4. SAB design inputs

Side airbag (SAB) design is an exercise in constrained optimization. The inflatable cushion must deploy through a defined gap, at a defined time, into a space that is already occupied by a seated human body, a seat frame, a door trim panel, and a dozen other components. Before a single mesh is generated in the CAE environment, the engineer must assemble a complete and consistent set of design inputs. This chapter defines those inputs, explains how each one constrains the SAB design space, and shows how they are translated into a CAE-ready model.

Vehicle Package Constraints

The vehicle package defines the physical envelope in which the SAB must operate. The most critical package dimensions are the lateral clearance between the seat and the door trim, the vertical position of the seat relative to the door beltline, and the longitudinal position of the B-pillar relative to the occupant's H-point.

In a typical sedan, the lateral gap between the outboard seat bolster and the door trim inner surface may be only 80–120 mm. This gap must accommodate the folded airbag module, the deployment path, and the inflated cushion thickness. If the gap is too small, the cushion will be squeezed against the trim during deployment, increasing the risk of trim detachment or cushion rupture. The CAE engineer must therefore extract the minimum lateral clearance from the CAD assembly and use it as a hard constraint in the airbag folding and inflation model.

Another key package constraint is the door trim material and its attachment strategy. A rigid trim panel with metal clips will behave differently from a soft trim with plastic retaining tabs. The CAE model must represent the trim as a deformable body with appropriate material properties and boundary conditions, not as a rigid wall. This is because the trim's compliance directly affects the cushion's deployment trajectory and the resulting occupant loading.

Seat Structure and Trim Interfaces

The seat structure provides the reaction surface for the SAB module. In a seat-mounted SAB, the module is typically attached to the outboard side of the seat back frame, often on the recliner mechanism or a dedicated bracket. The seat frame stiffness and the module mounting points determine how the reaction force is transmitted during deployment.

For CAE, the seat frame is usually represented as a combination of shell elements for sheet metal and solid elements for castings or welds. The module mounting bracket is modeled with rigid elements or a tied contact interface. The seat foam and trim cover are modeled with low-density foam material models (e.g., *MAT_LOW_DENSITY_FOAM in LS-DYNA) to capture the cushioning effect during occupant interaction.

The trim interface is equally important. The seat trim cover has a tear seam that must open at a specific pressure. In the CAE model, this is often represented by a seam line with reduced element strength or a tied contact that releases at a defined force. The tear seam location and strength are design inputs that come from the trim supplier and must be validated against the airbag deployment pressure curve.

A common mistake in CAE setup is to model the seat trim as a continuous shell without a tear seam. This leads to unrealistic pressure buildup and an over-prediction of deployment force. The correct approach is to define the tear seam as a separate part with a failure criterion, or to use a *CONSTRAINED_TIE_BREAK keyword with a force-based release condition.

Occupant Packaging Data

Occupant packaging data defines the position and posture of the ATD (Anthropomorphic Test Device) or human body model relative to the SAB. The primary inputs are the H-point coordinates, the torso angle, the seat back angle, and the occupant size percentile (e.g., 5th percentile female, 50th percentile male, 95th percentile male).

For side impact CAE, the occupant is typically positioned according to the FMVSS 214 or Euro NCAP side impact test protocol. The H-point is set to the seat track position specified in the test procedure, and the torso angle is adjusted to match the seat back angle. The lateral gap between the occupant's thorax and the door trim is then measured and used as a key input for the airbag inflation timing.

The occupant packaging data also includes the arm position. In side impact, the occupant's arm may be positioned between the thorax and the door trim, which can interfere with airbag deployment. The CAE model must include the arm as a deformable body with appropriate joint stiffness. The arm position is typically defined by the test protocol or by the vehicle manufacturer's internal standard.

Translating Inputs into a CAE Model

The process of building a SAB CAE model from these inputs follows a logical sequence. First, the vehicle package and seat structure are imported as CAD geometry and meshed. The door trim is meshed with shell elements and assigned material properties. The seat frame is meshed with a combination of shells and solids. The occupant is positioned using the H-point and torso angle data.

Next, the airbag module is modeled. The folded airbag geometry is created using a folding simulation or a predefined fold pattern. The inflator is represented by a *AIRBAG_PARTICLE or *AIRBAG_HYBRID keyword with a mass flow rate curve. The module housing is modeled as a rigid body or a deformable shell, depending on its material.

The contact interfaces are then defined. The airbag-to-occupant contact uses a *CONTACT_AUTOMATIC_SURFACE_TO_SURFACE definition with a friction coefficient of 0.2–0.3. The airbag-to-trim contact uses a similar definition but with a lower friction coefficient to account for the sliding of the cushion against the trim. The tear seam is defined using a *CONSTRAINED_TIE_BREAK with a force-based release criterion.

Finally, the boundary conditions and initial velocity are applied. The vehicle is given an initial lateral velocity corresponding to the side impact test speed (e.g., 50 km/h for FMVSS 214). The occupant is given an initial velocity matching the vehicle. The simulation is run for 120–150 ms, which is sufficient to capture the airbag deployment and occupant interaction.

Practical Considerations and Common Pitfalls

One of the most frequent issues in SAB CAE is inconsistent units. Package dimensions may be in millimeters, material properties in MPa, and inflator mass flow rates in kg/s. A unit mismatch can cause the simulation to run but produce meaningless results. Always verify that the unit system is consistent before running the model.

Another common pitfall is neglecting the seat trim's tear seam. As mentioned earlier, a continuous trim model will over-constrain the airbag and produce an unrealistic pressure spike. The tear seam must be modeled explicitly, either as a separate part with a failure criterion or as a tied contact with a release force.

Finally, the occupant packaging data must be validated against the actual vehicle. A small error in H-point position or torso angle can significantly change the airbag-to-occupant gap, which in turn affects the deployment timing and injury outcomes. It is good practice to run a sensitivity study on the H-point position to understand how much variation the design can tolerate.

Summary of Input Data Requirements

With these inputs properly defined and translated into the CAE model, the simulation can proceed with confidence. The next step is to set up the airbag folding and inflation parameters, which will be covered in the following chapter.