A Numerical Investigation of Biomechanical Response and Injury of Occupant Lower Extremities in Automotive Frontal Impact Scenario

Few countermeasures currently exist for lower limb injuries sustained in automobile crashes despite the lower extremity being one of the most frequently injured regions. One factor limiting countermeasure development has been the lack of a biofidelic surrogate. The objective of this research was to develop numerical tools, namely an advanced Finite Element (FE) human model and a THOR-Metric FE dummy model, to better enable the development and assessment of countermeasure designs. As an example of their utility, these tools were then applied in a vehicle frontal crash environment to develop a better understanding of the injury risk assessment capabilities of each surrogate, to identify differences and similarities in their response, and to recommend a systematic approach for interpreting the differences as they relate to countermeasure design.
A detailed and biofidelic occupant Lower Extremity (LEX) FE model was developed based on the medical image data of a 50th percentile male volunteer in a sitting posture. Appropriate constitutive material models were assigned to each component with the corresponding parameters identified as within the ranges of published test data. Loading cases were simulated and the model injury prediction capabilities were validated at both regional and global levels to the latest corresponding cadaveric test data. These validations focused on the predictions of frontal Crash-Induced Injuries (CII) recorded in vehicle crashes, which included the femoral mid-shaft/head fractures, tibia distal-third section fractures, and knee ligament (e.g. Posterior Cruciate Ligament (PCL)) ruptures. Sensitivity studies were performed using the validated model for investigation of injury surfaces as a function of the joint angle in frontal impacts.
The knee-thigh region of the THOR-NT FE dummy was updated to the THOR-Metric V2.0.6 beta specification. The certification and biofidelity validations in the knee-thigh region were conducted and the model responses matched the test data well in terms of response curve shape and magnitude.
The human and dummy model LEX injury responses in vehicle frontal crash scenarios were compared numerically in an identical buck environment with the same sitting position. The force/moment histories from the pelvis, femur and tibial load cells and the knee displacements were extracted from both surrogates and used to calculate the injury risks. Results showed the LEX kinematics differed between the two surrogates in the severe scenario (with toepan intrusion) due to the modeling and stiffness discrepancies in the abdomen and the pelvis flesh, the knee joint and the ankle areas with a load response difference of 17~150%, while both surrogates shared similar kinematics in the less-severe scenario (without toepan intrusion) with a load response difference of 9~66%. A proposed systematic approach of using both surrogates for countermeasure designs and interpreting their responses for better understanding of the injuries a human would sustain was demonstrated. Several discrepancies between the two surrogates revealed opportunities for future improvement in order to better align the surrogate injury responses. Finally, the complementary use of both the GHBMC and the THOR surrogates is recommended to improve the understanding of occupant response and injury for research, development, and certification purposes.