A Study On The Biomechanical Of Occupant Lower Extremity Injuries In Car Frontal Impact Based On Human Body Model

Extensive use of seat belts and airbags in passenger cars has greatly reduced thenumber and severity of injuries to the occupant’s head and chest in traffic accidents.Based on the survey of the NASS database from1999to2001, lower extremityinjuries account for36%of all moderate-to-fatal injuries (AIS2+) injuries resultingfrom frontal crashes and46%of the life-years lost to injury (LLI) of occupants inairbag-equipped vehicles. The lower extremity injuries are the most frequently injuredbody region of AIS2+in frontal crashes. Lower extremity injuries are usually not lifethreatening, but they require long recovery times, the lower extremity injuries oftenresult in great pain and significant loss of body function and even in long term orpermanent disability. This problem can not be ignored in vehicle safety research.In order to better understand the injury mechanism and tolerance of lowerextremity in frontal impact, a FE model of the lower extremity in sitting posture wasdeveloped based on the anatomy of a50th percentile male adult’s lower extremityusing Hyperworks11.0code. The model consisted of bones (pelvis, sacrum, femur,patella, tibia/fibula, foot) and soft tissues (knee ligaments, tendons, hip joint capsule,articular cartilage, fresh and skin, etc.). The resulting model consists of97components,65,626elements,40,155deformable solid elements,25,263shellelements, and208discrete elements. Nine PMHS tests were used to validate againstthe FE model from component to complex. According to the comparison of the resultsbetween simulations and PMHS tests, the bio-fidelity of this FE model is acceptableand can be used to study the injury tolerance of the lower extremity in frontal impact.Based on the validated lower extremity FE model, several researches wasconducted to study the injury mechanism，tolerance and injury prevention of occupantlower extremity in car frontal impact. Such as, femur fracture has been carried out tolearn the femur injury mechanism and tolerance under axial compression-externalbending loading; A parameter analysis was conducted to investigate the influence ofhip posture on the pelvis injury tolerance under knee axial impact loading; Theorthogonal experimental design was conduct to parameters analysis the foot/ankleinjuries in frontal impact; a study of the occupant lower extremity injurycharacteristics in three overlap frontal impact.Firstly, validated lower extremity FE model was used to study the influence of femur physiological characteristics to injure tolerance in bending test. Then a study offemur fracture failure under axial compression-external bending loading has beencarried out using an analytical model of curved beam and virtual tests. The resultsshow that: the location of bony fractures and tolerance of femur depend on the levelsof both external bending and axial compression. Under axial compression-condylebending load, two of six virtual tests occurred femoral neck fracture in loading of8.0kN-100Nm and8.9kN-0Nm, and the tolerance of femur is285and296Nmrespectively. In a lower pre-axial compression (below6kN), other four virtual testsoccurred fracture at the femur shaft segment with a distance of171to134.9mm fromthe distal femur, and the tolerance of the femur are381Nm~443Nm. while, underaxial compression-mid-shaft impact load, two of six virtual tests occurred femoralneck fracture in loading of8.0kN-0.64kN and8.9kN-0kN, and the tolerance of femuris307.2Nm and296Nm respectively. Other four virtual tests occurred femoralmid-shaft fracture in a lower pre-axial compression (below6kN), and the tolerance ofthe femur are382Nm-400.7Nm. The results indicated that the femur fractures alwaysoccurred at femoral neck in axial impact tests on the knee-thigh complex (Rupp2002),but in real world car frontal impacts the femoral shaft fractures can be observedfrequently under lower axial compression than10kN.A parameter analysis was conducted to investigate the influence of hip postureon pelvis injury tolerance under knee axial impact load using the validated lowerextremity model. The results show that pelvis fracture location and failure valuedepend on the hip gesture under the knee axial impact load. This is because of thevaried strength of contact interface between the femoral head and acetabulum wall.With the increasing of hip flexion and abduction angles, pelvis fractures locationtransferred from ilium to the acetabulum. Moreover, the failure value of pelvisincreases in a range of13.5%~34.4%as the hip flexion angle α increasing, while thefailure value of pelvis varies in a range of6.0%~20.9%with increasing of the hipabduction angle.The foot/ankle has been reported as the most vulnerable region of lowerextremity. Combined the FE lower extremity model and upper torso of ATD model, aoccupant-restraint system FE model was developed based on a Prototype vehicle ofChery Automotive company. The orthogonal experimental design was conduct toparameters analysis the foot/ankle injuries in frontal impact, the parameters includeangel of instrument panel, pedal intrusion in X direction, pedal intrusion in Zdirection, pedal inversion/eversion angle and pedal dorsiflexion angle. The results show that: limb and foot/ankle injuries are occurred in eleven of sixteen virtual tests.the the most sensitive parameter to tibial axial force is pedal intrusion in Z direction;As the pedal intrusion changes from0~150mm in X direction, the tibia resultantbending moment and Tibia index has the largest fluctuation; With the increasing ofpedal dorsiflexion angle and pedal intrusion in X direction, the dorsiflexion angle ofankle grows. While, with the expanding of instrument panel angel, the dorsiflexionand injury risk of ankle will decrease. Furthermore, the phenomenon was found thatthe Tibia index could not reflect the risk foot/ankle injuries, and the directly cause offoot/ankle injury are over-angle of ankle inversion/eversion and dorsiflexion.To investigate the occupant lower extremity injury characteristics in differentoverlap frontal impact, above all, three car frontal impact simulations (full widthfrontal impact,40%and25%offset frontal impact) was calculated by a validatedwhole car FE model. Many difference of crashworthiness was observed in terms of caracceleration, intrusion magnitude and location of passenger compartment in three typefrontal impacts. The less overlap in frontal impact, the bigger intrusion of passengercompartment and the smaller car acceleration will occur. Furthermore, the results ofcar acceleration, passenger compartment intrusion in three type frontal impacts wereused to study the occupant lower extremity injury characteristics as boundarycondition. The results show that：Different overlap frontal impact will generateddifferent lower extremity injuries. In25%offset frontal impact, the huge intrusion ofinstrument panel and rest pedal result in the fracture of left femoral neck and anklePilon fractures in both sides; In full width frontal impact, there is posterior talotibialligament avulsion in the tibial medial condyle. While non injury was observed in40%offset frontal impact. Moreover, the result imply that: the car accelerator impulse iscorrelative to the lower extremity injuries; the influence of passenger compartmentintrusion to leg and foot/ankle is nonlinear; and intrusion of the accelerator pedal mayresult in bigger dorsiflexion than rest pedal, and easier to generate the injury of talarfracture and posterior talotibial ligament avulsion.To sum up, the lower extremity FE model was proofed to be a efficient tool tolower extremity biomechanical research. Research results of femur, pelvis andfoot/ankle provide useful reference to the injury mechanism, tolerance and injuryprevention of lower extremity in car frontal impact.