3. Oblique Pole Impact Angle Sweep: Injury Response Envelope

Why an Angle Sweep Defines the Envelope

A single oblique pole impact at one angle tells you what happens at that angle. It does not tell you where the far-side occupant is worst off, nor how quickly the response degrades as the impact vector rotates. The injury response envelope is the mapping from impact angle to injury metric, and its shape is what matters for restraint design: a restraint tuned at 30° may be marginal at 20° or 40° if the envelope is non-monotonic. This case builds that envelope by sweeping the pole impact angle from 15° to 45° in 5° increments, holding all other model parameters fixed, and extracting far-side head, chest, and pelvis metrics at each point.

The sweep is deliberately narrow. Below roughly 15° the loading is close to a pure lateral pole strike and the far-side occupant is driven into the near-side interior; above roughly 45° the impact becomes dominated by longitudinal deceleration and the pole contact geometry changes character. The 15°–45° band is where oblique far-side loading is most sensitive to small angle changes, which is exactly the region a restraint calibration must survive.

Sweep Definition and Fixed Parameters

To make the envelope interpretable, every parameter except the impact angle must be frozen. The table below is the sweep control set; any deviation from these values invalidates cross-angle comparison.

Executing the Sweep

The sweep is a scripted loop over the angle parameter, with each run writing its metrics to a separate result file. The key is that the angle is applied as a rotation of the impact velocity vector, not as a repositioning of the pole, so the contact geometry relative to the occupant stays comparable.

# Parametric angle sweep driver (pseudo-runnable structure)
angles = [15, 20, 25, 30, 35, 40, 45]   # degrees, lateral-to-longitudinal
results = []

for theta in angles:
    deck = load_template("pole_impact_baseline.k")
    # Rotate impact velocity vector about vertical axis by theta
    set_velocity_vector(deck, speed=8.89, angle_deg=theta, axis="Z")
    # Keep pole, seat, belt, and occupant definitions untouched
    write_deck(deck, f"run_theta_{theta:02d}.k")
    run_solver(f"run_theta_{theta:02d}.k", output=f"out_theta_{theta:02d}")

    metrics = extract_metrics(f"out_theta_{theta:02d}")
    results.append({
        "theta": theta,
        "HIC36": metrics["HIC36"],
        "head_amax_g": metrics["head_amax_g"],
        "chest_defl_mm": metrics["chest_defl_mm"],
        "chest_vc_ms": metrics["chest_vc_ms"],
        "pelvis_force_kN": metrics["pelvis_force_kN"],
    })

write_csv("angle_sweep_envelope.csv", results)

Two implementation details decide whether the envelope is trustworthy. First, the velocity vector rotation must be applied in the vehicle coordinate frame, and the pole must remain fixed in that frame; rotating the pole instead changes which body region contacts first and contaminates the angle effect with a geometry effect. Second, the metric extraction window must be identical across runs — a fixed simulation end time and a fixed filtering class (e.g., CFC 1000 for HIC, CFC 180 for chest) — otherwise apparent envelope curvature can be an artifact of post-processing rather than occupant response.

Reading the Envelope

The sweep produces a set of curves, one per metric, plotted against impact angle. The shape of each curve is the deliverable. A monotonic rise toward 45° indicates that longitudinal loading dominates and the restraint should be biased toward the high-angle end. A peak near 25°–30° with a fall-off on both sides indicates a resonance-like interaction between the occupant's lateral excursion and the pole contact timing, and that peak is the critical angle the restraint must be validated against.

The figure below shows the envelope structure the sweep is meant to reveal: the angle axis, the three metric families, and the critical-angle identification logic that converts raw curves into a design constraint.

The envelope above is illustrative of the structure the sweep is designed to expose, not a measured result. In an actual run, the head curve's peak location is the primary output: it identifies the critical impact angle. The chest curve typically peaks later and broader because chest deflection integrates belt and pole loading over a longer window. The pelvis curve often rises monotonically toward 45° because longitudinal loading drives the occupant forward into the belt path, increasing lap-belt force. When the three peaks do not coincide, the critical angle is the one that produces the first metric to exceed its injury assessment reference value, not the angle of the highest single peak.

Identifying the Critical Angle

Convert the envelope into a decision by comparing each curve against its reference value at every angle. The critical angle is the smallest θ at which any metric crosses its threshold, because that is the first angle at which the restraint is no longer adequate. A compact way to record this is a pass/fail matrix keyed by angle:

Verification and Common Failure Symptoms

Before trusting the envelope, verify three things. First, confirm the velocity vector rotation is applied about the correct axis and that the pole position is unchanged between runs — a drifting contact point shows up as an implausible jump between adjacent angles. Second, confirm the metric extraction window and filter class are identical across all seven runs; a mismatch produces a stepped or jagged envelope that does not reflect occupant physics. Third, confirm the occupant remains seated and belted through the event at every angle; a belt slip or occupant excursion anomaly at one angle is a model defect, not an envelope feature.

Typical failure symptoms and their causes: an envelope that is flat across all angles usually means the angle parameter never reached the solver (the template was reloaded after the rotation was applied); a single outlier point usually means a contact definition changed at that angle; and a head curve that peaks at 15° rather than mid-band usually means the pole offset drifted, so the occupant contacts the pole earlier at low angles. Each of these is diagnosable from the envelope shape alone, which is why the sweep is run before any restraint optimization.