Influence Of Service Conditions On Knee Ligament Injury Response In Car-to-Pedestrian Collisions

Pedestrian impact protection is an important topic in road traffic safety.Knee ligament injuries,as a type of lower extremity injury,are common in car-to-pedestrian collisions.Using human body models to simulate car-to-pedestrian collisions,injury characteristics of human knee ligaments can be studied at the tissue level,which is important for injury mechanism understanding and virtual testing.To improve the biofidelity of human body models for future pedestrian impact protection,this study focused on the mechanical properties of knee ligaments,conducting experimental and simulation studies around service conditions.On the experimental side,this study selected the porcine stifle joint as a substitute for medial collateral ligament tests.First,the mechanical properties of different fiber bundles of the medial collateral ligament were compared experimentally.It was found that the anterior and posterior bundles had significantly different elastic modulus and failure strain.A method was further proposed to estimate the material properties of different fiber bundles.Hence,subsequent experiments can measure only the entire medial collateral ligament without dealing with the anterior and posterior bundles separately.Then,to obtain the ligament material properties under quasi-real service conditions,the mechanical properties of the medial collateral ligament were measured under four test conditions considering the temperature and cyclic loading variations caused by walking.It was found that temperature and cyclic loading had a significant interaction effect.Based on the structure of ligament fibrils,a microstructural model considering the lateral force transmission between fibrils was proposed.The influence of service conditions on fibril structure was analyzed,explaining the mechanism of the interaction effect.The reasonableness of the microstructural model was also verified on the experimental results of different fiber bundles.On the simulation side,this study used the finite element model of a car-to-pedestrian collision to investigate the effect of the material property changes of ligaments induced by service conditions on the risk of knee ligament injury in pedestrians.First,the differences in simulation results under the influence of temperature,cyclic loading,and strain rate were compared.It was found that the effect of strain rate was related to the collision speed,with the maximum effect on the risk of ligament injury at 25 km/h being8% and dropping to less than 2% at 40 km/h.While the effect of temperature and cyclic loading was less related to speed,and the maximum effect on the risk of ligament injury was 10%.Therefore,the effect of temperature and cyclic loading should be of more concern in high-speed collisions.There are large errors in biomaterial test results.Simulations using the upper and lower boundary values of the test results revealed that the impact of experimental error on the risk of ligament injury was comparable to the crash speed.To improve the reliability of simulation results,further uncertainty analysis was performed on car-to-pedestrian collision simulations.The uncertainty in ligament material parameters due to individual differences and service conditions,and the uncertainty in crash conditions due to emergency braking were considered.The results showed that failure strain and collision speed had a large influence on pedestrian knee kinematics and deserved attention.The results of uncertainty analysis can facilitate the reconstruction of typical accident scenarios and the calculation of injury risk functions,further improving the reliability of crash safety analysis and virtual testing.