4. Multi-impact load case definition

Multi-impact vehicle design optimization begins with a precise, machine-readable definition of every load case that the body structure must survive. In PAM-CRASH, a load case is not merely a barrier position and a velocity; it is a complete bundle of impactor geometry, initial kinematics, contact definitions, boundary conditions, sequencing logic, and output requests. If any of these elements is ambiguous, the resulting CAE predictions become non-comparable across design iterations, and optimization loops will chase numerical noise rather than structural performance. This chapter establishes a consistent framework for defining frontal, side, rear, and pole impacts, and for chaining them into multi-impact sequences with controlled dwell time and boundary conditions.

4.1 Reference Frame and Sign Conventions

Before any load case is written, fix the global coordinate system. The convention used throughout this course is: X points forward along the vehicle longitudinal axis (positive toward the front bumper), Y points to the vehicle's left, and Z points upward. All impactor velocities, displacements, and force outputs are reported in this frame unless a local coordinate system is explicitly declared. When a load case requires a reversed direction (for example, a rear impact where the barrier moves toward the vehicle), the velocity vector is negated rather than redefining the axis. This single rule prevents the most common source of sign errors in multi-impact decks.

4.2 Frontal Impact Definition

A frontal load case typically uses a rigid wall or an offset deformable barrier (ODB). The essential inputs are the barrier surface, the initial vehicle velocity, the contact thickness, and the friction coefficient. A representative full-frontal setup is shown below.

/LOADCASE/FRONTAL_56
  IMPACTOR = RIGID_WALL
  VELOCITY = (16.67, 0.0, 0.0)   ! 60 km/h along +X
  CONTACT_THICKNESS = 1.0
  FRICTION = 0.3
  GRAVITY = (0.0, 0.0, -9.81)
  TERMINATION_TIME = 0.120
  OUTPUT = (ENERGY, FORCE, DISPLACEMENT)

The velocity magnitude of 16.67 m/s corresponds to 60 km/h. The termination time must be long enough for the vehicle to rebound and for the kinetic energy to plateau; 120 ms is a practical starting value for a full-frontal case. The contact thickness controls when nodes are detected as penetrating the barrier and directly affects the initial force spike. A value of 1.0 mm is typical for shell-based body models, but it must be consistent with the smallest element edge length in the front structure.

4.3 Side Impact Definition

Side impact introduces a moving deformable barrier (MDB) that strikes the vehicle laterally. The barrier is usually modeled as a deformable honeycomb block mounted on a rigid trolley. The critical parameters are the barrier mass, the impact velocity, the vertical alignment of the barrier face with the vehicle's sill, and the yaw angle of the barrier relative to the vehicle.

/LOADCASE/SIDE_MDB
  IMPACTOR = MDB_HONEYCOMB
  BARRIER_MASS = 950.0
  VELOCITY = (0.0, -13.89, 0.0)   ! 50 km/h along -Y
  YAW_ANGLE = 0.0
  ALIGNMENT_Z = 0.0               ! barrier center at sill height
  CONTACT_THICKNESS = 1.5
  FRICTION = 0.2
  TERMINATION_TIME = 0.100

Because the barrier moves along −Y, the vehicle's left side is loaded first. The alignment height is referenced to the barrier's geometric center; a mismatch of even 20 mm can shift the load path from the sill to the door beam and change the intrusion pattern significantly. The yaw angle is kept at zero for a perpendicular side impact, but a non-zero value is used when simulating oblique pole or car-to-car side scenarios.

4.4 Rear Impact Definition

Rear impact is structurally similar to frontal impact but with reversed velocity and a different barrier stiffness. A moving rigid wall or a deformable rear barrier is positioned behind the vehicle. The key difference is that the rear structure is typically less stiff than the front, so the termination time may need to be longer to capture the full deformation and rebound.

/LOADCASE/REAR_80
  IMPACTOR = RIGID_WALL
  VELOCITY = (-22.22, 0.0, 0.0)   ! 80 km/h along -X
  CONTACT_THICKNESS = 1.0
  FRICTION = 0.3
  TERMINATION_TIME = 0.150

The 80 km/h rear impact is a severe case used to evaluate fuel system integrity and seat back strength. The longer termination time of 150 ms reflects the softer rear crush zone and the need to observe the rebound phase.

4.5 Pole Impact Definition

Pole impact is a localized loading case that produces high intrusion over a small area. The pole is modeled as a rigid cylinder with a defined radius, usually 254 mm for side pole and 150 mm for frontal pole. The pole is fixed in space, and the vehicle is given an initial velocity toward it. The contact between the pole and the vehicle shell must be carefully defined because the small contact area can cause numerical instabilities if the mesh is too coarse.

/LOADCASE/SIDE_POLE
  IMPACTOR = RIGID_CYLINDER
  RADIUS = 254.0
  VELOCITY = (0.0, -8.33, 0.0)   ! 30 km/h along -Y
  CONTACT_THICKNESS = 2.0
  FRICTION = 0.1
  TERMINATION_TIME = 0.080

The pole radius and contact thickness are the two parameters that most strongly influence the intrusion depth. A larger contact thickness reduces the initial penetration spike but can artificially soften the contact. For side pole, a 2.0 mm contact thickness is a reasonable compromise when the door outer panel has an element size of 5–8 mm.

4.6 Sequencing and Dwell Time

Multi-impact sequences require explicit ordering and a dwell time between events. In PAM-CRASH, this is handled by restarting the simulation or by using a multi-stage load case definition. The dwell time is the period during which the vehicle is allowed to settle after one impact before the next impact begins. A typical sequence is frontal impact, 50 ms dwell, then side impact. The dwell time must be long enough for the vehicle to reach a quasi-static state but short enough to remain physically realistic.

/SEQUENCE/MULTI_IMPACT
  STAGE_1 = FRONTAL_56
  DWELL_1 = 0.050
  STAGE_2 = SIDE_MDB
  DWELL_2 = 0.030
  STAGE_3 = REAR_80

During the dwell phase, the vehicle retains its residual velocity and deformation. Boundary conditions must be updated between stages: for example, the frontal barrier is removed, and the side barrier is activated. If the vehicle is constrained by a sled or a fixture, those constraints must remain consistent across all stages.

4.7 Boundary Condition Setup

Boundary conditions in multi-impact analysis fall into three categories: fixed constraints, moving constraints, and contact interfaces. Fixed constraints are applied to nodes that represent the test fixture, such as the rear mounting points during a frontal impact. Moving constraints are used when the vehicle is on a sled that accelerates or decelerates. Contact interfaces define how the vehicle interacts with the ground, the barrier, and itself.

A common mistake is to apply a fixed constraint to a node that later becomes part of a contact interface. This creates an artificial stiffening effect. The recommended practice is to define boundary conditions on a dedicated set of nodes that are not part of any deformable body, or to use a tied interface that can be released between stages.

4.8 Visual Overview of the Multi-Impact Workflow

The following diagram summarizes the relationship between the four impact types, their sequencing, and the boundary condition transitions. It is intended as a reference for building and checking a multi-impact deck.

4.9 Practical Checklist for Load Case Definition

The following checklist is intended to be used directly when preparing a multi-impact deck. It captures the parameters that most frequently cause errors.

Use this table as a starting point, not as a fixed prescription. The correct values depend on the vehicle class, the mesh density, and the specific regulatory or internal target. Always document the rationale for any deviation from these defaults, because the optimization loop will treat every parameter as a design variable and will exploit any inconsistency.

With the load cases defined and sequenced, the next step is to integrate them into the optimization workflow, where each design iteration must satisfy all impact cases simultaneously. The quality of that optimization depends directly on the precision of the definitions established here.