4. Injury criteria and targets for side impact

Side impact loading is fundamentally different from frontal loading. The struck-side occupant has very little structural crush space, the door trim and armrest intrude directly into the torso, and the restraint system — typically a side airbag plus a thorax/pelvis bag — must deploy within roughly 10–20 ms to be useful. Because the load path is short and the loading is highly localized, the injury criteria used for side impact are not simply frontal criteria applied with different limits. Each criterion measures a specific physical mechanism, and each has its own transducer, its own signal processing chain, and its own threshold in Euro NCAP and FMVSS 214.

HIC: Head Injury Criterion

HIC is defined from the resultant translational acceleration at the head centre of gravity:

HIC = max over t1,t2 of [ (t2 - t1) * ( 1/(t2-t1) * ∫ a_res dt )^2.5 ]
      with (t2 - t1) ≤ 15 ms
a_res in g, time in seconds

In side impact, the head is usually loaded by the intruding B-pillar, the side curtain airbag, or contact with the struck-side occupant. The 15 ms window is retained, but the acceptance value is lower than the frontal 700 because the head is often struck by a stiffer, more localized surface. Euro NCAP applies HIC₁₅ ≤ 500 for the head in the side mobile deformable barrier (MDB) test, with a lower performance limit of 700 for the "weak" boundary. FMVSS 214 does not use HIC as a pass/fail criterion in the same way; it relies on the Head Injury Criterion only indirectly through the ES-2re dummy's head acceleration, and the primary head-related requirement is that the head must not contact the intruding structure in a way that produces a HIC above the specified limit. In practice, the head is often the least critical region in side impact because the curtain airbag and the head-to-airbag contact geometry are well controlled; the thorax and pelvis dominate.

Chest Deflection and Rib Fracture Metrics

Chest deflection is measured by a potentiometer or IR-TRACC (Infra-Red Telescoping Rod for Assessment of Chest Compression) inside the ribcage of the ES-2re or WorldSID dummy. The deflection is the inward displacement of the rib relative to the spine box, and it is the single most important side impact injury parameter. Euro NCAP requires chest deflection ≤ 42 mm for the ES-2re and ≤ 55 mm for WorldSID, with lower performance limits of 50 mm and 65 mm respectively. FMVSS 214 specifies a maximum chest deflection of 60 mm for the ES-2re in the MDB test.

Rib fracture metrics go beyond peak deflection. The Rib Fracture Criterion (RFC) and the Thoracic Trauma Index (TTI) are used to estimate the number of rib fractures. TTI is defined as:

TTI = 1.4 * AGE + 0.5 * (G_R + G_L) * (MASS / 75)^0.5
G_R, G_L = peak lateral acceleration of the right and left rib (g)
AGE in years, MASS in kg

TTI is a human-surrogate criterion used in FMVSS 214 for the older Hybrid III-based side impact dummy; the current FMVSS 214 uses the ES-2re and the Rib Deflection and VC (Viscous Criterion) as the primary thoracic metrics. VC combines deflection and deflection rate:

VC = max over time of [ dD(t)/dt * D(t) / D_max ]
D = chest deflection (m), D_max = 1.0 m for normalization
VC in m/s

Euro NCAP applies VC ≤ 1.0 m/s as the thoracic viscous criterion limit. The physical meaning is that a fast, shallow compression can be as injurious as a slow, deep compression because the viscous response of the tissue is what tears the rib cage and underlying organs.

Abdominal Pressure and Pelvic Force

Abdominal injury in side impact is dominated by compression and shear of the liver, spleen, and kidneys. The ES-2re and WorldSID measure abdominal pressure via a fluid-filled or pressure-sensing abdomen. Euro NCAP requires abdominal pressure ≤ 2.5 bar (approximately 250 kPa) for the ES-2re, with a lower performance limit of 3.5 bar. FMVSS 214 specifies a maximum abdominal pressure of 2.5 bar as well. The abdomen is particularly sensitive to the armrest and door trim geometry, and a common failure mode is a localized pressure spike from a stiff armrest that is not covered by the side airbag.

Pelvic force is measured by load cells in the iliac wing of the dummy. Euro NCAP requires pelvic force ≤ 3.0 kN for the ES-2re and ≤ 4.0 kN for WorldSID, with lower performance limits of 4.0 kN and 5.0 kN. FMVSS 214 specifies a maximum pelvic force of 4.0 kN. The pelvis is the primary load path into the occupant in side impact, and pelvic force is often the first criterion to be exceeded when the door trim is too stiff or the side airbag does not cover the pelvis.

Euro NCAP and FMVSS 214 Thresholds at a Glance

From Criteria to Simulation Targets

In a MADYMO side impact model, each criterion is extracted from a specific output request. The head acceleration is taken from the head rigid body's accelerometer node; chest deflection from the IR-TRACC spring element between the rib and spine box; abdominal pressure from the abdomen pressure sensor; pelvic force from the pelvic load cell. A practical workflow is to define the target values as a table in the model input, run the baseline simulation, and then compare the peak values against the thresholds. If the chest deflection exceeds 42 mm, the first design levers are the side airbag pressure, the vent size, and the armrest stiffness; if the pelvic force exceeds 3.0 kN, the door trim stiffness and the pelvis bag coverage are the primary levers.

The criteria above are not independent. A stiffer door trim raises pelvic force and chest deflection simultaneously; a larger side airbag reduces chest deflection but can increase abdominal pressure if the bag loads the abdomen directly. The engineering task is to find the combination of door trim stiffness, airbag pressure, vent size, and bag geometry that keeps all five criteria below their performance limits with margin. In MADYMO, this is typically done with a parameter study or an optimization loop, using the criteria table as the objective function and the design variables as the inputs.