3. Crash dynamics for child occupants

Child occupants do not behave like scaled-down adults. Their mass distribution, skeletal ossification, ligament laxity, and geometric proportions shift both the kinematics of the body during a crash and the load paths through which energy reaches vulnerable tissues. This chapter examines frontal, side, and rear impact scenarios, tracing how the restraint system, seat structure, and occupant anatomy interact to determine injury outcome.

3.1 Reference Frame and Sign Conventions

All descriptions in this chapter use the vehicle coordinate system defined by ISO 8855: the x-axis points forward (direction of travel), the y-axis points to the driver's left, and the z-axis points upward. Acceleration is positive when it acts in the direction of the positive axis. In a frontal crash, the vehicle decelerates, so the occupant experiences a negative x-acceleration relative to the vehicle (forward excursion). In a rear impact, the vehicle accelerates forward, producing positive x-acceleration and rearward occupant excursion. Side impacts are described along the y-axis, with the struck side defined by the impact direction. These conventions are maintained consistently in the discussion and in the accompanying figure.

3.2 Frontal Impact Kinematics

In a frontal collision, the vehicle undergoes rapid deceleration—typically 20–40 g at the sill over 80–120 ms for a 56 km/h full-frontal rigid barrier test. The occupant, restrained by the seat belt and possibly a child restraint system (CRS), continues forward until the restraint couples the body to the vehicle's deceleration. The sequence unfolds in distinct phases:

1. Pre-impact and initial deceleration (0–30 ms): The occupant is still moving at the pre-crash velocity. The belt begins to engage, and the CRS harness or vehicle belt starts to load.
2. Restraint coupling (30–70 ms): Belt forces rise, and the occupant's torso and pelvis are progressively decelerated. The head, being the most massive and least restrained segment in a young child, lags behind, creating a differential velocity between head and torso.
3. Head excursion and possible contact (70–120 ms): The head rotates forward and downward, potentially striking the CRS shell, the vehicle seatback, or an adjacent surface. Neck loads peak during this phase.
4. Rebound (120–200 ms): The restraint forces reverse, and the occupant moves rearward into the seatback.

For a child in a rear-facing CRS, the kinematics differ fundamentally: the CRS shell distributes deceleration over the child's entire back, and the head is supported by the shell, reducing neck tension. For a forward-facing child, the harness loads the shoulders and pelvis, but the head remains free to move forward, concentrating bending moments in the cervical spine.

3.3 Injury Mechanisms in Frontal Impact

The primary injury mechanisms for child occupants in frontal crashes are:

• Head injury: Contact with the CRS shell or vehicle interior produces focal skull fracture or diffuse brain injury. In forward-facing children, the immature neck allows greater head excursion, increasing the risk of contact.
• Neck injury: Tensile and bending loads on the cervical spine can cause ligamentous injury or, in severe cases, spinal cord injury without radiographic abnormality (SCIWORA). The child's proportionally larger head and weaker neck musculature amplify these loads.
• Thoracic and abdominal injury: Improperly positioned belt or harness can load the abdomen, causing liver or spleen laceration. The "seat belt syndrome" (chance fracture of the lumbar spine plus intra-abdominal injury) is a classic pattern in older children using adult belts without a booster.
• Lower extremity injury: Femur fractures can occur from knee contact with the dashboard or from loading through the CRS.

3.4 Side Impact Kinematics and Load Paths

Side impacts account for a disproportionate share of child fatalities because the distance between the occupant and the striking object is small, leaving little crumple zone. In a near-side impact (struck side adjacent to the child), the vehicle door intrudes into the occupant compartment. The child's kinematics depend on the restraint type:

• CRS with side wings: The head is contained by the wing, and the shell transfers load to the vehicle seat structure. However, the wing itself may contact the child's head, causing focal injury.
• Booster seat with belt positioning: The child's torso is restrained by the belt, but the head is free to move laterally. The head may strike the intruding door or the window.
• No restraint: The child is ejected or strikes the door directly, with high risk of head and thoracic injury.

Load paths in side impact run from the door through the CRS shell or the child's body to the seat structure and floor. The seat's ability to resist lateral bending and the presence of side airbags significantly influence outcome. For a child in a CRS, the shell's stiffness and the harness's ability to keep the child contained are critical.

3.5 Rear Impact Kinematics and Injury Mechanisms

Rear impacts are less frequent but can produce severe neck injuries, particularly in young children. In a rear impact, the vehicle accelerates forward, and the occupant's torso is pushed forward by the seatback. The head, due to inertia, lags behind, causing the neck to extend (whiplash). For a child in a forward-facing CRS, the CRS shell moves with the vehicle seat, and the child's head extends relative to the torso. The immature cervical spine, with its shallow facet joints and weak ligaments, is vulnerable to hyperextension injury.

For a rear-facing CRS, the child's back is against the shell, and the head is supported, so the neck is not subjected to the same extension moment. This is one reason why rear-facing restraints are recommended for as long as possible. In a rear impact, the primary load path is from the vehicle seatback through the CRS shell to the child's back and head.

3.6 Comparative Summary of Impact Directions

3.7 Engineering Example: Estimating Head Excursion in a Frontal Sled Test

Consider a forward-facing CRS subjected to a 50 km/h frontal sled pulse with a peak deceleration of 30 g. The child dummy has a head mass of 3.5 kg, and the harness stretches 15 mm under load. The head excursion can be approximated by integrating the relative velocity between head and torso. Assuming the torso decelerates with the sled and the head follows a pendulum motion with a pivot at the neck, the head's forward displacement d can be estimated from the energy balance:

0.5 * m_head * v_rel^2 = 0.5 * k_eff * d^2
where v_rel is the relative velocity at the onset of restraint coupling,
k_eff is the effective stiffness of the neck and harness system.

For a typical v_rel of 8 m/s and k_eff of 2000 N/m, d ≈ 0.25 m. This exceeds the clearance to the front seatback in many vehicles, indicating a high risk of head contact. This simplified calculation illustrates why rear-facing restraints, which eliminate the pendulum motion, are preferred for young children.

3.8 Load Path Visualization

The following SVG illustrates the load paths and occupant kinematics for the three impact directions. It shows the child occupant within a CRS, the vehicle seat, and the primary force vectors and deformation paths.

The figure shows how the same child occupant, when placed in different impact directions, experiences distinct motion patterns and load paths. In frontal impact, the head moves forward relative to the torso, loading the neck in tension and bending. In side impact, the head moves laterally toward the intruding door, and the CRS shell must absorb and redirect the intrusion load. In rear impact, the head extends rearward, loading the neck in extension. These differences drive the design of CRS side wings, harness geometry, and the recommendation for extended rear-facing use.

3.9 Key Takeaways for Validation

When validating child occupant protection, engineers must consider the specific impact direction and the corresponding injury criteria. Frontal validation focuses on head excursion, neck tension, and chest deflection. Side validation emphasizes head containment and thoracic compression. Rear validation examines neck extension moments and head-to-seatback contact. The load paths identified in this chapter inform both the design of restraint systems and the selection of appropriate test dummies and instrumentation. Understanding these dynamics is essential for interpreting crash test results and for making informed decisions about child restraint selection and vehicle design.