1. Build a Baseline Abdomen-Pelvis FE Model for Submarining Assessment

Why a Baseline Model Must Be Built Before Any Countermeasure

Submarining is a kinematic failure mode: the occupant's pelvis slides forward and downward beneath the lap belt, allowing the belt to migrate from the iliac crest onto the compliant abdomen. Every countermeasure study — anti-submarining ramps, seat pan geometry changes, pretensioner strategies, belt-anchorage relocation — is only meaningful if it is compared against a baseline that already exhibits this failure. The task in this case is therefore not to optimize anything, but to assemble a seated-occupant abdomen-pelvis finite element (FE) model, route the lap and shoulder belts correctly, include a deformable seat cushion, and confirm that the baseline loading produces a measurable submarining response.

Two properties define a usable baseline. First, it must be validated in the sense that the pelvis, belt, and cushion interact through a stable contact chain with no numerical artifacts dominating the response. Second, it must be discriminating: the baseline must produce a clear, quantifiable submarining signature so that a later countermeasure can be shown to reduce it. A model that is too stiff, over-constrained, or frictionless will either suppress submarining entirely or produce a response driven by penetration and hourglassing rather than by kinematics.

Model Assembly: Objects, Interfaces, and Units

The baseline is assembled from four functional groups. Keep them in separate include files so that countermeasure variants can be swapped without touching the occupant mesh.

Use a consistent unit system throughout — millimetre, tonne, second, and newton (mm-t-s-N) is the conventional choice for occupant safety models, giving stress in MPa and density in t/mm³. Mixing grams with millimetres is the single most common cause of a "model that runs but behaves like rubber."

Belt Routing and Contact Chain

The lap belt must pass over the anterior superior iliac spines, not over the mid-abdomen. Route the belt through explicit routing nodes placed just outboard of each ASIS, then to the buckle and anchor points. The initial belt-to-pelvis gap should be small but non-zero (roughly 1–3 mm) so that the belt engages the pelvis early in the pulse rather than after a free-flight phase.

Define the contact chain explicitly and in the correct order of importance:

• Belt–pelvis: the load path that must remain engaged for the occupant to be restrained. Friction coefficient here is the dominant parameter for submarining onset.
• Belt–abdomen: the path that opens once submarining begins; it must be soft enough to allow migration.
• Pelvis–cushion: controls sink-in and the forward slide that precedes submarining.
• Belt–cushion / belt–seat frame: prevents the belt from passing through the seat; a missing contact here produces non-physical belt travel.

Baseline Load Case and Expected Response

Apply a frontal deceleration pulse to the seat and floor — a simplified half-sine or a standard sled pulse is sufficient for a baseline. The occupant is restrained by the belt only; no pretensioner and no airbag, so that the submarining mechanism is isolated. Gravity and an initial seated posture (torso reclined a few degrees, thighs horizontal) complete the setup.

The baseline is expected to show the following sequence, which is the signature you will later quantify:

1. Belt engages the pelvis and the lap-belt force rises.
2. Pelvis rotates forward (posterior tilt) and slides forward on the cushion.
3. The lap belt migrates from the iliac crest toward the abdomen; belt–abdomen contact force grows while belt–pelvis force falls.
4. Lap-belt force drops as the belt loses purchase on the pelvis.

These four observable events are exactly what the next case will turn into detection metrics, so the baseline must make all four visible in the output. If any of them is absent, the model is not yet a valid baseline.

Verification: Confirming the Baseline Is Usable

Run the baseline and check the following before accepting it. These are pass/fail gates, not suggestions.

• Energy balance: hourglass and contact energy should remain a small fraction of total energy. A large hourglass component means the pelvis or cushion mesh is too coarse or the penalty stiffness is wrong.
• Belt penetration: no belt node should pass through the pelvis shell. Inspect the belt–pelvis gap over time; a negative gap indicates tunneling.
• Contact force history: belt–pelvis force should rise, then fall as belt–abdomen force rises. If belt–pelvis force never falls, the model is over-constrained and will not show submarining.
• Pelvis kinematics: record pelvis forward displacement and rotation. A baseline that shows no forward slide cannot produce submarining.
• Lap-belt force drop: the force–time curve should show a measurable drop after the initial peak. This is the primary discriminator for later countermeasure comparison.

Common Failure Symptoms and Fixes

Baseline Acceptance Checklist

Use this checklist to sign off the model before handing it to countermeasure studies. Each item is a concrete, checkable condition.

Once all seven items pass, the model is a valid baseline: it restrains the occupant through the intended load path, it exhibits the submarining mechanism under baseline loading, and it produces the force and kinematic histories that the next case will convert into quantitative detection metrics.