Seat Structure Strength and Comfort Pressure Study

The problem

Seats must survive braking, bumps and misuse loads while staying comfortable on long rides. Strength is often checked only with a simple static load, and comfort is judged by sitting on a sample. Linking the frame stress, the foam stiffness and the pressure under the occupant gives designers a measured way to trade mass, strength and comfort.

Objectives

• Model a seat frame, base and foam as an assembly suitable for analysis.
• Check frame strength for occupant, braking and bump load cases with linear static studies.
• Estimate contact pressure on the foam with a nonlinear study using a hyperelastic material model.
• Compare predicted pressure with a pressure-mat measurement and study foam thickness and density options.

What you need

Key concepts

Hyperelastic material

A material model, such as Mooney-Rivlin or Ogden, for large elastic strains. Its constants are fitted to test data, and flexible foams are often approximated this way, though foam-specific models exist in other solvers.

Contact pressure

The normal stress between the occupant and the seat surface. Peak pressure and pressure distribution are common comfort measures.

Inertial load case

Braking or bumps accelerate the occupant relative to the seat, adding loads that are a multiple of body weight; the multiple is a design assumption to be justified.

Indenter

A simplified rigid body shaped like a seated buttock and thigh, used in place of a full human model to load the foam repeatably.

Step-by-step guide

1. 1

   Define the seat and load cases

   Choose an example scooter or car seat and write its requirements: occupant mass, an example 100 kg rider, and load cases such as static seated, a braking load as a stated multiple of g acting forwards, and a vertical bump load as a stated multiple of body weight. Note that vehicle seats have strength requirements in the current AIS and CMVR documents, which you should check for your vehicle class, and that your load factors are assumptions to be justified.

   Checkpoint: A load-case table with each factor, its source or justification and the acceptance criterion such as a minimum factor of safety.

2. 2

   Model the seat for analysis

   Model the frame with Weldments or Sheet Metal, the moulded base and the foam as a solid body. Remove cosmetic details that do not affect stiffness, such as logos and small fillets, in a Simulation configuration. Model a rigid indenter shaped from published buttock-thigh contours or a simple ellipsoid scaled to the rider. Keep bodies separate so you can assign different materials and contacts.

   Checkpoint: A Simulation configuration where every body is a clean solid or sheet, with no tiny faces or slivers that would spoil the mesh.

3. 3

   Hand-calculate the frame

   Before meshing, hand-calculate the bending stress in the main frame member under the seated and braking loads using simple beam theory. Treat the seat mounts as supports and the occupant load as distributed. Also estimate the deflection at the seat front. These numbers give a target to compare with the finite element results and catch unit or load errors early.

   Checkpoint: Hand-calculated stress and deflection for at least two load cases, written down with assumptions.

4. 4

   Run frame static studies

   In a static study, assign steel or aluminium from the material library to the frame, a filled polymer to the base and fix the mounting holes with bolt connectors or fixed hinges as appropriate. Apply each load case as a remote load or distributed force on split-line patches. Use Load Case Manager if available, or one study per case. Use beam elements for weldment members or solid mesh with mesh controls at welds.

   Checkpoint: Each load case solves, reaction forces sum to the applied load and the stress agrees with the hand calculation within a stated margin, for example 15 per cent.

5. 5

   Check mesh convergence

   Refine the mesh in steps, using mesh controls at the highest-stress regions, and plot peak stress at a chosen point away from sharp corners against element size. Identify any stress that keeps rising with refinement, which signals a singularity at a re-entrant corner or point restraint rather than a real stress. Report converged values or stresses read a small distance from singular points.

   Checkpoint: A convergence plot where the chosen stress changes by less than about 5 per cent between the last two meshes.

6. 6

   Fit the foam model

   Compress foam samples in a universal testing machine to large strain and record force and displacement. Convert to stress and stretch and fit Mooney-Rivlin constants in Excel or MATLAB, or use the curve-fitting option in the SOLIDWORKS material dialog if your version provides it. Compare the fitted curve with test data over the strain range seen in seating. Note that hysteresis and rate effects are ignored.

   Checkpoint: A fitted curve that matches the test stress-strain data within a stated error over the seating strain range.

7. 7

   Run the nonlinear comfort study

   Set up a nonlinear static study with large displacement on, the base fixed, the foam hyperelastic and the indenter rigid or very stiff, with a surface-to-surface contact set between indenter and foam. Push the indenter down with a force ramp to the occupant's share of body weight on the seat. Use a fine mesh in the contact zone and check the solver converges at each step.

   Checkpoint: The study converges to full load, and the indenter reaction force equals the applied force within 1 per cent.

8. 8

   Validate with a pressure mat

   Build or obtain the physical seat, place the pressure mat on it and record pressure maps for a few volunteers near the example mass, following your institute's ethics process for human participants. Compare measured peak pressure, mean pressure and contact area with the model. Expect differences, since the indenter is rigid and the body is not, and discuss where they come from.

   Checkpoint: A table comparing predicted and measured peak pressure and contact area, with the error stated and explained.

9. 9

   Study design variants

   Use configurations to vary foam thickness and fitted foam stiffness for two or three candidate foams, and frame gauge or tube size. Run the frame and comfort studies for each variant and record mass, minimum factor of safety, peak pressure and contact area. Plot mass against factor of safety and peak pressure to show trade-offs, and recommend a design. Keep the load cases and mesh settings identical across variants so differences come from the design only.

   Checkpoint: A variant table and trade-off plot, with a recommended design that meets the strength criterion and lowers peak pressure.

10. 10

    Report the study

    Report the load cases with justifications, hand calculations, convergence results, frame stress plots, foam fitting, contact pressure maps and validation comparison. State the limits clearly: quasi-static loads only, rigid indenter, no foam hysteresis, ageing or temperature effects. Recommend physical strength tests for the chosen design and further comfort trials with more participants. Include the foam test data, fitted constants and every study setting in an appendix so the work can be repeated by another student.

    Checkpoint: A report with traceable inputs, validation evidence and stated limits, suitable for a design review.

Testing & validation

• Frame stresses agree with hand calculations within a stated margin.
• Mesh convergence shown for frame stress and contact pressure.
• Foam model fitted to measured compression data.
• Predicted contact pressure compared with pressure-mat measurements.

Expected results

A validated seat analysis that links frame strength, foam properties and occupant contact pressure, with a trade-off between mass, strength and comfort and a recommended design.

Take it further

• Add a dynamic study with a measured seat-base acceleration to study vibration comfort.
• Replace the rigid indenter with a layered deformable buttock model.
• Compare two foam densities in long-duration sitting trials.

Viva and review questions

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