor of 2
The average and maximum pressure, the size of the contact area, the SPD% (a measure for the seat cushion ability to uniformly distribute pressure (2)) and the shear stresses at the contact interface are the evaluation parameters used in the study.
Figure 4 show the values of the average pressure, the maximum pressure, the contact area, the maximum and minimum shear stresses and the SPD% for all conditions. All parameters have been normalised to the values of the reference seat simulation.
From all geometric variations the influence of the cushion thickness on the human-seat interaction the largest. The average pressure and the contact area are less sensitive to variations in cushion thickness. The influence of cushion width and depth was more limited than variations in cushion thickness.
0 20 40 60 80 100 120 140 160 Average
pressure Maximumpressure Contactarea Maximumshear stress Minimum shear stress SPD % Reference Width+ Width-0 20 40 60 80 100 120 140 160 Average pressure Maximum pressure Contact area Maximum shear stress Minimum shear stress SPD % Reference Stiffness+ Stiffness-0 20 40 60 80 100 120 140 160 Average pressure Maximum pressure Contact area Maximum shear stress Minimum shear stress SPD % Reference Depth+ Depth-0 20 40 60 80 100 120 140 160 Average pressure Maximum pressure Contact area Maximum shear stress Minimum shear stress SPD % Reference Friction+ Friction-0 20 40 60 80 100 120 140 160 Average pressure Maximum pressure Contact area Maximum shear stress Minimum shear stress SPD % Reference Thick+
Thick-Fig. 5: The results of the design study on seat pressure distributions: the values for the average pressure, the maximum pressure, the contact area and the maximum and minimum shear stresses have been expressed as a ratio of the results of the reference seat simulation
The foam stiffness has a major influence on the human-seat interaction. An increase or decrease in cushion stiffness results in a increase and a decrease in the average pressure. The maximum pressure increases due to the increase and decrease in cushion stiffness respectively. The contact area varies with respect to the reference simulation due to variations in cushion stiffness. The shear stresses increase due to an increase in cushion stiffness, while an decrease in cushion stiffness leads to a reduction of the shear stresses. The SPD% increases due to both an increase and a decrease in cushion stiffness.
The influence of the friction on the shear stress distribution at the contact interface between human and seat is limited.
5.
CONCLUSIONS
A finite element model of the human buttocks has been developed for prediction of seat pressure distributions at the contact interface between human and seat. The model has been based on an average seating male and the geometry was modelled in detail. The following can be concluded:
• A validation study based on volunteer experiments on a rigid and soft surface shows that the FE pelvis model is able to predict realistic seat pressure distributions. The
seat pressure distribution and the values for the maximum pressure, average pressure and contact area agree reasonably well with the volunteer data.
• The sensitivity study on seat pressure distributions showed that the FE buttocks model is able to predict the influence of variations in seat geometry and seat properties on the seat pressure distribution at the contact interface between human and seat.
6.
REFERENCES
1. Adams D., Morgan, G.B., Nghi, T., Salloum, M.J. O’Bannon, T. (1999), Creating a biofidelic seating surrogate. SAE Conference 1999, SAE no 1999-01-0627
2. Ahmadian et al., 2002
3. Bader, D.L., Hawken, M.B. (1990), Ischial pressure distribution under the seated person. In: Pressure Sores – Clinical Practice and Scientific Approach, Edited by D.L. Bader, pp. 223-233.
4. Chow , W.W., Odell, E.I. (1978), Deformations and stresses in soft body tissues of a sitting person. Journal of Biomechanical Engineering, Vol. 100, pp. 79-87.
5. Hoof, J. van, Happee, R., Meijer, R., Bours, R. (2001), MADYMO FE human body model for automotive impact conditions. Proceedings of the JSAE Spring Convention 2001, No. 20015336.
6. Lizee, E., Robin, S., Song, E., Bertholon, N., Le Coz, J.Y., Besnault, B., Lavaste, F. (1998), Development of a 3D finite element model of the human body. Proceedings of the 42nd Stapp Car Crash Conference 1998, SAE no 983152.
7. Moens & Horváth (2002), Using finite elements model of the human body for shape optimization of seats: optimization material properties. Proceedings of the International Design Conference – Design 2002.
8. TNO Automotive (2001), MADYMO 6.0 Theory Manual.
9. TNO Automotive (2001). Manual: MADYMO Human body models.
10. Todd, B.A., Thacker, J.G. (1994), Three-dimensional computer model of the human buttocks in vivo. Journal of Rehabilitation Research and Development, Vol. 31(2), pp. 111-119.
11. Verver, M.M. (2004). Numerical tools for comfort analyses of automotive seating. PhD-thesis Eindhoven University of Technology, ISBN 90-386-2855-2
12. Yamada, H. (1970), Strength of biological materials. Edited by F. Gaynor Evans. Robert E. Krieger Publishing Company (NewYork), ISBN #0-88275-119-0
