📚 Course ContentsChapter 3 Leg impact simulation workflow

3. Leg impact simulation workflow

Leg impact simulation in LS-DYNA addresses two distinct injury mechanisms that occur at different heights on the vehicle front-end. The lower legform evaluates bumper-level loading, where the dominant risks are tibia fracture and knee ligament rupture. The upper legform evaluates bonnet leading-edge loading, where the dominant risk is femur fracture. Because the two models differ in geometry, mass, impact energy, and output metrics, they are treated as separate load cases with separate acceptance criteria. This section walks through the complete workflow for both, from model selection to metric extraction.

Lower Legform: Model and Impact Conditions

The lower legform represents a pedestrian's leg from knee to ankle. In the standard configuration it consists of two rigid cylindrical segments (representing the tibia and femur) connected by a deformable knee joint, with a total mass of approximately 13.1 kg and a length of about 926 mm. The model is positioned vertically and propelled horizontally into the vehicle bumper at a specified velocity.

Key impact parameters for the lower legform:

Impact velocity: typically 40 km/h (11.1 m/s), representing the vehicle's forward speed at the moment of pedestrian contact.
Impact height: the legform centerline is aligned with the bumper reference line, usually 25–50 mm above the ground clearance plane.
Impact direction: perpendicular to the vehicle's longitudinal axis, in the horizontal plane.
Knee joint: modeled with a rotational spring-damper to capture ligament behavior; the knee shear and bending stiffness are calibrated to represent human knee biomechanics.

In LS-DYNA, the legform is typically represented by a combination of rigid bodies and discrete elements. The tibia and femur are *MAT_RIGID parts, while the knee joint uses *CONSTRAINED_JOINT_STIFFNESS or a set of *ELEMENT_DISCRETE springs. The vehicle front-end is modeled with shell elements and a validated material model such as *MAT_PLASTIC_KINEMATIC for the bumper fascia and *MAT_RIGID for the bumper beam.

Contact is defined using *CONTACT_AUTOMATIC_SURFACE_TO_SURFACE between the legform and the vehicle. A friction coefficient of 0.3–0.5 is typical for the bumper surface. The simulation runs for 50–80 ms, which is sufficient for the legform to rebound and for all injury metrics to peak.

Upper Legform: Model and Impact Conditions

The upper legform represents the thigh and pelvis region. It is a single rigid cylinder with a mass of approximately 9.5 kg and a length of about 350 mm, with a hemispherical or cylindrical impactor face. It is used to assess the bonnet leading edge, where the femur is at risk of fracture from the concentrated contact force.

Key impact parameters for the upper legform:

Impact velocity: typically 40 km/h, same as the lower legform.
Impact height: the legform centerline is aligned with the bonnet leading edge, which is determined by the vehicle's geometry (typically 600–800 mm above ground).
Impact direction: horizontal, perpendicular to the vehicle's longitudinal axis.
Impactor face: the contact surface is a rigid cylinder with a radius of 50 mm, representing the femur's cross-section.

The upper legform is modeled as a single *MAT_RIGID part with a defined mass and initial velocity. The bonnet leading edge is modeled with shell elements and a material model that captures both elastic and plastic deformation, such as *MAT_PIECEWISE_LINEAR_PLASTICITY. Contact is defined with *CONTACT_AUTOMATIC_SURFACE_TO_SURFACE, and the simulation duration is typically 40–60 ms.

Output Metrics and Acceptance Criteria

The two load cases produce different injury metrics, and each has its own threshold. The table below summarizes the primary outputs and their typical acceptance limits based on Euro NCAP and GTR No. 9 requirements.

Load Case	Metric	Symbol	Typical Limit	Measurement Location
Lower legform	Tibia acceleration	a_tibia	≤ 150 g	Upper tibia accelerometer
Lower legform	Knee bending angle	θ_knee	≤ 15°	Knee joint rotation
Lower legform	Knee shear displacement	d_shear	≤ 6 mm	Knee joint translation
Upper legform	Femur force	F_femur	≤ 5 kN	Contact force at impactor
Upper legform	Bending moment	M_bend	≤ 300 N·m	Impactor moment arm

In LS-DYNA, these metrics are extracted using *DATABASE_ASCII_option or *DATABASE_BINARY_D3PLOT and post-processed in LS-PrePost. For the lower legform, tibia acceleration is read from an accelerometer element or a *ELEMENT_SEATBELT_ACCELEROMETER attached to the tibia part. Knee bending angle and shear displacement are computed from the relative motion of the femur and tibia rigid bodies using *DATABASE_HISTORY_NODE or *DATABASE_HISTORY_RIGID_BODY. For the upper legform, the contact force is obtained from the contact interface using *DATABASE_FORCE or *CONTACT_FORCE_TRANSDUCER, and the bending moment is calculated from the force and the moment arm defined by the impactor geometry.

Practical tip: Always verify that the legform's initial velocity is applied to the correct rigid body and that the contact definition includes both the legform and the vehicle's front-end. A common error is missing contact between the legform and the bumper beam, which leads to unrealistically low forces and no injury metrics.

Workflow Summary and Common Pitfalls

The complete workflow for both load cases follows a consistent sequence: define the legform model, position it relative to the vehicle, apply initial velocity, define contact, run the simulation, and extract metrics. However, the two cases differ in model geometry, impact height, and output metrics, so they must be set up as separate LS-DYNA input decks.

Common pitfalls include:

Incorrect impact height: If the legform is positioned too high or too low, the contact force and injury metrics will not represent the intended load case. Always verify the height against the vehicle's reference lines.
Missing knee joint stiffness: The knee joint must be calibrated to represent human ligament behavior. Using a rigid joint will produce unrealistically high bending angles and shear displacements.
Insufficient simulation time: If the simulation ends before the legform rebounds, the peak metrics may be missed. Run for at least 60 ms for the lower legform and 50 ms for the upper legform.
Contact penetration: Check for excessive penetration between the legform and the vehicle. If penetration exceeds the element thickness, adjust the contact stiffness or refine the mesh.

The SVG below illustrates the two load cases side by side, showing the legform models, impact heights, and the primary output metrics for each.

The diagram above contrasts the two load cases: the lower legform is a two-segment model with a knee joint, positioned at bumper height, while the upper legform is a single rigid cylinder positioned at the bonnet leading edge. The output metrics differ accordingly—tibia acceleration and knee angles for the lower legform, femur force and bending moment for the upper legform.

Important: The acceptance limits shown in the table and SVG are based on Euro NCAP and GTR No. 9 requirements. Always confirm the applicable regulation for your target market, as limits may vary slightly between standards. Additionally, the legform models must be validated against physical test data before use in production simulations.

When setting up both load cases in LS-DYNA, it is good practice to use a single input deck with two separate *INCLUDE files—one for each legform—and a shared vehicle model. This ensures consistency in the vehicle representation and reduces the risk of modeling discrepancies between the two cases. The simulation results should be post-processed in LS-PrePost, where you can plot the time histories of the injury metrics and compare them against the acceptance limits.