4. Correlate Knee Bolster and Lower Extremity Loading in Frontal Impact
Correlation Target: Femur Axial Force and Knee Slider Travel
In a frontal impact sled or full-vehicle test, the driver's lower extremities load the knee bolster through two distinct paths: the femur axial force transmitted along the leg into the knee slider mechanism, and the bolster's own deformation as foam and substrate crush under knee contact. The CAE correlation task is to make the simulated femur axial force–time history and the knee slider displacement–time history match the physical test within a defined tolerance, typically ±10% on peak force and ±5 mm on peak slider travel, with the timing of the peak within ±3 ms. These two channels are coupled: if the bolster is too stiff, the femur force peaks early and high while slider travel is suppressed; if the foam crushes too easily, the femur force is under-predicted and the slider bottoms out later than observed.
Decomposing the Load Path
Before touching any material card, map the load path in the model. The knee slider is a rigid or deformable body constrained to translate along a defined axis, with a spring or force–displacement curve representing the column-mounted energy absorber. The bolster assembly sits between the knee impactor (or dummy knee) and the slider. Three calibration levers dominate the correlation:
- Bolster substrate stiffness — the structural shell behind the foam, usually modeled as a shell or solid with an elastic–plastic material. Its stiffness sets the initial force rise rate.
- Foam crush behavior — the low-density energy-absorbing foam, modeled with a crushable foam material (e.g., *MAT_CRUSHABLE_FOAM or *MAT_LOW_DENSITY_FOAM) whose stress–strain curve has a plateau stress and densification strain.
- Contact friction — the coefficient of friction between the knee/impactor surface and the bolster face, and between the slider and its guide. Friction delays slider travel and raises the measured femur force for a given penetration.
Calibration Sequence and Parameter Bounds
Work the levers in order of sensitivity, not alphabetically. The foam plateau stress controls the mid-phase force level and the onset of slider travel; the substrate stiffness controls the initial rise; friction controls the residual offset between simulated and measured slider displacement. A practical sequence:
- Match the foam plateau stress first. Extract the plateau stress from the test's femur force plateau (force divided by knee contact area). Adjust the foam stress–strain curve's plateau value within ±15% of the nominal datasheet value. If the simulated plateau is high and slider travel is short, reduce plateau stress before touching friction.
- Tune substrate stiffness second. If the initial force rise (first 5–8 ms) is too steep, reduce the substrate's elastic modulus or yield stress. Keep the substrate within the supplier's material card range; do not invent stiffness outside the validated envelope.
- Adjust friction last. Friction is the least physically certain parameter. Set μ_knee–bolster in the range 0.2–0.5 and μ_slider in 0.05–0.15. If slider travel is under-predicted after foam and substrate are matched, reduce μ_slider; if femur force is under-predicted while travel matches, increase μ_knee–bolster.
Worked Example: Reading the Mismatch
Suppose the test shows a femur force peak of 4.2 kN at 28 ms and a slider travel of 62 mm, while the baseline CAE gives 5.1 kN at 24 ms and 48 mm. The pattern — high force, early peak, short travel — points to an over-stiff bolster. The diagnosis order:
| Symptom | Likely cause | Corrective action |
|---|---|---|
| Force peak high, travel short, peak early | Foam plateau too high or substrate too stiff | Reduce foam plateau stress 10–15%; re-run |
| Force peak low, travel long, peak late | Foam crushes too easily | Raise plateau stress; check densification strain |
| Force matches, travel short | Slider friction too high | Reduce μ_slider toward 0.05 |
| Travel matches, force low | Knee–bolster friction too low | Raise μ_knee–bolster toward 0.5 |
| Both channels oscillate | Contact stiffness or time step | Check contact penalty, reduce time step |
After reducing the foam plateau stress by 12% and μ_slider from 0.12 to 0.07, a typical outcome is a force peak near 4.3 kN at 27 ms and slider travel near 60 mm — within the ±10% force and ±5 mm travel tolerances. These numbers are illustrative; the actual values depend on the specific bolster geometry and foam grade.
Verification and Injury Criteria Check
Once both channels correlate, verify the lower extremity injury criteria. The femur force criterion (FFC) is the peak axial femur force normalized to the 10 kN reference; confirm the simulated FFC is within the same tolerance band as the test. Also check knee slider bottom-out: if the slider reaches its mechanical stop in CAE but not in test (or vice versa), the force trace will show a sharp spike that no material tuning can remove — this indicates a geometry or stroke-length discrepancy, not a stiffness problem. Document the final calibrated parameters (foam plateau stress, substrate modulus, μ values) and the resulting force and travel peaks in a correlation summary so the next build inherits a traceable baseline.